Sex Determination Principles of Inheritance and Variation PDF

© www.bankofbiology.com Contact us SEX DETERMINATION Autosomes and Sex chromosomes (allosomes) The chromosomes that are involved in sex determination are called sex chromosomes (allosomes). They include X & Y chromosomes. Autosomes are chromosomes other than sex chromosomes....

© www.bankofbiology.com Contact us SEX DETERMINATION Autosomes and Sex chromosomes (allosomes) The chromosomes that are involved in sex determination are called sex chromosomes (allosomes). They include X & Y chromosomes. Autosomes are chromosomes other than sex chromosomes. Number of autosomes is same in males and females.  SEX DETERMINATION Autosomes and Sex chromosomes (allosomes) Henking (1891) studied spermatogenesis in some insects and observed that 50% of sperm  received a nuclear structure after spermatogenesis, and other 50 % sperm did not receive it. Henking called this structure as the X body (now it is called as X chromosome). SEX DETERMINATION XX – XO  Mechanism Mechanism ZZ – Mechanism of sex ZW determination Mechanism XX – XY SEX DETERMINATION  Mechanism of Sex Determination 1. XX-XO Mechanism Here, male is heterogametic, i.e. XO (Gametes with X and gametes without X) and female is homogametic, i.e. XX (all gametes are with X-chromosomes). E.g. Many insects such as grasshopper. 22 + 22 + X0 XX SEX DETERMINATION  Mechanism of Sex Determination 2. XX-XY Mechanism Male is heterogametic (X & Y) and female is homogametic (X only). E.g. Human & Drosophila.  SEX DETERMINATION Mechanism of Sex Determination 3. ZZ-ZW Mechanism Male is homogametic (ZZ) and female is heterogametic (Z & W). E.g. Birds. SEX DETERMINATIONSex Determination in Humans (XX-XY type)  Human has 23 pairs of chromosomes (22 pairs of autosomes + 1 pair of sex chromosomes). A pair of X-chromosomes (XX) is present in female. X & Y chromosomes (XY) are present in male. SEX DETERMINATIONSex Determination in Humans (XX-XY type)  During spermatogenesis, males produce 2 types of gametes- 50% with X chromosome and 50% with Y chromosome. Females produce only ovum with an X-chromosome. SEX DETERMINATIONSex Determination in Humans (XX-XY type)  There is an equal probability of fertilization of ovum with the sperm carrying either X or Y chromosome. The sperm determines whether the offspring male or female. SEX DETERMINATION Sex Determination in Honeybee It is based on the number of sets of chromosomes an individual receives. Fertilised egg develops as a female (queen or worker). An unfertilised egg develops as a male (drone). It is called parthenogenesis. SEX DETERMINATION Sex Determination in Honeybee Therefore, the females are diploid (32 chromosomes) and males are haploid (16 chromosomes). This is called as haplodiploid sex determination system. In this system, the males produce sperms by mitosis. They do not have father and thus cannot have sons, but have a grandfather and can have grandsons.  CLICK HERE FOR NEXT PART CLICK HERE FOR OTHER CHAPTERS