Sex Determination

Each zygote inherits twenty-three chromosomes from its mother and twenty-three chromosomes from its father. This does not produce forty-six different chromosomes, but rather twenty-three pairs of homologous chromosomes. The members of a homologous pair, with the important exception of the sex chromosomes, look like each other and contain similar genes (such as those coding for eye color, height, and so on). These homologous pairs of chromosomes can be photographed and numbered (as shown in fig. 20.2). Each cell that contains forty-six chromosomes (that is diploid) has two number 1 chromosomes, two number 2 chromosomes, and so on through pair number 22. The first twenty-two pairs of chromosomes are called autosomal chromosomes.

The twenty-third pair of chromosomes are the sex chromosomes. In a female, these consist of two X chromosomes, whereas in a male there is one X chromosome and one Y chromosome. The X and Y chromosomes look different and contain different genes. This is the exceptional pair of homologous chromosomes mentioned earlier.

When a diploid cell (with forty-six chromosomes) undergoes meiotic division, its daughter cells receive only one chromosome from each homologous pair of chromosomes. The

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■ Figure 20.2 Homologous pairs of chromosomes. These were obtained from diploid human cells. The first twenty-two pairs of chromosomes are called the autosomal chromosomes. The sex chromosomes are (a) XY for a male and (b) XX for a female.

gametes are therefore said to be haploid (they contain only half the number of chromosomes in the diploid parent cell). Each sperm cell, for example, will receive only one chromosome of homologous pair number 5—either the one originally contributed by the mother, or the one originally contributed by the father (modified by the effects of crossing-over; see chapter 3, fig. 3.34). Which of the two chromosomes—maternal or paternal—ends up in a given sperm cell is completely random. This is also true for the sex chromosomes, so that approximately half of the sperm produced will contain an X and approximately half will contain a Y chromosome.

The egg cells (ova) in a woman's ovary will receive a similar random assortment of maternal and paternal chromosomes. Since the body cells of females have two X chromosomes, however, all of the ova will normally contain one X chromosome. Because all ova contain one X chromosome, whereas some sperm are X-bearing and others are Y-bearing, the chromosomal sex of the zygote is determined by the fertilizing sperm cell. If a Y-bearing sperm cell fertilizes the ovum, the zygote will be XY and male; if an X-bearing sperm cell fertilizes the ovum, the zygote will be XX and female.

Although each diploid cell in a woman's body inherits two X chromosomes, it appears that only one of each pair of X chromosomes remains active. The other X chromosome forms a clump of inactive heterochromatin, which can often be seen as a dark spot, called a Barr body, in the nucleus of cheek cells (fig. 20.3). This provides a convenient test for chromosomal sex in cases where it is suspected that the chromosomal sex may differ from the apparent ("phenotypic") sex of the individual. Also, some of the nuclei in the neutrophils of females have a "drumstick" appendage not seen in neutrophils from males.

Formation of Testes and Ovaries

Following conception, the gonads of males and females are similar in appearance for the first forty or so days of development. During this time, cells that will give rise to sperm (called spermatogonia) and cells that will give rise to ova (called oogo-nia) migrate from the yolk sac to the developing embryonic go-nads. At this stage, the embryonic structures have the potential to become either testes or ovaries. The hypothetical substance that promotes their conversion to testes (fig. 20.4) has been called the testis-determining factor (TDF).

Although it has long been recognized that male sex is determined by the presence of a Y chromosome and female sex by the absence of the Y chromosome, the genes involved have only recently been localized. In rare male babies with XX genotypes, scientists have discovered that one of the X chromosomes contains a segment of the Y chromosome—the result of an error

Human Buccal Cells With Barr Bodies

■ Figure 20.3 Barr bodies. The nuclei of cheek cells from females (a) have Barr bodies (arrow). These are formed from one of the X chromosomes, which is inactive. No Barr body is present in the cell obtained from a male because males have only one X chromosome, which remains active. Some neutrophils obtained from females (b) have a "drumstick-like" appendage (arrow) that is not found in the neutrophils of males.

that occurred during the meiotic cell division that formed the sperm cell. Similarly, rare female babies with XY genotypes were found to be missing the same portion of the Y chromosome erroneously inserted into the X chromosome of XX males.

Through these and other observations, it has been shown that the gene for the testis-determining factor is located on the short arm of the Y chromosome. Evidence suggests that it may be a particular gene known as SRY (for sex-determining region of the Y). This gene is found in the Y chromosome of all mammals and is highly conserved, meaning that it shows little variation in structure over evolutionary time.

Notice that it is normally the presence or absence of the Y chromosome that determines whether the embryo will have testes or ovaries. This point is well illustrated by two genetic abnormalities. In Klinefelter's syndrome, the affected person has forty-seven instead of forty-six chromosomes because of the presence of an extra X chromosome. This person, with an XXY genotype, will develop testes and have a male phenotype despite the presence of two X chromosomes. Patients with Turner's syndrome, who have the genotype XO (and therefore have only forty-five chromosomes), have poorly developed ("streak") gonads and are phenotypically female.

The structures that will eventually produce sperm within the testes, the seminiferous tubules, appear very early in embryonic development—between 43 and 50 days following conception. The tubules contain two major cell types: germinal and nongerminal. The germinal cells are those that will eventually become sperm through meiosis and subsequent specialization. The nongerminal cells are called Sertoli (or sustentacular) cells. The Sertoli cells appear at about day 42. At about day 65, the Leydig (or interstitial) cells appear in the embryonic testes. The Leydig cells are clustered in the interstitial tissue that surrounds the seminiferous tubules. The interstitial Leydig cells constitute the endocrine tissue of the testes. In contrast to the rapid development of the testes, the functional units of the ovaries—called the ovarian follicles—do not appear until the second trimester of pregnancy (at about day 105).

Chromosomal Sex Determination

Seminiferous tubules

Interstitial cells

Develop in early embryo

■ Figure 20.4 The chromosomal sex and the development of embryonic gonads. The very early embryo has "indifferent gonads" that can develop into either testes or ovaries. The testis-determining factor (TDF) is a gene located on the Y chromosome. In the absence of TDF, ovaries will develop.

The early-appearing Leydig cells in the embryonic testes secrete large amounts of male sex hormones, or androgens (andro = man; gen = forming). The major androgen secreted by these cells is testosterone. Testosterone secretion begins as early as 8 weeks after conception, reaches a peak at 12 to 14 weeks, and then declines to very low levels by the end of the second trimester (at about 21 weeks). Testosterone secretion during embryonic development in the male serves the very important function of masculinizing the embryonic structures; similarly high levels of testosterone will not appear again in the life of the individual until the time of puberty.

Testosterone

No testosterone No MIF

Degenerates -

Penis, scrotum'

Müllerian inhibition factor (MIF)

Epididymides, Testosterone ductus deferentia, -

ejaculatory ducts

Testosterone

Paramesonephric duct

Mesonephric duct structures

(No testosterone)

Uterus, uterine tubes

Degenerates

Other embryonic (No testosterone) Vagina, labia, clitoris

■ Figure 20.5 The regulation of embryonic sexual development. In the presence of testosterone and mullerian inhibition factor (MIF) secreted by the testes, male external genitalia and accessory sex organs develop. In the absence of these secretions, female structures develop.

As the testes develop, they move within the abdominal cavity and gradually descend into the scrotum. Descent of the testes is sometimes not complete until shortly after birth. The temperature of the scrotum is maintained at about 35° C—about 3° C below normal body temperature. This cooler temperature is needed for spermatogenesis. The fact that spermatogenesis does not occur in males with undescended testes—a condition called cryptorchidism (crypt = hidden; orchid = testes)—demonstrates this requirement.

Associated with each spermatic cord is a strand of skeletal muscle called the cremaster muscle. In cold weather, the cremaster muscles contract and elevate the testes, bringing them closer to the warmth of the trunk. The cremasteric reflex produces the same effect when the inside of a man's thigh is stroked. In a baby, however, this stimulation can cause the testes to be drawn up through the inguinal canal into the body cavity. The testes can also be drawn up into the body cavity voluntarily by trained Sumo wrestlers.

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Responses

  • Jennifer
    Are the two x chromosomes a homologous pair?
    5 years ago
  • Hobson
    How to develope xxy chromosome?
    5 years ago
  • ANNA
    What is important about the twentythird pair of chromosomes?
    5 years ago
  • Bilal
    How many barr bodies in klinefelter syndrome?
    5 years ago
  • venanzio
    What are the genetic implications of having cheek cell with barr body?
    5 years ago
  • charlotte
    What causes the neutrophils to have a drumstick appendage on the nuclei?
    5 years ago
  • Sarah
    When a male undergoes meiosis, how many of the sperm will contain an x?
    5 years ago
  • Andrea
    Do sperm cell go through barr body formation?
    5 years ago
  • mebrat abdullah
    Is gender determined by the twenty third chromosome?
    5 years ago
  • awet
    What the Barr body used in six determination in human?
    1 year ago
  • Berylla
    How many barr bodies are present in a zygote?
    8 months ago
  • eglantine
    How are neutrophils used in sex determination?
    7 months ago
  • keijo
    How to sex in human step with fig?
    3 months ago

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