le X chromosome from his mother, all sons of white-eyed females also have white eyes. A female inherits one X chromosome from her mother and the other X from her father. Red-eyed females may have genes for red eyes in both of their X chromosomes (homozygotes) or may have one X with the gene for red and the other for white (heterozygotes). In the progeny of heterozygous females one half of the sons will receive the X chromosome with the gene for white and will have white eyes, and the other half will receive the X with the gene for red eyes. The daughters of the heterozygous females crossed with white-eyed males will have either two X chromosomes with the gene for white and hence white eyes or will have one X with white and the other X with the gene for red eyes and will be red-eyed heterozygotes.In humans, the red-green colour blindness and hemophilia are among many traits showing sex-linked inheritance and consequently are due to genes borne in the X chromosome.In some animals--birds, butterflies and moths, some fish, and at least some amphibians and reptiles--the chromosomal mechanism of sex determination is a mirror image of that described above. The male has two X chromosomes and the female an X and Y chromosome. Here the spermatozoa all have an X chromosome; the eggs are of two kinds, some with X and others with Y chromosomes, usually in equal numbers. The sex of the offspring is then determined by the egg rather than by the spermatozoon. Sex-linked inheritance is altered correspondingly. A male homozygous for a sex-linked recessive trait, crossed to a female with the dominant one, gives in the F1 generation daughters with the recessive trait and heterozygous sons with the corresponding dominant trait. The F2 generation has recessive and dominant females and males in equal numbers. A male with a dominant trait crossed to a female with a recessive trait gives uniformly dominant F1 and a segregation in a ratio of 2 dominant males : 1...
