Understanding Genetics - Determining Gender
chromosome males females sex
From the moment of fertilization, the new organism has been assigned a gender, and its growth will proceed to develop either a male or a female organism. The first clues that prompted scientists to consider that the determination of gender was influenced by genetics came from two key observations. The first was the fact that there is a general tendency toward a one-to-one ratio of males to females in all species. The second was the realization that the determination of gender, or sex, followed the principles of Mendelian genetics—gender was predictable as expected when individuals pure for a recessive trait were crossed with individuals that were hybrid.
The determination of gender occurs in all complex organisms, but the processes vary, even among animals. In humans twenty-two of the twenty-three pairs of chromosomes are as likely to be found in males as in females. These twenty-two chromosomes are known as the autosomes, and the twenty-third pair is the sex chromosome. The sex chromosomes of females are identical and are called X chromosomes. In males the pair consists of an X chromosome and a smaller Y chromosome. (See Figure 2.23.)
The Chromosome Theory of Sex Determination
The genetic influence on gender is called the chromosome theory of sex determination, which states that:
- Gender is determined by the sex chromosome.
- In females the sex chromosomes are identical—both are X chromosomes.
- Since females have an XX genotype, all egg cells contain an X chromosome.
- In males the sex chromosomes are not identical; one is X and one is Y.
- Since males have an XY genotype, half of all sperm cells contain an X chromosome and half contain a Y chromosome.
- Upon fertilization, the egg may receive either an X or a Y chromosome from the sperm. Since all egg cells contain an X chromosome, the determination of gender is wholly dependent on the chromosomal composition of the sperm. Sperm carrying the Y chromosome are known as androsperm; those containing the X chromosome are called gynosperm. If the sperm carries the Y chromosome, the offspring will be male (XY); if it carries an X chromosome, the offspring will be female (XX).
The determination of gender occurs at conception with the designation of chromosomal composition that is either XX or XY. But a number of other genetic and environmental influences determine sex differentiation—the way in which the genetically predetermined gender becomes a reality. Differentiation translates the genetically coded message for gender into the physical traits, such as the hormones that influence development of male and female genitalia, body functions, and behaviors associated with gender identity.
Interestingly, humans have an inherent tendency toward female development. Research conducted during the 1940s and 1950s confirmed that in many animals individuals with just a single X chromosome developed as females, although in many instances they did not develop completely and were sterile (unable to reproduce). The absence of the Y chromosome results in female development, while the presence of the Y chromosome sets in motion the series of events that result in male development. These findings led to the premise that female development is the "default option" in the process of gender determination.
The Ratio of Males to Females in the Population
Since human males produce equal numbers of sperm bearing either the X or the Y chromosome, and fertilization is a random event, then it stands to reason that in each generation equal numbers of males and females should be born. An examination of birth statistics in the United States and in other countries where reliable statistics have been compiled over time show that every year there are more births of males than females. For example, according to the Centers for Disease Control and Prevention (CDC), in the United States in 2000 there were 2,076,969 live male births, compared to 1,981,845 live female births, a ratio of 1,048 males per 1,000 females. The CDC reports that the annual sex ratio of births has remained essentially unchanged over the past sixty years, varying by less than 1%.
For years, this difference was attributed to the idea that males were inherently stronger than females and better able to survive pregnancy and birth. This theory was dispelled when researchers found that nearly three times as many male fetuses spontaneously abort (dying before birth). In fact, male life expectancy is less than female life expectancy at every age, from conception to adulthood. (See Table 2.1.) The explanation for the higher proportion of male births appears to be that more male offspring are conceived—possibly as many as 125 to every 100, however, since the prenatal death rate for males is so high; at birth the gap closes to about 105 to 100.
One explanation for the higher number of males conceived is that the smaller and stronger Y sperm are better able to swim quickly and successfully to reach the egg cell. Along with androsperm size and mobility, environmental conditions influence gender determination and the chances of fetal survival. For example, the mother's age and general health are strongly linked to favorable outcomes of conception and pregnancy, and are less strongly linked to but are associated with gender. Younger mothers are more likely to conceive male offspring, by a ratio as high as 120 to 100, and unfavorable prenatal conditions such as poor health or maternal illness are more likely to compromise the survival of the male fetus than the female.
The two sex chromosomes also differ in terms of the genes they contain, which relate to many traits other than gender. The Y chromosome is quite small and carries few
genes other than the one that determines male gender. One of the few confirmed traits linked to the Y chromosome is the hairy ear trait, a characteristic that is distinctive but unrelated to health. Since this trait is located exclusively on the Y chromosome, the trait only appears in males.
The X chromosome is larger and holds many genes that are as necessary for males as they are for females. The genes on the X chromosome are termed X-linked, and characteristics or conditions arising from these genes are called X-linked traits or conditions. Most people, male and female, very likely have several "defective" genes with the potential to produce harmful characteristics or conditions, but these genes are usually recessive and are not expressed in the phenotype unless they are combined with a similar recessive gene on the corresponding chromosome. For this to occur, both parents would have to contribute the same defective gene. Species are also protected from the harmful effects of single defective genes by virtue of the fact that most traits are multigenic (controlled by more than one gene).
Although X-linked dominant alleles affect males and females, males are more strongly affected because they inherit just one X chromosome and do not have a counterbalancing normal allele. Huntington's disease, an inherited neuropsychiatric disease that affects the
body and mind, is an example of a disorder caused by an X-linked dominant allele. Males are affected more frequently and more severely than females by X-linked recessive alleles. The male receives his X chromosome from his mother. Since males have just one X chromosome, all the alleles it contains are expressed, including those that cause serious and sometimes lethal medical disorders. Examples of disorders caused by X-linked recessive alleles range from relatively harmless conditions such as red-green color blindness to the always-fatal Duchenne muscular dystrophy (DMD), one of a group of muscular dystrophies characterized by the enlargement of muscles. DMD is one of the most prevalent types of muscular dystrophy and involves rapid progression of muscle degeneration early in life. All are X-linked and affect mainly males—an estimated one in 3,500 boys worldwide. Another X-linked recessive allele causes Tay-Sachs disease, a disease that is most common among persons of Jewish descent and results in neurological disorders and death in childhood.
All of the male offspring of females who carry X-linked recessive alleles will be affected by the recessive allele. Female children are not as likely to express harmful recessive X-linked traits because they have two X chromosomes. Fifty percent of the female offspring will receive the recessive allele from a mother who carries the allele and an unaffected father. (See Figure 2.22.)
Common Misconceptions about Inheritance
There are many myths and misunderstandings about genetics and inheritance. For example, some people mistakenly believe that in any population dominant traits are inevitably more common than recessive traits. This is simply untrue, as evidenced by the observation that, among humans, the allele that produces six fingers and six toes is dominant over the allele for five fingers and five toes, but the incidence of polydactyly (extra digits) is actually quite low.
Another lingering misconception is that sex-linked diseases occur only in males. This is untrue but it is easy to understand the source of the misunderstanding. For years it was thought that hemophilia (a disease characterized by uncontrolled bleeding) did not occur in females. The observation seemed reasonable since there were no reported cases of the disease among females. While it was true that there were no females with the disease, the reasoning was incorrect. For a female to suffer from hemophilia, she would require a defective recessive gene on both of her X chromosomes, meaning her mother was carrying the gene and the disease affected her father. Since most persons with hemophilia died young, few lived to produce offspring. In other words, female hemophiliacs were rare because the pairings that might produce them were infrequent. During the 1950s the first cases of hemophilia in females were documented, and the theory was discarded.
Finally, the idea that humans are entirely unique in their genetic makeup is false. In fact, human beings share much of their genetic composition with other organisms in the natural world. Furthermore, most human genetic variation is relatively insignificant. Even variations that alter the sequence of amino acids in a protein often produce no discernable influence on the action of the protein. Differences in portions of DNA with as yet unknown functions appear to have no impact at all.