• What are chromosomes? Where are they seated? • What is a sex chromosome? • Explain the mechanism of sex determination in human beings.

• In case of asexually reproducing organisms, there is no gametogenesis and fertilization. Chance separation and chance pairing of genes and their chromosomes are absent. Therefore, asexually developed individual carries the same genes and their chromosomes as are present in its parent.
• Allosomes (Gk. alios— other, soma—boay) or sex chromosomes are those chromosomes which determine the sex of the individual in unisexual organisms. Human beings have 23 pairs of chromosomes.
• Establishment of male and female individuals through differential development of their sex organs is called sex determination. In some organisms sex is determined by environmental conditions. In others including human beings, it is determined genetically.

Environmental Determination of Sex:

• Crepidula (marine mollusc) and Bonellia (marine worm) develop into females if growing alone. In the company of a female, they develop into males.
• In turtle, Chrysema picta an incubation temperature above 33°C produces females while a temperature below 28°C produces only males.
• In lizard, Agama agama, high incubation temperature produces male offspring.
• Annelid Ophryotrocha is male in young state and female later on. Snails are also known to change sex.