Sex Chromosomes
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Contents |
[edit] Introduction
Sex chromosomes are chromosomes which by their presence or absence determine the sex of the organism carrying them.
If you are unclear as to what a chromosome is, or forgetful of the terminology used to discuss them, you should read our introductory article on chromosomes before reading on.
[edit] Sex determination in mammals
A female mammal will have two homologous sex chromosomes, known as X chromosomes because (at least in humans) during mitosis and meiosis they take on the same stubby X shape as other human chromosomes. A male mammal, on the other hand, will have one X chromosome and one Y chromosome. The Y chromosome is a little stub of a chromosome carrying a genetic instruction saying "be male" and very little else; genetic instructions on how to develop as a male are carried on other chromosomes, with the Y chromosome merely acting as a switch, so that the animal will develop as male if it is present and female if it is absent.
During reproduction, a female must pass on one X chromosome to her haploid ovum. In the male, on the other hand, the X and Y chromosome behave, during anaphase I of meiosis, as though they were homologues, and separate to opposite ends of the cell, which then divides. After the second round of cell division, the result is that for every cell that undergoes meiosis, two of the haploid cells produced will have an X chromosome, and two will have a Y chromosome. So on fertilization, a zygote has a 50:50 chance of getting an X from the mother and an X from the father, determining sex as female, or an X from the mother and a Y from the father, determining sex as male.
[edit] Variations
A similar "XY" system of sex determination is found in some insects, such as fruit flies. In other insects, such as grasshoppers, there is what is known as an "XO" system, where the female has two X chromosomes and the male has one X chromosome with nothing at all to pair with it.
It is sometimes said that ants and bees have no sex chromosomes. Another way of looking at this is to say that in ants and bees, every chromosome is a sex chromosome: females have two homologues of every chromosome, whereas males are haploid: that is, they have only one copy of each chromosome. So this is like the "XO" system used by grasshoppers, except that it applies to every chromosome.
In some insects, and in birds, a so-called "ZW" system is used, where males have two homologous "Z" chromosomes, and females have one "Z" and one "W" chromosome.
Not all animals have their sex determined by sex chromosomes: in some groups of animals sex is determined by environmental factors: for example, in reptiles it is determined by the temperature of incubation of the eggs.
[edit] Sex chromosomes and Mendelian genetics
In this section we shall discuss the relationship of sex chromosomes to the Mendel's principles of genetics: readers unfamiliar with these principles should consult our article on Mendelian Genetics before attempting to read on.
In an XY system of sex determination, as found in humans and fruit flies, amongst others, the Y chromosome contains little in the way of genes. This is not true of the X chromosome, which, besides its role in sex determination, also functions as a perfectly normal chromosome. This affects the statistics of Mendelian-style breeding experiments in a way that was unknown to Mendel.
In fruit flies, for example, a gene controlling eye color lies on the X chromosome and has no homologous gene on the Y chromosome. The allele of this gene for white eye color (which we shall call r) is recessive to the allele for red eye color (which we shall call R).
This means that a female can have genotype RR, Rr, or rr, whereas a male's possible genotypes might be written as RY and rY, where Y stands for the Y chromosome.
We can detect such sex-linked genes by breeding experiments. If we breed female fruit flies of a true-breeding white-eyed line with males from a true-breeding red-eyed line, then the females in the parental generation will have genotype rr and the males have genotype RY. Hence, by the law of segregation, 50% of the generation produced by a first cross will have genotype Rr, will be female (since they have no Y chromosome) and will have red eyes (since R is dominant to r); while the other 50% will have genotype rY, will be male, and will have white eyes, since although the r allele is recessive there is nothing on the Y chromosome to dominate it.
If the gene was not sex-linked, so that we were crossing RR with rr, then we should get a very different result, since then all the products of a first cross would have genotype Rr and so would uniformly be red-eyed.
So sex chromosomes provide an exception to what we should expect from Mendel's laws as they were first formulated, but these results still fit nicely into Mendel's overall framework: once we have accepted that the Y chromosome determines sex and is an exception to the principle that every gene has two copies, Mendel's other principles then explain what we observe.
