Sex-Linked Disorders

Recall that non-_____________________ disorders can occur in which gametes receive more or less than the proper number of sex chromosomes (or autosomes). A _____________________ chart can often be used to identify these disorders before a baby is born. Complete the table below:

The sex chromosomes contain genes (stretches of __________) which code for some traits such as production of female sex hormones (_______________), male sex hormones (__________________), etc. They also carry genetic disorders such as _________________ (blood fails to clot properly), colour blindness, high blood pressure (______________________), muscular dystrophy, etc.

Because a female has 2 X chromosomes, she has 2 sex-linked genes (one on each X chromosome). But a male can only have 1 sex-linked gene because the Y chromosome does not carry a copy of the genes located on the X chromosome. Therefore, males often suffer from more sex-linked disorders. Also, sex-linked disorders are usually caused by _______________ (not dominant) genes.

Example: Hemophilia (recessive sex-linked disorder)

Because the male only has sex-linked genes on the ________ chromosome, there is nothing on the ______ chromosome to mask the disorder on the _______ chromosome. So if a male has even 1 copy of the disordered gene, he suffers from the disorder. A female can carry 1 disordered gene on one _______ chromosome but still have a normal gene to mask it on the other ________ chromosome. A "carrier female" usually doesn't suffer from the disorder, she just carries it and can pass it on .... usually to her SONS as you'll soon see!

Question 3:
Cross a carrier female (for hemophilia) with a normal male. Of all their offspring, what is the probability they will produce a hemophiliac son? (H = normal blood, h = hemophilia)


Question 4:
Cross a hemophiliac female with a normal male. Of all their offspring, what is the probability they will produce a hemophiliac daughter? (H = normal blood, h = hemophilia)


Question 5:
Cross a carrier female (for colour blindness) with a normal male. Of all their offspring, what is the probability they will produce a carrier daughter? (B = normal vision, b = colour blind)

How Disorders Can Skip a Generation

You've all heard how some disorders can skip a generation and then show up. Here's how it works!

Question 6:
Cross a normal female (grandma) with a hemophiliac male (grandpa). Of all their offspring, what is the probability they will produce a hemophiliac son? A hemophiliac daughter? A carrier daughter? Now cross one of the daughters (mom) with a normal male (dad). What sex is the child that gets the disorder? See how the disorder skipped a generation? (H = normal blood, h = hemophilia)

The Dash Method

You can use the dash method to figure out someone's genotype if you know the genotypes of the offspring.

Question 7:
Cross a dad with inherited high blood pressure with a mom that doesn't have inherited high blood pressure but who might be a carrier (use dash for the unknown gene). If half of the sons have high blood pressure, what must the dash represent? Therefore, what is mom's genotype? (T = normal b.p., t = high b.p.)

Dominant Sex-Linked Disorders

So what happens if a sex-linked disorder is caused by a dominant gene instead of a recessive gene? The presence of a single dominant gene on the X chromosome causes the disorder. Let's "see" if you can do the problem below...

Question 8:
Mom suffers from a disorder of the eye called retinoblastoma caused by a dominant sex-linked gene (D). Dad does not. Half of their daughters and half of their sons do not inherit the disease. Figure out the cross and the Punnett square.