Introduction to sex chromosomes: it’s not just X and Y

Sex chromosomes were historically distinguished from autosomal chromosomes because they were easily visible under the microscope, and depending on the species of the organism being studied, these distinctive chromosomes tracked with sex through generations.  Geneticists later found that these chromosomes carry genes associated with the development of sex structures, called sex-determination genes.

In humans and other mammals, males typically have one X and one Y chromosome and are said to be heterogametic. Females typically have two X chromosomes and are homogametic.

Note 1: In this text, you’ll see the XX genotype described as “typically” or “most commonly” associated with a female phenotype and the XY genotype with males. The reason for the “typically” or “most commonly” description is that there are many exceptions to this, both in humans and other organisms. For example, there are individuals with sex chromosome aneuploidies (SCAs), with only one X chromosome (described sometimes as XO) or more than two sex chromosomes (XXX, XXY, or XXXX, for example).  There are also many individuals for whom their sex chromosome pair (XY or XX) does not align with either the appearance of sex-associated traits or gender.

Did I Get It?

Note 2: Although humans and many other species (including some plants^[1]!) use X and Y sex chromosomes, other organisms use different systems of sex determination.  Birds, for example, use Z and W sex chromosomes.  In yet other species, sex can be determined by environmental conditions or even the ploidy of an organism.  In honeybees, for example, males are typically haploid, and females are diploid^[2].

The X and Y chromosomes share similar DNA sequences at the ends

During meiosis I, the autosomes are paired with their homolog. For both autosomes and sex chromosomes, the pairing happens due to sequence similarities between the sister homologs.  This pairing is discussed in more detail in the chapter on meiosis and mitosis. The pairing is possible, even when that “pairing” is between XY or ZW chromosomes because the very ends of the sex chromosomes contain homologous sequences. These are called pseudoautosomal (PAR) regions because both males and females have two copies of all the genes in those regions.  The pseudoautosomal regions of the X and Y chromosomes are shown in Figure 1.

During meiosis, chromosome pairs – including XX or XY pairs – are separated into different daughter cells.  This results in haploid daughter cells.  Each daughter cell ultimately contains one copy of each autosome and one sex chromosome.  For mammals, eggs typically carry an X chromosome and sperm carry either an X or Y chromosome.  For birds, eggs typically contain either a Z or a W chromosome, while all sperm contain a Z chromosome.

Sex is a phenotype, like any other observable trait. Sex-determination genes are genes that control the development of sex-associated traits.

In mammals, fruit flies, and some flowering plant embryos, females usually (but not always) have an XX genotype, while males usually (but not always) have an XY genotype. In birds and some reptiles and amphibians, males typically have a ZZ genotype, and females have a ZW genotype. Note that females are not always the homogametic sex – they don’t always have two of the same chromosome.

These genotypes are associated with either male or female phenotypes due to the presence of sex-determination genes located on the sex chromosomes. However, the mechanism for the development of sex-associated phenotypes is different depending on species. For example, although humans and Drosophila both have X and Y sex chromosomes, they have different mechanisms for determining sex.

Table 1 lists chromosomal methods of sex determination in several species. In humans and other mammals, the SRY gene on the Y chromosome triggers the development of testes in the early embryo.

Fruit flies and certain other insects also use an XY system of sex determination, but the mechanism of sex determination is different. For fruit flies, the ratio of X chromosomes to autosomes determines sex phenotype due to the expression of autosomal genes that, in turn, influence the expression of sex-determination genes on the X chromosome.

In other insects, there’s no Y chromosome at all! The number of X chromosomes influences maleness. In birds and some other species, the DMRT1 gene on the Z chromosome initiates the process of sex development. Still, it is haploinsufficient: one copy of the gene does not produce enough gene product to trigger maleness, so ZW individuals will typically develop female anatomy.

Note: some species do not use sex chromosomes! Sex in some species can be determined by autosomes or even environmental conditions. In honeybees, sex is determined by whether eggs are fertilized: unfertilized eggs develop into males, while fertilized eggs develop into females. In many turtles, sex is determined by environmental temperature: cooler temperatures are associated with male development, and warmer temperatures with female development^[3].

In all organisms, the sex chromosomes are associated with sex determination because they house some, but not all, of the genes responsible for determining sex. Other sex-determination genes are located on the autosomes. The sex chromosomes also house genes not responsible for sex determination at all. These are called sex-linked genes, but they have nothing to do with sex beyond their chromosomal locus. Some examples in humans are a gene linked with color blindness and a gene that affects the strength of tooth enamel.

In the next video, hear a short talk from evolutionary biologist Joan Roughgarden on sexual diversity in nature. Then check out the article On the Originality of Species from Stanford Magazine to learn more about Roughgarden and her work!

Did I Get It?