Question

Compare and contrast the three methods of sex determination.

Answer 1

Establishment of male and female individuals or male and female organs of an individual is called sex determination. It is of three types ;

• environmental.
• genic.
• chromosomal.

Environmental

Environmental sex determination is a mechanism in which an individual's sex is decided after conception, according to its immediate environment. A previous theory proposed that environmental sex determination is adaptive in certain life histories by allowing control of sex in response to environmental effects on fitness. Although plausible, this theory did not explain how environmental sex determination evolves from an alternative mechanism.

A model is presented here to describe the evolution of environmental sex determination from genotypic sex determination with male heterogamety (XX♀/XY♂). As environmental sex determination evolves, both sexes become XX and male heterogamety disappears. Except for certain types of X/Y sex chromosome effects, a pre-existing genotypic mechanism of sex determination does not intrinsically interfere with the evolution of environmental sex determination, and the adaptation theory gives the correct conditions for its evolution. The widespread occurrence of XX♀/XO♂ sex chromosome systems in nematodes suggests that genotypic sex determination is possibly ancestral to environmental sex determination in this group, but convincing evidence is lacking.

Genic

In simpler terms, the sex determination is achieved by a balance of female determinants on the X chromosome (X) and male determinants on the autosomes (A). This type of sex determination is called Genic Balance System. Instead of XY chromosomes, sex is determined by the genic balance or ratio between X-chromosomes and autosome genomes. It means that expression of maleness is not controlled by Y- chromosome but is instead localised on autosomes.

Heterochromatisation of one X-chromosome is maintained by a gene Xist which is expressed only in the inactive chromosome. Heterochromatisation of one X-chromosome provides for dosage compensation in females as it equalises the X-linked genes in the two sexes (males have only one X-chromosome). The X-chromosome is reactivated in meiotic prophase.

Chromosomal.

In mammals, primary sex determination is strictly chromosomal and is not usually influenced by the environment. In most cases, the female is XX and the male is XY. Every individual must have at least one X chromosome. Since the female is XX, each of her eggs has a single X chromosome. The male, being XY, can generate two types of sperm: half bear the X chromosome, half the Y. If the egg receives another X chromosome from the sperm, the resulting individual is XX, forms ovaries, and is female; if the egg receives a Y chromosome from the sperm, the individual is XY, forms testes, and is male. The Y chromosome carries a gene that encodes a testis-determining factor. This factor organizes the gonad into a testis rather than an ovary.

A male mammal has a penis, seminal vesicles, and prostate gland. A female mammal has a vagina, cervix, uterus, oviducts, and mammary glands. In many species, each sex has a sex-specific size, vocal cartilage, and musculature. These secondary sex characteristics are usually determined by hormones secreted from the gonads. However, in the absence of gonads, the female phenotype is generated.

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