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The Sex of Offspring Is Determined by Particular Chromosomes

A photograph shows a basic light microscope on a flat surface. The microscope has a single eyepiece and three lenses of different magnifications.
In humans and many other animal species, sex is determined by specific chromosomes. How did researchers discover these so-called sex chromosomes? The path from the initial discovery of sex chromosomes in 1891 to an understanding of their true function was paved by the diligent efforts of multiple scientists over the course of many years. As often happens during a lengthy course of discovery, scientists observed and described sex chromosomes long before they knew their function.

An idea inspired by the "X element"

By the 1880s, scientists had established methods for staining chromosomes so that they could be easily visualized using a simple light microscope. With this staining method, scientists were able to observe cell division and to identify the steps that occurred during both mitosis and meiosis (Figure 1).

A photomicrograph of chromosomes in a dividing cell is shown beside a schematic illustration of a cell in anaphase. In the photomicrograph, chromosomes are being pulled apart toward the cell's opposite poles, leaving an empty space between the two sets of chromosomes. The chromosomes resemble worms. In the illustration, the two sister chromatids of each chromosome have become separated due to the action of the spindle fibers and are shown migrating to opposite poles of the cell. There are two sets of green chromatids and two sets of orange chromatids. The spindle fibers are white. Arrows at the top and bottom of each cell indicate the axis along which the cell divides.
Figure 1: Cell division observed through the microscope (left) is redrawn to show the action of chromosomes (right). Arrows indicate the axis along which the cell divides.

The first indication that sex chromosomes were distinct from other chromosomes came from experiments conducted by German biologist Hermann Henking in 1891. While using a light microscope to study sperm formation in wasps, Henking noticed that some wasp sperm cells had 12 chromosomes, while others had only 11 chromosomes. Also, during his observation of the stages of meiosis leading up to the formation of these sperm cells, Henking noticed that the mysterious twelfth chromosome looked and behaved differently than the other 11 chromosomes. Accordingly, he named the twelfth chromosome the "X element" to represent its unknown nature. Interestingly, when Henking used a light microscope to study egg formation in female grasshoppers, he was unable to spot the X element.

Based on his observations, Henking hypothesized that this extra chromosome, the X element, must play some role in determining the sex of insects. However, he was unable to gather any direct evidence to support his hypothesis.

Before Her Time
A illustration shows the dorsal side of a black beetle. The beetle has a body composed of a head, thorax, and elongated abdomen. The abdomen has long ridges running from the front to the back. Four segmented legs are attached to the abdomen, and two segmented legs are attached to the thorax. Two antennae are attached to the front of the head.
Figure 2: The darkling beetle, Tenebrio molitor.

More than a decade after Henking's work, Nettie Stevens surveyed multiple beetle species and examined the inheritance patterns of their chromosomes. In 1905, while studying the gametes of the beetle Tenebrio molitor (Figure 2), Stevens noted an unusual-looking pair of chromosomes that separated to form sperm cells in the male beetles. Based on her comparisons of chromosome appearance in cells from male and female beetles, Stevens proposed that these accessory chromosomes were related to the inheritance of sex.

Over time, other scientists studied the appearance of chromosomes in a wide variety of animal species, and it became clear that there was a relationship between the physical appearance and number of chromosomes in gametes and somatic cells from males and females of a given species.

The variety of sex determination systems

A photomicrograph shows 46 chromosomes from a somatic cell. The chromosomes are shown scattered against a black background. The 46 chromosomes correspond to 23 chromosome pairs, which include 22 pairs of autosomes and one pair of sex chromosomes (X and Y). Forty-four of the chromosomes are dyed in a fluorescent blue color; the blue chromosomes are not uniformly stained and show some banding patterns. The blue chromosomes differ in their orientations and sizes, but each has a partner that is similar in size. The X and Y sex chromosomes, shown near each other in the lower left side, are also fluorescently labeled in blue, but also have secondary colors. The larger X chromosome is labeled in a green, speckled pattern along its entire length. The Y chromosome, which is about one-third the length of the X chromosome, is labeled in pink at one end, and is blue at its other end.
Figure 3: Example set of male human chromosomes. In the image, the X and Y chromosomes are indicated by arrows.
In humans, females inherit an X chromosome from each parent, whereas males always inherit their X chromosome from their mother and their Y chromosome from their father. Consequently, all of the somatic cells in human females contain two X chromosomes, and all of the somatic cells in human males contain one X and one Y chromosome (Figure 3). The same is true of all other placental mammals — males produce X and Y gametes, and females produce only X gametes (Figure 4). In this system, referred to as the XX-XY system, maleness is determined by sperm cells that carry the Y chromosome.

A Punnett square diagram shows sex determination in humans. A female parent with the genotype XX is crossed with a male parent with the genotype XY. Half of the resulting offspring are males with the genotype XY, and half of the resulting offspring are females with the genotype XX.
Figure 4: Sex determination in humans.

A Punnett square diagram shows sex determination in insects. A female parent with the genotype XX is crossed with a male parent with the genotype X-. Half of the resulting offspring are females with the genotype XX, and half of the resulting offspring are males with the genotype X-.
Figure 5: Sex determination in insects.
Many people do not realize, however, that the XX-XY sex determination system is only one of a variety of such systems within the animal kingdom. In fact, sex determination can be very different between different organisms. For example, in the XX-XO system found in crickets, grasshoppers, and some other insects, sperm cells that lack an X chromosome (referred to as O) determine maleness. Here, females carry two X chromosomes (XX) and only produce gametes with X chromosomes. Males, on the other hand, carry only one X chromosome (XO) and produce some gametes with X chromosomes and some gametes with no sex chromosomes at all (Figure 5).

A Punnett square diagram shows sex determination in birds. A female parent with the genotype ZW is crossed with a male parent with the genotype ZZ. Half of the resulting offspring are males with the genotype ZZ, and half of the resulting offspring are females with the genotype ZW.
Figure 6: Sex determination in birds.
Despite the previous examples, males are not always the sex with the mismatched chromosome pair. For example, the ZZ-ZW sex determination system used in birds, snakes, and some insects relies upon females to carry the mismatched chromosome pair (ZW) and males to carry the identical pair (ZZ) (Figure 6).

If the three systems discussed above are compared in side-by-side Punnett squares (Figure 7), it is easy to see that sex determination is simply a matter of gamete assortment. Determinations of male and female character arise from a variety of different gamete combination patterns, all of which are the result of gender coding in sexually reproducing organisms.

Three Punnett square diagrams show the ratio of female to male offspring in a cross between a female parent with a male parent in humans, insects, and birds. In each scenario, half of the resulting offspring are males, and half of the resulting offspring are females. In humans, homozygotes are female, and heterozygotes are male. In insects, offspring with two alleles are female, and offspring with one allele are male. In birds, homozygotes are male, and heterozygotes are female.
Figure 7: A side-by-side comparison of sex determination systems in humans, insects, and birds.

The variety of inheritance patterns described in this article illustrate that sex determination is a complex and varied feature among organisms. The XX-XY, XX-XO, and ZZ-ZW systems are only a sample of the wide variety of sex determination systems that scientists have documented in the wide world of living beings, however.

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