Mendelian Genetics is a set of biological inheritance which highlights the laws proposed by Gregor Mendel in 1866 and rediscovered in 1900. These laws faced a few controversies before Mendel’s theories got integrated with the chromosome theory of inheritance and it soon became the heart of classical genetics
Sex determination in Humans
Sex determination is the mechanism of determining the gender of the offspring during reproduction in an organism. It was first discovered in insects by Henking. He observed at a nuclear level that fifty percent of the sperms got a specific characteristic during spermatogenesis while the rest did not. He named this character as X body which later came to be known as the X chromosome. Thus, the sperm containing the X chromosome when fertilized the egg, gave rise to a female offspring and the sperm which did not have the X chromosome gave rise to a male offspring on fertilization. Some insects follow this mode of determination of gender which is otherwise called XO type of sex determination.
It could be observed from this example that the X chromosome determines the gender and hence such chromosomes are termed as the sex chromosomes and the rest of the chromosomes in the cell are called autosomes.
In all the cases of sex determination, the number of sex chromosomes and the types of chromosomes determining the sex might differ in either of the genders viz., it may show
- Male heterogamety – The unequal number of male chromosomes or heterozygous chromosomes. Example – Humans
- Female heterogamety- The unequal number of female chromosomes or heterozygous chromosomes. Example – Birds
The number of autosomes, however, remains constant in all the cells.
Sex determination in humans is represented by the XY type of sex determination. In humans, XX chromosomes determine the female features and the XY chromosomes determine the male features of the baby. The presence of an X chromosome or Y chromosome in the sperm determines whether the baby would be a male or a female.
The total genome in humans is represented by
- Female chromosomes – 44+XX where 44 are the autosomes and XX chromosomes are the sex chromosomes.
- Male chromosomes – 44+XY, where 44 are the autosomes and XY chromosomes, are the sex chromosomes.
In humans, the sex determination could be represented by the following figure:
This shows a fifty percent probability to get a male or a female baby. Thus, during reproduction, the gender of a baby could be predicted during pregnancy.
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