Genes: Genes On The X or Y Chromosome

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A gene is a unit of heredity in a living organism. Each cell in the human body contains between 2,500 and 25,000 genes. These genes contain information which determines one’s traits as they are the basic physical and functional unit of heredity. Traits are characteristics inherited from your parents such as eye color or height. These inherited characteristics, or outward appearances of a trait, are also known as phenotypes. All living organisms, including plants, animals and humans, are made up of many genes. Genes are made up of DNA. Genes on each chromosome are arranged in a particular sequence and each gene has a particular location on the chromosome called its locus.

Geneticists use maps to describe the location of a particular gene on a chromosome. One type of map uses the molecular location while the other map uses the cytogenetic location. The cytogenetic location is based on a distinctive pattern of bands created when chromosomes are stained with certain chemicals while the molecular location uses the precise description of a gene’s position on a chromosome and is based in the sequence of DNA building blocks that make up the chromosome.

Biologists thought until very recently that the Y chromosome has only a few active genes. The reason they believed this was because there is no other Y to serve as a pairing partner in meiosis. A gene which is known to be found on the Y chromosome is the key sex determination gene. The SRY gene is known as the “sex determining region y” gene. It is located on the short arm of the Y chromosome at position 11.3. To be more precise, the SRY gene is located from the base pair 2,654,895 to base pair 2,655,791 on the Y chromosome. The main function of this gene is to provide instructions for making a transcription factor called the sex-determining region Y protein. This protein ultimately causes a fetus to develop as a male.

All human embryos develop in a similar way for the first 40 days after conception. At the seventh week, the SRY gene becomes functional. The product of the SRY gene switches the gonad cells of the early human embryo into testes. This in turn triggers the formation of male sexual organs. If the appearance of SRY is blocked, the embryo’s gonad cells continue to become ovaries. If ovaries remain, female sexual organs will develop. In essence, this means that all human embryos will develop into females unless they are made masculine by the product of the SRY gene. The gene has also been studied to understand inconsistencies and rarities in particular people. Such rarities include females who have an XXY or XY karyotype or individuals with a male phenotype and female karyotype.

The X chromosome carries hundred of genes but few have anything directly to do with sex. This is due to the fact that males have only a single X chromosome. Almost all the genes on the X have no counterpart on the Y and thus any gene on the X will be expressed in males. Genes that are passed down in this fashion are described as sex-linked or, more precisely, x-linked.

Sex linkage of the phenotypic expression of an allele relates to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes where both sexes have the same probability of expressing the trait. Because humans have many more genes on the X than the Y, there are many more x-linked traits than y-linked traits.

Resources about the X and Y Chromosome

Info about the X Chromosome
Info about the Y Chromosome

Resource by T.Price

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