Principles of Inheritance and Variation

Statement in option (a) is incorrect about breeding. The correct form of statement is as follows

Inbreeding depression decrease the productivity. Inbreeding refers to mating of closely related individuals for 4-6 generation. Increasing homozygosity due to the inbreeding results in decrease in the variation within the group and stabilisation of a particular type (i.e. pureline). Continued inbreeding specially, usually reduces fertility of animals and even their productivity. This condition is called inbreeding depression.

Pleiotropic gene example is skin colour in humans.

Co-dominance example is blood groups in human.

Epistasis example is fruit colour in cucurbita.

Mutation example is sickle-cell anaemia.

(b) When blood group of father is A and of mother is B then the blood group of progeny will be A, B, AB and O.

The problem does not specify whether the parents are homozygous or heterozygous for the alleles of gene I.

Thus, the possible genotype of

parent could be :

(i) When only one parent is homozygous

(ii) When both parents are homozygous

(iii) When both parents are heterozygous

Therefore, in either of the three cases, the alleles that could be contributed from male parent is either '$\mathrm{I^A}$' or '$\mathrm{i}$' likewise the alleles that could be contributed from female parent is either '$\mathrm{I}^{\mathrm{B}}$' or '$\mathrm{i}$'.

Assertion is true, but reason is false.

Down's syndrome, Klinfelter's syndrome and Turner syndrome are chromosomal disorders. In Klinfelter's syndrome males are sterile as individual possess 47 chromosomes ($44+\mathrm{XXY}$).

Down's syndrome It is also called 21 trisomy. It occurs due to the non-disjunction of 21 chromosome during meiosis. It is an autosomal aneuploidy caused by presence of extra chromosomes number $21,(45+\mathrm{XY}$ in male, $45+\mathrm{XX}$ in female).

Turner' syndrome It is due to monosomy. The individual has $(44+\mathrm{XO})$ instead of 46 chromosomes. The females become sterile.

Mendel’s law of independent assortment states that when the parents differ from each other in two or more pair of contrasting characters, the inheritance of one pair of factor is independent of the other. For the character to assort independently they should be located on separate non-homologous chromosomes. Genes present on the same chromosome show linkage. It means that these characters remain together and thus less number of combinations are formed. This phenomenon is called linkage and such genes are called linked genes.

$\begin{aligned} X^{\mathrm{C}} X & =(50 \% \text { carrier daughters }) \\\\ X X & =(50 \% \text { normal daughters }) \\\\ X^C Y & =(50 \% \text { colourblind sons }) \\\\ X Y & =(50 \% \text { normal sons }) \end{aligned}$

XO type of sex chromosomes determine male sex in grasshoppers. This type of sex-determination comes under XX-XO type. It is common in cockroaches, grasshoppers and bugs. The female has two homomorphic sex chromosomes XX and is homogametic. It produces similar eggs, each with one X-chromosome. The male has one chromosome only and is heterogametic. It produces two types of sperms–gynosperms with X and androsperms without X.

Other statements can be corrected as follows

In birds, the females are heterozygous and represented as ZW.

Female sex in humans is represented as XX.

In Drosophila, males are heterogametic, i.e. XY.

Phenylketonuria occurs due to the deficiency of liver enzyme phenylalanine hydroxylase which converts phenylalanine into tyrosine. It occurs in people who are homozygously recessive. It results in a higher level of phenylalanine in blood, tissue fluids and urine.

Genes situated in the same chromosomes are said to be linked. All genes in a single chromosome form a linkage group and usually pass it into the same gamete and are inherited together as a result of linkage. These genes do not show independent assortment, e.g. genes for red hair and prickles are linked genes.