Problem 2 - Campbell Biology

Question

A man with type A blood marries a woman with type B blood. Their child has type O blood. What are the genotypes of these three individuals? What genotypes, and in what frequencies, would you expect in future offspring from this marriage?

Step-by-Step Solution

Answer

Man: IAi, Woman: IBi, Offspring possible genotypes with 25% frequency: IAIB, IAi, IBi, ii.

Step 1: Interpret Blood Types

Understand the blood types: Blood type A can have genotypes IAIA or IAi, while blood type B can have genotypes IBIB or IBi. Blood type O genotype is ii.

Step 2: Determine Parents' Genotypes

Since the child has type O blood (genotype ii), each parent must carry one i allele. Therefore, the man must have genotype IAi and the woman must have genotype IBi.

Step 3: Create Punnett Square

Create a Punnett square to find the possible genotypes of future offspring. The square combines IAi (man) and IBi (woman).

Step 4: Fill Out Punnett Square

Fill the Punnett square: The combinations are IAIB, IAi, IBi, and ii.

Step 5: Determine Genotype Frequencies

Determine the frequencies for each genotype: IAIB (25%), IAi (25%), IBi (25%), and ii (25%).

Key Concepts

Blood Types

Blood types are an important aspect of genetics. They are determined by proteins called antigens found on the surface of red blood cells. The major blood groups are A, B, AB, and O.
Blood type A has the A antigen, type B has the B antigen, type AB has both, and type O has neither. These antigens are determined by alleles you inherit from your parents.

Genotype Determination

Genotypes refer to the genetic makeup of an individual regarding a particular trait. For example, in blood types, the possible genotypes are:

• Type A: IAIA or IAi
• Type B: IBIB or IBi
• Type AB: IAIB
• Type O: ii

The genotypes are combinations of alleles inherited from an individual's parents. In our exercise:

• The man with type A blood could be IAIA or IAi.
• The woman with type B blood could be IBIB or IBi.
• Their child with type O blood must be ii, meaning each parent must carry an i allele.

Therefore, based on the child’s type O blood, the parents must have genotypes IAi and IBi.

Punnett Square

A Punnett square is a diagram used to predict the genotypes of offspring from a particular cross or breeding experiment. It helps in visualizing how alleles from each parent combine. To solve the exercise, we can use a Punnett square to combine the genotypes IAi (father) and IBi (mother):
When we fill out the Punnett square, we look at all the possible combinations of alleles from both parents. The possible outcomes for their child would be:

• IAIB: AB blood
• IAi: A blood
• IBi: B blood
• ii: O blood

Inheritance Patterns

Inheritance patterns describe how genes and traits are passed from parents to their offspring. In our exercise, the ABO blood type exhibits simple Mendelian inheritance with multiple alleles. Each parent passes one allele to each child:

• Mother's alleles: IB or i
• Father's alleles: IA or i

When you combine these alleles, you get the following inheritance patterns in their children:
25% chance for blood type IAIB (AB blood),
25% for IAi (A blood),
25% for IBi (B blood),
and 25% for ii (O blood).

Alleles

Alleles are different forms of a gene. Each individual has two alleles for each gene, one inherited from each parent. For blood types, there are three alleles: IA, IB, and i.
IA and IB are co-dominant, meaning that if both are present, they both express. That’s why individuals with IAIB have AB blood. The i allele is recessive, meaning it only expresses when two i alleles are present, resulting in type O blood. Understanding how alleles interact is key to predicting genotypes and phenotypes in offspring.