In a population of asexually reproducing species, the chances of appearance of new traits due to variations are very low. The trait which is already present in the population is likely to be in higher percentage and would have been arisen earlier. Therefore, the trait B present in 60% of the population is the trait which has arisen earlier.

• Reproductive processes, especially sexual reproduction, give rise to new individuals that are similar, but subtly different. Thus, different variants emerge in a species.
• The variants which are suitable to prevailing environment survive by natural selection. Other members of a species may vanish.
• Environment also goes on changing. Now, individuals (variants) which are suitable to the changed environment will survive. Thus continuity of the species will be maintained.
• For example, Bacteria variants which can tolerate heat will survive better in a heat wave than variant which cannot withstand heat wave. It proves that creation of variations in a species promote survival.

Mendel crossed a pure tall pea plant (TT) with pure dwarf pea plant (tt) and observed that all the progeny were hybrid tall (Tt), i.e. only one of the traits was able to express itself in the F, -generation, which is the dominant trait. The other trait is called the recessive trait which remains suppressed.

However, when he self-crossed plants of F₁ -generation, he observed that one-fourth of the plants were dwarf and three-fourth were tall.

The expressed trait T for the tallness is dominant trait, while the trait 't' of dwarfness is recessive. Thus, Mendel's experiments show that trait may be dominant or recessive.

Mendel took Dihybrid (two pairs of alternative expression of two traits) of a pea plant and crossed them. The F₁ progeny showed only the dominant characteristics among each pair. The F₂ progeny had phenotypes similar to parents but also new phenotypes that did not exist in the parents. This indicates that pair of alternate characteristics behave independently of the other pair and are thus inherited independent of each other.

The information is insufficient to tell whether the trait 'A' or 'O' is dominant. As a person having blood group 'A' may have genotypes I^A I^A or I^A I^O. We can find out by assuming the following cases.

In case I : Let us assume that father has I^A I^O and mother has I^O I^O. In this case, 50% of the progeny will have blood group 'A' and 50% of the progenies will have blood group 'O'.

In case II : Let us assume that father has I^A I^A and mother has I^A I^O. In this case 100% progeny will have blood group 'A'.

So, in case daughter to have blood group 'O', father must have genotype I^A I^O. From this we can infer that I^A is dominant over I^O, as it masks the expression of I^O.

• In human, 23 pairs (46) of chromosomes are present in each cell. Out of these 22 pairs of chromosomes which determine the characters other than sex are called autosomes. The 23rd pair of chromosomes is called sex chromosome. Women have perfect pair of sex chromosomes, both called XX chromosomes. Men have a mismatched pair of chromosomes-one is a normal size X chromosome and a short one called Y chromosome.
• So, women have XX sex chromosomes and men have XY sex chromosomes.
• All children will inherit X chromosome from their mother regardless of whether they are boys or girls. It means that the sex of the children will be determined by what sex chromosome they inherit from their father. A child who inherits an X chromosome from father will be a girl, and one who inherits a Y chromosome from father will be a boy.

(c) TtWW

Where tall (T) and violet (W) are dominant characters while short (t) and white (w) are recessive characters.

TtWW (tall, violet) x ttww (short, white)

Genotypic ratio : TtWw : ttWw = 1:1

Phenotypic ratio : Tall, Violet : Short, Violet = 1 : 1

From this study, we cannot make any inference whether light eye colour trait is recessive or dominant, because both the parents have light colour eye. Thus, all the children will definitely have light colour eye (though certain variations may occur).

In order to find a dominant coat colour in dogs, homozygous black (BB) male dog and a homozygous white (bb) female dog are allowed to cross-breed in order to produce offspring (F₁-generation).

If all the offspring in the F₁-generation are black, this concludes that black colour coat is dominant over white colour coat in dogs and if all the offspring are white, the dominant colour will be white.

• The equal genetic contribution of male and female parents in the progeny is ensured by the equal inheritance of chromosome from each parent.
• Human possesses 23 pairs of chromosomes, of which 22 pairs are autosomes and 1 pair is the sex-chromosomes.
• The two sex-chromosomes in human are X and Y. Females have two X-chromosomes and males have a X and Y-chromosome.
• During the process of fertilisation, a haploid sperm fuses with a haploid ovum to produce a diploid zygote.
• The zygote receives equal amount of genetic material from each parent and thus, retains the diploid nature on fertilisation.