Match List – I with List – II.

Choose the correct answer from the options given below:

(A)-(III), (B)-(I), (C)-(IV), (D)-(II)

The correct answer is Option (4) → (A)-(III), (B)-(I), (C)-(IV), (D)-(II)

A. The failure of segregation of chromatids during cell division, resulting in the gain or loss of chromosomes, is known as Aneuploidy. For example, Down’s syndrome results in the gain of extra copy of chromosome 21. Similarly, Turner’s syndrome results due to loss of an X chromosome in human females.

B. Co-dominance occurs when both alleles in a heterozygous individual contribute to the phenotype. In the case of the ABO blood group system, the I^A and I^B alleles are co-dominant. This means that if an individual has both I^A and I^B alleles (genotype I^AI^B), both alleles are expressed, and the individual has blood type AB, which has both A and B antigens on the surface of the red blood cells. A good example of co-dominance is different types of red blood cells that determine ABO blood grouping in human beings.

C. When a single gene can exhibit multiple phenotypic expression such a gene is called a pleiotropic gene. The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways which contribute towards different phenotypes. An example of this is the disease phenylketonuria, which occurs in humans. The disease is caused by mutation in the gene that codes for the enzyme phenyl alanine hydroxylase (single gene mutation). This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.

D. Colour Blidness : It is a sex-linked recessive disorder due to defect in either red or green cone of eye resulting in failure to discriminate between red and green colour. This defect is due to mutation in certain genes present in the X chromosome. It occurs in about 8 per cent of males and only about 0.4 per cent of females. This is because the genes that lead to red-green colour blindness are on the X chromosome.