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 single 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 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 dominant female : 1 recessive female.
Observations on pedigrees or experimental crosses show that certain traits exhibit sex-linked inheritance; the behaviour of the X chromosomes at meiosis is such that the genes they carry may be expected to exhibit sex-linkage. This evidence still failed to convince some skeptics that the genes for the sex-linked traits were in fact borne in certain chromosomes seen under the microscope. An elegant experimental proof was furnished in 1916 by the U.S. geneticist Calvin Blackman Bridges. As stated above, white-eyed Drosophila females crossed to red-eyed males usually produce red-eyed female and white-eyed male progeny. Among thousands of such “regular” offspring there are occasionally found exceptional white-eyed females and red-eyed males. Bridges constructed the following working hypothesis. Suppose that during meiosis in the female, gametogenesis occasionally goes wrong, and the two X chromosomes fail to disjoin. Exceptional eggs will then be produced carrying two X chromosomes and eggs carrying none. An egg with two X chromosomes coming from a white-eyed female fertilized by a spermatozoon with a Y chromosome will give an exceptional white-eyed female. An egg with no X chromosome fertilized by a spermatozoon with an X chromosome derived from a red-eyed father will yield an exceptional red-eyed male. This hypothesis can be rigorously tested. The exceptional white-eyed females should have not only the two X chromosomes but also a Y chromosome, which normal females do not have. The exceptional males should, on the other hand, lack a Y chromosome, which normal males do have. Both predictions were verified by examination under a microscope of the chromosomes of exceptional females and males. The hypothesis also predicts that exceptional eggs with two X chromosomes fertilized by X-bearing spermatozoa must give individuals with three X chromosomes; such individuals were later identified by Bridges as poorly viable “superfemales.” Exceptional eggs with no Xs, fertilized by Y-bearing spermatozoa, will give zygotes without X chromosomes; such zygotes die in early stages of development.
Two general types of chromosomal abnormalities occur: numerical and structural. Numerical aberrations result from nondisjunction; that is, from the failure of a pair of homologous chromosomes or a pair of sister chromatids to separate during cell division. As described above, when nondisjunction occurs during meiosis two types of germ cells will be formed, those with an extra chromosome and those with a missing chromosome. If one of the former combines with a normal germ cell, the new fertilized egg and all the cells of the individual it produces will have an extra chromosome ; if one of the latter combines with a normal germ cell, the fertilized egg will lack a chromosome. If nondisjunction occurs after fertilization, the resulting individual will be a mosaic and will have two or more populations of cells differing in chromosomal number.
Structural aberrations result from chromosome breakages. Chromosomes may break spontaneously, or they may be broken by such environmental agents as radiation, viruses, and toxic chemicals. If a chromosomal segment breaks off and is not rejoined, it may be lost entirely in the gametes or somatic cells that derive, respectively, from meiosis or mitosis. Such a loss is called a deletion. In other instances, the broken-off segment may rejoin its chromosome but with its position inverted 180?; such inversions can alter the sequence of genetic information along the chromosome. In other cases, the segment may become translocated; that is, it may become attached to a different chromosome. When such a rearrangement occurs between two nonhomologous chromosomes without net loss or gain of chromosomal material, it is called a balanced, or reciprocal, translocation, and the individual is not phenotypically affected. If, however, the translocation results in the deletion or duplication of chromosomal material in gametes or somatic cells, the effects may be severe. This is especially true in the event of gametes that carry autosomal translocations; such chromosomal aberrations often produce lethal phenotypic effects.