II. Sex determination in mammals.

III. X-chromosome inactivation.

IV. Sex and gene expression.

Terms

Notes Page 188: Pseudohermaphrodite should be defined as any person in whom the phenotypic sex does not agree with the genetic sex. Page 189: Not all genes in the "inactive" region of the X chromosome are inactive. Figure 7.22 and several places in text: Persons with XY chromosome constitution are referred to as "males," even though they are females phenotypically. The common practice among professionals is to assign gender on the basis of phenotype rather than genotype. Therefore, these persons should be considered females.

I. Sex can be defined as a means whereby individuals can exchange genes, leading to new combinations of alleles.

A. There are several different patterns of sex, although the most common is two sexes.
1. In some species, different individuals belong to one sex or the other, i.e. male or female, + or –, a or alpha.

2. In other species, a single individual may have two different sexual structures, e.g. many plants have both male and female organs.

B. A number of different factors determine whether an individual develops as male or female.
1. The chromosome constitution, e.g. mammals, birds, fruit flies, many plants.

2. Body size, e.g. some fish are male when little and female when big.

3. Environment, e.g. incubation temperature in the case of some turtles.

C. Mammals have XX/XY sex determination.
1. Females are XX and produce only X-bearing gametes; they are homogametic.

2. Males are XY and produce X-bearing and Y-bearing sperm; they are heterogametic.

3. The sex ratio (ratio of males to females) is approximately but not quite 1.0.

D. The phenotypic differences between males and females is called sexual dimorphism. In many species, sexual dimorphism is small. In humans, sexual dimorphism is obvious, though not so much so as in many other species.

A. Presence of the SRY (sex-determining region on the Y) gene on the short arm of the Y chromosome causes a gonad to develop as a testis; otherwise it develops as an ovary. This gene is also referred to as TDF (testis-determining factor).
1. The interstitial cells of the testis secrete testosterone ( an androgen or male hormone), which stimulates embryonic Wolffian ducts to develop into internal male structures (epidydimis, seminal vesicles, vas deferens). Testosterone also enhances development of male genitalia and other secondary sex characteristics (muscle mass, body hair, etc.).

2. The seminiferous tubules of the testis secrete anti-Mullerian hormone (AMH), called Mullerian-inhibiting hormone in the textbook, which suppresses Mullerian duct development. In the absence of AMH, the Mullerian duct develops into Fallopian tubes, uterus, and the upper third of the vagina.

3. The ovary secretes estrogen, which enhances development of female genitalia.

B. Some abnormalities of sexual development are due to defective genes.
1. In androgen insensitivity (testicular feminization), cells are completely unresponsive to testosterone, due to lack of an active steroid receptor. Therefore, Wolffian duct development does not occur and genitalia are not stimulated to form male structures. However, AMH response is normal and internal female organs do not develop.

2. In 5-alpha reductase deficiency (identified in text as "one form of pseudohermaphroditism - p. 188), development of male genitalia does not occur until puberty. This disorder is caused by inherited inability to convert testosterone to dihydrotestosterone (DHT). Apparently DHT is critical for development of male genitalia during embryonic and fetal development. However, the great increase in testosterone that occurs at puberty causes male genitalia to form.

3. XX males occur rarely, most often due to translocation of the SRY gene onto an X chromosome. The individuals are effectively XXY.

4. XY females have also been observed. In these cases, the SRY gene is nonfunctional or is lost from the Y chromosome.

5. Congenital adrenal hyperplasia (CAH) is an autosomal recessive defect in the synthesis of adrenal hormones. These share the same precursor as testosterone, and the adrenal gland, trying to respond to the need for adrenal hormones, shunts the production to testosterone instead. This causes masculinization of young females and premature sexual development in young males. Correction of the adrenal problem corrects the aberrant sexual development.

A. Once an X chromosome is inactivated, it remains inactive through subsequent mitotic divisions. The inactive X is seen in the nucleus as a Barr body or sex chromatin body.

B. Much of the short arm of the X chromosome is not inactivated. For genes in this region, females produce twice as much gene product as do males.

C. Because of the small number of cells present in the embryo during inactivation, chance can cause either of the X chromosomes to be overrepresented among the active X's. This sometimes leads to expression of an X-linked recessive trait in a heterozygous female.

A. Sex-influenced traits can occur in both males and females, but expression is different. Pattern baldness is the classical example of a sex-influenced trait, but other traits may be expressed somewhat differently in males and females.

B. Sex-limited traits can appear only in one sex. Examples are genetic differences in expression of a feature that occurs only in one sex. An example is "precocious puberty," which is due to unregulated production of testosterone in young boys, who enter puberty at 1-2 years of age. It is inherited as an autosomal dominant trait that has no effect in heterozygous females.

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