Principles of Inheritance and Variation - Class 12 Biology - Chapter 4 - Notes, NCERT Solutions & Extra Questions
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Extra Questions - Principles of Inheritance and Variation | NCERT | Biology | Class 12
A monohybrid cross between two plants, one having 22 cm long internodes and the other having 12 cm long internodes, produced F1 hybrids all of 18 cm long internodes. This is a case of:
A. incomplete dominance
B. complete dominance
C. multiple allelism
D. recessive dominance
The correct option is A. incomplete dominance.
Incomplete dominance is a genetic scenario where neither allele is completely dominant over the other, leading to a blended phenotype in the offspring. In this specific example, the F1 hybrids showing 18 cm long internodes (a value between the two parent internode lengths of 22 cm and 12 cm) is a classic indication of incomplete dominance. This blend of traits results from neither of the parent alleles being able to exert full dominance, resulting in a phenotype that is intermediate to those of the parents.
A cross is made between true breeding lines of pea plants with axillary (A) and terminal (a) flowers. What is the ratio between the number of possible genotypes of homozygous and heterozygous plants in $\mathrm{F}_{2}$ progeny?
A) $3: 1$ B) $1: 2: 1$ C) $1: 1$ D) $1 : 1 : 2$
The correct option is C) $1: 1$
Parental Genotypes:
Axillary flowers parent has genotype: $$AA$$
Terminal flowers parent has genotype: $$aa$$
$F_{1}$ Generation Cross: $$AA \times aa$$
All offspring in the $F_1$ generation are heterozygous: $$Aa$$.
$F_{2}$ Generation Cross: $$Aa \times Aa$$
The $F_2$ progeny genotypic distribution in a monohybrid cross is:
Homozygous dominant (AA): $$\frac{1}{4}$$
Heterozygous (Aa): $$\frac{1}{2}$$
Homozygous recessive (aa): $$\frac{1}{4}$$
The total proportion of homozygous individuals (combining AA and aa) is:
$$\frac{1}{4} + \frac{1}{4} = \frac{1}{2}$$
The proportion of heterozygous individuals is:
$$\frac{1}{2}$$
Thus, the ratio of homozygous to heterozygous individuals in the $F_2$ generation is:
$$1 : 1$$
Thus, the correct option is C) $1: 1$.
If two persons with 'AB' blood group marry and have a sufficiently large number of children, these children could be classified as 'A' blood group, 'AB' blood group, and 'B' blood group in a 1:2:1 ratio. Modern techniques of protein electrophoresis reveal the presence of both 'A' and 'B' type proteins in 'AB' blood group individuals. This is an example of:
A Codominance
B Incomplete dominance
C Partial dominance
D Complete dominance
The correct answer is A) Codominance.
In codominance, both alleles in a gene pair are fully expressed in a heterozygous organism. This results in a phenotype where both traits can be seen distinctly. This scenario differs markedly from incomplete dominance, where the heterozygote exhibits a blended phenotype of the two alleles, not showing either one fully.
Complete dominance is observed when only one of the alleles is visible in the phenotype of the heterozygote, with the other allele being completely masked.
In the given question, the fact that individuals with an 'AB' blood group exhibit both 'A' and 'B' proteins through techniques like protein electrophoresis supports the concept of codominance. This is because both the 'A' and 'B' alleles express themselves fully, resulting in the manifestation of both types of proteins, rather than a blend or suppression of one.
Assertion (A): Without variations, evolution is impossible. Reason (R): Only useful variations are transmitted to the next generation.
A. Both $A$ and $R$ are true, and $R$ is the correct explanation of $A.
B. Both $A$ and $R$ are true, but $R$ is not the correct explanation of $A.
C. $A$ is true, but $R$ is false.
D. $A$ is false, but $R$ is true.
The correct answer is C. $A$ is true, but $R$ is false.
Assertion (A): Variations are essential for evolution because they introduce genetic diversity within a population. This diversity is the raw material for natural selection, which can lead to evolutionary changes. Without variations, there would be no differences among individuals for natural selection to act upon, making evolution impossible.
Reason (R): The statement that only useful variations are transmitted to the next generation is incorrect. In reality, all genetic variations, whether beneficial, neutral, or detrimental, are passed to the next generation. Natural selection then acts on these variations. Beneficial variations may increase in frequency within the population over time, while detrimental ones may decrease, but initially, all variations are transmitted.
Based on the observations of the monohybrid cross, Mendel postulated the law which distinguished dominant and recessive characters:
A) dominance
B) recessiveness
C) segregation
D) assortment
The correct answer is A) dominance
Gregor Mendel, based on his detailed monohybrid cross experiments, formulated several fundamental genetic laws. Among them, the Law of Dominance is critical for explaining how different traits are expressed in offspring. According to this law, when two differing inherited traits (alleles) are present in an organism, the trait that is visible or expressed is termed the dominant trait, and the trait that does not show up is termed recessive. The recessive trait can only manifest in the phenotype if both alleles for that trait are recessive.
Furthermore, Mendel's Law of Segregation states that each individual has two alleles for each trait which segregate (separate) during gamete formation, ensuring that each gamete receives only one allele from each pair. This is distinct from the Law of Dominance but is equally foundational in understanding genetic inheritance.
Mendelian recombinations are mainly due to:
A) Linkage
B) Independent assortment of genes
C) Mutations
D) Dominant characters
The correct answer is B) Independent assortment of genes
Mendelian recombinations refer to the genetic variations that emerge in the progeny due to the way genes are assorted and passed on from parents. These recombinations manifest notably in the offspring of a dihybrid cross observed in the $F_{2}$ generation.
For instance, consider the dihybrid cross between two plants, one homozygous for two dominant traits ($\text{AABB}$) and the other homozygous for two recessive traits ($\text{aabb}$). In the $F_{2}$ generation, the combinations are not restricted to just the parental types ($\text{AABB}$ and $\text{aabb}$), but also include recombinant types such as $\text{AAbb}$, $\text{AaBb}$ (having the dominant trait $A$ and the recessive trait $b$) and $\text{aaBB}$, $\text{aaBb}$ (with dominant trait $B$ and recessive trait $a$). This is enabled by the alleles for traits $A$ and $B$, which are located on different chromosomes and thus assort independently during the formation of gametes in meiosis. Independent assortment is a key principle of Mendelian genetics, explaining the genetic diversity observed in the $F_{2}$ populations.
The number of phenotypes and genotypes of blood groups in human beings are:
A) 4 phenotypes and 4 genotypes B) 6 phenotypes and 6 genotypes C) 6 phenotypes and 4 genotypes D) 4 phenotypes and 6 genotypes
Solution:The correct option is D: 4 phenotypes and 6 genotypes.
In humans, blood groups exhibit four different phenotypes: $A$, $B$, $AB$, and $O$.
For phenotype $A$, there are two possible genotypes: $I^A I^A$ and $I^A i$.
For phenotype $B$, there are also two possible genotypes: $I^B I^B$ and $I^B i$.
Phenotype $AB$ has a single genotype: $I^A I^B$.
Phenotype $O$ has a single genotype: $ii$.
Thus, the total number of genotypes adds up to 6.
Genetics is the branch of science which deals with the study of
A. cell function B. cell structure C. heredity and variation D. relation between plant and environment
The correct answer is C. heredity and variation.
Genetics is the scientific discipline concerned with the study of genes, genetic variations, and heredity in living organisms. On the other hand, Cytology is focused on the study of cell structure and function, while Ecology deals with the interactions between organisms and their environment.
"What is variety? What is the difference between variety and species?"
In botanical nomenclature, variety (abbreviated as var.; in Latin: varietas) is a taxonomic rank that is positioned below species and subspecies but above the rank of form. Thus, it is assigned a three-part infraspecific name.
The key difference between 'variety' and 'species' lies in their taxonomic distinction. A species is defined as a group of organisms that share common characteristics and are capable of interbreeding to produce fertile offspring. Species can encompass various naturally occurring varieties, which are minor variations within the species that do not change the basic characteristics to an extent that they constitute a separate species.
To illustrate, consider the species Cercis canadensis (commonly known as redbud). A particular variety of redbud with white flowers is classified under the infraspecific name Cercis canadensis var. alba. This variety predominantly produces seeds that yield white flowers, which demonstrates the minor yet distinct variation within the same species.
Explain the line - 'change in gene frequency'.
Change in gene frequency refers to the alteration in the relative frequency of an allele at a specific locus within a population over time. This concept is seminal in understanding microevolution, which is essentially the evolutionary changes that happen at the genetic level within a population.
Key components to understand here include:
Allele frequency: This is the proportion of a specific allele among all alleles at a particular genetic locus in a population. It is expressed as a fraction or percentage.
Ploidy and Population Structure: Assume a population consists of $N$ individuals, each having $n$ copies of each chromosome. If a particular allele is present on $i$ chromosomes across the entire population, then the allele frequency for that variant is given by the formula: $$ \text{Frequency} = \frac{i}{nN} $$ where $i$ is the number of occurrences of the allele and $nN$ is the total number of chromosome copies in the population.
Relation to Genotype Frequency: Although allele frequency is different from genotype frequency, the two are interrelated. Allele frequencies can indeed be computed from genotype frequencies.
In population genetics, allele frequencies serve a crucial role in assessing the amount of genetic variation at a specific locus or across many loci. The spread of these frequencies across a population or species is often depicted in an allele frequency spectrum.
In simpler terms, "change in gene frequency" refers to how the occurrences of specific genetic variants (alleles) shift within the genetic makeup of a population, which can be influenced by factors like natural selection, genetic drift, migration, and mutation.
Is it necessary that all mutations in genes will lead to abnormalities in a child? Is there any difference between sickle cell anemia and beta-cell anemia?
Mutations in genes do not necessarily lead to abnormalities in a child. Genes contain the instructions needed for the development and functioning of the body, influencing characteristics like height, hair texture, and eye color. These genes are inherited from your parents.
Sometimes, changes occur in these instructions — mutations. Such mutations can be passed down to offspring and might result in various health conditions, including single gene disorders like cystic fibrosis and sickle cell disease, or birth defects that affect the shape or function of body parts.
Regarding sickle cell anemia and beta-cell anemia, there is indeed a distinction between them:
Sickle cell anemia is a genetic disorder where red blood cells, which transport oxygen throughout the body, are crescent-shaped due to a mutation in the hemoglobin gene. This shape makes the cells prone to break down and can block blood flow, leading to pain and potential organ damage.
Beta-cell anemia, also known as sickle beta thalassemia or simply thalassemia, is another inherited condition affecting hemoglobin. This disorder combines sickle cell disease with beta thalassemia, another blood disorder that reduces hemoglobin production.
In thalassemia, the individual carries two different hemoglobin gene mutations: one causing the red blood cells to “sickle” and another affecting hemoglobin production due to beta thalassemia. The severity of sickle beta thalassemia can vary, depending on whether the beta thalassemia mutation results in no normal hemoglobin (sickle beta zero thalassemia) or reduced levels of it (sickle beta plus thalassemia). Symptoms may include anemia, repeated infections, and frequent episodes of pain, generally manifesting in early childhood and varying in severity.
Treatment for these conditions tends to be supportive, tailored to alleviate the individual symptoms experienced by the sufferer. Sickle beta thalassemia is inherited in an autosomal recessive manner, requiring both parents to pass on the faulty gene for a child to be affected.
The number of linkage groups in man are
A) 46
B) 23
C) 22
D) 24
The correct answer is B) 23.
Linkage refers to the phenomenon where genes located close to each other on the same chromosome are inherited together across generations. Each chromosome represents a linkage group. The number of linkage groups in any organism is typically equivalent to the haploid number of its chromosomes. Humans have 23 haploid chromosomes, so they therefore have 23 linkage groups.
Go through the following statements:
(i) Chromosomal aberrations are commonly observed in cancer cells. (ii) The possibility of a female becoming haemophilic is extremely rare because the mother of such a female has to be at least a carrier, and the father should be haemophilic. (iii) All the daughters of a couple exhibiting sickle-cell trait will suffer from sickle-cell anemia. (iv) Myotonic dystrophy is an autosomal recessive trait.
Find out the correct statements:
A (i) & (ii)
B (i), (ii), & (iii)
C (ii) & (iv)
D All are correct
The accurate answer to the question is Option B (i), (ii), & (iii).
Explanation:
Statement (i) indicates that chromosomal aberrations are commonly observed in cancer cells, which is a correct observation as cancer often involves chromosomes that are altered or damaged.
Statement (ii) explains that the chance of a female becoming haemophilic is extremely rare since it requires her mother to be at least a carrier and her father to be haemophilic. This scenario correctly describes the inheritance pattern of X-linked recessive disorders like hemophilia, making the statement true.
Statement (iii) is incorrect as it states all daughters of a couple exhibiting the sickle-cell trait will suffer from sickle-cell anemia. Sickle-cell anemia requires two copies of the sickle-cell gene. Daughters of parents both exhibiting the trait have only a $1/4$ chance of actually suffering from sickle-cell anemia. They might also inherit one normal gene and one defective gene (sickle-cell trait), or two normal genes.
Statement (iv) describes myotonic dystrophy as an autosomal recessive trait which is incorrect because myotonic dystrophy is an autosomal dominant trait.
Thus, statements (i) and (ii) are the only correct ones, making Option B the right choice.
The part of the brain which controls the balance and posture of the body is:
A. Cerebellum
B. Pons
C. Medulla
D. Cerebellum
The correct answer to the question regarding which part of the brain controls the balance and posture of the body is:
A. Cerebellum
The cerebellum is a critical region of the brain that manages balance and posture. It occupies approximately 80% of the brain's volume dedicated solely to these functions. This significant portion makes the cerebellum responsible for maintaining the body's position and stability, crucial for coordinated movement.
Not only does the cerebellum handle balance and posture, but it is also involved in other functions like motor skills, coordination, and possibly contributing to cognitive processes such as attention and language. Thus, making it integral in multiple aspects of neural function beyond just physical balance.
Other parts of the brain, like the medulla oblongata, are involved in different essential functions such as heartbeat regulation and respiration but not specifically in balance or posture. Hence, the best answer to the question is Option A: Cerebellum.
Changes in the non-reproductive tissues caused by environmental factors: A) Are inheritable B) Are not inheritable C) Both (A) and (B) D) None of these
The question pertains to whether changes in non-reproductive tissues caused by environmental factors are inheritable. Non-reproductive tissues refer to somatic cells in the body, which are distinct from reproductive cells or gametes.
The key factor to understand here is that changes in genetic material must occur within the gametes for traits to be passed on to the next generation. When environmental factors lead to changes in somatic cells, these changes do not alter the genetic material passed on through the gametes. Therefore, these changes cannot be inherited by offspring.
For example, consider a person who builds significant muscle mass through rigorous bodybuilding. The muscle mass, which is a change in the non-reproductive (somatic) tissues, is a result of environmental factors like exercise. However, this muscle mass cannot be genetically passed down to this person's children. The children would need to engage in their own exercise regimen to develop similar muscle mass because the trait (muscle build) does not alter the genetic material in the gametes.
Hence, the correct answer to the question is:B) Are not inheritable.
Which of the following are pairs of analogous organs? I. Forelimbs of horse - Wings of bat II. Wings of bat - Wings of butterfly III. Forelimbs of horse - Wings of butterfly IV. Wings of bird - Wings of bat
A) I & III B) II & IV C) III & IV D) II & III
Analogous organs are ones that have similar functions but different structures and evolutionary origins. To identify pairs of analogous organs from the given options, it's crucial to ascertain if the organs share similar roles despite different developmental backgrounds.
Here's the analysis for each pair:
Forelimbs of a horse vs. Wings of a bat: These do not perform similar functions. The forelimbs of a horse are used for walking while the wings of a bat are used for flying. Hence, they are not analogous.
Wings of a bat vs. Wings of a butterfly: Both are used for flying, but their structural compositions and origins differ significantly. Bats have wings made of skin stretched over their fingers whereas butterflies have wings composed of chitin. They are great examples of analogous organs.
Forelimbs of a horse vs. Wings of a butterfly: Like the first pair, these organs serve entirely different purposes, with the former used for walking and the latter for flying.
Wings of a bird vs. Wings of a bat: Although both are used for flying, their structural designs are different enough to not be considered analogous. The wing of a bird is covered with feathers, while the wing of a bat is more like a stretched membrane.
From the analysis, the pairs of analogous organs, which perform similar functions but are structurally different, are:
Wings of a bat and wings of a butterfly.
Thus, the correct answer is: D) II & III
The total volume of air accommodated in the lungs at the end of a forced inspiration is called ____.
A. Residual volume
B. Expiratory reserve volume
C. Vital capacity
D. Functional residual capacity
The question asks for the term that describes the total volume of air accommodated in the lungs at the end of a forced inspiration. Let's explore the options:
Total Lung Capacity (TLC): This is the total volume of air in the lungs after a maximum inspiration effort. In healthy adults, this is typically around 6 liters.
Vital Capacity (VC): This represents the maximum amount of air a person can expel after a maximum inhalation. It is the sum of inspiratory reserve volume (IRV), tidal volume (TV), and expiratory reserve volume (ERV).
Functional Residual Capacity (FRC): This is the volume of air remaining in the lungs after a normal, passive exhalation.
Expiratory Capacity (EC): This is the total volume of air that can be exhaled after a normal tidal inhalation, and is calculated as ( \text{ERV} + \text{TV} ).
Given the definitions:
The correct answer to the question "The total volume of air accommodated in the lungs at the end of a forced inspiration" is Total Lung Capacity (TLC).
Thus, Option 1: Total Lung Capacity (TLC) is the correct answer.
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Mention the advantages of selecting pea plant for experiment by Mendel.
Gregor Mendel chose pea plants (Pisum sativum) for his experiments on inheritance for several significant advantages:
Variety of Traits: Pea plants have many distinct, easily observable traits such as flower color, seed shape, and pod color, which made it easier to track and record the inheritance patterns.
Short Life Cycle: Pea plants grow, flower, and produce seeds quickly, allowing Mendel to observe multiple generations within a relatively short period.
Controlled Pollination: Pea plants can self-pollinate and also be easily cross-pollinated by human intervention. This feature allowed Mendel to control which plants bred with each other.
Large Number of Offspring: Each pea pod contains multiple seeds, providing a large sample size for statistical analysis and more accurate results.
True-Breeding Varieties: Pea plants have many true-breeding varieties that consistently exhibit the same traits, making it easier to predict and analyze the inheritance patterns.
Ease of Cultivation: Pea plants are easy to grow and manage, making them a practical choice for repetitive and large-scale experiments.
These advantages helped Mendel to meticulously study and eventually formulate the foundational Laws of Inheritance.
Differentiate between the following -
(a) Dominance and Recessive
(b) Homozygous and Heterozygous
(c) Monohybrid and Dihybrid.
(a) Dominance and Recessive
Dominance:
Definition: The phenomenon in which one allele masks the expression of another allele at the same locus.
Example: In pea plants, the allele for tall height $(\mathbf{T})$ is dominant over the allele for dwarf height $(\mathbf{t})$.
Characteristics: The dominant allele expresses itself in both homozygous $(\mathbf{TT})$ and heterozygous $(\mathbf{Tt})$ forms.
Recessive:
Definition: The phenomenon in which the expression of one allele is masked by the presence of a dominant allele at the same locus.
Example: In pea plants, the allele for dwarf height $(\mathbf{t})$ is recessive.
Characteristics: The recessive allele expresses itself only in the homozygous form $(\mathbf{tt})$.
(b) Homozygous and Heterozygous
Homozygous:
Definition: An organism is homozygous for a gene when it has identical alleles on both homologous chromosomes.
Example: For the gene controlling plant height in pea plants, $\mathbf{TT}$ (tall) or $\mathbf{tt}$ (dwarf) are homozygous genotypes.
Characteristics: True-breeding with respect to a trait, resulting in offspring with identical traits when self-pollinated or crossed with another homozygous organism.
Heterozygous:
Definition: An organism is heterozygous for a gene when it has two different alleles on both homologous chromosomes.
Example: For the gene controlling plant height in pea plants, $\mathbf{Tt}$ is a heterozygous genotype.
Characteristics: The heterozygous condition often results in the expression of the dominant allele, masking the expression of the recessive allele.
(c) Monohybrid and Dihybrid
Monohybrid:
Definition: A cross between two organisms involving a single pair of contrasting traits.
Example: A monohybrid cross between tall $(\mathbf{TT})$ and dwarf $(\mathbf{tt})$ pea plants results in $\mathbf{Tt}$ in the $\mathbf{F}_1$ generation.
Characteristics: It investigates the inheritance of a single trait and typically results in a phenotypic ratio of 3:1 in the $\mathbf{F}_2$ generation.
Dihybrid:
Definition: A cross between two organisms involving two pairs of contrasting traits.
Example: A dihybrid cross between a pea plant with yellow round seeds $(\mathbf{RRYY})$ and a pea plant with green wrinkled seeds $(\mathbf{rryy})$ results in $\mathbf{RrYy}$ in the $\mathbf{F}_1$ generation.
Characteristics: It investigates the inheritance of two traits and typically results in a phenotypic ratio of 9:3:3:1 in the $\mathbf{F}_2$ generation.
A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
Given that the organism is heterozygous at 4 different loci, each locus can have two different alleles. The number of possible combinations of these alleles in the gametes can be calculated using the formula (2^n), where (n) is the number of heterozygous loci.
Thus, the number of types of gametes produced will be $2^4$.
[ 2^4 = 16 ]
Therefore, the diploid organism can produce 16 types of gametes.
Explain the Law of Dominance using a monohybrid cross.
Law of Dominance
The Law of Dominance states that when two organisms with contrasting traits are crossed, the trait that appears in the first filial generation (F1) is the dominant trait. The recessive trait, although present, is not expressed in the F1 generation but can reappear in subsequent generations.
Example: Monohybrid Cross in Peas
Step-by-Step Explanation
Selection of Traits:Mendel selected a pair of contrasting traits. For example, he chose tall (T) and dwarf (t) pea plants.
Parental Generation (P):The pure-breeding parent plants were:
Tall plant (TT)
Dwarf plant (tt)
First Filial Generation (F1):When the TT (Tall) and tt (Dwarf) plants were crossed, the F1 generation received one allele from each parent:
All F1 plants were Tt (Tall).
The tall trait dominates over the dwarf trait in the heterozygous condition.
Self-Pollination of F1 Generation:When the F1 plants (Tt) were self-pollinated, the following gametes were produced during meiosis:
50% T and 50% t
Second Filial Generation (F2):The F2 generation exhibited both traits in a specific ratio due to random fertilization:
75% of the plants were Tall (TT and Tt).
25% of the plants were Dwarf (tt).
This led to a phenotypic ratio of 3:1 (Tall to Dwarf).
Conclusion:
Based on these observations, Mendel proposed the Law of Dominance. The key points are:
Characters are controlled by discrete units called "factors" (now known as genes).
Factors occur in pairs.
In a pair of dissimilar factors, one member is dominant (expressed) and the other is recessive (hidden in F1 but can reappear in F2).
Mendel's Findings:
F1 Generation: Only the dominant trait appears (Tall).
F2 Generation: Both traits appear in a 3:1 ratio (3 Tall: 1 Dwarf).
This principle was used by Mendel to explain why one trait can mask the presence of another in hybrids: dominant trait masks the recessive trait in heterozygous states.
Define and design a test-cross.
Test Cross Definition
A test cross is a way to explore the genotype of an organism. It's performed to determine whether an organism displaying a dominant trait is homozygous dominant (having two identical dominant alleles) or heterozygous (having one dominant and one recessive allele). The organism in question is crossed with an organism that is homozygous recessive for the trait in question.
How a Test Cross Works
P (Parental) Generation:
The organism with the unknown genotype (showing the dominant trait) is crossed with an organism that is homozygous recessive for that trait.
Possible Outcomes:
If all offspring show the dominant trait, the unknown genotype is likely homozygous dominant.
If some offspring show the recessive trait, the unknown genotype is heterozygous.
Designing a Test Cross
Let's design a test cross for a pea plant with an unknown genotype which exhibits the dominant trait of tall height (T).
Identify Genotypes:
Homozygous dominant tall plant: TT
Heterozygous tall plant: Tt
Homozygous recessive short plant: tt
Cross the Unknown Genotype with Homozygous Recessive:
Let's denote the unknown genotype as T_ (it could be TT or Tt).
Cross T_ with tt.
Punnett Square Representation:
Case 1: Unknown is TT$$ \begin{array}{c|c} & t & t \ \hline T & Tt & Tt \ T & Tt & Tt \ \end{array} $$
All offspring (100%) will be tall (Tt).
Case 2: Unknown is Tt$$ \begin{array}{c|c} & t & t \ \hline T & Tt & Tt \ t & tt & tt \ \end{array} $$
50% offspring will be tall (Tt) and 50% will be short (tt).
Outcome Interpretation
Homozygous Dominant Outcome (TT): All offspring tall (Tt).
Heterozygous Outcome (Tt): 50% tall (Tt) and 50% short (tt).
Conclusion
The test cross effectively reveals whether the dominant trait showing organism is homozygous dominant or heterozygous by observing the phenotypic ratio of the offspring. If no recessive traits appear in the offspring, the unknown genotype is homozygous dominant. If some offspring exhibit the recessive trait, the unknown genotype is heterozygous.
Figure: Diagrammatic representation of a test cross
Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
To work out the distribution of phenotypic features in the first filial generation (F₁) after a cross between a homozygous female and a heterozygous male for a single locus, we need to represent this cross using a Punnett Square.
Let's use the following variables for the alleles:
Dominant allele: A
Recessive allele: a
Assume the genotypes:
Homozygous female: aa
Heterozygous male: Aa
Here's the Punnett Square for this cross:
A | a | |
---|---|---|
a | Aa | aa |
a | Aa | aa |
Phenotypic Distribution
Aa: This genotype exhibits the dominant phenotype. aa: This genotype exhibits the recessive phenotype.
Outcome:
50% Aa: Dominant phenotype
50% aa: Recessive phenotype
So, the phenotypic distribution in the F₁ generation is:
50% Dominant phenotype
50% Recessive phenotype
Conclusion
In the F₁ generation following this cross, there is an equal distribution of phenotypes: 50% will show the dominant trait, and 50% will show the recessive trait.
When a cross in made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy), what proportions of phenotype in the offspring could be expected to be
(a) tall and green.
(b) dwarf and green.
To address this question, since the cross involves two traits (height and seed color), we'll use a Punnett square to determine the proportions of different phenotypes.
Given:
Tall (T) is dominant over dwarf (t).
Yellow seeds (Y) is dominant over green seeds (y).
The parents are:
Parent 1: TtYy (tall and yellow seeds)
Parent 2: Ttyy (tall and green seeds)
We need to find the proportions of: (a) Tall and green (b) Dwarf and green
Let's set up the Punnett square for the given cross ( TtYy × Ttyy ).
Step 1: Determine the possible gametes
From TtYy: TY, Ty, tY, ty
From Ttyy: Ty, ty, ty, ty (Note: both Ty and ty reoccur because ( yy ) is fixed)
Step 2: Complete the Punnett square
Below is the completed Punnett square for the cross:
Ty | ty | Ty | ty | |
---|---|---|---|---|
TY | TYTy (tall, yellow) | TYty (tall, yellow) | TYTy (tall, yellow) | TYty (tall, yellow) |
Ty | TyTy (tall, yellow) | Tyty (tall, green) | TyTy (tall, yellow) | Tyty (tall, green) |
tY | tYTy (tall, yellow) | tYty (tall, yellow) | tYTy (tall, yellow) | tYty (tall, yellow) |
ty | tyTy (tall, green) | tyty (dwarf, green) | tyTy (tall, green) | tyty (dwarf, green) |
Step 3: Analyze the phenotypes
Tall and Green (T_yy: Ty_ or ty_ depending on the tall allele):
Tyty, Tyty, tyTy, tyTy
Proportion: ( \frac{4}{16} = \frac{1}{4} ) or 25%
Dwarf and Green (ttyy):
tyty, tyty
Proportion: ( \frac{2}{16} = \frac{1}{8} ) or 12.5%
Conclusion
(a) The proportion of offspring that are tall and green is 25% (or 1 out of 4).
(b) The proportion of offspring that are dwarf and green is 12.5% (or 1 out of 8).
Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in $\mathrm{F}_{1}$ generation for a dibybrid cross?
In a linkage scenario, the two loci are located on the same chromosome and tend not to assort independently. This is different from the classical 9:3:3:1 phenotypic ratio typically observed in a dihybrid cross when the two loci assort independently. Instead, the distribution of phenotypic features in the $\mathrm{F}_{1}$ generation will depend on the degree of linkage between the loci.
Steps to Determine F₁ Generation Phenotypic Features in Linked Loci:
Parental Genotypes: Identify the genotype of the heterozygous parents. For simplicity, consider the parental genotypes as $\mathrm{RrYy}$, where:
$\mathrm{R}$ and ($\mathrm{r}$ represent alleles of one gene (e.g., seed shape),
$\mathrm{Y}$ and $\mathrm{y}$ represent alleles of another gene (e.g., seed color).
Linked Genes: Assume the genes are linked such that (\mathrm{R}) and (\mathrm{Y}) are on one chromosome, and $\mathrm{r}$ and $\mathrm{y}$ are on the homologous chromosome.
Gamete Formation: Parental gametes will largely be $\mathrm{RY}$ and $\mathrm{ry}$, with very few recombinants $(\mathrm{Ry}$ and $\mathrm{rY})$ depending on the degree of crossing over.
F₁ Generation Cross: Crossing $\mathrm{RrYy} \times \mathrm{RrYy}$.
Expected Phenotypic Ratio in F₁ Generation:
Parental Combinations (No Crossing Over):
RY (dominant traits): Round and Yellow
ry (recessive traits): Wrinkled and Green
Possible Recombinants (Crossing Over):
Ry: Round and Green
rY: Wrinkled and Yellow
Phenotypic Classes in F₁ Generation:
Round Yellow (RY)
Wrinkled Green (ry)
Round Green (Ry)
Wrinkled Yellow (rY)
Frequencies of Phenotypes:
No Crossing Over (Parental type):
Higher frequency
With Crossing Over (Recombinant type):
Lower frequency
Conclusion:
In F₁, the phenotypic distribution will be skewed towards the parental types with a small proportion of recombinant types, depending on the recombination frequency. The exact ratio deviates from the classic (9:3:3:1) and leans more towards a higher presence of $\textbf{Round Yellow}$ and $\textbf{Wrinkled Green}$.
For highly linked genes, parental phenotypes will dominate, while for loosely linked genes, there will be a significant presence of recombinant phenotypes.
Briefly mention the contribution of T.H. Morgan in genetics.
Thomas Hunt Morgan made significant contributions to the field of genetics through his work with the fruit fly *Drosophila melanogaster*. Some key contributions include:
Experimental Verification of Chromosomal Theory of Inheritance: Morgan's experiments provided concrete evidence that chromosomes are the carriers of genes, thereby supporting Sutton and Boveri's Chromosomal Theory of Inheritance.
Discovery of Sex-linked Inheritance: He discovered that certain traits, such as white eye color in fruit flies, were linked to sex chromosomes, leading to the understanding of sex-linked inheritance patterns.
Linkage and Recombination: Morgan and his team found that genes located on the same chromosome tend to be inherited together, a phenomenon known as linkage. He also studied genetic recombination and established the concept of genetic maps showing the order of genes on chromosomes.
Mapping Genes: His student Alfred Sturtevant used the frequency of recombination between gene pairs to create genetic maps, which indicated the positions of genes on chromosomes.
Morgan's work laid the foundation for modern genetics and significantly advanced our understanding of heredity and chromosome behavior.
What is pedigree analysis? Suggest how such an analysis, can be useful.
Pedigree analysis is a method used in human genetics to study the inheritance patterns of specific traits, abnormalities, or diseases across multiple generations within a family. It involves creating a family tree that outlines the genetic relationships and shows which individuals express or carry a particular trait or disease.
Symbols Used in Pedigree Analysis
Squares represent males.
Circles represent females.
Shaded symbols indicate individuals expressing the trait being studied.
Unshaded symbols indicate individuals not expressing the trait.
Half-shaded symbols often indicate carriers of a recessive trait.
Horizontal lines connecting a square and a circle represent mating.
Vertical lines descending from a couple represent their offspring.
Utility of Pedigree Analysis
Identifying Genetic Disorders:Pedigree analysis can help identify whether a particular trait is dominant or recessive, autosomal or sex-linked. This helps in understanding the inheritance pattern and predicting the likelihood of the trait appearing in future generations.
Disease Diagnosis:Pedigree charts can be used by healthcare professionals to diagnose inherited genetic disorders by spotting patterns that indicate carriers and affected individuals within a family.
Genetic Counseling:Couples seeking to have children can use pedigree charts to assess the risk of passing on genetic disorders. Genetic counselors use these charts to provide information on potential genetic risks to couples.
Research and Treatment:Researchers use pedigree analysis to track the transmission of genetic conditions, which can lead to the development of targeted treatments and therapies.
Educational Tool:Pedigree charts are used in education to teach students about the principles of inheritance and how genetic traits are passed down through generations.
Example of Pedigree Analysis
Consider a family where the pedigree chart is used to trace the inheritance of a genetic disorder like hemophilia. By analyzing the chart, one can determine:
How the disorder is transmitted (e.g., X-linked recessive).
Which individuals are carriers.
Which offspring are at risk.
Such analysis helps in preemptive measures and better management of genetic conditions.
Representative Pedigree Symbol Example
Figure 4.13: Symbols used in the human pedigree analysis.
Figure 4.14: Representative pedigree analysis indicating (a) Autosomal dominant trait, (b) Autosomal recessive trait.
Pedigree analysis thus serves as a critical tool in both clinical and research settings to understand and manage hereditary conditions effectively.
How is sex determined in human beings?
Sex determination in human beings follows the XY sex-determination system. Here's a detailed explanation:
Chromosomal Basis
Humans have 23 pairs of chromosomes. Out of these, 22 pairs are autosomes and 1 pair are sex chromosomes.
Females have two X chromosomes (XX).
Males have one X chromosome and one Y chromosome (XY).
Gamete Formation
During spermatogenesis (formation of sperm) in males, two types of sperm are produced:
50% of the sperm carry an X chromosome.
50% of the sperm carry a Y chromosome.
During oogenesis (formation of the ova) in females, all ova carry:
Only X chromosomes.
Fertilization
Sex is determined at fertilization, which is the moment when an ovum from the female is fertilized by a sperm from the male:
If an X-carrying sperm fertilizes the ovum (X), the resulting zygote will be XX, leading to a female offspring.
If a Y-carrying sperm fertilizes the ovum (X), the resulting zygote will be XY, leading to a male offspring.
Equal Probability
There is an equal probability (50%) of the offspring being either male (XY) or female (XX).
Conclusion
Thus, it is the sperm from the father that determines the sex of the child, as it can carry either an X or a Y chromosome. The ovum from the mother always carries an X chromosome.
It's important to note that social misconceptions have often wrongly blamed mothers for the sex of the child, while scientifically, it's the father's sperm determining it.
A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.
To work out the genotypes, we need to understand the inheritance pattern of the ABO blood group system, which is determined by the presence of three alleles: $I^A$, $I^B$, and $i$.
Given:
Blood group O means the genotype is $ii$.
Blood group A can have the genotype $I^A I^A$ (homozygous) or $I^A i$ (heterozygous).
Blood group B can have the genotype $I^B I^B$ (homozygous) or $I^B i$ (heterozygous).
Since the child has blood group O ($ii$), both parents must carry the recessive allele $i$. Therefore, the father's genotype must be $I^A i$ and the mother's genotype must be $I^B i$.
Now, let's work out the possible genotypes of other offspring using a Punnett square.
Cross: $I^A i$ × $I^B i$
Using a Punnett square to visualize:
$I^B$ | $i$ | |
---|---|---|
$I^A$ | $I^A I^B$ | $I^A i$ |
$i$ | $I^B i$ | $ii$ |
The possible genotypes and their corresponding blood groups are:
$I^A I^B$ – Blood group AB
$I^A i$ – Blood group A
$I^B i$ – Blood group B
$ii$ – Blood group O
Possible Genotypes and Phenotypes of Offspring:
$I^A I^B$ – Blood group AB
$I^A i$ – Blood group A
$I^B i$ – Blood group B
$ii$ – Blood group O
Summary of Probabilities:
Each genotype has an equal probability of occurring:
Blood group A: 25%
Blood group B: 25%
Blood group AB: 25%
Blood group O: 25%
Therefore, the genotypes of the parents are $I^A i$ and $I^B i$, and the possible genotypes of their offspring are $I^A I^B$, $I^A i$, $I^B i$, and $ii$, corresponding to blood groups AB, A, B, and O, respectively.
Explain the following terms with example
(a) Co-dominance
(b) Incomplete dominance
(a) Co-dominance
Definition: Co-dominance occurs when both alleles in a heterozygous organism are fully expressed, resulting in a phenotype that simultaneously exhibits traits from both alleles. In co-dominance, neither allele is recessive or masked by the other.
Example: The ABO blood group system in humans is a classic example of co-dominance. The gene for blood type (denoted as 'I') has three alleles: $I^A$, $I^B$, and (i).
The $I^A$ and $I^B$ alleles are co-dominant.
The 'i' allele is recessive to both $I^A$ and $I^B$.
When a person inherits the (I^A) allele from one parent and the (I^B) allele from the other (genotype $I^A I^B)$, they have:
Blood type AB: This genotype expresses both A and B antigens on the surface of red blood cells, resulting in the AB blood type.
Illustration:
| Allele from Parent 1 | Allele from Parent 2 | Offspring Genotype | Blood Type |
|-----------------------|-----------------------|---------------------|--------------|
| \(I^A\) | \(I^B\) | \(I^A I^B\) | AB |
(b) Incomplete Dominance
Definition: Incomplete dominance is a form of inheritance where the phenotype of a heterozygous individual is intermediate between the phenotypes of the homozygous parents. Neither allele is completely dominant over the other, and the heterozygous phenotype is a blend of the two homozygous phenotypes.
Example: The flower color in snapdragons (Antirrhinum majus) is a famous example of incomplete dominance. When a plant with red flowers (RR) is crossed with a plant with white flowers (rr), the heterozygous offspring (Rr) have pink flowers.
Red flowers: (RR)
White flowers: (rr)
Pink flowers: (Rr)
Illustration:
$$ \begin{array}{c|c} \text{Parent 1} & \text{Parent 2} \ (R) & (r) \ \hline RR & rr \ \end{array} $$ \begin{array}{c|c} \text{F1} & \text{Phenotype} \ Rr & \text{Pink} \ \end{array} $$
So, in incomplete dominance, the heterozygous genotype (Rr) produces a pink phenotype, which is intermediate between the red ((RR)) and white ((rr)) phenotypes.
What is point mutation? Give one example.
A point mutation is a type of genetic mutation where a single nucleotide base is changed, inserted, or deleted from a DNA or RNA sequence. This small change can have significant effects on the gene's function and the organism's phenotype.
Example: Sickle Cell Anemia
A classical example of a point mutation is sickle cell anemia. In this disease, the amino acid glutamic acid (Glu) is substituted by valine (Val) at the sixth position in the beta chain of hemoglobin. This substitution occurs due to a single nucleotide change in the DNA sequence of the hemoglobin gene, specifically a GAG to GTG mutation in the sixth codon.
This change results in abnormal hemoglobin molecules, which causes red blood cells to take on a sickle shape under low oxygen conditions, leading to various health issues such as pain, anemia, and fatigue.
Summary
Point mutation: Change in a single nucleotide base in DNA or RNA. Example: Sickle cell anemia (Glu to Val substitution in hemoglobin).
Who had proposed the chromosomal theory of the inheritance?
The chromosomal theory of inheritance was proposed by Walter Sutton and Theodore Boveri.
Mention any two autosomal genetic disorders with their symptoms.
Two examples of autosomal genetic disorders are Sickle-cell Anemia and Thalassemia.
1. Sickle-cell Anemia
Symptoms:
Anemia: Due to the sickle-shaped red blood cells breaking down prematurely.
Pain Crises: Episodes of severe pain due to blockages in blood flow caused by the irregularly shaped red blood cells.
Swelling: In the hands and feet due to blocked blood flow, often seen as the first symptom in infants.
Frequent Infections: Damaged spleen makes it harder for the body to fight infections.
Delayed Growth: Lack of healthy red blood cells can slow growth in children.
Vision Problems: Blocked blood vessels in the eye can lead to vision issues.
2. Thalassemia
Symptoms:
Fatigue and Weakness: Caused by the decreased production of hemoglobin.
Pale Skin or Jaundice: Due to reduced hemoglobin and destruction of red blood cells.
Facial Bone Deformities: Particularly in the cheeks and forehead, due to bone marrow trying to produce more blood cells.
Slow Growth: Lower supply of oxygen can impede a child's growth.
Abdominal Swelling: Enlarged spleen and liver due to increased demand for blood cell production.
Dark Urine: Due to the breakdown of red blood cells releasing hemoglobin into the bloodstream and then the urine.
These disorders result from mutations in single genes that affect the blood's functionality and efficiency in oxygen transport. Sickle-cell anemia is especially notable for its unique red blood cell shape, while thalassemia affects the production of hemoglobin chains.
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Comprehensive Class 12 Notes on the Principles of Inheritance and Variation
Introduction to Principles of Inheritance and Variation
Inheritance and variation are fundamental concepts in genetics that help us understand how traits are passed down from parents to offspring and how variations occur within a species. These principles were first systematically studied by Gregor Mendel through his experiments with pea plants.
Fundamentals of Genetics
Overview
Genetics is a branch of biology that deals with the study of genes, genetic variation, and heredity in organisms. Understanding genetics is crucial for comprehending how traits are inherited and how genetic disorders are passed down through generations.
Importance of Understanding Genetics
Knowledge of genetics is essential in fields like medicine, agriculture, and evolutionary biology. It helps in developing treatments for genetic disorders, improving crop yields, and understanding the evolutionary processes that shape biodiversity.
Mendel's Laws of Inheritance
Introduction to Gregor Mendel
Gregor Mendel, a monk in the 19th century, is known as the father of genetics. Through his meticulous experiments with pea plants, he discovered the basic principles of heredity.
Mendel’s Experiments with Pea Plants
Mendel selected pea plants that exhibited distinct, heritable traits such as tall vs. dwarf plants and yellow vs. green seeds. By cross-pollinating these plants, he observed how traits were passed from one generation to the next.
Mendel’s Laws:
Law of Dominance
Characters are controlled by discrete units called factors (now known as genes).
Factors occur in pairs.
In a pair of dissimilar factors, one member of the pair dominates (dominant) the other (recessive).
Law of SegregationThis law states that the two alleles for a trait separate during the formation of gametes so that each gamete carries only one allele for each trait.
Law of Independent AssortmentAccording to this law, the factors for different traits segregate independently of one another during the formation of gametes.
Genetic Terminology
GeneA gene is the basic unit of heredity, made up of DNA. It dictates the expression of specific traits in an organism.
Allele: Dominant and RecessiveAlleles are different forms of the same gene. Dominant alleles mask the expression of recessive alleles.
Genotype vs PhenotypeGenotype refers to the genetic makeup of an individual, while phenotype is the visible expression of the genotype.
Homozygous vs HeterozygousAn organism is homozygous for a trait if it has two identical alleles and heterozygous if it has two different alleles.
Crosses and Inheritance Patterns
Monohybrid Cross
A monohybrid cross involves the study of inheritance of a single trait. Mendel’s experiments showed that crossing pure-bred tall and dwarf pea plants yielded tall plants in the first generation (F1). However, when these F1 plants were self-pollinated, both tall and dwarf plants appeared in a 3:1 ratio in the second generation (F2).
Dihybrid Cross
A dihybrid cross involves the study of two traits simultaneously. Mendel found that traits are inherited independently of each other, leading to a phenotypic ratio of 9:3:3:1 in the F2 generation.
Extensions of Mendelian Genetics
Incomplete DominanceIn incomplete dominance, the F1 hybrid shows a phenotype that is intermediate between the two parental traits.
Co-dominanceCo-dominance occurs when both alleles in a heterozygous organism are fully expressed, as seen in the ABO blood group system.
Multiple AllelesSome traits are controlled by more than two alleles. For example, the ABO blood group is determined by three alleles: IA, IB, and i.
Polygenic InheritanceTraits like skin colour and height are controlled by multiple genes, each contributing to the phenotype.
PleiotropyA single gene can affect multiple traits. For instance, the genetic disorder phenylketonuria affects both mental development and skin pigmentation.
Sex Determination Mechanisms
Sex Determination in Humans (XY System)In humans, sex is determined by the presence of X and Y chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
Sex Determination in Birds and Other Organisms (ZW System)In birds, the sex chromosomes are Z and W. Females have ZW, and males have ZZ.
Haplodiploid Sex Determination (e.g., Honey Bees)In honey bees, males develop from unfertilised eggs and are haploid, while females develop from fertilised eggs and are diploid.
Chromosomal Theory of Inheritance
Discovery and DevelopmentThe Chromosomal Theory of Inheritance emerged from the studies of scientists like Sutton and Boveri, who observed that genes are located on chromosomes.
Contributions of Sutton and BoveriThey noted that chromosomes and genes both segregate independently during meiosis, providing a physical basis for Mendel’s laws.
Chromosome Behaviour during MeiosisDuring meiosis, chromosomes undergo segregation and independent assortment, explaining how genetic variation occurs.
graph TD;
A(Mitosis) --> B(Meiosis I);
B --> C(Meiosis II);
C --> D(Gametes);
D --> E(Fertilisation);
E --> F(Zygote);
click B "https://en.wikipedia.org/wiki/Meiosis"
Linkage and Recombination
Definition and ImportanceLinkage refers to the physical association of genes on a chromosome. Genes that are closely located tend to be inherited together.
Morgan’s ContributionsThomas Hunt Morgan’s experiments with fruit flies demonstrated that genes located on the same chromosome exhibit linkage and recombination.
Mutations and Genetic Disorders
Types of Mutations (Point, Frame-shift)Mutations are changes in the DNA sequence. Point mutations involve a change in a single nucleotide, while frame-shift mutations involve insertions or deletions that alter the reading frame of the gene.
Mendelian DisordersThese are genetic disorders caused by mutations in a single gene. Examples include sickle cell anaemia, cystic fibrosis, and haemophilia.
Chromosomal DisordersThese are caused by abnormalities in chromosome number or structure. Examples include Down’s syndrome, Turner’s syndrome, and Klinefelter’s syndrome.
Summary and Conclusion
In conclusion, the principles of inheritance and variation form the cornerstone of genetics. Mendel’s laws provided the foundation for understanding how traits are passed down through generations. With advancements in molecular biology, we now have a deeper understanding of the genetic mechanisms underlying inheritance and variation.
Illustration of Mendel's pea plant experiments showing different pea plant traits.
Diagram showcasing the structure and composition of chromosomes.
A simple illustration of the Punnett square method used for predicting genetic outcomes.
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