The male of many animals has one chromosome pair, the sex chromosomes, consisting of unequal members called X and Y. At meiosis the X and Y chromosomes first pair, then disjoin and pass to different cells. One-half of the gametes formed contain the X and the other half the Y chromosome. The female has two X chromosomes, and all egg cells normally carry a single X. The eggs fertilized by X-bearing spermatozoa give females (XX), and those fertilized by Y-bearing spermatozoa give males (XY).The genes located in the X chromosomes exhibit what is known as sex-linkage or crisscross inheritance. This is due to a crucial difference between the paired sex chromosomes and the other pairs of chromosomes. The members of the autosome pairs are truly homologous; that is, each member of a pair contains a full complement of the same genes (albeit, perhaps, in different allelic forms). The sex chromosomes, on the other hand, do not constitute a homologous pair, as the X chromosome is much larger and carries far more genes than does the Y. Consequently, many recessive alleles carried on the X chromosome of a male will be expressed just as if they were dominant, for the Y chromosome carries no genes to counteract them. The classic case of sex-linked inheritance, described by Morgan in 1910, is that of the white eyes in Drosophila. White-eyed females crossed to males with the normal red eye colour produce red-eyed daughters and white-eyed sons in the F1 generation and equal numbers of white-eyed and red-eyed females and males in the F2 generation. The cross of red-eyed females to white-eyed males gives a different result: both sexes are red eyed in F1, the females in the F2 generation are red eyed, half of the males are red eyed, and the other half white eyed. As interpreted by Morgan, the gene that determines the red or white eyes is borne on the X chromosome, and the allele for red eye is dominant over that for white eye. Since a male receives its sing...
