Assertion (A): In Drosophila melanogaster, genes for yellow body and white eyes are very tightly linked. Reason (R): These genes showed a high recombination frequency of 37.2%, indicating a large distance between them.
A is True: The genes white and yellow were indeed found to be very tightly linked. R is False: They showed a very low recombination frequency of only 1.3%, not 37.2%. 37.2% recombination was observed for white and miniature wing genes.
Question 402 of 519
📘 CLASS XII
Assertion (A): In polygenic inheritance, human skin colour is controlled by three or more genes. Reason (R): Besides multiple genes, polygenic inheritance also takes into account the influence of environment on the phenotype.
Human skin colour is a classic polygenic trait controlled by multiple genes. Polygenic inheritance systems generally involve multiple genes and environmental influence. However, the involvement of the environment (R) is a feature of polygenic inheritance, but does not explain why it is controlled by multiple genes (A).
Question 403 of 519
📘 CLASS XII
Assertion (A): The X body discovered by Henking (1891) in insects was found to be received by only 50% of the sperm. Reason (R): The mechanism of sex determination in such insects is the XO type, where the male has only one X-chromosome besides autosomes.
Henking observed that 50% of the sperm received the X body during spermatogenesis. This observation led to the understanding of the XO type sex determination (e.g., in grasshopper), where males produce sperm with X or no X chromosome.
Question 404 of 519
📘 CLASS XII
Assertion (A): Mendel’s second law, the Law of Segregation, states that alleles do not show blending. Reason (R): The Law of Segregation is evidenced by the recovery of both parental traits in the F2 generation, even though one was unexpressed in F1.
The Law of Segregation is based on the fact that alleles do not show any blending. The fact that both characters are recovered in the F2 generation supports this, even though one was masked in F1.
Question 405 of 519
📘 CLASS XII
Assertion (A): In birds, the female is the heterogametic sex. Reason (R): The mechanism of sex determination in birds is the ZW type, where females have ZW chromosomes and produce two types of gametes (Z and W).
In birds, female heterogamety (ZW type) is observed. The female produces two different types of gametes (Z and W), making the female responsible for determining the sex of the offspring.
Question 406 of 519
📘 CLASS XII
Assertion (A): Klinefelter’s Syndrome results in an individual with a karyotype of 47, XXY. Reason (R): Such individuals have overall masculine development but often express feminine development like gynaecomastia and are sterile.
Klinefelter’s Syndrome is caused by the presence of an additional X chromosome, leading to the karyotype 47, XXY. These individuals show masculine development but also feminine traits (gynaecomastia) and are sterile.
Question 407 of 519
📘 CLASS XII
Assertion (A): Turner’s Syndrome is characterised by the karyotype 45, X0. Reason (R): This disorder is caused due to the absence of one of the X chromosomes, and affected females are sterile with rudimentary ovaries.
Turner’s Syndrome is caused by the absence of one X chromosome, resulting in a 45, X0 karyotype. Affected females are sterile due to rudimentary ovaries and lack secondary sexual characters.
Question 408 of 519
📘 CLASS XII
Assertion (A): In the F2 generation of a monohybrid cross, 3/4th of the plants are tall. Reason (R): The tall plants include 1/4th homozygous dominant (TT) and 1/2 heterozygous (Tt) genotypes, due to the Law of Dominance.
At F2, 3/4th of the plants are tall. This is because the tall phenotype includes both the TT (1/4th) and Tt (1/2) genotypes, as T is dominant over t.
Question 409 of 519
📘 CLASS XII
Assertion (A): In a dihybrid cross, the F2 phenotypic ratio 9:3:3:1 confirms the Law of Independent Assortment. Reason (R): This ratio is derived as a combination series of the 3:1 ratio for each independent trait, confirming that the segregation of one pair of characters is independent of the other.
The 9:3:3:1 ratio observed in the F2 generation of a dihybrid cross supports the Law of Independent Assortment. This is because the ratio is derived from the combination of two independent 3:1 ratios, indicating independent segregation of the traits.
Question 410 of 519
📘 CLASS XII
Assertion (A): Walter Sutton and Theodore Boveri proposed the chromosomal theory of inheritance. Reason (R): They noted that the behavior of chromosomes during meiosis was parallel to the behavior of Mendel’s genes (factors).
Sutton united the knowledge of chromosomal segregation with Mendelian principles to propose the chromosomal theory of inheritance. This synthesis was based on their observation that the behaviour of chromosomes was parallel to the behaviour of genes.
