Describe chromosomal theory of sex determination.

• The process of sex determination by allosomes (sex chromosomes) is called genetic (or) chromosomal sex determination.
• The sex of the young one is determined at the time of fertilisation.

Male heterogamety: In this male produces dissimilar gametes that decides sex of the offspring and female produces similar gametes.

XX-XO type: In some insects such as bugs, grasshopper & cockroaches females are with
two X chromosomes and males are with one X-chromosome.

• Male produces 2 kinds sperms half of them with X and other half with out sex chromosomes.
• Females (homogametic) produce only one type of ova with 'AX' complement.
  

XX-XY type: This method is seen in human beings and some insects such as Drosophila.

• Males produce two kinds of sperms, half of them with X-chromosome and the other half with Y-chromosome.
• Females produce similar ova with one X-chromosome.
• The sex of the off-spring depends on the fertilizing sperm.
  

Female heterogamety: In this type, males produce similar gametes while females produce dissimilar gametes. Sex of the offspring depends on the type of ovum that is fertilised.

ZO-ZZ type: 

• Female produces two kinds of ova half of them with A Z chromosome and the other half with no sex chromosome.
• Male produces similar types of sperms with 'AZ'.
• Example: Fumea moths and some butterflies
  

ZW - ZZ type: 

• Female produces two kinds of ova. Half of them with allosome 'Z' and other half with allosome-'W'.
• Males produce similar sperms with allosome-Z.
• Example: Fishes, reptiles, birds