Sex Linked Genes Definition
Sex linked genes are genes that are in the sex chromosomes and that are therefore inherited differently between males and females. In mammals, where the female has two X chromosomes (XX) and the male has one X and one Y chromosome (XY), recessive genes on the X chromosome are more often expressed in males because their only X chromosome has this gene, while females may carry a defective recessive gene on one X chromosome that is compensated by a healthy dominant gene on the other X chromosome. Common examples of sex linked genes are those that code for colorblindness or those that code for hemophilia (inability to make blood clots) in humans. In birds, on the other hand, where the female has two different chromosomes (ZW) and the male has two Z chromosomes (ZZ), it is the female who has higher chances of expressing recessive genes on the Z chromosome because they cannot compensate with the dominant gene on the W chromosome.
In species in which males and females are clearly differentiated, sex chromosomes determine the sex of the organism. In mammals, females have two X chromosomes (XX) and males have one X chromosome and one Y chromosome (XY) (see below for a different pattern of sex chromosome inheritance in birds). The other non-sex chromosomes (called autosomal chromosomes) are the same for males and females, i.e. they code for the same genes. The cells of each individual have two copies of each chromosome although each copy may contain different alleles. In other words, cells have pairs of chromosomes, each pair coding for the same genes (e.g. eye color) but each copy of the chromosome may have a different allele (e.g. one copy may code for blue eyes and the other copy for brown eyes). Humans have 23 pairs of chromosomes, i.e. 46 chromosomes: 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes.
The way sex chromosomes are inherited is quite straightforward. Each organism has two copies of each chromosome; in the case of sex chromosomes this can be either XX (female) or XY (male). Females can thus only transfer X chromosomes to their offspring (because they only have X chromosomes), while males can transfer either one X chromosome or one Y chromosome to their offspring. From the offspring perspective, a female will have inherited one X chromosome from the mother (the only chromosome mothers can transfer to offspring) and the other X chromosome from the father; a male will have inherited one X chromosome from the mother and the Y chromosome from the father.
Sex chromosomes are different from autosomal chromosomes in that the X chromosome is larger than the Y chromosome and, not surprisingly, the distinct sizes entail that each sex chromosome contains different genes (even though there are some genes that are coded in both X and Y chromosomes, but these are not considered sex linked genes). This means that a gene that is coded on the Y chromosome will only be expressed in males, whereas a gene that is coded on the X chromosome could be expressed in males and in females.
Importantly, recessive genes—genes that need two copies to be expressed, otherwise the dominant gene is expressed—have specific consequences on each sex. When a recessive gene is expressed on the X chromosome, it more likely to be expressed in males than in females. This is because males have only one X chromosome, and will therefore express the gene even if it is recessive, whereas females have two X chromosomes and carrying a recessive gene may not be expressed if the other X chromosome carries another dominant gene. This is the reason these genes are called sex linked genes: because they are inherited differently depending on the sex of the organism. Let us look at one example that will make things easier to understand.
An Example: Colorblindness
An example of sex linked genes is colorblindness. Colorblindness is a recessive gene that is only expressed on the X chromosome (let’s use X* for the X chromosome carrying the recessive colorblind gene). If a male receives the colorblind gene from the mother, this individual will be colorblind (X*Y). If, on the other hand, a female receives one colorblind gene (either from the mother or the father) and another healthy gene (not colorblind, either from the mother or the father), then this female organism (XX*) will not be colorblind because the healthy gene is dominant and the recessive colorblind gene will not be expressed. She will be however a carrier, which implies that she can pass on the colorblind gene to her offspring. Finally, if a female receives a colorblind gene from the mother and another colorblind gene from the father, this female will be colorblind (X*X*).
In other words, females can be healthy (XX), carriers (XX*) without being colorblind, and colorblind (X*X*) while males can either be healthy (XY) or colorblind (X*Y). Therefore, the chances of males being colorblind are extremely higher than the chances of females being colorblind. In fact, around 1 in 20 men is colorblind and only 1 in 400 women is.
In birds, the sex of the organisms is also determined by two different chromosomes but instead of the females having two equal chromosomes (XX) and males having to different chromosomes (XY), female birds have two different chromosomes (ZW) and male birds have two equal chromosomes (ZZ).
In pigeons, for instance, an example of a sex linked gene is the one that codes for the color of the feathers. This gene is coded on the Z chromosome, so that whichever allele (ash-red, blue or brown) is expressed on the Z chromosome will determine the feather color of the female. For males, it will depend on both Z chromosomes (ash-red is dominant to blue, and blue is dominant to brown).
Genetic Linkage During Homologous Recombination
When an individual has two copies of the same chromosome (any autosomal chromosome, two X chromosomes in the case of female mammals, or two Z chromosomes in the case of male birds), these chromosomes can recombine during meiosis in a processed called homologous recombination, resulting in swaps of some portions of the chromosomes. To put it simply, the two copies of a chromosome are cut at random places and the cut portion is swapped between both copies. If two genes sit physically close together on the chromosome, they are very likely to be inherited together because the cut during homologous recombination is not likely to happen in between them. Therefore, female mammals (XX) and male birds (ZZ) can show genetic linkage of sex linked genes.
An example of this would be feather color and color intensity in pigeons, both of which are always inherited together in females (ZW) and quite often in males too (ZZ). In males, because color and color intensity sit close together, they are likely to be inherited together because the chromosome cut during recombination is not likely to take place in between, although they can also be mixed and recombined.