Human Sex Chromosomes
Humans have 23 pairs of chromosomes and therefore a total of 46 chromosomes. The first 22 pairs are autosomes. They possess the same structure and genes. The last 23rd pair are the sex chromosomes and they determine gender.
Human sex chromosomes are signified by an X or a Y.
Women are XX, meaning they possess two X chromosomes.
Men are XY meaning they possess one X chromosome and one Y chromosome.
This shows that ultimately it is the man who decides the gender of the offspring since females must give an X chromosome while a man can give either a Y chromosome or an X chromosome. Since the X and Y chromosomes are different sizes, they cannot undergo crossing over.
Human sex chromosomes are signified by an X or a Y.
Women are XX, meaning they possess two X chromosomes.
Men are XY meaning they possess one X chromosome and one Y chromosome.
This shows that ultimately it is the man who decides the gender of the offspring since females must give an X chromosome while a man can give either a Y chromosome or an X chromosome. Since the X and Y chromosomes are different sizes, they cannot undergo crossing over.
Genes on the Sex Chromosomes
There are some genes that are on the X chromosome and absent from the Y chromosome and vice versa.
The Y chromosome is smaller than the X chromosome. The X chromosome includes genes for color blindness and hemophilia while the Y chromosome contains the gene for hairy ears.
Sex Linkage
Sex linkage is when a gene controlling a trait is found on a sex chromosome. Sex linked traits are usually more on the X chromosome since it's bigger than the Y chromosome and thus has more genes.
Example: Good old colorblindness
Colorblindness is an X-linked recessive trait that predominantly features in males. Females have two sets of the X chromosome so if by chance one chromosome has the color blind allele it is fine because the other chromosome might not have that recessive allele so the trait won't manifest. Thus, women have the ability to be carriers.
For men however, they only have one X chromosome so if their mother gives them a chromosome with the color blind allele on it, then the males automatically have colorblindness because they do not have another X chromosome to potentially counteract that trait like females. This means that men can never be carriers and that is why more men are color blind.
Hemophilia is also an X-linked recessive trait.
Example: Good old colorblindness
Colorblindness is an X-linked recessive trait that predominantly features in males. Females have two sets of the X chromosome so if by chance one chromosome has the color blind allele it is fine because the other chromosome might not have that recessive allele so the trait won't manifest. Thus, women have the ability to be carriers.
For men however, they only have one X chromosome so if their mother gives them a chromosome with the color blind allele on it, then the males automatically have colorblindness because they do not have another X chromosome to potentially counteract that trait like females. This means that men can never be carriers and that is why more men are color blind.
Hemophilia is also an X-linked recessive trait.