Incomplete Dominance Practice Problems Answer Key

Incomplete dominance, where neither allele is fully dominant, results in a blended phenotype in heterozygotes, offering a fascinating departure from Mendelian genetics. Mastering incomplete dominance requires practice; let's dive into practical problems and their solutions, ensuring a solid grasp of this concept.

Understanding Incomplete Dominance: The Basics

Before tackling practice problems, it's crucial to understand the underlying principles of incomplete dominance. In Mendelian genetics, a dominant allele completely masks the presence of a recessive allele in a heterozygote. However, in incomplete dominance, the heterozygous genotype results in a phenotype that is a mix of the two homozygous phenotypes.

• Key Characteristics:

  • Neither allele is dominant over the other.
  • The heterozygous phenotype is intermediate.
  • Phenotypic and genotypic ratios often coincide.

• Example:

  • In snapdragons, a cross between a red-flowered plant (RR) and a white-flowered plant (WW) produces pink-flowered plants (RW).

Understanding these basics will pave the way for effectively solving incomplete dominance practice problems.

Incomplete Dominance Practice Problems: A Step-by-Step Guide

Let's work through a series of practice problems, breaking down each one into manageable steps. Each problem will illustrate a different facet of incomplete dominance.

Problem 1: Snapdragon Flower Color

In snapdragons, flower color is governed by incomplete dominance. Red flowers (RR) crossed with white flowers (WW) produce pink flowers (RW). What are the genotypic and phenotypic ratios of the offspring if you cross two pink-flowered snapdragons?

• Step 1: Define the Alleles

  • R = Red allele
  • W = White allele

• Step 2: Determine the Parental Genotypes

  • Both parents are pink-flowered, so their genotypes are RW.

• Step 3: Set Up the Punnett Square

  	R	W
  R	RR	RW
  W	RW	WW

• Step 4: Determine Genotypic Ratios

  • RR: 1/4
  • RW: 2/4 or 1/2
  • WW: 1/4

• Step 5: Determine Phenotypic Ratios

  • Red: 1/4
  • Pink: 1/2
  • White: 1/4

Answer: The genotypic ratio is 1 RR : 2 RW : 1 WW. The phenotypic ratio is 1 red : 2 pink : 1 white.

Problem 2: Feather Color in Chickens

In chickens, feather color is an example of incomplete dominance. Black feathered chickens (BB) crossed with white feathered chickens (WW) produce blue feathered chickens (BW). If you cross a blue feathered chicken with a white feathered chicken, what percentage of the offspring will also be blue feathered?

• Step 1: Define the Alleles

  • B = Black allele
  • W = White allele

• Step 2: Determine the Parental Genotypes

  • Blue feathered chicken: BW
  • White feathered chicken: WW

• Step 3: Set Up the Punnett Square

  	B	W
  W	BW	WW
  W	BW	WW

• Step 4: Determine Genotypic Ratios

  • BW: 2/4 or 1/2
  • WW: 2/4 or 1/2

• Step 5: Determine Phenotypic Ratios

  • Blue: 1/2
  • White: 1/2

• Step 6: Calculate Percentage

  • Percentage of blue feathered offspring: (1/2) * 100% = 50%

Answer: 50% of the offspring will be blue feathered.

Problem 3: Tail Length in Mice

Tail length in mice exhibits incomplete dominance. Long tails (LL) crossed with short tails (SS) result in medium tails (LS). If you cross two medium-tailed mice, what is the probability of having a long-tailed mouse?

• Step 1: Define the Alleles

  • L = Long tail allele
  • S = Short tail allele

• Step 2: Determine the Parental Genotypes

  • Both parents are medium-tailed, so their genotypes are LS.

• Step 3: Set Up the Punnett Square

  	L	S
  L	LL	LS
  S	LS	SS

• Step 4: Determine Genotypic Ratios

  • LL: 1/4
  • LS: 2/4 or 1/2
  • SS: 1/4

• Step 5: Determine Phenotypic Ratios

  • Long-tailed: 1/4
  • Medium-tailed: 1/2
  • Short-tailed: 1/4

• Step 6: Determine Probability

  • Probability of having a long-tailed mouse: 1/4

Answer: The probability of having a long-tailed mouse is 1/4 or 25%.

Problem 4: Fruit Color in Eggplants

In eggplants, fruit color is controlled by incomplete dominance. Purple fruits (PP) crossed with white fruits (WW) produce violet fruits (PW). A farmer has a field of violet eggplants and wants to know the percentage of offspring that will have purple fruits if the violet eggplants are allowed to self-pollinate.

• Step 1: Define the Alleles

  • P = Purple allele
  • W = White allele

• Step 2: Determine the Parental Genotypes

  • Violet eggplants: PW

• Step 3: Set Up the Punnett Square

  	P	W
  P	PP	PW
  W	PW	WW

• Step 4: Determine Genotypic Ratios

  • PP: 1/4
  • PW: 2/4 or 1/2
  • WW: 1/4

• Step 5: Determine Phenotypic Ratios

  • Purple: 1/4
  • Violet: 1/2
  • White: 1/4

• Step 6: Calculate Percentage

  • Percentage of purple fruits: (1/4) * 100% = 25%

Answer: 25% of the offspring will have purple fruits.

Problem 5: Flower Height in Plants

Consider a plant where height is determined by incomplete dominance. Tall plants (TT) crossed with short plants (SS) produce medium-height plants (TS). If you cross a medium-height plant with a tall plant, what are the expected genotypic and phenotypic ratios of the offspring?

• Step 1: Define the Alleles

  • T = Tall allele
  • S = Short allele

• Step 2: Determine the Parental Genotypes

  • Medium-height plant: TS
  • Tall plant: TT

• Step 3: Set Up the Punnett Square

  	T	S
  T	TT	TS
  T	TT	TS

• Step 4: Determine Genotypic Ratios

  • TT: 2/4 or 1/2
  • TS: 2/4 or 1/2
  • SS: 0/4

• Step 5: Determine Phenotypic Ratios

  • Tall: 1/2
  • Medium: 1/2
  • Short: 0/4

Answer: The genotypic ratio is 1 TT : 1 TS : 0 SS. The phenotypic ratio is 1 tall : 1 medium : 0 short.

Advanced Incomplete Dominance Problems

Now, let's explore more complex problems that require integrating multiple concepts.

Problem 6: Coat Color in Horses

Coat color in horses sometimes displays incomplete dominance. Chestnut horses (CC) crossed with white horses (WW) produce palomino horses (CW). A breeder crosses two palomino horses. If she wants to produce chestnut horses, what percentage of the offspring will be chestnut? If she breeds a palomino with a chestnut, what percentage will be palomino?

• Part 1: Palomino x Palomino

  • Step 1: Define the Alleles

    • C = Chestnut allele
    • W = White allele

  • Step 2: Determine the Parental Genotypes

    • Both parents are palomino, so their genotypes are CW.

  • Step 3: Set Up the Punnett Square

    	C	W
    C	CC	CW
    W	CW	WW

  • Step 4: Determine Genotypic Ratios

    • CC: 1/4
    • CW: 2/4 or 1/2
    • WW: 1/4

  • Step 5: Determine Phenotypic Ratios

    • Chestnut: 1/4
    • Palomino: 1/2
    • White: 1/4

  • Step 6: Calculate Percentage of Chestnut Offspring

    • (1/4) * 100% = 25%

• Part 2: Palomino x Chestnut

  • Step 1: Define the Alleles

    • C = Chestnut allele
    • W = White allele

  • Step 2: Determine the Parental Genotypes

    • Palomino horse: CW
    • Chestnut horse: CC

  • Step 3: Set Up the Punnett Square

    	C	W
    C	CC	CW
    C	CC	CW

  • Step 4: Determine Genotypic Ratios

    • CC: 2/4 or 1/2
    • CW: 2/4 or 1/2
    • WW: 0/4

  • Step 5: Determine Phenotypic Ratios

    • Chestnut: 1/2
    • Palomino: 1/2
    • White: 0/4

  • Step 6: Calculate Percentage of Palomino Offspring

    • (1/2) * 100% = 50%

Answer: When crossing two palomino horses, 25% of the offspring will be chestnut. When breeding a palomino with a chestnut, 50% of the offspring will be palomino.

Problem 7: Spine Development in Cacti

In cacti, spine development exhibits incomplete dominance. Cacti with long spines (LL) crossed with those having no spines (NN) produce cacti with short spines (LN). If a cactus with short spines is crossed with another cactus with short spines, what proportion of the offspring will have no spines?

• Step 1: Define the Alleles

  • L = Long spine allele
  • N = No spine allele

• Step 2: Determine the Parental Genotypes

  • Both parents have short spines, so their genotypes are LN.

• Step 3: Set Up the Punnett Square

  	L	N
  L	LL	LN
  N	LN	NN

• Step 4: Determine Genotypic Ratios

  • LL: 1/4
  • LN: 2/4 or 1/2
  • NN: 1/4

• Step 5: Determine Phenotypic Ratios

  • Long spines: 1/4
  • Short spines: 1/2
  • No spines: 1/4

• Step 6: Determine Proportion

  • Proportion of offspring with no spines: 1/4

Answer: The proportion of offspring with no spines is 1/4.

Problem 8: Plant Pigment Production

In a certain plant, pigment production is governed by incomplete dominance. Plants with alleles AA produce a lot of red pigment, plants with alleles BB produce no pigment, and plants with alleles AB produce a small amount of red pigment, appearing pink. A pink plant is crossed with a plant that produces a lot of red pigment. What are the possible genotypes and phenotypes of their offspring?

• Step 1: Define the Alleles

  • A = Produces a lot of red pigment
  • B = Produces no pigment

• Step 2: Determine the Parental Genotypes

  • Pink plant: AB
  • Plant that produces a lot of red pigment: AA

• Step 3: Set Up the Punnett Square

  	A	B
  A	AA	AB
  A	AA	AB

• Step 4: Determine Genotypic Ratios

  • AA: 2/4 or 1/2
  • AB: 2/4 or 1/2
  • BB: 0/4

• Step 5: Determine Phenotypic Ratios

  • A lot of red pigment: 1/2
  • Small amount of red pigment (pink): 1/2
  • No pigment: 0/4

Answer: The possible genotypes are AA and AB. The possible phenotypes are plants producing a lot of red pigment and pink plants.

Problem 9: Comb Shape in Chickens

Comb shape in chickens can be influenced by incomplete dominance. Chickens with a walnut comb (WW) crossed with chickens having a single comb (SS) result in chickens with a pea comb (WS). If you cross a pea-combed chicken with a walnut-combed chicken, what percentage of the offspring will have a pea comb?

• Step 1: Define the Alleles

  • W = Walnut comb allele
  • S = Single comb allele

• Step 2: Determine the Parental Genotypes

  • Pea-combed chicken: WS
  • Walnut-combed chicken: WW

• Step 3: Set Up the Punnett Square

  	W	S
  W	WW	WS
  W	WW	WS

• Step 4: Determine Genotypic Ratios

  • WW: 2/4 or 1/2
  • WS: 2/4 or 1/2
  • SS: 0/4

• Step 5: Determine Phenotypic Ratios

  • Walnut comb: 1/2
  • Pea comb: 1/2
  • Single comb: 0/4

• Step 6: Calculate Percentage

  • Percentage of pea-combed offspring: (1/2) * 100% = 50%

Answer: 50% of the offspring will have a pea comb.

Problem 10: Flower Size in Roses

Flower size in roses exhibits incomplete dominance. Large flowers (LL) crossed with small flowers (SS) produce medium-sized flowers (LS). A gardener has a rose garden with only medium-sized flowers. If these roses self-pollinate, what proportion of the offspring will have large flowers?

• Step 1: Define the Alleles

  • L = Large flower allele
  • S = Small flower allele

• Step 2: Determine the Parental Genotypes

  • Medium-sized flowers: LS

• Step 3: Set Up the Punnett Square

  	L	S
  L	LL	LS
  S	LS	SS

• Step 4: Determine Genotypic Ratios

  • LL: 1/4
  • LS: 2/4 or 1/2
  • SS: 1/4

• Step 5: Determine Phenotypic Ratios

  • Large flowers: 1/4
  • Medium flowers: 1/2
  • Small flowers: 1/4

• Step 6: Determine Proportion

  • Proportion of offspring with large flowers: 1/4

Answer: The proportion of offspring with large flowers is 1/4.

Common Mistakes and How to Avoid Them

• Confusing Incomplete Dominance with Codominance:

  • Incomplete dominance results in a blended phenotype, while codominance results in both alleles being expressed simultaneously.

• Incorrectly Assigning Alleles:

  • Always define alleles clearly before setting up the Punnett square.

• Misinterpreting Ratios:

  • Ensure you understand the difference between genotypic and phenotypic ratios.

• Forgetting to Define Parental Genotypes:

  • Accurately determine the genotypes of the parents before attempting to solve the problem.

Conclusion

Incomplete dominance is a fundamental concept in genetics that illustrates how alleles interact to produce diverse phenotypes. By understanding the principles of incomplete dominance and working through practice problems, you can master this essential concept. Remember to define alleles, determine parental genotypes, set up Punnett squares, and interpret the resulting ratios accurately. Consistent practice will solidify your understanding and enable you to tackle even the most complex incomplete dominance problems with confidence.