Name: Genetic Mechanisms of Sex Determination
Uploaded: 2019-07-19
Duration: 4 min 34 s
Description: In many species, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology

Question

Discuss the chromosomal theory of sex determination.

Accepted Answer

The chromosome theory of sex determination proposed by McClung states that chromosomes play a major role in the determination of sex in most dioecious organisms.

In diploid organisms, a pair of chromosomes determines the sex of the individual. They are called the sex chromosomes or allosomes. Sex determination can be defined as the process by which the fertilized egg develops the characters of either the male or female.

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. Following conditions will explain the mechanism:

a. XX-XY Condition:

In man, there are two types of sex chromosomes X and Y. The female has a pair of X chromosomes and is expressed as XX, while the male is expressed as XY. 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.

b. 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.

a. ZW-ZZ Condition:

In vertebrates, such as fishes, reptiles and birds, the female has heteromorphic sex chromosomes and males have homomorphic. They are represented as ZW and ZZ, respectively.

b. 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 by the process of parthenogenesis.

D. Genic Balance Theory of Sex-Determination:

Bridges' Genie balance theory for sex determination in Drosophila, states that 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.

E. Environmental Determination of Sex:

In some animals, environmental cues play an important role in the determination of sex.

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. In crocodiles, low temperature induces femaleness and high-temperature maleness.
c. In turtles, temperature below $28^{0} C$ induce maleness and above $33^{0} C$ femaleness.

Discuss the chromosomal theory of sex determination.

Important Questions on Sex Determination

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