Sex Determination (NCERT) - Free MCQ Practice Test with solutions, NEET

In XO type and XY type sex determining mechanisms, males produce two different types of gametes, (a) either with or without X-chromosome (XO type), (b) some gametes with X-chromosome and some with Y-chromosome (XY type). Such type of sex determination mechanism is designated to be the example of male heterogamety. In both, females are homogametic and produce X type of gametes in both the cases and have XX genotype.

In ZW - ZZ type of sex determination, the male has two homomorphic sex chromosomes (ZZ) and is homogametic, and the female has two heteromorphic sex chromosomes (ZW) and is heterogametic. There are, thus, two types of eggs : with Z and with W, and only one type of sperms, i.e., each with Z. Fertilisation of an egg with Z chromosome by a sperm with Z chromosome gives a zygote with ZZ chromosomes (male). Fertilisation of an egg with W chromosome by a sperm with Z chromosome yields a zygote with ZW chromosomes (female). This mechanism operates in some vertebrates (fishes, reptiles and birds).

The possibility of having a girl or boy child is equal i.e., 50% as 50% male gametes are Y type and 50% are X type. Fusion of egg with X type sperm will produce a girl child.

In humans, number of autosomes are 2n = 44 or 22 pairs regardless of the sex.

Honeybees have a haplodiploid sex-determination system, which means that females develop from fertilized eggs and are therefore diploid, while males develop from unfertilized eggs and are therefore haploid. This system is different from the XY sex-determination system found in humans and many other animals, where males have one X and one Y chromosome and females have two X chromosomes. In honeybees, fertilized eggs give rise to females, which have two sets of chromosomes (16 chromosomes from the queen and 16 chromosomes from the drone), while unfertilized eggs give rise to males, which have only one set of chromosomes (16 chromosomes from the queen).