Principles of Inheritance & Variation Test 6

Turtles are one of the organisms which rely on environmental factors for sex determination completely. They usually depend upon temperature for this purpose.

The sex of a grasshopper is determined genetically. Some experiments were carried out by some of the scientists on insects to study the genetic or chromosomal mechanism of sex determination.

The foundation of the chromosomal basis of sex determination was laid down by Henking in 1891. He studied a specific nuclear structure though spermatogenesis in a few insects.

Stevens was the scientist who first discovered Y-chromosome. Many scientists concluded that the ‘X body’ which was discovered by Henking was a chromosome. Therefore, in 1905, X and Y chromosomes were named as sex chromosomes by Stevens and Wilson.

Birds do not show male heterogametic conditions. Instead, they show the female heterogametic condition. Male heterogametic condition is characterised by male individuals producing two different types of gametes.

When the condition of male heterogamety is present, then sperm determines the sex of the offspring because two kinds of sperms are present, one with X-chromosome and the other with Y-chromosome. Eggs of an individual are the same as both of them contain 22+ X chromosomes.

In a grasshopper, male and female individuals have a different number of chromosomes because grasshoppers exhibit XO type of male heterogamety where 2 kinds of sperms are formed- one with X-chromosome and the other with no sex chromosome.

Grasshoppers do not show XY type of male heterogametic condition. XY type of male heterogametic condition is shown by a number of insects like Drosophila and mammals including human beings and elephants, the males in this type bear two types of sex chromosomes- X and Y type.

The probability of having a girl child in the sixth pregnancy, where the first five progenies were male is 50%. As humans show XY type of male heterogamety, where 50% sperms carry 22+ X chromosomes and the other 50% carry 22+Y chromosomes, therefore in each pregnancy there is 50% probability of either a male or a female child.

TDF stands for testis determining factor. TDF is essential for the development of male sex organs and if it is not produced in the body then it leads to the formation of the female sex organ.