Sex Linkage Sex Linkage Sex linkage applies to genes that are located on the sex chromosomes. These genes are considered sex-linked because their expression and inheritance patterns differ between males and females. While sex linkage is not the same as genetic linkage , sex-linked genes can be genetically linked see bottom of page.
Sex Chromosomes Sex chromosomes determine whether an individual is male or female. In humans and other mammals, the sex chromosomes are X and Y. Females have two X chromosomes, and males have an X and a Y. Non-sex chromosomes are also called autosomes. Autosomes come in pairs of homologous chromosomes. Homologous chromosomes have the same genes arranged in the same order.
So for all of the genes on the autosomes, both males and females have two copies. So females have two copies of every gene, including the genes on sex chromosomes. The X and Y chromosomes, however, have different genes. So for the genes on the sex chromosomes, males have just one copy.
The Y chromosome has few genes, but the X chromosome has more than 1, Well-known examples in people include genes that control color blindness and male pattern baldness. These are sex-linked traits. Inheritence of Sex Chromosomes in Mammals Meiosis is the process of making gametes, also known as eggs and sperm in most animals. During meiosis, the number of chromosomes is reduced by half, so that each gamete gets just one of each autosome and one sex chromosome. Female mammals make eggs, which always have an X chromosome.
And males make sperm, which can have an X or a Y. Egg and sperm join to make a zygote, which develops into a new offspring. An egg plus an X-containing sperm will make a female offspring, and an egg plus a Y-containing sperm will make a male offspring. Female offspring get an X chromsome from each parent Males get an X from their mother and a Y from their father X chromosomes never pass from father to son Y chromosomes always pass from father to son Sex Chromosomes in Pigeons The way sex determination works in birds is nearly the reverse of how it works in mammals.
Male birds have two Z chromosomes, and females have a Z and a W. Male birds make sperm, which always have a Z chromosome. Female gametes eggs can have a Z or a W. The W-chromosome is small with few genes. But the Z-chromosome has many sex-linked genes, including genes that control feather color and color intensity. Some animals can even change from one sex to another. To learn more, visit Sex Determination.
Inheritance of Sex-Linked Genes For genes on autosomes, we all have two copies—one from each parent. The two copies may be the same, or they may be different. Genes code for proteins, and proteins make traits.
Female pigeons ZW have just one Z chromosome, and therefore just one allele for each of the genes located there. One gene on the Z chromosome affects feather color ; three different alleles make feathers blue, ash-red, or brown. In a female bird ZW , her single color allele determines her feather color.
But in males ZZ , two alleles work together to determine feather color according to their dominance. That is, 'ash-red' is dominant to 'blue', which is dominant to 'brown'. A functional second copy can often work well enough on its own, acting as a sort of back-up to prevent problems. With sex-linked genes, male mammals and female birds have no back-up copy. In people, a number of genetic disorders are sex-linked, including Duchenne muscular dystrophy and hemophilia.
These and other sex-inked disorders are much more common in boys than in girls. You need at least one working copy of the gene to be able to see red and green. Since boys have just one X-chromosome, which they receive from their mother, inheriting one defective copy of the gene will render them colorblind. Girls have two X-chromosomes; to be colorblind they must inherit two defective copies, one from each parent.
Consequently, red-green colorblindness is much more frequent in boys 1 in 12 than in girls 1 in The differences in sex chromosomes between males and females leads to specific inheritance patterns for sex-linked genes. Above Female pigeons inherit their color allele from their father. Males inherit one allele from each parent. In humans below , the pattern is reversed. Recombination and Sex-Linked Genes When gametes egg and sperm form, chromosomes go through a process called recombination.
During recombination, homologous chromosomes pair up and exchange stretches of DNA. Recombination makes new allele combinations, which can then be passed to offspring. But when sex chromosomes do have a homologue as in XX female mammals and ZZ male birds , the sex chromosomes recombine to make new allele combinations.
In pigeons, color and dilute color intensity are controlled by two genes on the Z chromosome. In males, recombination between homologous Z chromosomes can make new combinations of color and dilute alleles by chance, some offspring will still receive the same allele combination as the father.
But in females, where the Z chromosome does not recombine, the two alleles always pass to offspring together. The closer together the linked genes are, the less likely it is that a recombination event will happen between them. Gene 3 is more closely linked to Gene 2 than to Gene 4.