- The exploration of sex determination mechanisms owes much to early cytological observations in various species. A significant breakthrough was made by biologist Henking in 1891.
- Henking meticulously traced a specific nuclear structure during spermatogenesis in certain insects, which became known as the "X body."
- Interestingly, he noticed that approximately 50% of sperm cells possessed this structure after spermatogenesis, while the remaining 50% did not.
- Although Henking couldn't decipher the significance of the X body at the time, further investigations by scientists confirmed that it was indeed a chromosome. Hence, it earned the name X-chromosome.
- In numerous insects, a specific type of sex determination known as XO-type was uncovered. In this mechanism, all eggs contain an additional X-chromosome in addition to their autosomes. However, some sperm carry the X chromosome, while others do not.
- In XO-type sex determination, fertilization by an X-carrying sperm results in female offspring, while fertilization by a sperm without an X-chromosome leads to males.
- This groundbreaking work initiated the exploration of sex determination in various species. Subsequently, XY-type sex determination was discovered in mammals, including humans, where both males and females have an equal number of chromosomes.
- Sex determination mechanisms govern whether an organism develops as a male or female.
- In humans and fruit flies (Drosophila), the genetic basis of sex determination relies on the presence of specific sex chromosomes.
- Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
- During reproduction, males contribute either an X or a Y chromosome-bearing sperm, determining the offspring's sex.
- The union of an X-bearing sperm with an X-bearing egg results in a female (XX), while the combination of a Y-bearing sperm with an X-bearing egg produces a male (XY).
ZZ/ZW System (Birds):
- Birds employ a ZZ/ZW system, where males have two Z chromosomes (ZZ), and females have one Z and one W chromosome (ZW).
- Female birds determine their offspring's sex because they can produce eggs with either Z or W chromosomes.
- If a Z-bearing sperm fertilizes the egg (ZZ), it develops into a male, whereas a W-bearing sperm results in a female (ZW).
Grasshoppers use the XO system, where males have one X chromosome (XO), and females have two (XX).
Sex Determination in Honey Bees:
- Honey bees exhibit a unique haplodiploid sex-determination system.
- Females are diploid, possessing 32 chromosomes.
- Males are haploid, having 16 chromosomes, which is half the number of female chromosomes.
- This haplodiploid system results in different reproductive mechanisms.
- Females arise from fertilized eggs, which develop as either queens or workers.
- Males, on the other hand, originate from unfertilized eggs through parthenogenesis.
- Due to their haploid state, males can't have sons but can have grandsons.
- This remarkable system ensures a specialized approach to sex determination in honey bees.
Male Heterogamety and Female Heterogamety:
Male heterogamety refers to a sex determination mechanism where males produce two different types of gametes with distinct sex chromosomes (e.g., XY).
In some organisms, such as birds, a different mechanism exists, known as female heterogamety.
In female heterogamety, both males and females have the same number of chromosomes.
However, females produce two different types of gametes with different sex chromosomes, such as Z and W in birds.
The specific combination of these sex chromosomes in fertilization determines the offspring's sex.