How is sex determined in human beings?
In human beings, sex determination follows the XY type mechanism. Out of 23 pairs of chromosomes, 22 pairs are autosomes which are exactly the same in both males and females. Females have a pair of X-chromosomes (XX), whereas males have one X and one Y chromosome (XY) as the determinant of male characteristic.
During spermatogenesis among males, two types of gametes are produced. 50 per cent of the total sperm carry the X-chromosome and the rest 50 per cent carry the Y-chromosome besides the autosomes. Females, however, produce only one type of ovum with an X-chromosome.
There is an equal probability of fertilisation of the ovum with a sperm carrying either X or Y chromosome. In case the ovum fertilises with a sperm carrying X-chromosome, the zygote develops into a female (XX). The fertilisation of ovum with Y-chromosome carrying sperm results into a male offspring.
Explanation
The textbook explains the chromosomal basis of sex determination in humans. The key mechanism is the XY type where males are heterogametic (produce two types of gametes) and females are homogametic (produce one type of gamete). The sex of the offspring is determined by which chromosome (X or Y) the sperm contributes at fertilization. This creates an equal 50-50 probability for male or female offspring.
Solution Steps
Step 1: State the type of sex determination - XY type in humans with 23 pairs of chromosomes (22 autosomes + 1 sex chromosome pair)
Step 2: Describe the chromosomal difference - Females have XX, Males have XY sex chromosomes
Step 3: Explain gamete production - Males produce 50% X-bearing and 50% Y-bearing sperm; Females produce only X-bearing ova
Step 4: Conclude with fertilization outcome - X sperm + X ovum = Female (XX); Y sperm + X ovum = Male (XY)