### All Genetics Resources

## Example Questions

### Example Question #2 : Understanding Punnett Squares And Test Crosses

In a certain species of bird, yellow beaks are dominant to orange beaks, and blue feathers are dominant to black feathers.

Two heterozygous birds are crossed. What fraction of the offspring would be expected to have yellow beaks and blue feathers?

**Possible Answers:**

**Correct answer:**

This question requires us to do a dihybrid cross. We can represent the gene for beak color with the symbol "A" for dominant yellow and "a" for the recessive orange. Likewise, for feather color, we can use "B" for blue feathers and "b" for black.

The problem states that both birds are heterozygous for each trait, implying that our cross is between two birds with the genotype AaBb.

Now we look at the gametes that can be produced by these parents: AB, Ab, aB, and ab. These gametes can then be used to make a punnet square.

Offspring: 1 AABB, 3 Aabb, 8 AaBa, 3 aaBb, 1 aabb

There are 16 total offspring. 12 of them carry the dominant A allele, giving them the yellow beak phenotype.

Yellow beaks: 1 AABB, 3Aabb, 8 AaBb

Finally, of these 12, 9 carry the dominant B allele for blue feathers.

Yellow beaks and blue feathers: 1 AABB, 8 AaBb

This gives a total of 9 out of the 16 offspring that will express both the yellow beak and blue feather phenotypes.

You should be familiar with the 9:3:3:1 phenotypic ratio resulting from dihybrid crosses.

### Example Question #1 : Mendelian And Population Genetics

Consider two traits in pea plants that exhibit complete dominance. Smooth peas are dominant to wrinkled peas, and purple flowers are dominant to white flowers.

A pure breeding plant with purple flowers and wrinkled peas is crossed with a pure breeding plant with white flowers and smooth peas. The first generation is self-pollinated to produce the second generation.

What fraction of the second generation will be heterozygous for both traits?

**Possible Answers:**

**Correct answer:**

When dealing with two traits, it helps to approach each trait separately. The question asks for the fraction of plants in the second generation that are heterozygous for both traits.

First, we will look at the first generation. The parents are both pure breeding, meaning they are homozygous for each trait. One is purple (dominant) and wrinkled (recessive), while the other is white (recessive) and round (dominant). The result will be dihybrid offspring.

Parent cross: *PPrr* x *ppRR*

Offspring: All offspring will be *PpRr* and exhibit both dominant phenotypes (purple and round).

Now we will look at the first generation self-cross for each trait.

First generation: *PpRr* x *PpRr*

Offspring for color: 1 *PP*, 2 *Pp*, 1 *pp*; 3 purple and 1 white.

Offspring for seeds: 1 *RR*, 2 *Rr*, 1 *rr*; 3 round and 1 wrinkled.

We can see that half of the offspring will be heterozygous for color, and half of the offspring will be heterozygous for seed shape.

Since we are looking for plants that have both of these traits, we multiply these two probabilities together.

### Example Question #2 : Mendelian And Population Genetics

Consider a plant with the following characteristics.

Round leaves (R) are dominant to pointed leaves (r).

White flowers (W) are dominant to pink flowers (w).

Plants heterozygous for both traits are crossed.

What is the probability of obtaining a plant with pointed leaves and white flowers?

**Possible Answers:**

**Correct answer:**

The genotype to obtain pointed leaves is rr, while the genotypes to obtain white flowers are WW or Ww. Determine the frequence of obtaining WWrr or Wwrr using a Punnett Square.

You should be familiar with the ratios represented in a dihybrid cross: 9:3:3:1. There will be nine inidividuals with both dominant traits (pointed leaves and white flowers), three individuals dominant for one trait (round leaves and pink flowers), three individuals dominant for the other trait (pointed leaves and white flowers), and only one individual recessive for both traits (pointed leaves and pink flowers).

### Example Question #1 : Mendelian And Population Genetics

In a population of deer mice, the allele for white hair is recessive and the allele for brown hair is dominant. If the population consists of 500 individuals and the frequency of homozygous brown mice is 49%, what is the frequency of the recessive allele? Assume the population is in Hardy-Weinberg equilibrium.

**Possible Answers:**

**Correct answer:**

In Hardy-Weinberg equilibrium, the sum of the dominant allele frequency (p) and the recessive allele frequency (q) is equal to 1.

The question says that 49% of the population consists of mice with the homozygous dominant gene, therefore, the dominant genotype frequency is equal to 0.49.

The question asks us to find the frequency of the recessive allele (q). In order to find the frequency of the recessive allele, we must first find the frequency of the dominant allele (p). According to the Hardy-Weinberg principle, the square root of the homozygous genotype frequency is equal to the allele frequency.

The dominant allele frequency is 0.7. Using this, we can solve for the recessive allele frequency.

### Example Question #2 : Mendelian And Population Genetics

A rare recessive mutation causes rabbits that are normally white to be pink. If one out of every 625 rabbits is pink, what percentage of the population is heterozygous?

**Possible Answers:**

**Correct answer:**

We can use the Hardy-Weinberg equilibrium formulas to calculate the allele frequencies.

We know that the frequency of homozygous recessive (pink) rabbits is . This is equal to in the Hardy-Weinberg calculation. We can use this information to solve for , the recessive allele frequency.

Now that we know the value of , we can solve for the value of .

The frequency of heterozygotes is equal to in the Hardy-Weinberg calculation. Now that we know the frequency of each allele, we can complete this calculation.

### Example Question #4 : Mendelian And Population Genetics

Eye color in a certain species is decided by a single gene locus. Only two alleles influence eye color in a population of this species that exists in Hardy-Weinberg equilibrium. The dominant allele codes for brown eyes, while the recessive allele codes for blue eyes.

If the frequency of the brown allele is , what percent of the population is heterozygous at this locus?

**Possible Answers:**

**Correct answer:**

For problems of this type, we need to understand the Hardy-Weinberg equations:

Here, represents the frequency of the dominant allele, while c refers to the frequency of the recessive allele. and denote the proportion of homozygous dominant and recessive phenotypes, respectively. Finally, the proportion of heterozygotes is denoted by .

We already know that , and if only two alleles are present in the population, must be equal to .

Using the values for and , we can solve for the proportion of heterozygotes using the term of the Hardy-Weinberg equation.

### Example Question #3 : Mendelian And Population Genetics

In a population of deer mice, the allele for white hair is recessive and the allele for brown hair is dominant. If the population consists of 500 individuals and the frequency of homozygous brown mice is 49%, what is the frequency of the recessive allele?

Assume the population is in Hardy-Weinberg equilibrium.

**Possible Answers:**

**Correct answer:**

In Hardy-Weinberg equilibrium, the sum of the dominant allele frequency (p) and the recessive allele frequency (q) is equal to 1.

The question says that 49% of the population consists of mice with the homozygous dominant gene, therefore, the dominant genotype frequency is equal to 0.49.

The question asks us to find the frequency of the recessive allele (q). In order to find the frequency of the recessive allele, we must first find the frequency of the dominant allele (p). According to the Hardy-Weinberg principle, the square root of the homozygous genotype frequency is equal to the allele frequency.

The dominant allele frequency is 0.7. Using this, we can solve for the recessive allele frequency.

### Example Question #1 : Understanding Biological Fitness

There are two very different reproductive strategies in nature: r-selection and k-selection. These strategies are so extreme, we typically observe organisms somewhere in between these two strategies.

Which of the following characteristics is not indicative of r-selection?

**Possible Answers:**

High brood mortality rate

Small brood size

Very little parental care

Very fast maturation of organisms

**Correct answer:**

Small brood size

The r-selection strategy for reproduction is typically seen in environments that are very volatile and unpredicatable. It has a variety of characteristics including high brood sizes with a high mortality rate, and fast maturation with very little parental assistance. Low brood sizes are typically seen in the k-selection strategy for reproduction.

### Example Question #4 : Mendelian And Population Genetics

Black fur (A) is dominant to brown fur (a) and brown eyes (B) are dominant to blue eyes (b) in mice. Two mice are heterozygous for both traits. If one of the offspring shows a phenotype for green eyes, what is most likely the best explanation?

**Possible Answers:**

Independent assortment must have occurred

A new mutation must have occurred

Codominance must have occurred in this particular offspring

The parental genotypes reported initially must have been incorrect

**Correct answer:**

A new mutation must have occurred

The best choice is that the individual has a mutation that accounts for his/her green eyes. This mutation would yield a new phenotype that was not a result of direct inheritance from the parent generation.

Codominance cannot only occur in one individual offspring; rather it would occur in some form in all offspring with respect to the given trait. Independent assortment always occurs in cases of typical Mendelian genetics.

### Example Question #7 : Mendelian And Population Genetics

In a dihybrid cross (AaBb x AaBb), how many total genotypes are possible in the offspring?

**Possible Answers:**

**Correct answer:**

The alleles for gene *A* assort independently from the alleles of gene *B*, meaning that the genotype for one does not affect the genotype of the other. Even though there are two genes, we can solve this problem by answering the question separately for the two genes.

There are three possible genotypes with respect to the *A* gene (*AA*,* Aa*, *aa*) and three possible genotypes with respect to the *B* gene (*BB*, *Bb*, *bb*). Since genes *A* and *B* assort independently, the possible offspring will be the product of the possibilities for each separate gene.

Listed out, these genotypes are: *AABB, AABb, AaBB, AAbb, AaBb, aaBB, Aabb, aaBb, aabb*.

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