Do boys have different genes than girls?
-A middle school student from California July 10, 2008 Yes they do. But the number of different genes is very small. Humans have over 21,000 different genes. Boys have around 80 genes that girls don't have. These genes are critical for turning a fertilized human egg into a boy. This isn't the only difference between boys and girls, though. Boys and girls also use many of the other 21,000 or so genes they share in different ways. It is this combination of extra genes and how we use some of the rest of our genes that makes boys different from girls. Differences Usually Don't Mean Different Genes Of course not all boys look alike (nor for that matter do all girls). And yet everyone of the same sex has pretty much the same genes. So why does a boy look different from other boys? Because he has different versions of the same genes. For example, because my eyes are brown and yours are blue, it doesn't mean we have different genes. We both have the genes that give our eyes color. But, I have a brown version of an eye color gene and you have a blue version of the same gene. The differences between boys and girls are a different story though. A big reason why they are different comes from the extra 80 male genes that I talked about earlier. These genes are found on an extra chromosome that only boys have, the Y chromosome. The most important of these genes is called the SRY gene. It's got a weird name so let's just call it the "boy gene". This gene is responsible for making a boy different than a girl. So, if you have this gene, you grow up as a boy. And if you don't, you grow up a girl. How the SRY Gene Makes a Boy The "boy gene" turns a fertilized human egg into a boy by causing the development of testes. These organs make a lot of the hormone testosterone. It is the high levels of this special chemical that then makes a person a boy. Testosterone works by affecting how a boy uses his genes. It will cause some genes to be turned on and others to be turned off. It will also cause some genes to be turned up and others turned down. These differences in how genes are used prevent the formation of breasts, and cause growth of facial hair, change in voice, and other characteristics typical of boys and men. (A different hormone, estrogen, makes a girl a girl.) What do you think would happen if someone didn't have a Y Chromosome? Well, you have learned that the Y Chromosome is found only in boys and it contains genes that make a person a boy. The answer is that the person will grow up to be a girl only because this one chromosome and its few genes (including the "boy gene") are not present. This is why girls normally do not have a Y Chromosome. But since all people have the same number of chromosomes, what is there in place of the Y Chromosome in girls? All people have a pair of what are called the sex chromosomes. There are two kinds of these chromosomes: the X chromosome and the Y Chromosome you've come to know so well. Girls have two X Chromosomes while boys have one X Chromosome and, as you now know, one Y Chromosome. So girls are XX and boys are XY. Now, what do you think would happen if a person has the Y Chromosome? Easy! The person will grow to be a boy. Except when something happens to the "boy gene." When Things Happen Differently It turns out that there are a few rare cases where the whole Y Chromosome or the single "boy gene" are either absent or present in a person by mistake. Let's see a few examples of what happens when things don't go the usual way by trying to use what you've learned so far. What do you think would happen if someone does NOT have a Y Chromosome BUT has the "boy gene" in another chromosome by mistake? Think about it. The person will grow to be a boy. However, this person would not have all of the other genes in the Y Chromosome and would suffer symptoms such as underdeveloped testis and sterility. Now, what do you think would happen if a person HAS a Y Chromosome BUT the "boy gene" is broken and not working? Think about the answer to the previous question and you will see that this person will grow to be a girl. However, this person will suffer symptoms such as sterility and underdevelopment of the breasts. This is mainly because a girl is more than just a person without the Y Chromosome (or without the "boy gene"). Two copies of the X Chromosome are necessary for a girl to develop properly. Where Did Sex Chromosomes Come From? Strong evidence suggests that the X and Y Chromosomes came from an identical pair of ancient chromosomes. Eventually, one of the genes in one of these chromosomes evolved (changed) and became the "boy gene". As time went on, the chromosome with the "boy gene" began to shed lots of its genes. It also accumulated new versions of existing genes. At this point, it became the Y chromosome we know today. The remaining chromosome is what we now call the X Chromosome. When did all of this happen? Scientists believe that the Y Chromosome found in humans and other mammals originated 170-310 million years ago! We believe this to be true because species that are very similar to us such as mice and monkeys have the "boy gene" just like humans while older ancestors such as birds do not. If we think about it, this means that our ancient ancestors didn't make the boy/girl decision by having different sex chromosomes. They must have used some other method. Perhaps they did what some species that lack sex chromosomes do today. For example, reptiles such as alligators and turtles make the boy/girl decision based on the temperature in which the eggs develop. There are animals such as certain coral reef fish that actually change sex throughout their lifespan. Other organisms such as certain worms have both male and female reproductive organs. So, in a way, they are both boys and girls at the same time. Want to Know More? Now you know that boys and girls have a few different genes and that they are responsible for some of the differences between us. If you want to find out more, check out the following websites: Jose Morillo

More Information

The sex of some reptiles like this tuatara
is determined by the temperature at
which their eggs are incubated.

Boys have an X and a Y
chromosome. Girls have
two X chromsomes.

Extra testosterone causes a boy's
genes to be read differently
from a girl's.