NEET Practice Questions (MCQs) with Answers & Solutions

Assertion (A) is true because chromosomes do undergo segregation and independent assortment according to Mendel's laws. Reason (R) is false because the reduction in chromosome number by half occurs during meiosis, not mitosis. Therefore, (A) is true but (R) is false.

Colourblindness and hemophilia are indeed both X-linked recessive traits, meaning they are carried on the X chromosome and manifest primarily in males. The pedigrees for these conditions will look similar because they follow the same inheritance pattern. Therefore, both the Assertion (A) and Reason (R) are true, and (R) is the correct explanation of (A).

Lack of independent assortment of two genes in Drosophila is due to linkage. Linkage refers to the phenomenon where genes that are located close to each other on the same chromosome tend to be inherited together because they do not assort independently during meiosis.

To determine the proportion of sperms with the genotype abh, we need to consider the segregation of each gene during meiosis. The male is heterozygous for genes A and B (genotype AaBb) and hemizygous for the haemophilic gene (genotype h). The possible combinations for the sperms are AB, Ab, aB, and ab. Since the male is hemizygous for the haemophilic gene, each combination will either have H or h. Therefore, the proportion of sperms with the genotype abh is calculated as follows:
Probability of 'a' = 1/2,
Probability of 'b' = 1/2,
Probability of 'h' = 1/2.
Combining these probabilities: (1/2) * (1/2) * (1/2) = 1/8.

Recessive genes located on the X-chromosome in humans are always expressed in males because males have only one X-chromosome (XY). There is no corresponding allele on the Y-chromosome to mask the effect of a recessive gene on the X-chromosome. In contrast, females have two X-chromosomes (XX), so a recessive gene on one X-chromosome can be masked by a dominant gene on the other X-chromosome.

To find the different types of gametes produced by a pea plant with the genotype AaBb, it should be crossed with a plant that is homozygous recessive for both traits (genotype aabb). This is known as a test cross. The progeny of this cross will display the different combinations of alleles from the AaBb parent, allowing us to observe the different types of gametes produced. Therefore, the plant should be crossed with a plant with the genotype aabb.

Cretinism is not a hereditary disease. It is a condition arising from a deficiency of thyroid hormone, which is often due to a lack of iodine in the diet. Unlike hereditary diseases such as Cystic Fibrosis, Thalassaemia, and Haemophilia, cretinism is not passed down genetically.

A woman with 47 chromosomes due to 3 copies of chromosome 21 is characterized by Down's Syndrome. This condition is also known as Trisomy 21, where the presence of an extra chromosome 21 leads to the characteristic symptoms of Down's Syndrome.

The disease described in the question is likely inherited in a sex-linked recessive pattern. This is suggested by the fact that the disease affects only the sons (males) and not the daughters (females), indicating that the gene responsible for the disease is located on the X chromosome. Males have only one X chromosome, so a single recessive allele on this chromosome will result in the disease, whereas females have two X chromosomes, so they would need two copies of the recessive allele to express the disease.