Biology Principles of inheritance and variation class 12 Part 10 Notes

Female Heterogamety

In this type female individuals produces two different types of gametes. Thus the egg determines the sex of offspring.
It involves two types of sex determining mechanism ZW type and ZO type.

i) ZW type

While males possess have homomorphic sex chromosomes.Unlike human being the female contain heteromorphic sex chromosomes.
While males possess have homomorphic sex chromosomes
Because of having heteromorphic sex chromosomes osomes the the femalegre female are heterogametic.

ii) ZO type

It is observed in butterflies, sex determination is exactly opposite the condition found in grasshopper.
In this type, the females have only one sex chromosome and hence represented as ZO.
Females are heterogametic. Males carry two identical sex chromosomes designated ZZ.
Males are homogametic. On fertilization by a Z-carrying sperm the Z-carrying ovum would develop into a male (ZZ) and on fertilization, the Z-lacking ovum would develop into a female.

SEX- DETERMINATION IN HUMANS :-

Human beings have 22 pairs of autosomes and one pair of sex chromosomes.
All the ova (haploid) formed by female are similar in their chromosomes type (22 + x) f Therefore females are homogametic.
Male individuals produces two types of sperm during the process of spermatogenesis.
50% of the total sperm produced possess the X- chromosomes and rest 50% has Y chromosomes beside autosomes.
There is an equal probability of fertilization of ovum overline (22 + X) with the sperm carrying either X and Y chromosomes.
If ovum fertilizes with ((22 + x) type sperm, the zygote develops into a female (44 + XX) .
If the ovum fertilizes with (22 + Y) type sperm, the zygote develops into a male (44+ XY).
Thus, the genetic make up 22 + X determines the sex of child.
It is also clear that in each case of pregnancy there is always 50% of probability of either a male or female child.

Honeybees show haplodiploid method of sex determination.
Haplodiploidy is a sex determination mechanism in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid.
This mode of sex determination was first discovered by Johann, a Catholic priest, in 1845.
In honeybees, sex is normally determined by the fertilization or non-fertilization of the eggs, rather than the presence or absence of sex chromosomes.
In honeybees, the male progeny normally develops from unfertilized eggs, which are haploid and have just one set of chromosomes.
The fertilized honeybee eggs, which are diploid and have two sets of chromosomes, differentiate into queens and worker bees.