Medium Punnett Square Problems Practice Questions

1. Concept Explanation

Medium Punnett Square problems involve using a grid-based diagram to predict the probability of offspring genotypes and phenotypes by analyzing the segregation of alleles from two parents. While basic problems focus on single-gene (monohybrid) crosses with simple dominance, medium-level problems often incorporate inheritance patterns such as incomplete dominance, codominance, and dihybrid crosses involving two independent traits. Mastering these requires an understanding of Gregor Mendel's laws, specifically the Law of Segregation and the Law of Independent Assortment. According to Khan Academy, the Law of Segregation ensures that each gamete receives only one of the two gene copies present in an organism. In a medium-level scenario, you might be asked to calculate the phenotypic ratio of a cross between two heterozygous individuals for two different traits (a dihybrid cross), which typically results in a 9:3:3:1 ratio if the genes are not linked. Understanding these genetics practice questions is essential for biology students preparing for standardized exams.

2. Solved Examples

Review these step-by-step solutions to understand how to approach more complex genetic crosses.

1. Dihybrid Cross: In pea plants, Round seeds (R) are dominant to wrinkled (r), and Yellow seeds (Y) are dominant to green (y). Cross two plants that are heterozygous for both traits (RrYy x RrYy).
   1. Determine the gametes for each parent: RY, Ry, rY, ry.
   2. Set up a 4x4 Punnett Square.
   3. Fill in the genotypes: You will find 1 RRYY, 2 RRYy, 2 RrYY, 4 RrYy, 1 RRyy, 2 Rryy, 1 rrYY, 2 rrYy, and 1 rryy.
   4. Calculate Phenotypes: 9 Round/Yellow, 3 Round/green, 3 wrinkled/Yellow, 1 wrinkled/green.
2. Incomplete Dominance: In snapdragons, flower color shows incomplete dominance. Red (RR) and White (WW) produce Pink (RW). Cross two Pink snapdragons.
   1. Identify parent genotypes: RW x RW.
   2. Set up a 2x2 square.
   3. Results: 1 RR (Red), 2 RW (Pink), 1 WW (White).
   4. The phenotypic ratio is 1:2:1.
3. Codominance (Blood Typing): A mother has Type A blood (genotype AO) and a father has Type B blood (genotype BO). What are the possible blood types of their children?
   1. Identify gametes: Mother (A, O) and Father (B, O).
   2. Fill the square: AB, AO, BO, OO.
   3. Determine phenotypes: 25% Type AB, 25% Type A, 25% Type B, 25% Type O.

3. Practice Questions

Test your skills with these Medium Punnett Square problems. Ensure you read the trait descriptions carefully before setting up your squares.

1. In guinea pigs, black fur (B) is dominant to white fur (b), and short hair (S) is dominant to long hair (s). A guinea pig that is homozygous dominant for fur color and heterozygous for hair length is crossed with a white, long-haired guinea pig. What is the probability of having offspring with black, short hair?
2. In a certain species of birds, feather color is controlled by codominance. Blue feathers (B) and White feathers (W) produce "Speckled" birds (BW). If a Speckled bird mates with a Blue bird, what percentage of the offspring will be White?
3. A man with Type AB blood marries a woman with Type O blood. They are curious about the potential blood types of their future children. What is the phenotypic ratio of their offspring?

4. In tomato plants, tall stems (T) are dominant to dwarf stems (t) and round fruits (R) are dominant to pear-shaped fruits (r). If a TtRr plant is crossed with a ttrr plant, what is the expected phenotypic ratio?
5. Hypercholesterolemia is an example of incomplete dominance in humans. Individuals with genotype HH have normal cholesterol, Hh have mild disease, and hh have severe disease. If two parents with mild disease have children, what is the chance a child will have severe disease?
6. In fruit flies, gray bodies (G) are dominant to black bodies (g) and normal wings (N) are dominant to vestigial wings (n). Cross a fly that is GgNn with another GgNn fly. How many offspring out of 16 are expected to have black bodies and normal wings?
7. A woman who is a carrier for Hemophilia (an X-linked recessive trait, $X^HX^h$) has a child with a man who does not have hemophilia ($X^HY$). What is the probability that a male child will have hemophilia?
8. In cattle, the poll gene determines the presence of horns. Polled (P) is dominant to horned (p). A heterozygous polled bull is crossed with a horned cow. What percentage of the offspring are expected to be horned?
9. Radish shape can be long, round, or oval. This is an incomplete dominance trait where LL is long, RR is round, and LR is oval. If you cross two oval radishes, what is the probability of getting a long radish?
10. A breeder crosses a pure-breeding black Lab (BBEE) with a yellow Lab (bbee). All F1 offspring are black (BbEe). If two F1 Labs are crossed, what fraction of the F2 generation will be yellow (genotype __ee)? (Note: This is an example of epistasis).

4. Answers & Explanations

1. Answer: 50%. Parent 1 is BBSs (gametes: BS, Bs). Parent 2 is bbss (gamete: bs). The cross results in 50% BbSs (Black, short) and 50% Bbss (Black, long).
2. Answer: 0%. Crossing BW (Speckled) with BB (Blue) results in 50% BB (Blue) and 50% BW (Speckled). No offspring will be WW (White).
3. Answer: 1 Type A : 1 Type B. The cross is AB x OO. The resulting genotypes are 50% AO (Type A) and 50% BO (Type B).
4. Answer: 1:1:1:1. This is a test cross (TtRr x ttrr). The offspring will be 25% Tall/Round, 25% Tall/Pear, 25% Dwarf/Round, and 25% Dwarf/Pear.
5. Answer: 25%. The cross Hh x Hh yields 1 HH, 2 Hh, and 1 hh. The "hh" genotype represents severe disease.
6. Answer: 3 out of 16. In a standard 9:3:3:1 dihybrid ratio, the "recessive-dominant" combination (black body, normal wings) occurs 3 times.
7. Answer: 50% of males. The male offspring receive either $X^H$ or $X^h$ from the mother and $Y$ from the father. There is a 1 in 2 chance a son is $X^hY$.
8. Answer: 50%. Crossing Pp (heterozygous) with pp (homozygous recessive) results in 50% Pp and 50% pp.
9. Answer: 25%. Crossing LR x LR results in 25% LL (long), 50% LR (oval), and 25% RR (round).
10. Answer: 4/16 (or 1/4). In this epistatic interaction, any dog with the "ee" genotype will be yellow regardless of the B/b alleles. In a dihybrid cross, the "ee" combinations are: 1 BBee, 2 Bbee, and 1 bbee, totaling 4/16.

5. Quick Quiz

6. Frequently Asked Questions

What is the difference between a monohybrid and a dihybrid cross?

A monohybrid cross tracks the inheritance of a single trait, such as flower color, usually resulting in a 3:1 ratio. A dihybrid cross tracks two independent traits simultaneously, such as seed shape and seed color, typically resulting in a 9:3:3:1 ratio.

How do you determine the gametes for a dihybrid Punnett Square?

Use the FOIL method (First, Outer, Inner, Last) to distribute alleles. For a genotype like RrYy, the gametes would be RY (First), Ry (Outer), rY (Inner), and ry (Last).

What makes a Punnett Square problem "medium" difficulty?

Medium problems move beyond simple dominance and single traits to include dihybrid crosses, multiple alleles (like blood types), or non-Mendelian patterns like incomplete dominance. They require more steps to set up and analyze the resulting ratios.

Why is the 9:3:3:1 ratio important in genetics?

This ratio is a hallmark of Mendelian genetics, demonstrating the Law of Independent Assortment. It shows that the alleles for one trait separate into gametes independently of the alleles for another trait, provided the genes are on different chromosomes.

Can a Punnett Square predict the exact number of offspring?

No, a Punnett Square only predicts the probability or likelihood of specific genotypes and phenotypes. Like flipping a coin, actual results in small sample sizes may deviate from the expected statistical ratios.

Enjoyed this article?

Share it with others who might find it helpful.

Share Article