DECODE The Script of LIFE

  Sex Determination Mechanism Finalization of sex at the time of zygote formation is called sex determination. Two types of chromosomes are present in individuals – sex chromosomes (which determine the sex of individuals) and autosomes. 1. XX-XY TYPE Seen in many insects and mammals including humans, Drosophila melanogaster. Males have X and Y chromosomes along with autosomes [A] and females have a pair of X chromosomes. 2. XX-XO TYPE Seen in grasshopper. Males have only one X chromosomes besides autosomes and females have a pair of X chromosomes. 3. ZZ-ZW TYPE Seen in birds, fowl and fishes. Females have one Z and one W chromosomes whereas males have a pair of Z chromosomes. 4. XX-XY TYPE Seen in cockroaches. Females have only one Z chromosomes besides autosomes and males have a pair of Z chromosomes.

  Chromosomal Theory of Inheritance The chromosomal theory of inheritance was proposed independently by Walter Sutton and Theodore Boveri in 1902. They stated that behavior of chromosomes was parallel to behavior of genes and used chromosome movement to explain Mendel’s laws. The hereditary factors are carried in the nucleus. Like the Mendelian alleles, chromosomes are also found in pairs. The sperm and eggs having haploid sets of chromosomes fuse to re-establish the diploid state. Morgan extensively worked on fruit flies, Drosophila melanogaster and provided experimental evidence to support the chromosomal theory of inheritance. Comparison between the behavior of Genes and Chromosomes Genes Chromosomes Occurs in Pairs. Occurs in Pairs. Segregate at the time of gamete formation such that only one of each pair is transmitted to a gamete. Segregate at gamete formation and only one of each pair is transmitted to a gamete. It has independent pairs segregate independently of each other. It