Sex Linked Gene Names

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Lesson 5 - Sex Linkage

Autosomes and Sex chromosomes

So far all our examples deal with autosomes. Autosomes exist in pairs for all diploid organisms. With our nomenclature system, we've separated one homolog from the other using a slash (/).

There are several mechanisms to determine sex in organisms. Some organisms switch sex over their lifetime. Others rely on the environment.

For this module, we're going with the mammalian system that has homogametic and heterogametic sexes. Males are typically determined by having a "Y" chromosome that is the partner to the single "X" chromosome. Because male gametes are haploid, they can contain either an "X" or a "Y" chromosome; they are heterogametic.

Females have a pair of "X" chromosomes and therefore all eggs will have one or the other version of the "X" chromosome, making her the homogametic gender.

Let's investigate one more term based on this situation. If an organism has two copies of the same allele, it is "homozygous". If it has different alleles, it is "heterozygous". What about genes on the "X" chromosome of a male? It can't be either of those because they are comparing a gene represented on homologous chromsomes. Because there's only one, the genes on the "X" are hemizygous.

Now it's getting confusing! How can we know if a gene is on the X chromosome or not?

Nomenclature to the rescue!

Let's add just one more convention for a gene symbol. Symbols are intended to convey as much information as possible about a locus, so having a handy way to distinguish sex chromosomes from autosomes can help predict inheritance patterns.

We will still use three descriptive letters for the gene symbol and we will capitalize the first letter if the mutant allele is dominant. However, these are placed as a superscript beside a capital letter "X".

In males, the X is hemizygous, but the partner chromosome is simply called "Y". Typically there are no genes complementary to those on the X, so a superscript designation is seldom necessary.

The symbol must include the "X" - the symbol is not just the superscript letters: if the letters are presented, they will refer to an autosome.

As before, the symbol is italicized or underlined - this includes the letters "X" and "Y".

e.g. the white gene in Drosophila melanogaster is on the X chromosome. The mutant allele is recessive and is given the symbol X^wht; the wild-type allele is dominant to the mutation and is given the symbol X^wht+.

Exercise: Writing genotypes for autosomal and sex-linked chromosomes.

Background image credit greyloch via flickr/ https://creativecommons.org/licenses/by-sa/2.0/