Humans have 23 pairs of Chromosomes and in each pair one Chromosome comes from the father and the other comes from the mother. So in all we have 46 Chromosomes in every cell, of which 23 come from the mother and 23 from the father.
Of these 23 pairs, there is one pair called the Sex Chromosomes which decide the gender of the person. During conception, if the resultant cell has XX sex chromosomes then the child will be a girl and if it is XY then the child will be a boy. X chromosome decides the female attributes of a person and Y Chromosome decides the male attributes of a person.
When the initial embryonic cell has XY chromosome, the female attributes get suppressed by the genes in the Y Chromosome and the embryo develops into a male child. Since only men have Y Chromosomes, son always gets his Y Chromosome from his father and the X Chromosome from his mother. On the other hand daughters always get their X Chromosomes, one each from both father and mother.
So the Y Chromosome is always preserved throughout a male lineage (Father . Son - Grandson etc) because a Son always gets it from his father, while the X Chromosome is not preserved in the female lineage (Mother, Daughter, Grand Daughter etc) because it comes from both father and mother.
A mother will pass either her mother’s X Chromosome to her Children or her father’s X Chromosome to her children or a combination of both because of both her X Chromosomes getting mixed (called as Crossover). On the other hand, a Son always gets his father’s Y Chromosome and that too almost intact without any changes because there is no corresponding another Y chromosome in his cells to do any mixing as his combination is XY, while that of females is XX which hence allows for mixing as both are X Chromosomes.Y Chromosome and the Vedic Gotra System
By now you might have got a clue about the relation between Y Chromosome and the Hindu Vedic Gotra System
Y Chromosome is the only Chromosome which gets passed down only between the men in a lineage. Women never get this Y Chromosome in their body. And hence Y Chromosome plays a crucial role in modern genetics in identifying the Genealogy ie male ancestry of a person. And the Gotra system was designed to track down the root Y Chromosome of a person quite easily. If a person belongs to Angirasa Gotra then it means that his Y Chromosome came all the way down over thousands of years of timespan from the Rishi Angirasa! And if a person belongs to a Gotra (say Bharadwaja) with Pravaras (Angirasa, Bhaarhaspatya, Bharadwaja), then it means that the person’s Y Chromosome came all the way down from Angirasa to Bhaarhaspatya to Bharadwaja to the person.
This also makes it clear why females are said to belong to the Gotra of their husbands after marriage. That is because women do not carry Y Chromosome, and their Sons will carry the Y Chromosome of the Father and hence the Gotra of a woman is said to be that of her husband after marriage. Pretty neat isn’t it?
All iz well so far, we now know the science behind the Gotra System. The ancient vedic Rishis hence very well knew the existence of the Y Chromosome and the paternal genetic material that was passed almost intact from father to Son, and hence created the Gotra system to identify their male lineages. Lord Buddha for instance belonged to Gautama Gotra which means that Buddha was a direct descendant of Rishi Gautama.
But then what is the reason to prevent marriages between individuals belonging to the same Gotra? Before we get into that, let us understand a bit more about the Y Chromosome.