Heredity - Class 10 Science - Chapter 8 - Notes, NCERT Solutions & Extra Questions
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Back Questions - Heredity | NCERT | Science | Class 10
How do Mendel’s experiments show that traits may be dominant or recessive?
Mendel's experiments, particularly his work with pea plants, demonstrated the concepts of dominant and recessive traits clearly. By cross-breeding pea plants with different characteristics (such as flower color and seed shape), he observed patterns in the offspring's traits. For example, when he crossed a plant with purple flowers (a dominant trait) with one with white flowers (a recessive trait), the first generation (F1) consistently showed purple flowers. Only when these F1 plants were self-pollinated did the recessive white flower trait reappear in the second generation (F2), but in a predictable 3:1 ratio (three purple to one white). Through this and similar experiments, Mendel showed that traits are inherited as discrete units (genes), with dominant ones masking recessive ones in heterozygous combinations.
How do Mendel’s experiments show that traits are inherited independently?
Mendel's experiments, particularly those involving dihybrid crosses, demonstrated the principle of independent assortment. By cross-breeding pea plants with two distinct traits (such as seed shape and color), Mendel observed that the offspring inherited these traits in combinations that showed the traits were inherited independently of each other. For instance, crossing plants with round yellow seeds and wrinkled green seeds resulted in offspring with all possible combinations of these traits (round yellow, round green, wrinkled yellow, wrinkled green) in specific ratios (9:3:3:1). This pattern of inheritance indicated that the genes for seed shape and color were not linked and were passed on to offspring independently, supporting the idea that traits are inherited independently according to the laws of probability.
A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant? Why or why not?
Yes, this information is sufficient to determine that the blood group A trait is dominant over blood group O. Blood groups are determined by the presence of certain antigens on the surface of red blood cells. The ABO blood group system is controlled by a gene called the ABO gene, which has three alleles: (I^A), (I^B), and (i). The alleles (I^A) and (I^B) are co-dominant, and both are dominant over (i). Given that a man with blood group A (which could be (I^A I^A) or (I^A i)) marries a woman with blood group O ((ii)), their daughter having blood group O ((ii)) reveals she inherited an (i) allele from both parents. This demonstrates that the A blood group is dominant, as the man must carry at least one (I^A) allele but it wasn't passed to the daughter.
How is the sex of the child determined in human beings?
The sex of a child in human beings is determined by the combination of sex chromosomes inherited from the parents. Humans have two types of sex chromosomes: X and Y. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). During reproduction, the mother contributes an X chromosome, while the father contributes either an X or a Y chromosome, depending on the sperm that fertilizes the egg. If the sperm carrying an X chromosome fertilizes the egg, the offspring will be female (XX). If a sperm with a Y chromosome fertilizes the egg, the offspring will be male (XY). Thus, the genetic sex of the child is determined by the type of sperm that fertilizes the egg, making the father's genetic contribution decisive in determining the sex of the offspring.
A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as
(a) TTWW
(b) TTww
(c) TtWW
(d) TtWw
In a Mendelian experiment as described, where tall pea plants with violet flowers are crossed with short pea plants bearing white flowers, and the progeny all had violet flowers but almost half of them were short, it suggests that the trait for violet flowers is dominant over white flowers, and the trait for tallness is dominant over shortness. However, because almost half of the progeny were short, it indicates that the tall parent must have carried the allele for shortness as well.
Given the options: (a) TTWW (b) TTww (c) TtWW (d) TtWw
The correct genetic makeup of the tall parent bearing violet flowers would be (c) TtWW. This is because 'T' represents the dominant tall allele and 't' the recessive short allele. 'W' represents the dominant violet flower allele, and since violet is dominant, no 'w' allele (which would represent white flowers) is necessary in the parent for its progeny to display violet flowers. The presence of 'Tt' allows for the expression of the short phenotype in the progeny due to the tall parent having one copy of the recessive allele ('t'), which can pair with the 't' allele from the short parent.
A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?
The observation that children with light-colored eyes often have parents with light-colored eye color suggests a potential genetic linkage but does not conclusively indicate whether the trait for light eye color is dominant or recessive. Genetic traits are determined by alleles, with dominant alleles expressing the trait if present and recessive alleles expressing the trait only if both alleles are recessive. Without knowing the genotypes of the parents (whether they are homozygous or heterozygous for the trait) and considering the possible combinations that could produce light-colored eyes in children, it is not possible to definitively state whether the trait is dominant or recessive based solely on parental and child eye color. Genetic inheritance patterns are more complex and require additional information for accurate determination.
Outline a project which aims to find the dominant coat colour in dogs.
The project aims to determine the dominant coat color in dogs within a specific geographic area. It will involve collecting data from a representative sample of the dog population, assessing the variety of coat colors, and categorizing them accordingly. Data collection methods may include surveys of dog owners, visits to local dog parks, and consultations with veterinarians and animal shelters. The data will be analyzed statistically to identify the most prevalent coat color, considering potential factors such as breed, age, and environmental influences. The project's objective is to contribute to the understanding of genetic and environmental factors influencing coat color in dogs, which could be beneficial for studies in genetics and breeding practices.
How is the equal genetic contribution of male and female parents ensured in the progeny?
The equal genetic contribution of male and female parents to their progeny is ensured through the process of sexual reproduction. In humans and many other organisms, each parent possesses two sets of chromosomes. During gamete formation (spermatogenesis in males and oogenesis in females), a process called meiosis occurs, reducing the chromosome number by half. This results in gametes (sperm and egg cells) each carrying a single set of chromosomes. When these gametes fuse during fertilization, the resulting zygote (the first cell of the new offspring) receives one set of chromosomes from each parent, thus restoring the original chromosome number. This process ensures that both male and female parents contribute equally to the genetic makeup of their progeny. Genetic variation is further enhanced by mechanisms such as crossing-over and independent assortment, which occur during meiosis.
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A homozygous purple flower variety of pea plant [PP] is crossed with a white flower variety of pea [pp]. Answer the following questions by using Punnett square:
i) Mention the phenotype and genotype of the F1 generation offspring.
ii) If the offspring of the F1 generation are crossed, what will be the phenotypic and genotypic ratios of the F2 generation?
iii) State Mendel's Law of Dominance.
iv) What is the scientific name of the pea plant? v) Name two genetic diseases in humans.
Find the solution with detailed analysis below:
Phenotype and Genotype of F1 Generation
When a homozygous purple flower (PP) is crossed with a homozygous white flower (pp), all the offspring in F1 generation are heterozygous purple flowers (Pp). Here, the phenotype (observable physical traits) of all F1 offspring is purple. Since they receive one allele for purple (P) and one for white (p), their genotype is heterozygous (Pp).
Here is the Punnett square:
P | P | |
p | Pp | Pp |
p | Pp | Pp |
Phenotypic and Genotypic Ratios of the F2 Generation
If we cross the F1 generation (all heterozygous, Pp) offspring with each other, the genotype in the next generation (F2) can be represented by the following Punnett square:
P | p | |
P | PP | Pp |
p | Pp | pp |
The Phenotypic Ratio for F2 is 3 purple : 1 white. The Genotypic Ratio is 1 homozygous purple (PP) : 2 heterozygous purple (Pp) : 1 homozygous white (pp), or 1:2:1.
Mendel’s Law of Dominance
Mendel's Law of Dominance states that in a heterozygote, the dominant allele manifests itself in the phenotype, completely masking the effect of the recessive allele. The allele that expresses itself is termed as dominant while the one which does not is termed as recessive.
Scientific Name of the Pea Plant
The scientific name of the garden pea is Pisum sativum.
Two Genetic Diseases in Humans
Thalassemia: This genetic disorder impairs the body's ability to produce hemoglobin, affecting the flow of oxygen.
Down's Syndrome: Caused by a trisomy of chromosome 21, leading to a total of 47 chromosomes instead of the usual 46.
This detailed analysis explains the concepts and answers for each part as requested.
The genetic composition of an organism is known as:
A) genotype
B) physiotype
C) phenotype
D) morphotype
The correct answer is A) genotype
Genotype refers to the genetic makeup of a cell, an organism, or an individual, particularly in relation to a specific trait. This includes the entire set of genes that an organism carries. For instance, the genetic code determining the height of a plant could be represented as $TT$, $Tt$, or $tt$. These genetic combinations exemplify different genotypes for the same characteristic (height in this case).
Transmission of parental characters to progeny is called inheritance.
A. variation
B. inheritance
C. genetics
D. aberrations
The correct option is $\mathbf{B}$ inheritance.
Inheritance is the process through which traits and characteristics are transferred from parents to their offspring. This phenomenon forms the foundation of heredity.
Genetic inheritance takes place because of the genetic material, primarily DNA.
During reproduction, all the information necessary for growth, survival, and reproduction is present in the DNA. This genetic information is transmitted from the parental generation to their offspring.
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