Sex Chromosomes

Learn about the Sex Chromosomes

Introduction to Sex Chromosomes

The nucleus of a cell contains many chromosomes, which are actually long strands of molecules called Deoxyribo Nucleic Acid (DNA).

In humans, every cell (except the sperm and egg cells) naturally contain 23 pairs of chromosomes in the nucleus, for an all together 46 chromosomes. The very first cell that we grew from, however, came from the fertilization of the sperm and egg. In order for our very first cell to obtain 46 chromosomes, it had to get half of the chromosomes (23) from the egg, and half of the chromosomes (23) from the sperm.

The egg contains 23 chromosomes...

Human Egg Chromosomes (23)

1 2 3 4 5 6

7 8 9 10 11 12

13 14 15 16 17 18

19 20 21 22 23 (X)

The sperm of the male also contains 23 chromosomes.

Human Sperm Chromosomes (23)

1 2 3 4 5 6

7 8 9 10 11 12

13 14 15 16 17 18

19 20 21 22 23 (X or Y)

or

Note the 23rd chromosome for the egg and sperm. These chromosomes are designated the sex chormosomes. The sex chromosomes determines the sex (male or female), and there are basically two types of sex chromosomes, labelled X and Y.

An X Sex Chromosome looks like this:
A Y Sex Chormosome looks like this:

The sex chromosome from the female egg is always X, and the sex chromosome from the male sperm can be either X or Y. If the sex chromosome from the sperm is a Y, then after fertilization we have a male, otherwise, we have a female.

When the sperm fertilizes the egg, the 23 chromosomes from the male pairs up with the 23 chromosomes from the female, making a total of 46 chromosomes. After fertilization, our very first cell contains these 46 chromosomes. From then on, the cell grows by splitting into two and with each having the exact same copy of the 46 chromosomes.

Human Cell Chromosomes (23 Pairs)

1 2 3 4 5 6

7 8 9 10 11 12

13 14 15 16 17 18

19 20 21 22 23 (X and Y)

Note that in each of the 23 pairs of chromosomes above, the left chromosome came from the egg, while the right chromosome came from the sperm. The Sex Chromosomes (23rd pair) indicates a male (XY) in this case because there is an existence of the Y Sex Chromosome from the sperm.

The sex chromosomes can be either XX: , which indicates a female;

Or it can be XY: , which indicates a male.

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