In Mendel’s work, reciprocal crosses always gave similar results; it did not matter whether a dominant allele was contributed by the female parent or the male parent. But in some cases, the parental origin of a chromosome does matter. For example, human males inherit a bleeding disorder called hemophilia from 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 DETERMINATION BY CHROMOSOMES In corn, every diploid adult has both male and female reproductive structures. The tissues in these two types of structure are genetically identical, just as roots and leaves are genetically identical. Organisms such as corn, in which the same individual produces both male and female gametes, are said to be monoecious (Greek, “one house”). Other organisms, such as date palms and most animals, are dioecious (“two houses”), meaning that some individuals can produce only male gametes and others can produce only female gametes. In other words, in dioecious organisms the different sexes are different individuals.

In mammals and birds, sex is determined by differences in the chromosomes, but such determination operates in different ways in different groups of organisms. For example, in many animals, including mammals, sex is determined by a single pair of sex chromosomes, which differ from one another. The remaining chromosomes, called autosomes, occur in pairs in males and females. For example, in humans there are 22 pairs of autosomes in males and females, and 1 pair of sex chromosomes. The chromosomal bases for sex determination in various groups of animals are summarized in Table 12.2.

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The sex chromosomes of female mammals consist of a pair of X chromosomes. Male mammals, by contrast, have one X chromosome and a sex chromosome that is not found in females, the Y chromosome. Females may be represented as XX (homogametic) and males as XY (heterogametic).

MALE MAMMALS PRODUCE TWO KINDS OF GAMETES Each gamete produced by a male mammal has a complete set of autosomes, but just as each gamete carries one copy of each pair of autosomes, 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.

SEX CHROMOSOME ABNORMALITIES REVEALED THE GENE THAT DETERMINES SEX Can we determine which chromosome, X or Y, carries the sex-determining gene, and can the gene be identified? One way to determine cause (e.g., the presence of a gene on the Y chromosome) and effect (e.g., maleness) is to look at cases of biological error, in which the expected outcome does not happen.

We can learn something about the functions of X and Y chromosomes from abnormal sex chromosome arrangements resulting from nondisjunction during meiosis or mitosis (see Key Concept 11.5). As you will recall, nondisjunction occurs when a pair of homologous chromosomes (in meiosis I) or sister chromatids (in mitosis or meiosis II) fails to separate. As a result, a gamete may have one too few or one too many chromosomes. If this gamete fuses with another gamete that has the full haploid chromosome set, the resulting offspring will be aneuploid, with fewer or more chromosomes than normal.

In humans, XO individuals sometimes appear. The O indicates that a chromosome is missing—that is, individuals that are XO have only one sex chromosome (an X). Human XO individuals are females who are moderately abnormal physically but normal mentally; usually they are also sterile. The XO condition in humans is called Turner syndrome. It is the only known case in which a person can survive with only one member of a chromosome pair (here, the XY pair), although most XO conceptions are spontaneously terminated early in development. XXY individuals also occur; this condition, which affects males, is called Klinefelter syndrome and results in overlong limbs and sterility.

These observations suggested that the gene controlling maleness is located on the Y chromosome. Observations of people with other types of chromosomal abnormalities helped researchers pinpoint the location of that gene:

The Y fragments that are respectively missing and present in these two cases are the same and contain 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 gametes that an individual will produce and the organs that will make them (the male and female gonads). In the presence of the functional SRY protein, an embryo develops sperm-producing testes. (Notice that italic type is used for the name of a gene, but roman type is used for the name of a protein.) If the embryo has no Y chromosome, the SRY gene is absent, and thus the SRY protein is not made. In the absence of the SRY protein, the embryo develops egg-producing ovaries. In this case, a gene on the X chromosome called 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 is not present in females, DAX1 can act to inhibit maleness.

One function of the gonads is to produce hormones (such as testosterone and estrogen) that send signals to the rest of the body and control the development of secondary sex characteristics. These are outward manifestations of maleness and femaleness, such as differences in body type, breast development, body hair, and voice. Secondary sex characteristics distinguish males and females but are not directly part of the reproductive system.

SEX-LINKED INHERITANCE IN FRUIT FLIES As noted in Table 12.2, sex determination in fruit flies is based on the proportions of sex chromosomes, because the numbers of these chromosomes can vary. But most commonly, the fruit fly genome has four pairs of chromosomes: three pairs of autosomes and (as in humans) a pair of sex chromosomes that differ in size. In this case, the female fly has two X chromosomes and the male has only one, the other being the Y chromosome—so the female is XX and the male is XY. As in other organisms, the X and Y chromosomes are not true homologs of one another: many genes on the X chromosome are not present on the Y. The X chromosome of Drosophila was one of the first to have specific genes assigned to it.

Thomas Morgan identified a sex-linked gene that controls eye color in Drosophila. The wild-type allele of the gene confers red eyes, whereas a recessive mutant allele confers white eyes. Morgan’s experimental crosses demonstrated that this eye color locus is on the X chromosome. If we abbreviate the eye color alleles as R (red eyes) and r (white eyes), the presence of the alleles on the X chromosome is designated by X^R and X^r.

Morgan crossed a homozygous red-eyed female (X^RX^R) with a white-eyed male. The male is designated X^rY because the Y chromosome does not carry any allele for this gene. (Any gene that is present as a single copy in a diploid organism is called hemizygous.) All the sons and daughters from this cross had red eyes, because the red phenotype is dominant over white and all the progeny had inherited a wild-type X chromosome (X^R) from their mother (Figure 12.18A). This phenotypic outcome would have occurred even if the R gene had been present on an autosome rather than a sex chromosome. In that case, the male would have been homozygous recessive—rr.

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Heterozygous, X^RX^r. The female parent passed the X^r chromosome to the offspring, which had a Y chromosome from the male parent.

When Morgan performed the reciprocal cross, in which a white-eyed female (X^rX^r) was mated with a red-eyed male (X^RY), the results were unexpected: Instead of all offspring being heterozygous with red eyes, all the sons were white-eyed and all the daughters were red-eyed (Figure 12.18B). The sons from the reciprocal cross inherited their only X chromosome from their white-eyed mother and were therefore hemizygous for the white allele. The daughters, however, got an X chromosome bearing the r allele from their mother and an X chromosome bearing the R allele from their father; therefore they were red-eyed heterozygotes. When these 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.

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These and other experiments led to the term sex-linked inheritance: inheritance of a gene that is carried on a sex chromosome. (This term is somewhat misleading because “sex-linked” inheritance is not really linked to the sex of an organism—after all, both males and females carry X chromosomes.) In mammals, the X chromosome is larger and carries more genes than the Y. For this reason, most examples of sex-linked inheritance involve genes that are carried on the X chromosome.

Many sexually reproducing species, including humans, have sex chromosomes. As in most fruit flies, human males are XY, females are XX, and relatively few of the genes that are present on the X chromosome are present on the Y. Pedigree analyses of X-linked recessive phenotypes like the one in Figure 12.19 reveal the following patterns (compare with the pedigrees of non–X-linked phenotypes in Figure 12.7):