Non-sex chromosomes are also called autosomes. 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'. So for the genes on the sex chromosomes, males have just one copy. Other dioecious plants have no visibly different pair of chromosomes; they may still have sex chromosomes but not visibly distinguishable types.
During recombination, homologous chromosomes pair up and exchange stretches of DNA. The latter are called differential regions Figure Gene 3 is more closely linked to Gene 2 than to Gene 4. Other dioecious plants have no visibly different pair of chromosomes; they may still have sex chromosomes but not visibly distinguishable types. These and other sex-inked disorders are much more common in boys than in girls. Part a, Leslie Bohm; part b, Anthony Griffiths. The W-chromosome is small with few genes. A reciprocal cross between white-eyed females and red-eyed males gives an F1 in which all the females are red eyed, but all the males are white eyed. Female mammals make eggs, which always have an X chromosome. Homologous chromosomes have the same genes arranged in the same order. Of the species with nonidentical sex chromosomes, a large proportion have an XY system. While sex linkage is not the same as genetic linkage , sex-linked genes can be genetically linked see bottom of page. This inheritance pattern is explained by the alleles being located on the differential region of the X chromosome; in other words, by X- linkage. So females have two copies of every gene, including the genes on sex chromosomes. Some animals can even change from one sex to another. To learn more, visit Sex Determination. In humans and other mammals, the sex chromosomes are X and Y. Female pigeons ZW have just one Z chromosome, and therefore just one allele for each of the genes located there. 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. The genotypes are shown in Figure 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. But the Z-chromosome has many sex-linked genes, including genes that control feather color and color intensity. Egg and sperm join to make a zygote, which develops into a new offspring. The closer together the linked genes are, the less likely it is that a recombination event will happen between them.
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