Blood Types & Sex-Linked Traits

Biology Honors

Multiple Alleles

Blood Types

Multiple Alleles• Traits are the result of

more than 2 types of alleles

• Example: blood typing• There are 3 different

alleles for blood type (A, B, & O)

• A is dominant to O• B is also dominant to O• A and B are both

codominant

Blood Transfusions• Blood can only be transferred to a body of a person who's immune

system will "recognize" it. • Type A = A antigens• Type B = B antigens • Type O = No antigens• O is the universal donor

– a person can receive a transfusion from O blood without having an immune response

• AB is the universal recipient – because a person with AB blood has both the A and B antigens

already in the body

Multiple Alleles

Sample Crosses

Sex-Linked Traits

Sex-linked Traits A gene that is found only on

the X chromosome and not the Y chromosome

Examples: Colorblindness Hemophilia Duchenne Muscular

Dystrophy Male Pattern Baldness

Female Male

Sex-linked Traits More common in men

because they only have one X chromosome

Females need to have the gene on both X’s to have the trait

Females that are heterozygous for the trait are carriers

Carriers appear normal but can pass the trait to their offspring

Colorblindness Recessive trait on X

chromosome Condition in which certain

colors cannot be distinguished

Red/Green color blindness is most common (about 99%)

Causes problems in distinguishing reds and greens

No treatment Life is normal

Hemophilia Recessive trait on the X

chromosome Bleeding disorder in which

the blood does not clot normally

Persons with hemophilia may bleed for a longer time than others after an injury or accident

They also may bleed internally, especially in the joints

Live life cautiously, receive treatments to help blood clot normally

Duchenne Muscular Dystrophy

Caused by a recessive gene on the X chromosome

Rapidly-worsening muscle weakness that starts in the legs and pelvis, and later affects the whole body

No cure Treatment is aimed at control of

symptoms to maximize the quality of life

Death usually occurs by age 25

How to do a sex-linked cross

Sample Problem: Male Pattern Baldness

1. Unlike all the other crosses there is information that we know before we ever read the problem. We know that a XX female is going to mate with a XY male. So, you can set up your square ahead of time.

XX XY

XX XY

X Y X

X

2. Next, you have to read the problem and figure out what the sex-linked condition is and write down the allele you want to use for the problem and set up a key!

Baldness is a sex-linked trait. The allele for baldness is recessive and is carried on the X chromosome. The Griffin family lives in a town called Quahog. The father, Peter, has a gene for normal hair. The wife, Lois, is a carrier for baldness. Peter and Lois have 3 kids: Chris, Meg, and Stewie. The kids want to know what is the probability they will develop baldness?

Setting up your key What letters will Peter have since he is a male? XY What letters will Lois have since she is a female? XX Decide what letter you will use for the trait B = normal, b= bald

3. Reread the problem and find the parents genotypes using the key you just made

So, Peter is XY and Lois is XX Now you have to add the allele for

baldness to all of the X chromosomes as described in the problem

Peter XBY Lois XBXb

4. Now, set up the Punnett square and do the cross…Don’t forget the genotype and phenotype ratios!

XBXB XBY

XBXb XbY

XB Y

XB

XbPheno: ¼ female, normal

¼ female, carrier

¼ male, normal

¼ male, bald

Geno: ¼ XBXB

¼ XBXb

¼ XBY

¼ XbY

5. Answer the question: What is the probability

the children will develop baldness?

Females = 0% chance Males = 50% chance