Sex Determination
The chromosome involved in sex determination is called the sex chromosome (Allosome). Whereas the chromosome in an organism other than the sex chromosome is called somatic chromosomes (Autosomes). Henking (1891) studied spermatogenesis in some insects in which he observed that 50% of sperm received a nuclear structure after spermatogenesis, other 50% of sperm did not receive it, he named them as ‘X-body’ is now called X-chromosome. There are various mechanisms of sex determination, these are;
- XX-XY type sex determination: Male is heterogametic (X & Y) and female is homogametic (X only). The numbers of chromosomes are the same and the male determines the sex of the offspring. If sperm containing the Y chromosome fertilizes the egg then the offspring will be a male and if sperm containing the X chromosome fertilizes the egg the offspring will be female. E.g. Human & Drosophila.
- ZZ-ZW type sex determination: The male is homogametic (with two Z) and the female is heterogametic (Z & W) i.e. the female determines the sex of the offspring. If the female egg contains a Z chromosome then the offspring will be a male and if the egg contains a W chromosome the offspring will be a female. E.g. Birds, butterflies, reptiles, some fishes, etc.
- XX-XO type sex determination: Here, the male is heterogametic, i.e. XO (produce gametes with X and gametes without X) and the female is homogametic, i.e. XX (all gametes are with X chromosomes). The male determines the sex of the offspring i.e. if the male sperm contains an X chromosome then the offspring will be female and if the male sperm lacks any sex chromosome then the offspring will be a male. E.g. Many insects such as grasshoppers, cockroaches, spiders, bedbugs, etc.
- Haplo-Diploidy type sex determination: Here females are diploid (32 chromosomes) and males are Haploids (16 chromosomes). The males (Drone) produce sperm by mitosis thus they do not have fathers and thus cannot have sons, but have grandfathers and grandsons. Seen in Honey Bees.
Principles of Inheritance and Variation CBSE Notes for Chapter 4
Inheritance is the term given to the process by which characters are passed from parents to offspring which forms the basis of heredity. Heredity is the process of passing down genetic traits from parents to offspring. The degree of difference in characters between a parent and offspring is called variation. All these are studied under genetics which deals with the study of inheritance and variation of characters passed from parents to offspring. The first proper documented study of this inheritance and variation was done by G.J. Mendel which is why he is considered the father of genetics.
In a living cell, the chromosomes of a particular gene are present in a diploid manner and each one is called an allele. Genotype is the genetic make-up of an organism i.e. the TT or tt. Whereas phenotype is the external morphological appearance or expression of the genotype i.e. TT phenotype will be tall and for tt phenotype will be dwarf.
Table of Content
- Mendel’s Laws of Inheritance
- Inheritance of One Gene
- Inheritance of Two Genes
- Chromosomal Theory of Inheritance
- Linkage and Recombination
- Pleiotropy and Polygenic Inheritance
- Sex Determination
- Mutation
- Pedigree Analysis
- Genetic Disorders
- CBSE Previous Year Question Papers