A person has one of the four blood groups: A, B, AB or O. This blood group system is controlled by a gene which has three different forms denoted by the symbols IA, IB and 1°. The genes IA and IB show no dominance over each other, that is, they are co dominant. However, genes IA and IB both are dominant over the gene 1°. In other words, the blood gene I° is recessive in relation to genes IA and IB.

Although there are three gene forms (called alleles) for blood, but any one person can have only two of them. So, the blood group of a person depends on which two forms of the genes he possesses.

(i) If the genotype (gene combination) is IAIA, then the blood group of the person is A. And if the genotype is IAI° even then the blood group is A (because I° is a recessive gene).

(ii) If the genotype is IBIB, then the blood group of the person is B. And if the genotype is IB I° even then the blood group is B (because I° is a recessive gene).

ADVERTISEMENTS:

(iii) If the genotype is IAIB, then the blood group of the person is AB.

(iv) If the genotype is I0I°, then the blood group of the person is O.

Let us solve one problem now.

Sample Problem:

ADVERTISEMENTS:

A man having blood group A marries a woman having blood group O and they have a child. What will be the blood group of the child?

Answer. The answer to this question depends on whether the blood group A of the man has gene combination IAIA or IAI°.

(i) When the blood group A has genotype IAIA:

In this case the genotype of man’s blood is IAIA and that of woman’s blood is I°I°. So, the child will have blood group A (because the gene IA is dominant over gene I°).

ADVERTISEMENTS:

(ii) When the blood group A has genotype IAI°. Here the genotype of man’s blood is IAI° and that of woman’s blood is I°I°. So, in this case there is an equal chance that the genotype of child’s blood can be either IAI° or I°I°. Due to this, there is an equal chance of the child acquiring blood group A or blood group O.

Just as the blood group is inherited by a child from its parents, in the same way the sex of a child (boy or girl) is also inherited from the parents: mother and father. We will now describe the inheritance of sex by a child from the parents. Inheritance of sex is also known as sex determination. Please note that while discussing the determination of sex of a child, we use letter symbols to describe whole sex chromosomes rather than individual genes. The sex chromosomes are:

XX for a female (girl)

XY for a male (boy)

ADVERTISEMENTS:

Sex Determination:

A person can have a male sex or a female sex. The process by which the sex of a person is determined is called sex determination. Genetics is involved in the determination of the sex of a person. This can be explained as follows.

The chromosomes which determine the sex of a person are called sex chromosomes. There are two types of sex chromosomes, one is called X chromosome and the other is called Y chromosome.

(i) A male (man or father) has one X chromosome and one Y chromosome. This means that half the male gametes or half the sperms will have some and one Y chromosome chromosomes. X chromosomes and the other half will chromosomes.

ADVERTISEMENTS:

(ii) A female (woman or mother) has two X chromosomes (but no Y chromosomes). This means that all the female gametes called ova (or eggs) will have only X chromosomes.