Autosomes come in pairs of homologous chromosomes. Females have two X chromosomes, and males have an X and a Y. 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. Recombination and Sex-Linked Genes When gametes egg and sperm form, chromosomes go through a process called recombination.
These are sex-linked traits. Non-sex chromosomes are also called autosomes. Gene 3 is more closely linked to Gene 2 than to Gene 4. Male birds make sperm, which always have a Z chromosome. During recombination, homologous chromosomes pair up and exchange stretches of DNA. A functional second copy can often work well enough on its own, acting as a sort of back-up to prevent problems. Egg and sperm join to make a zygote, which develops into a new offspring. 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. 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 humans and other mammals, the sex chromosomes are X and Y. Males inherit one allele from each parent. 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. Inheritance of Sex-Linked Genes For genes on autosomes, we all have two copies—one from each parent. But in males ZZ , two alleles work together to determine feather color according to their dominance. Male birds have two Z chromosomes, and females have a Z and a W. Sex Chromosomes Sex chromosomes determine whether an individual is male or female. The differences in sex chromosomes between males and females leads to specific inheritance patterns for sex-linked genes. Well-known examples in people include genes that control color blindness and male pattern baldness. But the Z-chromosome has many sex-linked genes, including genes that control feather color and color intensity. 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, That is, 'ash-red' is dominant to 'blue', which is dominant to 'brown'. These and other sex-inked disorders are much more common in boys than in girls. 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.
Video about sex linked genes lesson:
Genetics - X-Linked Genes
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