Written Assignment 2

In humans, the sex of a baby is determined by its father. The sequence of events involved in sex determination is instigated by one special pair of chromosomes, the sex chromosomes, which carry information that directs a growing embryo to develop as a male or as a female. We’ve noted that there are 23 pairs of chromosomes in every somatic cell. These can be divided into two different types: 1 pair of sex chromosomes and 22 pairs of non-sex chromosomes. The human sex chromosomes are called the X and Y chromosomes (Figure 8-29). How do the X and Y chromosomes differ from the other chromosomes? All of the genetic information is stored on the chromosomes in all the cells of an organism’s body. But most of this information is not sex specific—that is, if you are building an eye or a neuron or a skin cell or a digestive enzyme, it doesn’t matter whether it is for a male or for a female; the instructions are the same for both sexes. Some genetic information, however, instructs the body to develop into one sex or the other. That information is found on the sex chromosomes. An individual has two copies of all the non-sex chromosomes (called autosomes). One copy is inherited from the mother, one from the father. Individuals also have two copies of the sex chromosomes, but not always two copies of the same kind. Males have one copy of the X chromosome and one copy of the Y chromosome. Females, on the other hand, don’t have a Y chromosome but instead have two copies of the X chromosome.

So how does the father determine the sex of the baby? During meiosis in females, the gametes that are produced carry only one copy of each chromosome, including the sex chromosomes. Half of the gametes receive a copy of one of the X chromosomes, and half receive a copy of the other X chromosome. Thus, every egg has an X for its one sex chromosome. During meiosis in males, the sperm that are produced also carry one copy of each chromosome, including the sex chromosomes, but in this case, half of the sperm inherit the X chromosome and the other half inherit the Y chromosome. At fertilization, an egg bearing a single X chromosome is fertilized by a sperm bearing either an X chromosome or a Y chromosome. When the sperm carries an X, the baby will have two X chromosomes and will develop as a female. When the sperm carries a Y, the baby will have an X and a Y and so will develop as a male (Figure 8-30). Females don’t have a Y chromosome in any of their cells, yet they are able to develop and live normal, healthy lives. For this reason, we know that nothing on the Y chromosome is absolutely necessary for the development of a normally functioning human. Physically, the X and Y chromosomes look very different from each other. The X chromosome is relatively large and carries a great deal of genetic information relating to a large number of non-sex-related traits. The Y chromosome is tiny and carries genetic information about only a very small number of traits. The genetic instructions on the Y chromosome set off a cascade of gene activity, instructing the fetal gonads to develop as testes rather than ovaries. (In the absence of these instructions from a Y chromosome, the fetus develops as a female.) Once this is done, very little additional genetic input from the Y chromosome is necessary. Instead, the hormones produced by the testes or, in the absence of the Y chromosome, by the ovaries generally direct the rest of the body to develop as a male or female. Later, we investigate some of the ramifications of males having two different sex chromosomes.

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A variety of other methods of sex determination are used in other species. In most plants, there aren’t even distinct male and female individuals. Every individual produces male and female gametes in separate reproductive parts on the same plant. The same goes for all earthworms and garden snails. Such organisms are called hermaphrodites, because male and female gametes are produced by a single individual. Among birds, it is the females that have one copy of two different sex chromosomes, while males have only one type, so the sex of bird offspring is determined by the female. In ants, bees, and wasps, sex is determined by the number of chromosome sets an individual possesses. Males are haploid and females are diploid. A female can fertilize an egg with sperm she has stored after mating, producing a female. Or she can lay the unfertilized egg, which develops into a male. And in some species, sex determination is controlled by the environment. In most turtles, for example, offspring’s sex is determined by the temperature at which the eggs are kept. We explore this in the next section.

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Unexpected observations may be a sign that our ideas about how the world works are not quite right. As such, they provide a great opportunity for scientists to solve a problem. Figuring out how to approach the problem can be like solving a puzzle, and the solution can have far-reaching implications and unexpected importance. In 1966, Madeleine Charnier reported a surprising observation in an obscure publication from West Africa. For a lizard species she observed, it seemed that the sex ratio of the offspring produced was influenced by the environment. In warmer temperatures, most of the eggs that hatched contained females. And in cooler temperatures, most of the hatched eggs contained males. As members of a species in which our sex is determined by the chromosomes we inherit from our parents, it isn’t surprising that we’d assume that sex determination would be the same for all animals. Charnier’s observation, however, which spurred biologists to notice similar patterns of sex determination in other reptiles and some amphibian species, suggested that our understanding of sex determination wasn’t quite right.

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To test whether incubation temperature really could influence offspring sex, researchers conducted a laboratory experiment. They set up incubators at two different temperatures: one cool (25°C) and one warm (30.5°C). They then collected turtle eggs. From each clutch of eggs, they put half of the eggs in the cool hatched, the results were as clear as could be. At 25°C, all 210 offspring produced were males. At 30.5°C, all 211 offspring produced were females. The researchers concluded that—at least in the lab—the temperature could influence the sex of the offspring.

In the initial lab experiment, the temperature was constant. In the turtles’ natural habitat, it fluctuates every day: it is cool at night and warmer in the day. To better approximate natural conditions, the researchers conducted two additional experiments. First, in the laboratory incubators they created fluctuating temperatures. In one, the temperature fluctuated from 20° to 30°C and back again each day. In the other, the temperature fluctuated similarly, but between 23°and 33°C. The results were identical to the first experiment. In the cooler incubator, 100% of the offspring hatching were male. In the warmer incubator, 100% were female.

In the final study, the eggs collected from each clutch were again divided evenly, but this time they were incubated in the turtles’ natural habitat. Half were buried at a shaded nesting site that received little sun exposure (and rarely exceeded 30°C). The other half were buried at a nesting site that was exposed to the sun (and often exceeded 30 C). Of the 100 eggs hatching at the shaded site, all were males. Of the 127 eggs hatching at the exposed site, 123 were females. Based on these results, would you be confident that incubation temperature does indeed influence the sex of the offspring? Can you think of any alternative explanations?

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Based on models of climate change, it is likely that the average temperature in North America will increase by about 4°C over the next 100 years. Scientists have predicted that such a rapid change could have a significant impact on biological systems. There has been relatively little direct empirical evidence for such impacts, however. One researcher, though, decided to test this prediction using temperature-dependent sex determination in turtles.

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