Punnett Square Practice Problems With Answers

Unlocking the secrets of inheritance becomes significantly easier with the Punnett square, a powerful tool used in genetics. This simple diagram helps predict the probability of offspring inheriting specific traits from their parents. Mastering the Punnett square requires practice, and this comprehensive guide provides numerous Punnett square practice problems with detailed answers to solidify your understanding.

What is a Punnett Square?

The Punnett square, named after Reginald Punnett, is a visual representation of all possible combinations of alleles (different forms of a gene) from the parents. It's a grid that allows you to calculate the probability of different genotypes (genetic makeup) and phenotypes (observable characteristics) in the offspring. Think of it as a roadmap of genetic possibilities.

Key Terminology:

• Gene: A unit of heredity that determines a specific trait.
• Allele: A variant form of a gene (e.g., B for brown eyes, b for blue eyes).
• Genotype: The genetic makeup of an individual (e.g., BB, Bb, bb).
• Phenotype: The observable characteristics of an individual, determined by their genotype (e.g., brown eyes, blue eyes).
• Homozygous: Having two identical alleles for a trait (e.g., BB or bb).
• Heterozygous: Having two different alleles for a trait (e.g., Bb).
• Dominant Allele: An allele that masks the expression of the recessive allele when present in a heterozygous state (e.g., B is dominant over b).
• Recessive Allele: An allele that is only expressed when present in a homozygous state (e.g., b is only expressed when the genotype is bb).

Setting Up a Punnett Square: A Step-by-Step Guide

Before diving into practice problems, let's review the steps involved in setting up and interpreting a Punnett square:

1. Determine the Genotypes of the Parents: Identify the alleles each parent carries for the trait you're analyzing.
2. Set Up the Punnett Square Grid: Draw a grid with the number of rows and columns determined by the number of alleles each parent can contribute. For a monohybrid cross (analyzing one trait), you'll typically have a 2x2 grid.
3. Place the Parental Alleles: Write the alleles of one parent across the top of the grid and the alleles of the other parent down the side.
4. Fill in the Grid: Combine the alleles from the top and side for each cell in the grid. This represents the possible genotypes of the offspring.
5. Determine the Genotype and Phenotype Ratios: Analyze the resulting genotypes in the Punnett square to determine the probability of each genotype and the corresponding phenotype.

Punnett Square Practice Problems: Monohybrid Crosses

Let's start with monohybrid crosses, where we're looking at the inheritance of a single trait.

Problem 1:

In pea plants, tallness (T) is dominant over shortness (t). A heterozygous tall plant (Tt) is crossed with a homozygous short plant (tt). What are the possible genotypes and phenotypes of the offspring?

Solution:

1. Parental Genotypes: Tt x tt

2. Punnett Square:

   	T	t
   t	Tt	tt
   t	Tt	tt

3. Genotype Ratio: 2 Tt : 2 tt (or 1:1)

4. Phenotype Ratio: 2 Tall : 2 Short (or 1:1)

Answer: The offspring have a 50% chance of being heterozygous tall (Tt) and a 50% chance of being homozygous short (tt). This translates to a 50% chance of being tall and a 50% chance of being short.

Problem 2:

In guinea pigs, black fur (B) is dominant over brown fur (b). Two heterozygous black guinea pigs (Bb) are crossed. What are the possible genotypes and phenotypes of their offspring?

Solution:

1. Parental Genotypes: Bb x Bb

2. Punnett Square:

   	B	b
   B	BB	Bb
   b	Bb	bb

3. Genotype Ratio: 1 BB : 2 Bb : 1 bb (or 1:2:1)

4. Phenotype Ratio: 3 Black : 1 Brown

Answer: The offspring have a 25% chance of being homozygous dominant black (BB), a 50% chance of being heterozygous black (Bb), and a 25% chance of being homozygous recessive brown (bb). This results in a 75% chance of having black fur and a 25% chance of having brown fur.

Problem 3:

A homozygous dominant purple flower (PP) is crossed with a homozygous recessive white flower (pp). What are the genotypes and phenotypes of the F1 generation (first generation of offspring)?

Solution:

1. Parental Genotypes: PP x pp

2. Punnett Square:

   	P	P
   p	Pp	Pp
   p	Pp	Pp

3. Genotype Ratio: 4 Pp

4. Phenotype Ratio: 4 Purple

Answer: All the offspring in the F1 generation will be heterozygous purple (Pp). Since purple is dominant, all the flowers will be purple.

Problem 4:

Now, let's cross two individuals from the F1 generation in Problem 3 (Pp x Pp). What are the genotypes and phenotypes of the F2 generation (second generation of offspring)?

Solution:

1. Parental Genotypes: Pp x Pp

2. Punnett Square:

   	P	p
   P	PP	Pp
   p	Pp	pp

3. Genotype Ratio: 1 PP : 2 Pp : 1 pp (or 1:2:1)

4. Phenotype Ratio: 3 Purple : 1 White

Answer: The F2 generation will have a 25% chance of being homozygous dominant purple (PP), a 50% chance of being heterozygous purple (Pp), and a 25% chance of being homozygous recessive white (pp). This results in a 75% chance of having purple flowers and a 25% chance of having white flowers.

Problem 5:

In humans, the ability to taste PTC is dominant (T) over the inability to taste PTC (t). A woman who is heterozygous for tasting PTC (Tt) marries a man who cannot taste PTC (tt). What is the probability that their child will be able to taste PTC?

Solution:

1. Parental Genotypes: Tt x tt

2. Punnett Square:

   	T	t
   t	Tt	tt
   t	Tt	tt

3. Genotype Ratio: 2 Tt : 2 tt (or 1:1)

4. Phenotype Ratio: 2 Taster : 2 Non-Taster (or 1:1)

Answer: There is a 50% probability that their child will be able to taste PTC (Tt).

Punnett Square Practice Problems: Dihybrid Crosses

Dihybrid crosses involve analyzing the inheritance of two traits simultaneously. This requires a larger Punnett square (4x4) to account for all possible allele combinations. Remember the principle of independent assortment, which states that alleles for different traits are inherited independently of each other.

Problem 6:

In pea plants, yellow seeds (Y) are dominant over green seeds (y), and round seeds (R) are dominant over wrinkled seeds (r). A plant that is heterozygous for both traits (YyRr) is crossed with another plant that is also heterozygous for both traits (YyRr). What are the possible genotypes and phenotypes of the offspring?

Solution:

1. Parental Genotypes: YyRr x YyRr

2. Possible Gametes: Each parent can produce four types of gametes: YR, Yr, yR, and yr.

3. Punnett Square:

   	YR	Yr	yR	yr
   YR	YYRR	YYRr	YyRR	YyRr
   Yr	YYRr	YYrr	YyRr	Yyrr
   yR	YyRR	YyRr	yyRR	yyRr
   yr	YyRr	Yyrr	yyRr	yyrr

4. Phenotype Ratio: 9 Yellow Round : 3 Yellow Wrinkled : 3 Green Round : 1 Green Wrinkled

Answer: The offspring will have a 9/16 probability of being yellow and round, a 3/16 probability of being yellow and wrinkled, a 3/16 probability of being green and round, and a 1/16 probability of being green and wrinkled. Calculating the exact genotype ratios is more complex but can be done by analyzing the Punnett square.

Problem 7:

In rabbits, black fur (B) is dominant over brown fur (b), and long ears (L) are dominant over short ears (l). A rabbit that is homozygous dominant for both traits (BBLL) is crossed with a rabbit that is homozygous recessive for both traits (bbll). What are the genotypes and phenotypes of the F1 generation?

Solution:

1. Parental Genotypes: BBLL x bbll

2. Possible Gametes: BL and bl

3. Punnett Square: (In this case, a 2x2 Punnett square is sufficient since there is only one type of gamete from each parent)

   	BL
   bl	BbLl

4. Genotype: All offspring will be BbLl

5. Phenotype: All offspring will be black with long ears.

Answer: The F1 generation will all be heterozygous for both traits (BbLl) and will all have black fur and long ears.

Problem 8:

Now, let's cross two rabbits from the F1 generation in Problem 7 (BbLl x BbLl). What are the genotypes and phenotypes of the F2 generation?

Solution:

1. Parental Genotypes: BbLl x BbLl

2. Possible Gametes: BL, Bl, bL, bl

3. Punnett Square:

   	BL	Bl	bL	bl
   BL	BBLL	BBLl	BbLL	BbLl
   Bl	BBLl	BBll	BbLl	Bbll
   bL	BbLL	BbLl	bbLL	bbLl
   bl	BbLl	Bbll	bbLl	bbll

4. Phenotype Ratio: 9 Black Long Ears : 3 Black Short Ears : 3 Brown Long Ears : 1 Brown Short Ears

Answer: The F2 generation will have the classic dihybrid cross phenotypic ratio: 9/16 black fur and long ears, 3/16 black fur and short ears, 3/16 brown fur and long ears, and 1/16 brown fur and short ears.

Problem 9:

In tomatoes, red fruit (R) is dominant over yellow fruit (r), and tall plants (T) are dominant over short plants (t). A tomato plant heterozygous for both traits (RrTt) is crossed with a tomato plant that is homozygous recessive for both traits (rrtt). What are the possible genotypes and phenotypes of the offspring?

Solution:

1. Parental Genotypes: RrTt x rrtt

2. Possible Gametes: RT, Rt, rT, rt (from RrTt) and rt (from rrtt)

3. Punnett Square:

   	RT	Rt	rT	rt
   rt	RrTt	Rrtt	rrTt	rrtt

4. Phenotype Ratio: 1 Red Tall : 1 Red Short : 1 Yellow Tall : 1 Yellow Short

Answer: The offspring will have an equal probability (25% each) of being red and tall, red and short, yellow and tall, and yellow and short.

Problem 10:

In fruit flies, gray body (G) is dominant over black body (g), and normal wings (N) are dominant over vestigial wings (n). A male fruit fly heterozygous for both traits (GgNn) is crossed with a female fruit fly that is homozygous recessive for black body and heterozygous for normal wings (ggNn). What are the possible phenotypes and their probabilities in the offspring?

Solution:

1. Parental Genotypes: GgNn x ggNn

2. Possible Gametes: GN, Gn, gN, gn (from GgNn) and gN, gn (from ggNn)

3. Punnett Square:

   	GN	Gn	gN	gn
   gN	GgNN	GgNn	ggNN	ggNn
   gn	GgNn	Ggnn	ggNn	ggnn

4. Phenotype Analysis:

   • Gray Body, Normal Wings: GgNN, GgNn (3/8)
   • Gray Body, Vestigial Wings: Ggnn (1/8)
   • Black Body, Normal Wings: ggNN, ggNn (3/8)
   • Black Body, Vestigial Wings: ggnn (1/8)

Answer: The offspring phenotypes and probabilities are: 37.5% gray body with normal wings, 12.5% gray body with vestigial wings, 37.5% black body with normal wings, and 12.5% black body with vestigial wings.

Beyond Monohybrid and Dihybrid Crosses: Expanding Your Punnett Square Skills

While monohybrid and dihybrid crosses are fundamental, the Punnett square can be adapted for more complex scenarios:

• Incomplete Dominance: When neither allele is completely dominant, the heterozygous genotype results in an intermediate phenotype (e.g., red flower + white flower = pink flower).
• Codominance: When both alleles are expressed equally in the heterozygous genotype (e.g., red fur + white fur = roan fur, where both red and white hairs are present).
• Multiple Alleles: When a gene has more than two alleles (e.g., human blood types: A, B, and O).
• Sex-Linked Traits: Traits located on sex chromosomes (usually the X chromosome), leading to different inheritance patterns in males and females.

These complex scenarios require modifying the Punnett square setup and interpretation to accurately reflect the inheritance patterns. However, the fundamental principle of using the grid to predict allele combinations remains the same.

Tips for Success with Punnett Squares

• Practice, Practice, Practice: The more problems you solve, the more comfortable you'll become with setting up and interpreting Punnett squares.
• Clearly Define Your Alleles: Use consistent and logical symbols for your alleles (e.g., upper case for dominant, lower case for recessive).
• Double-Check Your Work: Ensure you've correctly identified parental genotypes, possible gametes, and have filled in the Punnett square accurately.
• Understand the Underlying Concepts: Don't just memorize how to set up a Punnett square; understand the principles of Mendelian genetics that it represents.
• Draw it Out: Visualizing the problem with a Punnett square is often the best way to solve it.
• Break Down Complex Problems: For more complicated scenarios (e.g., multiple alleles, sex-linked traits), break the problem down into smaller, manageable steps.

Conclusion

The Punnett square is an invaluable tool for understanding and predicting inheritance patterns in genetics. By working through these Punnett square practice problems with answers, you've gained a solid foundation in applying this powerful diagram. Remember to continue practicing and expanding your knowledge to tackle even more complex genetic scenarios. With dedication and a clear understanding of the principles, you'll master the art of predicting the genetic future of offspring!