(i) Each human cell contains 23 pairs (46) chromosomes. Out of 23 pairs 22 pairs are autosomes. The last 23rd pair is allosome or sex chromosome.
(ii) The sex chromosomes in males are 2 types, i.e., X and Y. So sex chromosomes in males are XY. These two chromosomes determine the sex of an individual.
(iii) Females have two types of allosomes. They are X and X. Sex chromosomes of fema-les are XX.
(iv) All the gametes (ova) produced by a woman have only X-chromosomes. The gametes (sperm) produced by a man have both X and Y chromosomes.
(v) If the sperm carries Y-chromosome and fertilizes the ovum (X), then the baby will have XY. So the baby will be a boy.
(vi) If the sperm carries X chromosome and fertilises with X chromosome of ovum, the baby will be a girl. Hence, only father decides the sex of the baby.
