1. Environmental:

• In some animals, Zygote doesn’t differ genetically for sex determination.
• In such cases, Sexual differentiation is determined by environmental factors.
• Eg: Larvae of sea worm,

2. Chromosomal:

• Sex chromosome or allosome differs in number or kind between the male and female individuals of the same species and is involved in sex determination.
• There are two types of chromosomes i.e., X and Y chromosome and appear in both male and female but in different number.
• Y chromosome occurs either in male and female but never in both.
• Usually, the major portion of the Y chromosome is genetically inactive and may or mayn’t determine maleness depending on the species.

3. Genic:

• It states that the sex of an individual is determined by a balance between the genes for maleness and those for femaleness present in the individual.
• In drosophila, genes for maleness are present in autosomes, while those for femaleness are located in the X chromosome.
• The strength of genes for maleness and femaleness is so balanced that when the number of X chromosome and that of autosomal sets is equal in an individual, develops into female.
• An individual develops into a male only when the no. of its X chromosome is exactly half the number of its autosomal sets.
• The ratio of X chromosome to its autosomal sets is known as sex index.

i.e. Sex index = No. of X chromosome/ No. of Autosomal sets = X/A.

Sex expression in drosophila