Sex Determination and Sex-Linked Inheritance

In Mendel’s work, reciprocal crosses always gave identical results; it did not matter, in general, whether a dominant allele was contributed by the mother or by the father. But in some cases, the parental origin of a chromosome does matter. For example, as we saw at the beginning of this chapter, human males inherit hemophilia Afrom their mother, not from their father. To understand the types of inheritance in which the parental origin of an allele is important, we must consider the ways in which sex is determined in different species.

Sex is determined in different ways in different species

In corn, a plant much studied by geneticists, every diploid adult has both male and female reproductive structures. The tissues in these two types of structures are genetically identical, just as roots and leaves are genetically identical. Plants such as corn, in which the same individual produces both male and female gametes, are said to be monoecious (from the Greek, “one house”). Other plants, such as date palms and oak trees, and most animals are dioecious (“two houses”), meaning that some individuals can produce only male gametes and the others can produce only female gametes. In other words, dioecious organisms have two sexes. In most dioecious organisms, sex is determined by differences in the chromosomes, but such determination operates in different ways in different groups of organisms. For example, the sex of a honeybee depends on whether it develops from a fertilized or an unfertilized egg. A fertilized egg is diploid and gives rise to a female bee—either a worker or a queen, depending on the diet during larval life (again, note how the environment affects the phenotype). An unfertilized egg is haploid and gives rise to a male drone: In many other animals, including humans, sex is determined by a single sex chromosome, or by a pair of them. Both males and females have two copies of each of the rest of the chromosomes, which are called autosomes. Female grasshoppers, for example, have two X chromosomes, whereas males have only one. Female grasshoppers are described as being XX (ignoring the autosomes) and males as XO (pronounced “ex-oh”): Females form eggs that contain one copy of each autosome and one X chromosome. Males form approximately equal amounts of two types of sperm: One type contains one copy of each autosome and one X chromosome; the other type contains only autosomes. When an X-bearing sperm fertilizes an egg, the zygote is XX, and develops into a female. When a sperm without an X fertilizes an egg, the zygote is XO, and develops into a male. This chromosomal mechanism ensures that the two sexes are produced in approximately equal numbers. As in grasshoppers, female mammals have two X chromosomes and males have one. However, male mammals also have a sex chromosome that is not found in females: the Y chromosome. Females may be represented as XX and males as XY: Males produce two kinds of gametes. Each gamete has a complete set of autosomes, but half the gametes carry an X chromosome and the other half carry a Y. When an X-bearing sperm fertilizes an egg, the resulting XX zygote is female; when a Y-bearing sperm fertilizes an egg, the resulting XY zygote is male.

The X and Y chromosomes have different functions

Some subtle but important phenotypic differences show up clearly in mammals with abnormal sex chromosome constitutions. These conditions, which result from nondisjunctions, as described in Chapter 9, tell us something about the functions of the X and Y chromosomes. In humans, XO individuals sometimes appear. Human XO individuals are females who are physically moderately abnormal but mentally normal; usually they are also sterile. The XO condition in humans is called Turner syndrome. It is the only known case in which a human can survive with only one member of a chromosome pair (here, the XY pair), although most XO conceptions terminate spontaneously early in development. XXY individuals also occur; this condition is known as Klinefelter syndrome. People with this genotype are sometimes taller than average, always sterile and always male. These observations suggested that the gene that determines maleness is located on the Y chromosome. Observations of people with other types of chromosomal abnormalities helped researchers to pinpoint the location of that gene:

Some XY individuals are phenotypically women and lack a small portion of the Y chromosome.

Some men are genetically XX and have a small piece of the Y chromosome present but attached to another chromosome.

The Y fragment that is missing and present in these two examples, respectively, contains the maleness-determining gene, which was named SRY (sex-determining region on the Y chromosome).

The SRY gene encodes a protein involved in primary sex determination—that is, the determination of the kinds of gametesthat will be produced and the organs that will makethem. In the presence of functional SRY protein, the embryodevelops sperm-producing testes. (Notice that italic type isused for the name of a gene but roman type is used for thename of a protein.) If the embryo has no Y chromosome, the SRY gene is absent, and thus the SRY protein is not made. Inthe absence of the SRY protein, the embryo develops egg-producingovaries. In this case, a gene on the X chromosomecalled DAX1 produces an anti-testis factor. So the role of SRY in a male is to inhibit the maleness inhibitor encoded by DAX1. The SRY protein does this in male cells, but since it isnot present in females, DAX1 can act to inhibit maleness.Primary sex determination is not the same as secondary sex determination, which results in the outward manifestations ofmaleness and femaleness (body type, breast development,body hair, and voice). These outward characteristics are notdetermined directly by the presence or absence of the Y chromosome.Rather, they are determined by genes scattered onthe autosomes and X chromosome that control the actions ofhormones, such as testosterone and estrogen.The Y chromosome functions differently in Drosophila melanogaster. Superficially, Drosophila follows the same patternof sex determination as mammals—females are XX andmales are XY. However, XO individuals are males (ratherthan females as in mammals) and almost always are indistinguishablefrom normal XY males except that they are sterile.XXY Drosophila are normal, fertile females: Thus, in Drosophila, sex is determined by the ratio of X chromosomesto autosome sets. If there is one X chromosome foreach set of autosomes, the individual is a female; if there isonly one X chromosome for the two sets of autosomes, theindividual is a male. The Y chromosome plays no sex-determiningrole in Drosophila, but it is needed for male fertility. Caenorhabditis elegans is a favorite model organism forstudies of development . This tiny worm hastwo sexes: male and hermaphrodite (self-fertilizing). Asin fruit flies, sex is determined by the X:autosome ratioindividualswith a ratio below 0.67 are male.In birds, moths, and butterflies, males are XX and femalesare XY. To avoid confusion, these forms are usually expressedas ZZ (male) and ZW (female): In these organisms, the female produces two types of gametes,carrying Z or W. Whether the egg is Z or W determines the sex of the offspring, in contrast to humans and fruit flies, in which the sperm, carrying either X or Y, determines the sex.

Genes on sex chromosomes are inherited in special ways

Genes on sex chromosomes do not show the Mendelian patterns of inheritance we have described above. In Drosophila and in humans, the Y chromosome carries few known genes, but a substantial number of genes affecting a great variety of characters are carried on the X chromosome. Any such gene is present in two copies in females, but in only one copy in males. Therefore, females may be heterozygous for genes that are on the X chromosome but males will always be hemizygous for genes on the X chromosome — they will have only one copy of each, and it will be expressed. Thus, reciprocal crosses do not give identical results for characters whose genes are carried on the sex chromosomes, and these characters do not show the usual Mendelian ratios for the inheritance of genes located on autosomes. The first and still one of the best examples of inheritance of characters governed by loci on the sex chromosomes (sexlinked inheritance) is that of eye color in Drosophila. The wild-type eye color of these flies is red. In 1910, Morgan discovered a mutation that causes white eyes. He experimented by crossing flies of the wild-type and mutant phenotypes. His results demonstrated that the eye color locus is on the X chromosome. When a homozygous red-eyed female was crossed with a (hemizygous) white-eyed male, all the sons and daughters had red eyes because red is dominant over white and all the progeny had inherited a wild-type X chromosome from their mothers.

However, in the reciprocal cross, in which a white-eyed female was mated with a red-eyed male, all the sons were white-eyed and all the daughters were red-eyed.

The sons from the reciprocal cross inherited their only X chromosome from their white-eyed mother; the Y chromosome they inherited from their father does not carry the eye color locus.

The daughters, on the other hand, got an X chromosome bearing the white allele from their mother and an X chromosome bearing the red allele from their father; they were therefore red-eyed heterozygotes.

When heterozygous females were mated with red-eyed males, half their sons had white eyes, but all their daughters had red eyes. Together, these results showed that eye color was carried on the X chromosome and not on the Y.

Humans display many sex-linked characters

The human X chromosome carries about two thousand genes. The alleles at these loci follow the same pattern of inheritance as those for white eyes in Drosophila. One human X chromosome gene, for example, has a mutant recessive allele that leads to red-green color blindness, a hereditary disorder. Red-green color blindness appears in individuals who are homozygous or hemizygous for the mutant allele. Human mutations inherited as X-linked dominant phenotypes are rarer than X-linked recessives because dominant phenotypes appear in every generation, and because people carrying the harmful mutation, even as heterozygotes, often fail to survive and reproduce.

The small human Y chromosome carries several dozen genes. Among them is the maleness determinant, SRY. Interestingly, for some genes on the Y, there are similar, but not identical, genes on the X. For example, one of the proteins that make up ribosomes has a gene on the Y that is expressed only in male cells, while the X-linked counterpart is expressed in both sexes. This means that there are “male” and “female” ribosomes; the significance of this phenomenon is unknown. Y-linked alleles are passed only from father to son. (You can verify this with a Punnett square.)