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Sex Chromosomes

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Sex chromosomes are chromosomes that contain information that is used to determine the gender of an organism. The expression of a sex chromosomeon an allele can either be X or Y. Organisms with sex chromosomes XY are male while organisms with sex chromosomes XX are female. Because men only have one X chromosome, recessive traits on a man's X chromosome are always expressed.

The sex chromosomes are otherwise known as the x and y chromosome and those are the two chromosomes whose effect determine what gender you are. Whether you're going to be male or whether you're going to be female.

Let's take a quick look at this karyotype over here and we can see here are the 46 kinds of chromosomes or 46 chromosomes that you'll find in one human cell. Down here you can see the two x chromosomes. If you have those two x chromosomes, doesn't matter what's on these others, you're going to be a female. If you have an x and a y, you're going to be a male.

Now, what do we call all the other chromosomes? Those are called the autosomes. I've always thought it sounded kind of like a transformer. So chromosomes one through 22 are the autosome or chromosomes, the x and y chromosomes are the sex chromosomes.

Now this comes up in heredity because the sex link traits are the ones or genes that are located on the x chromosome and what that does is it winds up violating Mendel's basic rules of heredity because instead of you getting two copies of every gene, if you're a woman, yes you do because you have two copies of the x chromosome. But if you're a guy like me, I only have one copy of the x chromosome which means that if I get a trait such as color blindness or hemophilia, then I'm going to show it. I don't have another x chromosome to possibly cover up its effects with a dominant version of that gene.

There you go. The sex chromosomes.