SEX-LINKED TRAITS

AUTOSOMES= The first 22 pairs of homologous chromosomes.

All of the chromosomes surrounded by the red box are autosomes or autosomal chromosomes

Autosomes For genes on the autosomal

chromosomes, the pattern of inheritance is the same for males and females.

BUT- The last pair of chromosomes is a little bit different…

The Sex Chromosomes!!!!

The Sex Chromosomes

Remember! Males: XY

X chromosome from mom, Y chromosome from Dad

Females: XX One X chromosome from mom, one X

chromosome from Dad

Sex-Linked Traits

Sex-linked traits are on the X chromosome. Examples:

Hemophilia Muscular Dystrophy Color Blindness

DID YOU SEE A 25?

DID YOU SEE A 45?

DID YOU SEE A 29?

DID YOU SEE A 6?

DID YOU SEE A 56?

DID YOU SEE A 8?

The individual with normal color vision will see a 5 revealed in the dot pattern.

An individual with Red/Green (the most common) color blindness will see a 2 revealed in the dots.

An individual who can’t see the big circle at all needs glasses.

Why are males affected by sex linked traits more often than females???

In Order for a female to show a sex linked recessive condition, she needs two recessive alleles. One on each chromosome. B= Normal Vision b=Color blind

B B B B

Will she be color blind? Why?

I hope you said NO! She has two dominant alleles. As a result, she has normal vision.

B B b b

Will this female be color blind?

This female is known as a CARRIER. She is normal BUT she carries the recessive allele for the condition.

NOPE!!!

b b b b

Will this female be color blind???

How many of the recessive alleles did she need to inherit in order to be considered color blind?

Yes!!!

TWO

So, why are guys more likely to inherit sex linked recessive conditions?

Look at that “Y” chromosome

The male “X” chromosome is a normal sized chromosome just like the two female “X” chromosomes.

B B

See. It’s the same size.

The “Y” chromosome is not the same size as the “X” chromosome. As a result, there is/are no genes for color blindness or hemophilia, etc… on the “Y” chromosome.

B B

THE “Y” CHROMOSOME

LOOK! NO

GENES

As we just saw, females inherit two genes for color vision on the sex chromosomes. One from mom and one from dad.

How many genes for color vision do males inherit?

WHY??

JUST ONE!!!

WHY IS THIS BAD

FOR MALES????

WILL THIS MALE BE COLOR BLIND?

B B

NOPE! WHY?

He has the dominant allele on his “X” chromosome. As a result he has normal vision.

WILL THIS MALE BE COLOR BLIND?

b b

YEP! WHY?

He has the recessive allele. As a result, he is colorblind.

Conclusion:

Females need two copies of the recessive allele to be affected by a sex-linked trait.

Males only have one X chromosome so they only need one copy of the recessive allele to be affected.

Males CANNOT be carriers of sex-linked traits since they only have one X chromosome

Sex Linked Inheritance Practice

A recessive allele on the X chromosome is responsible for red-green color blindness in humans. A woman with normal vision whose father is color-blind marries a color-blind male. What is the probability that a son of this couple will be color-blind?