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According to the chromosome theory of sex determination proposed by McClung, chromosomes play a major role in determination of sex in most of the dioeciously organisms.
In diploid organisms, a pair of chromosomes determines the sex of individual. They are called the sex chromosomes or allosomes or heterosomes. Sex determination is the process by which the fertilised egg develops the characters of either the male or female. This process occurs at the time of fusion of egg and sperm.
Based on the chromosome theory of sex determination, the main types are:
a. Heterogametic Males:
When the males produce two types of gametes, such a male is called heterogametic male.
The heterogametic males may be of the following types:
i. XX-XY Condition:
In man and most insects, and among plants such as Melandrium album, Rumex, Populus, etc. there are two types of sex chromosomes. One is larger and is known as the X chromosome and the other is smaller and is known as the Y chromosome. The female has a pair of X chromosomes and is expressed as XX, while the male is expressed as XY. The female is therefore described as hemimorphic and the male as heteromorphic.
The X and the Y chromosomes in males act as homologous chromosomes during meiosis and pass into different gametes. Thus, males produce two types of gametes and are described as heterogametic while the female, which produces only one type of gamete, is homogametic (Fig. 9).
ii. XX-XO Condition:
In insects, such as cockroaches and some roundworms, the Y chromosome is missing in males and is expressed as XO, while the female is XX.
b. Heterogametic Females:
When the female produces two types of gametes, such a female is called heterogametic female.
i. ZW-ZZ Condition:
In vertebrates, such as fishes, reptiles and birds and insects such as butterflies and moths, the female has heteromorphic sex chromosomes and males have homomorphic. They are represented as ZW and ZZ, respectively.
ii. ZO-ZZ Condition:
In some butterflies and moths, the W chromosome is lacking and the female is ZO and the male is ZZ.
c. Haplodiploidy:
Haplodiploidy is a condition seen in honeybees, wasps, ants and termites. The female is diploid, while the male is haploid. The diploid female is formed by the fusion of the sperm and egg, while the male is formed directly from the egg.
Chromosomes of the body other than the sex chromosomes are called autosomes. The genes present on the autosomes are called autosomal genes. The inheritance of autosomal characters does not show variation with sex i.e., the results of reciprocal crosses are identical.
Genic Balance Theory of Sex-Determination:
C.B. Bridges in 1921 put forth the Genie balance theory for sex determination in Drosophila, according to which the X chromosomes are the carriers of genes for femaleness and autosomes are the carriers of genes for maleness.
According to this theory, sex is determined by the ratio of the X chromosomes and autosomes. If the value of X-chromosome is considered as 1½ (one and half), the value of Y is 0 and the value of one haploid autosome set is taken as 1.
Let A represent a haploid set of autosomes. When the value of the autosomes exceed the sex chromosomes, the organism is a male, while when the value of autosomes is lesser, then the organism is a female. In a normal male (AAXY) the male and female determinants are in the ratio of 2:1, the genie balance is in favour of maleness.
A normal female (AAXX) has a male and female ratio of 2:3, the balance is in favour of femaleness (Table 2) and summarise the various combinations as seen in Drosophila.
In man, the Y-chromosome carries a gene known as the sry gene, which codes for a protein called Testis-Determining Factor (TDF). TDF is required for the differentiating the gonads into the testes which in turn produce the hormone testosterone. Testosterone induces the development of the male reproductive tract.
Environmental Determination of Sex:
In some animals, environment plays an important role in the determination of sex.
A few examples are explained below:
a. The larva of Bonellia, a marine worm develops into a female, if it settles down alone. A larva coming in contact with the already grown female, changes into male and lives as a parasite in the uterus of the female.
b. The larva of Crepidula develops into a male in the company of a female.
c. In crocodiles, low temperature induces femaleness and high temperature maleness.
d. In turtles, temperature below 28°C induce maleness and above 33°C femaleness.