# AP Biology : Understanding Dominant and Recessive Traits

## Example Questions

### Example Question #1 : Understanding Dominant And Recessive Traits

Brown fur (B) in mice is dominant over white fur (bb). You have two brown mice, and when they are bred together you obtain 3 brown-haired offspring and one white-haired offspring. What must have been the genotypes of these brown-haired parents?

BB x BB

bb x bb

BB x bb

BB x Bb

Bb x Bb

Bb x Bb

Explanation:

Since B is completely dominant over b, and you obtained a 3:1 ratio of phenotypes, the parents must have been heterozygous, therefore you can represent their genotypes as Bb.

If either parent were BB, you would obtain only brown offspring. If either parent were bb, they would have been white (rather than bronw).

### Example Question #2 : Understanding Dominant And Recessive Traits

According to Mendel's experiments using pea plants, yellow color is autosomal dominant to green and round seeds are autosomal dominant to wrinkled seeds.

If you cross two plants that are each homozygous dominant for color and heterozygous for seed shape, what percentage of the F1 offspring would have round seeds?

25%

0%

75%

50%

100%

75%

Explanation:

Because the plants are homozygous dominant for color, you need only consider seed shape. Crossing a heterozygous plant with another heterozygous plant will always lead to 75% of offspring in the F1 generation displaying the dominant trait.

All of the offsrping will also be yellow, though this does not directly relate to the answer.

### Example Question #3 : Understanding Dominant And Recessive Traits

According to Mendel's experiments using pea plants, yellow color (allele B) is autosomal dominant to green (allele b) and round seeds (allele A) are autosomal dominant to wrinkled seeds (allele a).

What proportion of F1 offspring will be yellow with wrinkled seeds if you cross two plants that are heterozygous for both traits?

1/2

1/8

3/4

1/16

3/16

3/16

Explanation:

When examining two autosomal dominant traits, a cross between two heterozygous individuals will always produce F1 offspring in the same 9:3:3:1 ratio. Nine of the sixteen are individuals who are dominant for both traits, six are individuals who are dominant for one trait and recessive for the other, and the single individual is recessive for both traits. Three individuals will be yellow with wrinkled seeds, and three will be green with round seeds.

### Example Question #183 : Evolution And Genetics

Green antennae are dominant to blue antennae for a particular alien species. If an alien with green antennae mates with an alien with blue antennae, what is the chance that their offspring will have blue antennae?

Either  or

Either  or

Either  or

Explanation:

Since one of the parent has blue antennae, we know for that they have a homozygous recessive genotype (bb). The information given in the question does not tell us whether or not the parent with green antennae is homozygous dominant (BB) or heterozygous (Bb). The cross could be set up as either BB x bb or Bb x bb. These two crosses would give you a 0% chance and a 50% chance of having a child with blue antennae, respectively.

BB x bb: Offspring will all be Bb, and cannot show the blue phenotype.

Bb x bb: Half of the offspring will be Bb and half will be bb. Half of the offspring will show the blue phenotype.

### Example Question #1 : Understanding Dominant And Recessive Traits

In a certain breed of dog, yellow fur is dominant to black fur. A breeder of this breed is trying to figure out the genotype of one of her dogs with yellow fur. She decides to breed it with one of her dogs that has black fur.

What would be the ratio of offspring with yellow fur to offspring with black fur if the unknown parent was actually heterozygous for fur color?

75% yellow and 25% black

100% black

100% yellow

50% yellow and 50% black

50% yellow and 50% black

Explanation:

If the dog with the unknown genotype is heterozygous, our cross would be: Aa x aa. In this model, A represents the dominant yellow-fur allele and a represents the recessive black-fur allele. We the black furred dog has to be genotype aa because he displays the recessive phenotype. The offspring produced from this cross would be 2 Aa and 2 aa genotypes, based on a punnet square.

This cross would generate offspring that are 50% Aa and 50% aa. This corresponds to 50% yellow and 50% black.

### Example Question #1 : Understanding Dominant And Recessive Traits

A certain kidney disease is controlled by an allele that is autosomal recessive. A man's father has the condition, while his mother is healthy and homozygous for the normal allele. If the man marries a woman who is a carrier, what is the probability that their first child will have the disease?

Explanation:

First, we need to find the genotype of the man. We know that his father has the disease, and that his mother is homozygous dominant. For his father to display the autosomal recessive trait, he must be homozygous recessive. We can thus write the man's parents' genotypes, using A as the dominant allele and a as the recessive allele.

Parents: AA (mother) x aa (father)

Offspring: All offspring will be Aa.

The man must inherit one allele from each parent, meaning he must be heterozygous.

This man, we are told, marries a heterozygous woman and they have a child. Again, we can write out this cross.

Man and woman: Aa (man) x Aa (woman)

Offspring: 1 AA (dominant), 2 Aa (dominant), 1 aa (recessive)

The possible offspring from this cross show a three-to-one ratio for displaying the dominant phenotype. Only one of the four possible offspring will be homozygous recessive and display the recessive phenotype. This one-in-four chance correspond to a 25% chance.

### Example Question #181 : Evolution And Genetics

Two mice are crossed. The father is black and heterozygous, and the mother is white and homozygous. If they have twelve offspring, how many will most likely be white if the allele for color is autosomal?

Three

Zero

Twelve

Six

Nine

Six

Explanation:

We know that the allele is autosomal, the father is heterozygous and black, and the mother is homozygous and white. From this information, we can determine that black must be dominant to white since the heterozygote shows the black phenotype. We will use B to represent the dominant black allele and b to represent the recessive white allele.

Cross: Bb x bb

Offspring: Half Bb (black) and half bb (white)

We can see that roughly half of the offspring will show the white phenotype. If there are twelve offspring, six of them will be white.

### Example Question #1 : Understanding Dominant And Recessive Traits

Imagine a flower that can have two different colors and two different seed shapes. Purple is dominant to white, and round seeds are dominant to wrinkled seeds. A pure breeding purple flower with round seeds is crossed with a pure breeding white flower with wrinkled seeds. The Fgeneration is then self crossed.

What proportion of the Fgeneration will be purple with wrinkled seeds?

Explanation:

The first cross will produce a generation of double heterozygous flowers. Self-crossing this generation becomes a dihybrid cross problem.

In order to solve this problem, you can use either a punnett square and count the squares that display this phenotype, or you can use probability. Since purple is dominant, three out of four flowers will display the color purple. Since wrinkled is recessive to round, only one out of four flowers will display this phenotype. Since the flowers in question must express both of these traits, we can simply multiply these two fractions together.

You should be familiar with the 9:3:3:1 ratio of dihybrid cross phenotypes. A phenotype of one dominant trait and one recessive trait will always be observed with a  frequency.

### Example Question #188 : Evolution And Genetics

What is an allele?

A genetic feature unique to plants

A copy of a chromosome

One of several alternative versions of the same gene

A type of gene

One of several alternative versions of the same gene

Explanation:

Alleles are simply different versions of the same gene that encode for variations of the same characteristic. For example, different eye colors are encoded by different alleles of the same gene.

### Example Question #189 : Evolution And Genetics

Which of the following is an example of an somatic cell?

Ovum only

Sperm cell and ovum

Sperm cell only

Muscle cell