By Robert Hazen, George Mason University
Each species has a characteristic, even number of chromosomes in every cell. For example, humans have 46 chromosomes, dogs have 78, grasshoppers have 22. During sexual reproduction, they display fascinating behaviour. Chromosomes also play a crucial role in determining gender.
Chromosomes occur in matched pairs. They’re called homologous pairs. Thus, for example, a grasshopper with 22 chromosomes has 11 homologous pairs of chromosomes. That is, every cell except for the sex cells, and this is where we get an interesting difference.
Eggs and sperm have only half the usual number of chromosomes; in humans, there are only 23 single chromosomes. When the egg meets the sperm, the 23 singles from the male and 23 singles from the female, give you 23 pairs of chromosomes. These sex cells are called gametes, and two gametes then combine in the fertilized egg.
Many discoveries about this process were made in 1902 by American biologist Walter Sutton. He discovered that the 11 chromosomes from the egg of a female grasshopper have a matching 11 chromosomes in the sperm of the male. This is why ordinary cells really do have the pairs of chromosomes: half from the father, half from the mother.
Sutton made this important discovery while he was still a graduate student at Columbia University. He then decided that medicine was going to be his career, so he published one important paper in 1903: ‘The Chromosomes in Heredity’. it stands as a landmark in genetics.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
During the process of sexual reproduction, to obtain a set of chromosomes, sex cells have to undergo a special process called meiosis. The first step is just like mitosis, that is, all the chromosomes divide. So if we start with, for example, 23 pairs, we then end up with 46 pairs, which arrange themselves in these thick, X-like arrangements.
Just as in mitosis, those X-like arrangements of chromosomes line up around the equator of the nucleus; but meiosis deviates from mitosis at this point. Spindles form, just as in mitosis, but the X-shaped structures are not pulled apart to give new cells, each with 46 chromosomes.
Instead, what we find is that 23 of those X-shaped pairs are pulled to one pole, and 23 of the pairs are pulled to the other pole. There’s quite a subtle distinction here between meiosis and mitosis. Then another set of spindles forms, and the two cells, each with 23 pairs, now split into four cells—the four gametes, or sex cells—each of which now has 23 single chromosomes. In the female they’re the eggs; in the male they’re the sperm. Then, at the moment of fertilization, 23 chromosomes from the male are combined with 23 chromosomes from the female to get a new, unique combination of genes, half from the mother and half from the father.
What Determines Gender
Let’s focus on the gender in this process. A close examination of chromosomes shows that human males carry one pair of chromosomes that’s not exactly matched. That 23rd pair has a longer chromosome called X and a shorter one called Y. All males have this XY combination, whereas all females have the XX combination of the two longer chromosomes.
This seems to be the only genetic difference between males and females in the human species. All of the male sex traits have to lie on that Y chromosome, which is unique to males; so do other sex-linked traits—things like baldness and color blindness, which are much more common in men. These are traits that must also lie, somehow, along that short Y chromosome.
It is important to also understand what happens during the production of gametes. In females, every egg has to carry only X chromosomes, since that’s what the female has, two X’s. But in males, half the sperm carry X; half the sperm carry Y. When fertilization occurs, the decision regarding the gender, whether or not one has a male or a female, is really up to which sperm happens to fertilize the egg, whether it’s an X- or a Y-carrying sperm; and that’s why there’s a 50-50 chance, on average, of having a boy or a girl.
What Happens in Other Species?
What is amazing is that other species have a different strategy, that this isn’t universal among sexual reproduction. For example, female birds are XY, and the males are XX; so in birds it’s the female, the egg, that determines whether or not they have a male or a female offspring. In some insect species, there’s a completely different pattern. In some insect species, all males develop from unfertilized eggs, and therefore have only a single sex chromosome. It’s only the females that have two sex chromosomes—a different strategy altogether.
Then there are quite a number of species that can switch back and forth from male to female—depending on demand, or depending on environmental factors. For example, in several fish and amphibians, when there are too many males, some of the males will spontaneously switch to becoming females, and they can switch back again later in their lives.
One can clearly see that, when it comes to gender, there are amazing strategies, and that humans are only one representative of the different ways that genetic information is passed on and gender is determined.
Common Questions about How Chromosomes Help in Determining Gender
In 1902, Walter Sutton discovered that the 11 chromosomes from the egg of a female grasshopper have a matching 11 chromosomes in the sperm of the male. This is why ordinary cells really do have the pairs of chromosomes: half from the father, half from the mother.
When fertilization occurs, the decision regarding gender—whether or not one has a male or a female—is really up to which sperm happens to fertilize the egg, whether it’s an X- or a Y-carrying sperm; and that’s why there’s a 50-50 chance, on average, of having a boy or a girl.
Female birds are XY, and the males are XX; so in birds it’s the female, the egg, that determines whether or not they have a male or a female offspring.