Principles of Inheritance and Variation - Notes | Class 12 | Part 6: Sex Determination

• 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.
• The number of autosomes is the same in males and females.
• Henking (1891) studied spermatogenesis in some insects and observed that 50% of sperm received a nuclear structure after spermatogenesis, and the other 50% did not receive it. Henking called this structure the X body (now called the X-chromosome).

Mechanism of Sex Determination

• XX-XO mechanism: 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 grasshoppers.
• XX-XY mechanism: Male is heterogametic (X & Y), and female is homogametic (X only). E.g., humans & Drosophila.
• ZZ-ZW mechanism: Male is homogametic (ZZ), and female is heterogametic (Z & W). E.g., birds.

XX-XO & XX-XY mechanisms show male heterogamety.

ZZ-ZW mechanism shows female heterogamety.

Sex Determination in Humans (XX-XY Type)

• Humans have 23 pairs of chromosomes (22 pairs of autosomes and 1 pair of sex chromosomes).
• A pair of X-chromosomes (XX) is present in the female, whereas X and Y chromosomes are present in the male.
• 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.
• There is an equal probability of fertilization of the ovum with the sperm carrying either X or Y chromosome.

• The sperm determines whether the offspring is male or female.

Sex Determination in Honeybee

• It is based on the number of sets of chromosomes an individual receives.
• A fertilised egg develops as a female (queen or worker).
• An unfertilised egg develops as a male (drone). This is called parthenogenesis.
• Therefore, the females are diploid (32 chromosomes), and males are haploid (16 chromosomes). This is called a haplodiploid sex determination system.
• In this system, the males produce sperms by mitosis. They do not have a father and thus cannot have sons, but have a grandfather and can have grandsons.

Select a Topic 👇

Topic 1: Mendel's Experiments Topic 2: Inheritance of One Gene Topic 3: Inheritance of Two Genes Topic 4: Other Patterns of Inheritance Topic 5: Chromosomal Theory, Morgan's Experiment Topic 6: Sex Determination Topic 7: Mutation and Pedigree Analysis Topic 8: Genetic Disorders (Mendelian and Chromosomal)

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