KEY CONCEPT

The chromosomes on which genes are located can affect the expression of traits.

Two copies of each autosomal gene affect phenotype.

• Mendel studied autosomal gene traits (these genes are on autosomes), like hair texture.

• The “either-or” traits that were seen in your tutorial.

(dominant)

• Mendel’s rules of inheritance apply to autosomal genetic disorders.– A heterozygote for a recessive disorder is a carrier. – Disorders caused by dominant alleles are uncommon, but

possible.

• What about genes located on sex chromosomes? What are these traits called?

Males and females can differ in sex-linked traits.

• Genes on sex chromosomes are called sex-linked genes. – Y chromosome genes in mammals are responsible for male

characteristics.– X chromosome genes in mammals affect many traits.

• Male mammals have an XY genotype.

– All of a male’s sex-linked genes are expressed.

– Males have no second copies of sex-linked genes.

– Because of this, males can never be “carriers” of sex-linked traits or disorders.

– Ex: Colorblindness, Hemophilia, Muscular dystrophy

4 Sex-Linked Traits:

1. Normal Color Vision: A: 29, B: 45, C: --, D: 26

2. Red-Green Color-Blind: A: 70, B: --, C: 5, D: --

3. Red Color-blind: A: 70, B: --, C: 5, D: 6

4. Green Color-Blind: A: 70, B: --, C: 5, D: 2

Sample Colorblind Tests

Let’s take a look at…

Lynda is a healthy carrier of colorblindess and her husband, Jim, has no family

history of colorblindness. Lynda’s genotype:

XXd

Jim’s genotype:XY

What is the probability ofgetting:

• A colorblind daughter?0%

• Carrier child?25%

• A colorblind child?25%

• Two healthy children?¾ x ¾ = 9/16 (56%)

Jen is a healthy carrier of hemophilia and Adam has no history in his family.

What is the probability ofgetting:

• Daughter with hemophilia?0%

• Carrier child?25%

• Child with hemophilia?25%

• Two healthy children?¾ x ¾ = 9/16 (56%)

Kelly is a healthy carrier of hemophilia, but Joe suffers from the disease.

What is the probability of getting a:

• Healthy son?½ son x ½ hemophilia= ¼ or 25%

• Healthy daughter?½ girl x ½ hemophilia= ¼ or 25%

• Child with hemophilia?50%

• Carrier child?25%

We use PEDIGREES to trace: • Autosomal traits• Sex-linked traits• Autosomal disorders or

diseases• Sex-linked disorders or

diseases

A pedigree is a chart for tracing genes in a family.

• Phenotypes are used to infer genotypes on a pedigree.• Autosomal genes show different patterns on a pedigree

than sex-linked genes.

• If the phenotype is more common in males, the gene is likely sex-linked.

Creating a Pedigree

• 1) Manny and Rosa are married and both are healthy.• 2) They have 3 children.• 3) Their oldest child, Jacob, has a recessive disorder called

PKU.• 4) Their middle child Erica and their youngest Matthew are

healthy.• If healthy is dominant to PKU, fill in the missing genotypes.

(start with what you know)

hh

H H

H H

h h

? ?

Key

H = Healthy

h = PKU

The Next Generation

• 5) Erica meets Moe at college and the two become married.

• 6) Erica and Moe have 2 daughters.• 7) Their youngest daughter Jamie shows signs of PKU while

Amy does not.• Fill in the genotypes of the four individuals. (start with what

you know)

hhH

Hh

?

H h

Key

H = Healthy

h = PKU

Let’s try a sex-linked problem:A man and a woman marry.

The woman is a carrier of colorblindness, an X-linked disorder.

They have 4 children, 3 boys and one girl.

Two of the boys are colorblind, and the girl is a carrier.

The carrier daughter marries a healthy man.

a)What is the probability they will have carrier daughters?

b)What is the probability they will have sons that are colorblind?

?

a) ½ daughter x ½ carrier = 25%b) ½ son x ½ c.b. = 25%

Fill in the X-linked Pedigree

Sickle cell anemia (recessive) pedigree

hh

hh hh

hh

Hh

Hh HhHhH?

HhHh H?

H?

Key

H = Healthy

h = Sickle cell anemia

Hint: 3 unknown

Try this on your own…

Jack and Jill get married. As Jill falls down the hill, she cuts herself and bleeds profusely. She goes to the doctors and realizes that she has hemophilia, an X-linked blood disorder. They have 5 children, 3 boys and 2 girls, all of which get tested for hemophilia. They discover that 2 boys have hemophilia and one of the daughters is a carrier. One of the affected sons marries a healthy non carrier woman and has twin carrier children. What is the sex of these carrier children? Explain your answer.

A man and a woman marry. The man has DMD (a muscular dystrophy disease), which is an X-linked disease. They have 4 children, 2 carrier daughters and 2 completely healthy boys. One of the carrier daughters marries a healthy man and has 3 children. They have two boys with DMD, and a healthy, non carrier daughter. What is the probability that one of the boys will have sons with disorders if he marries a healthy, non carrier woman?

Now this…