Dihybrid Cross Practice Problems With Answers

The dihybrid cross, a cornerstone of Mendelian genetics, unveils the inheritance patterns of two different traits simultaneously. Understanding this concept is crucial for anyone delving into the world of genetics and heredity. This article will provide you with a comprehensive guide to dihybrid crosses, complete with practice problems and detailed solutions to solidify your understanding.

Understanding the Dihybrid Cross

A dihybrid cross involves tracking the inheritance of two distinct traits that are controlled by two different genes. Each gene has two alleles, resulting in different possible combinations. To successfully tackle dihybrid cross problems, a solid grasp of fundamental genetic principles is essential.

Key Concepts

• Genes and Alleles: Genes are the units of heredity, while alleles are the different versions of a gene.
• Dominant and Recessive Alleles: Dominant alleles mask the expression of recessive alleles.
• Genotype and Phenotype: Genotype refers to the genetic makeup of an organism, while phenotype refers to its observable characteristics.
• Homozygous and Heterozygous: Homozygous individuals have two identical alleles for a trait, while heterozygous individuals have two different alleles.
• Punnett Square: A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring in a genetic cross.

Setting Up a Dihybrid Cross

Before diving into practice problems, let's outline the general steps involved in setting up and solving a dihybrid cross:

1. Identify the Traits and Alleles: Determine the two traits being studied and their corresponding alleles. Assign symbols to represent each allele (e.g., R for round, r for wrinkled).
2. Determine the Parental Genotypes: Identify the genotypes of the parent individuals.
3. Determine the Gametes: Determine the possible gametes (sex cells) that each parent can produce. Remember that each gamete carries only one allele for each trait.
4. Construct the Punnett Square: Create a 4x4 Punnett square, with the gametes of one parent listed across the top and the gametes of the other parent listed down the side.
5. Fill in the Punnett Square: Fill in each cell of the Punnett square with the combination of alleles from the corresponding row and column. This represents the potential genotypes of the offspring.
6. Determine the Genotypic and Phenotypic Ratios: Analyze the Punnett square to determine the genotypic and phenotypic ratios of the offspring.

Dihybrid Cross Practice Problems

Now, let's put your knowledge to the test with a series of dihybrid cross practice problems. Each problem will be followed by a detailed solution to help you understand the process.

Problem 1:

In pea plants, round seeds (R) are dominant to wrinkled seeds (r), and yellow seeds (Y) are dominant to green seeds (y). A plant that is heterozygous for both seed shape and seed color is crossed with a plant that is homozygous recessive for both traits. What are the predicted phenotypic ratios of the offspring?

Solution:

1. Identify the Traits and Alleles:

   • Seed shape: Round (R), Wrinkled (r)
   • Seed color: Yellow (Y), Green (y)

2. Determine the Parental Genotypes:

   • Heterozygous for both traits: RrYy
   • Homozygous recessive for both traits: rryy

3. Determine the Gametes:

   • RrYy parent: RY, Ry, rY, ry
   • rryy parent: ry, ry, ry, ry (Since all gametes are the same, we only need to consider one)

4. Construct the Punnett Square:

   	RY	Ry	rY	ry
   ry	RrYy	Rryy	rrYy	rryy

5. Fill in the Punnett Square: (Completed in the table above)

6. Determine the Genotypic and Phenotypic Ratios:

   • Genotypes:
     • RrYy: 1
     • Rryy: 1
     • rrYy: 1
     • rryy: 1
   • Phenotypes:
     • Round, Yellow (RrYy): 1
     • Round, Green (Rryy): 1
     • Wrinkled, Yellow (rrYy): 1
     • Wrinkled, Green (rryy): 1

   Therefore, the predicted phenotypic ratio of the offspring is 1:1:1:1 (Round, Yellow : Round, Green : Wrinkled, Yellow : Wrinkled, Green).

Problem 2:

In guinea pigs, black coat color (B) is dominant to white coat color (b), and rough coat texture (R) is dominant to smooth coat texture (r). A guinea pig that is heterozygous for both traits is crossed with another guinea pig that is also heterozygous for both traits. What are the predicted phenotypic ratios of the offspring?

Solution:

1. Identify the Traits and Alleles:

   • Coat color: Black (B), White (b)
   • Coat texture: Rough (R), Smooth (r)

2. Determine the Parental Genotypes:

   • Both parents are heterozygous for both traits: BbRr

3. Determine the Gametes:

   • Both BbRr parents: BR, Br, bR, br

4. Construct the Punnett Square:

   	BR	Br	bR	br
   BR	BBRR	BBRr	BbRR	BbRr
   Br	BBRr	BBrr	BbRr	Bbrr
   bR	BbRR	BbRr	bbRR	bbRr
   br	BbRr	Bbrr	bbRr	bbrr

5. Fill in the Punnett Square: (Completed in the table above)

6. Determine the Genotypic and Phenotypic Ratios:

   • Phenotypes:
     • Black, Rough (BBRR, BBRr, BbRR, BbRr): 9
     • Black, Smooth (BBrr, Bbrr): 3
     • White, Rough (bbRR, bbRr): 3
     • White, Smooth (bbrr): 1

   Therefore, the predicted phenotypic ratio of the offspring is 9:3:3:1 (Black, Rough : Black, Smooth : White, Rough : White, Smooth). This is the classic dihybrid cross phenotypic ratio.

Problem 3:

In tomatoes, tall plants (T) are dominant to dwarf plants (t), and red fruit (R) is dominant to yellow fruit (r). A tall plant with red fruit that is heterozygous for both traits is crossed with a dwarf plant with yellow fruit. What are the predicted phenotypic ratios of the offspring?

Solution:

1. Identify the Traits and Alleles:

   • Plant height: Tall (T), Dwarf (t)
   • Fruit color: Red (R), Yellow (r)

2. Determine the Parental Genotypes:

   • Tall, Red (heterozygous for both): TrRr
   • Dwarf, Yellow: ttrr

3. Determine the Gametes:

   • TrRr parent: TR, Tr, tR, tr
   • ttrr parent: tr (all gametes are the same)

4. Construct the Punnett Square:

   	TR	Tr	tR	tr
   tr	TrRr	Trrr	ttRr	ttrr

5. Fill in the Punnett Square: (Completed in the table above)

6. Determine the Genotypic and Phenotypic Ratios:

   • Phenotypes:
     • Tall, Red (TrRr): 1
     • Tall, Yellow (Trrr): 1
     • Dwarf, Red (ttRr): 1
     • Dwarf, Yellow (ttrr): 1

   Therefore, the predicted phenotypic ratio of the offspring is 1:1:1:1 (Tall, Red : Tall, Yellow : Dwarf, Red : Dwarf, Yellow).

Problem 4:

In corn, purple kernels (P) are dominant to yellow kernels (p), and smooth kernels (S) are dominant to shrunken kernels (s). A corn plant heterozygous for both traits is crossed with a plant that is homozygous recessive for yellow kernels and heterozygous for smooth kernels. What are the predicted phenotypic ratios of the offspring?

Solution:

1. Identify the Traits and Alleles:

   • Kernel color: Purple (P), Yellow (p)
   • Kernel texture: Smooth (S), Shrunken (s)

2. Determine the Parental Genotypes:

   • Heterozygous for both traits: PpSs
   • Homozygous recessive for yellow, heterozygous for smooth: ppSs

3. Determine the Gametes:

   • PpSs parent: PS, Ps, pS, ps
   • ppSs parent: pS, ps, pS, ps (We only need to list unique gametes) pS, ps

4. Construct the Punnett Square:

   	PS	Ps	pS	ps
   pS	PpSS	PpSs	ppSS	ppSs
   ps	PpSs	Ppss	ppSs	ppss

5. Fill in the Punnett Square: (Completed in the table above)

6. Determine the Genotypic and Phenotypic Ratios:

   • Phenotypes:
     • Purple, Smooth (PpSS, PpSs): 3
     • Purple, Shrunken (Ppss): 1
     • Yellow, Smooth (ppSS, ppSs): 3
     • Yellow, Shrunken (ppss): 1

   Therefore, the predicted phenotypic ratio of the offspring is 3:1:3:1 (Purple, Smooth : Purple, Shrunken : Yellow, Smooth : Yellow, Shrunken).

Problem 5:

In fruit flies, gray body (G) is dominant to ebony body (g), and long wings (L) are dominant to short wings (l). A fly that is heterozygous for both traits is crossed with a fly that has an ebony body and short wings. What are the predicted phenotypic ratios of the offspring?

Solution:

1. Identify the Traits and Alleles:

   • Body color: Gray (G), Ebony (g)
   • Wing length: Long (L), Short (l)

2. Determine the Parental Genotypes:

   • Heterozygous for both traits: GgLl
   • Ebony body, short wings: ggll

3. Determine the Gametes:

   • GgLl parent: GL, Gl, gL, gl
   • ggll parent: gl (all gametes are the same)

4. Construct the Punnett Square:

   	GL	Gl	gL	gl
   gl	GgLl	Ggll	ggLl	ggll

5. Fill in the Punnett Square: (Completed in the table above)

6. Determine the Genotypic and Phenotypic Ratios:

   • Phenotypes:
     • Gray, Long (GgLl): 1
     • Gray, Short (Ggll): 1
     • Ebony, Long (ggLl): 1
     • Ebony, Short (ggll): 1

   Therefore, the predicted phenotypic ratio of the offspring is 1:1:1:1 (Gray, Long : Gray, Short : Ebony, Long : Ebony, Short).

Problem 6:

In rabbits, black fur (B) is dominant to brown fur (b), and long ears (L) are dominant to short ears (l). A rabbit homozygous dominant for black fur and heterozygous for long ears is crossed with a rabbit heterozygous for both traits. What are the predicted phenotypic ratios of the offspring?

Solution:

1. Identify the Traits and Alleles:

   • Fur color: Black (B), Brown (b)
   • Ear length: Long (L), Short (l)

2. Determine the Parental Genotypes:

   • Homozygous dominant for black fur, heterozygous for long ears: BB Ll
   • Heterozygous for both traits: BbLl

3. Determine the Gametes:

   • BBLl parent: BL, Bl
   • BbLl parent: BL, Bl, bL, bl

4. Construct the Punnett Square:

   	BL	Bl	bL	bl
   BL	BBLL	BBLl	BbLL	BbLl
   Bl	BBLl	BBll	BbLl	Bbll

5. Fill in the Punnett Square: (Completed in the table above)

6. Determine the Genotypic and Phenotypic Ratios:

   • Phenotypes:
     • Black, Long (BBLL, BBLl, BbLL, BbLl): 3/4 or 75% (6 outcomes out of 8)
     • Black, Short (BBll, Bbll): 1/4 or 25% (2 outcomes out of 8)
     • Because there is no possibility for a brown furred rabbit to appear, the ratio simplifies to:
     • Black Fur: 3
     • Brown Fur: 0
     • Long Ears: 1
     • Short Ears: 1

   Therefore, the predicted phenotypic ratio of the offspring is 3:1 (Black, Long : Black, Short).

Problem 7:

In dogs, black fur (B) is dominant to brown fur (b), and curly tails (C) are dominant to straight tails (c). A dog heterozygous for both traits is mated with a dog that is brown with a straight tail. Determine the possible genotypes and phenotypes of their puppies and the probability of each.

Solution:

1. Identify the Traits and Alleles:

   • Fur color: Black (B), Brown (b)
   • Tail type: Curly (C), Straight (c)

2. Determine the Parental Genotypes:

   • Heterozygous for both traits: BbCc
   • Brown with a straight tail: bbcc

3. Determine the Gametes:

   • BbCc parent: BC, Bc, bC, bc
   • bbcc parent: bc (all gametes the same)

4. Construct the Punnett Square:

   	BC	Bc	bC	bc
   bc	BbCc	Bbcc	bbCc	bbcc

5. Fill in the Punnett Square: (Completed in the table above)

6. Determine the Genotypic and Phenotypic Ratios:

   • Genotypes and Probability:
     • BbCc: 1/4
     • Bbcc: 1/4
     • bbCc: 1/4
     • bbcc: 1/4
   • Phenotypes and Probability:
     • Black fur, curly tail (BbCc): 1/4
     • Black fur, straight tail (Bbcc): 1/4
     • Brown fur, curly tail (bbCc): 1/4
     • Brown fur, straight tail (bbcc): 1/4

   Therefore, there is an equal probability of each genotype and phenotype appearing in the offspring (25% for each).

Problem 8:

Assume that in squirrels, gray color (G) is dominant to black color (g), and a bushy tail (B) is dominant to a curly tail (b). If a heterozygous gray, heterozygous bushy-tailed squirrel is mated with a homozygous gray, curly-tailed squirrel, what would be the expected phenotypic ratio of their offspring?

Solution:

1. Identify the Traits and Alleles:

   • Color: Gray (G), Black (g)
   • Tail: Bushy (B), Curly (b)

2. Determine the Parental Genotypes:

   • Heterozygous gray, heterozygous bushy-tailed: GgBb
   • Homozygous gray, curly-tailed: GGbb

3. Determine the Gametes:

   • GgBb parent: GB, Gb, gB, gb
   • GGbb parent: Gb, Gb, Gb, Gb. Simplified to Gb

4. Construct the Punnett Square:

   	GB	Gb	gB	gb
   Gb	GGBb	GGbb	GgBb	Ggbb

5. Fill in the Punnett Square:

6. Determine the Phenotypic Ratios:

   • Phenotype Breakdown:
     • Gray, Bushy: GGBb, GgBb (2 outcomes)
     • Gray, Curly: GGbb, Ggbb (2 outcomes)
   • Therefore, the expected ratio is 2:2, simplifying to 1:1.

   Therefore, the expected phenotypic ratio of their offspring would be approximately 1 Gray Bushy : 1 Gray Curly.

Beyond the Basics: Expanding Your Understanding

While these problems cover the fundamental principles of dihybrid crosses, there are several factors that can complicate inheritance patterns. These include:

• Incomplete Dominance: Neither allele is completely dominant, resulting in a blended phenotype.
• Codominance: Both alleles are expressed equally, resulting in a phenotype that displays both traits.
• Sex-linked Traits: Genes located on sex chromosomes (X and Y) exhibit different inheritance patterns in males and females.
• Linked Genes: Genes located close together on the same chromosome tend to be inherited together, deviating from the expected dihybrid cross ratios.

Tips for Solving Dihybrid Cross Problems

Here are some helpful tips to keep in mind when solving dihybrid cross problems:

• Read the problem carefully: Pay close attention to the information provided, including the traits being studied, the alleles involved, and the genotypes of the parents.
• Organize your work: Use a systematic approach, following the steps outlined earlier in this article.
• Double-check your work: Make sure you have correctly determined the gametes, constructed the Punnett square, and calculated the genotypic and phenotypic ratios.
• Practice regularly: The more you practice, the more comfortable you will become with solving dihybrid cross problems.

Conclusion

Mastering the dihybrid cross is a crucial step in understanding the complexities of genetics. By working through practice problems and understanding the underlying principles, you can gain a solid foundation in Mendelian genetics. Remember to approach each problem systematically, carefully considering the traits, alleles, and parental genotypes involved. With practice and perseverance, you'll be well on your way to mastering dihybrid crosses and unlocking the secrets of heredity.