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4.10. X-Linkage Describes Genes on the X Chromosome

In many animal and some plant species, one of the sexes contains a pair of unlike chromosomes that are involved in sex determination. In many cases, these are designated as the X and Y. For example, in both Drosophila and humans, males contain an X and a Y chromosome, whereas females contain two X chromosomes. While the Y chromosome must contain a region of pairing homology with the X chromosome if the two are to synapse and segregate during meiosis, a major portion of the Y chromosome in humans as well as other species is considered to be relatively inert genetically. While we now recognize a number of male-specific genes on the human Y chromosome, it lacks copies of most genes present on the X chromosome. As a result, genes present on the X chromosome exhibit unique patterns of inheritance in comparison with autosomal genes. The term X-linkage is used to describe such situations.

Below, we will focus on inheritance patterns resulting from genes present on the X, but absent from the Y chromosome. This situation results in a modification of Mendelian ratios, the central theme of this chapter.

X-Linkage in Drosophila

One of the first cases of X-linkage was documented in 1910 by Thomas H. Morgan during his studies of the white eye mutation in Drosophila (Figure 4-11). The normal wild-type red eye color is dominant to white eye color.

Figure 4-11. The F1 and F2 results of T. H. Morgan's reciprocal crosses involving the X-linked white mutation in Drosophila melanogaster. The actual data are shown in parentheses. The photographs show white eye and the brick-red wild-type eye color.

Morgan's work established that the inheritance pattern of the white-eye trait was clearly related to the sex of the parent carrying the mutant allele. Unlike the outcome of the typical Mendelian monohybrid cross where F1 and F2 data were very similar regardless of which P1 parent exhibited the recessive mutant trait, reciprocal crosses between white-eyed and red-eyed flies did not yield identical results. Morgan's analysis led to the conclusion that the white locus is present on the X rather than on one of the autosomes. As such, both the gene and the trait are said to be X-linked.

Results of reciprocal crosses between white-eyed and red-eyed flies are shown in Figure 4-11. The obvious differences in phenotypic ratios in both the F1 and F2 generations are dependent on whether or not the P1 white-eyed parent was male or female.

Morgan was able to correlate these observations with the difference found in the sex chromosome composition between male and female Drosophila. He hypothesized that in males with white eyes, the recessive allele for white eye is found on the X chromosome, but its corresponding locus is absent from the Y chromosome. Females thus have two available gene loci, one on each X chromosome, while males have only one available locus on their single X chromosome.

Morgan's interpretation of X-linked inheritance, shown in Figure 4-12, provides a suitable theoretical explanation for his results. Since the Y chromosome lacks homology with most genes on the X chromosome, whatever alleles are present on the X chromosome of the males will be directly expressed in the phenotype. Because males cannot be either homozygous or heterozygous for X-linked genes, this condition is referred to as hemizygous. In such cases, no alternative alleles are present, and the concept of dominance and recessiveness is irrelevant.

Figure 4-12. The chromosomal explanation of the results of the X-linked crosses shown in Figure 4-11.

One result of X-linkage is the crisscross pattern of inheritance, whereby phenotypic traits controlled by recessive X-linked genes are passed from homozygous mothers to all sons. This pattern occurs because females exhibiting a recessive trait must contain the mutant allele on both X chromosomes. Because male offspring receive one of their mother's two X chromosomes and are hemizygous for all alleles present on that X, all sons will express the same recessive X-linked traits as their mother. This pattern of inheritance is apparent in the pedigree in Figure 4-13.

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Figure 4-13. (a) A human pedigree of the X-linked color-blindness trait. (b) The most probable genotypes of each individual in the pedigree. The photograph is of an Ishihara color-blindness chart. Red-green color-blind individuals see a 3 rather than the 8 visualized by those with normal color vision.

In addition to documenting the phenomenon of X-linkage, Morgan's work has taken on great historical significance. By 1910, the correlation between Mendel's work and the behavior of chromosomes during meiosis had provided the basis for the chromosome theory of inheritance, as postulated by Sutton and Boveri. (See Chapter 3.) Morgan's work, and subsequently that of his student Calvin Bridges, providing direct evidence that genes are transmitted on specific chromosomes, is considered the first solid experimental evidence in support of this theory. In the ensuing two decades, the outcome of research inspired by these findings provided indisputable evidence in support of this theory.

How Do We Know?

How do we know that X-linkage exists and that specific genes are located on the sex-determining chromosomes?

X-Linkage in Humans

In humans, many genes and the traits controlled by them are recognized as being linked to the X chromosome. These X-linked traits can be easily identified in pedigrees, characterized by a crisscross pattern of inheritance. A pedigree for one form of human color blindness is shown in Figure 4-13. The mother in generation I passes the trait on to all her sons, but to none of her daughters. If the offspring in generation II have children by normal individuals, the color-blind sons will produce all normal male and female offspring (III-1, 2, and 3); the normal-vision daughters will produce normal-vision female offspring (III-4, 6, and 7), as well as color-blind (III-8) and normal-vision (III-5) male offspring.

Many X-linked human genes have now been identified, as shown in Table 4.3. For example, the genes controlling two forms of hemophilia and two forms of muscular dystrophy are located on the X chromosome. In addition, numerous genes whose expression yields enzymes are X-linked. Glucose-6-phosphate dehydrogenase and hypoxanthine-guanine-phosphoribosyl transferase are two examples. In the latter case, the severe Lesch-Nyhan syndrome (discussed later in this chapter) results from the mutant form of the X-linked gene product.

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Table 4.3. Human X-Linked Traits

Condition Characteristics

Color blindness, deutan type Insensitivity to green light

Color blindness, protan type Insensitivity to red light

Fabry's disease Deficiency of galactosidase A; heart and kidney defects, early death

G-6-PD deficiency Deficiency of glucose-6-phosphate dehydrogenase; severe anemic reaction following intake of primaquines in drugs and certain foods, including fava beans

Hemophilia A Classical form of clotting deficiency; deficiency of clotting factor VIII

Hemophilia B Christmas disease; deficiency of clotting factor IX

Hunter syndrome Mucopolysaccharide storage disease resulting from iduronate sulfatase enzyme deficiency; short stature, clawlike fingers, coarse facial features, slow mental deterioration, and deafness

Ichthyosis Deficiency of steroid sulfatase enzyme; scaly dry skin, particularly on extremities

Lesch-Nyhan syndrome Deficiency of hypoxanthine-guanine phosphoribosyltransferase enzyme (HPRT) leading to motor and mental retardation, self-mutilation, and early death

Muscular dystrophy Progressive, life-shortening disorder characterized by muscle degeneration and weakness; (Duchenne type) sometimes associated with mental retardation; deficiency of the protein dystrophin

Now Solve this

Problem 4.32 on page 96 asks you to determine if each of three pedigrees is consistent with X-linkage.

Hint: In X-linkage, because of hemizygosity, the genotype of males is immediately evident. Therefore, the key to solving this type of problem is to consider the possible genotypes of females that do not express the trait.

Because of the way in which X-linked genes are transmitted, unusual circumstances may be associated with recessive X-linked disorders in comparison to recessive autosomal disorders. For example, if an X-linked disorder debilitates or is lethal to the affected individual prior to reproductive maturation, the disorder occurs exclusively in males. This is because the only sources of the lethal allele in the population are heterozygous females who are "carriers" and do not express the disorder. They pass the allele to half of their sons, who develop the disorder because they are hemizygous but who rarely, if ever, reproduce. Heterozygous females also pass the allele to half of their daughters, who, like their mothers, become carriers but do not develop the disorder. Examples of such an X-linked disorder in humans include the Duchenne form of muscular dystrophy (DMD) and Lesch-Nyhan disease. (See the screened section on the next page.) DMD has an onset prior to age 6 and is often lethal around age 20. Affected males are unable to reproduce.

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Lesch-Nyhan Syndrome: The Molecular Basis of a Rare X-linked Recessive Disorder

Lesch-Nyhan syndrome (LNS) is a devastating disease that is first apparent in infants at age 3 to 6 months, when orange particles (sometimes referred to as orange sand) appear in the urine and discolor the affected infant's diaper. These urinary stones consist of urate crystals and they are a harbinger of many future difficulties that ultimately lead to premature death. LNS occurs only in males and is the result of the complete or nearly complete loss of activity of a critical enzyme, hypoxanthine-guanine phoshoribosyl-transferase (HPRT). This enzyme imparts the ability to metabolically recycle purines, one of the two major types of nitrogenous bases that make up nucleotides in DNA. While the purines adenine and guanine can be synthesized from basic chemical components, mammals have evolved the ability to extract them from DNA that is being degraded, recovering them in the form of the purine hypoxanthine. Under the direction of HPRT, hypoxanthine can be converted back to adenine and guanine-containing nucleotides. When this mechanism fails as a result of mutation, the excess hypoxanthine is converted to uric acid, which accumulates well beyond the body's ability to excrete it.

This so-called metabolic or biochemical disorder has numerous effects, the most severe being mental retardation, seizures, and aggressive, uncontrolled spastic movements (resembling cerebral palsy) that include self-mutilation of the fingers and lips. Patients require 24-hour care throughout their lives and almost always die prior to age 30, usually as a result of kidney failure. In February 2004, the oldest living LNS patient, Philip Barker, celebrated his thirty-third birthday in Bayview, New York.

The gene involved in LNS is located on the long arm of the X chromosome and consists of 44,000 base pairs (44 Kb). However, the HPRT gene product is only 218 amino acids long, thus requiring only 654 base pairs to encode it. Analysis of the cloned version of the gene reveals it to contain 9 exons and 8 introns. Mice have a nearly identical gene that is 95 percent homologous to its human counterpart.

In normal individuals the enzyme is ubiquitous in tissues throughout the body, but is present in greatest concentration in the basal ganglia of brain cells. No doubt this somehow relates to the behavioral phenotype characterizing LNS patients who lack enzyme activity in the brain and elsewhere in their bodies. In spite of extensive research efforts, there is no known cure. Because the responsible gene is recessive and X-linked and since affected males never reproduce, females, while they can be carriers of the mutant gene, never become homozygous and never develop LNS.