Practice Problems Incomplete Dominance And Codominance

Decoding Incomplete Dominance and Codominance: Practice Problems and Explanations

Genetics can sometimes feel like deciphering a secret code. While Mendelian genetics provides a strong foundation, the real world often presents more complex inheritance patterns. Two such patterns are incomplete dominance and codominance, where the relationship between alleles isn't a simple "one wins, one loses" scenario. Instead, we see blended or shared traits. This article dives deep into these concepts, providing practice problems and detailed explanations to solidify your understanding.

Understanding Alleles, Genotypes, and Phenotypes

Before we tackle incomplete dominance and codominance, let's quickly review some essential vocabulary.

• Allele: A variant form of a gene. For example, a gene for flower color might have alleles for red (R) and white (r).
• Genotype: The genetic makeup of an individual, represented by the combination of alleles they possess. For example, RR, Rr, or rr.
• Phenotype: The observable characteristics of an individual, resulting from the interaction of their genotype and the environment. For example, red flowers, pink flowers, or white flowers.
• Homozygous: Having two identical alleles for a particular gene (e.g., RR or rr).
• Heterozygous: Having two different alleles for a particular gene (e.g., Rr).

Incomplete Dominance: A Blending Act

Incomplete dominance occurs when the heterozygous genotype results in a phenotype that is intermediate between the two homozygous phenotypes. Neither allele is fully dominant over the other, leading to a blending effect.

Key Characteristics of Incomplete Dominance:

• The heterozygous phenotype is distinct from both homozygous phenotypes.
• There is a "blending" or "intermediate" effect.
• Phenotypic ratios in the F2 generation often mirror the genotypic ratios.

Example: Snapdragon Flower Color

A classic example of incomplete dominance is the flower color in snapdragons. Let's say:

• RR = Red flowers
• rr = White flowers
• Rr = Pink flowers (a blend of red and white)

Notice that the heterozygous genotype (Rr) doesn't produce red flowers (the dominant trait). Instead, it produces a new phenotype: pink flowers.

Codominance: Sharing the Spotlight

Codominance occurs when both alleles in a heterozygous genotype are fully expressed, and the phenotype exhibits both traits simultaneously. Unlike incomplete dominance, there is no blending; both alleles make their presence known.

Key Characteristics of Codominance:

• Both alleles are expressed equally.
• The heterozygous phenotype displays both traits associated with the homozygous phenotypes.
• There is no "blending" effect; both traits are visible.

Example: Human Blood Types (ABO System)

The ABO blood group system in humans provides an excellent example of codominance. The I gene controls the production of antigens on the surface of red blood cells. There are three alleles for this gene: I^A, I^B, and i.

• I^A allele: Produces A antigens
• I^B allele: Produces B antigens
• i allele: Produces no antigens

Here's how the genotypes and phenotypes break down:

• I^AI^A: Blood type A
• I^Ai: Blood type A (A is dominant over i)
• I^BI^B: Blood type B
• I^Bi: Blood type B (B is dominant over i)
• I^AI^B: Blood type AB (A and B are codominant; both antigens are produced)
• ii: Blood type O (no antigens are produced)

In individuals with the I^AI^B genotype, both A and B antigens are produced on the red blood cells. This is codominance because neither allele is masking the other; both are being expressed simultaneously.

Practice Problems: Incomplete Dominance

Let's work through some practice problems to solidify your understanding of incomplete dominance.

Problem 1:

In chickens, the allele for black feathers (B) is incompletely dominant to the allele for white feathers (W). Heterozygous chickens (BW) have blue feathers.

• a) What are the genotypes and phenotypes of the possible offspring if you cross a blue chicken with a white chicken?
• b) What phenotypic ratio would you expect from crossing two blue chickens?

Solution:

• a) Blue chicken (BW) x White chicken (WW)

  Let's create a Punnett square:

  	B	W
  W	BW	WW
  W	BW	WW

  • Genotypes: 50% BW (Blue), 50% WW (White)
  • Phenotypes: 50% Blue, 50% White

• b) Blue chicken (BW) x Blue chicken (BW)

  Punnett square:

  	B	W
  B	BB	BW
  W	BW	WW

  • Genotypes: 25% BB, 50% BW, 25% WW
  • Phenotypes: 25% Black, 50% Blue, 25% White
  • Phenotypic Ratio: 1 Black : 2 Blue : 1 White

Problem 2:

Radishes exhibit incomplete dominance for root color. RR plants have red roots, WW plants have white roots, and RW plants have purple roots. A farmer crosses a purple radish plant with a white radish plant.

• a) What are the possible genotypes and phenotypes of the offspring?
• b) What percentage of the offspring will have purple roots?

Solution:

• a) Purple radish (RW) x White radish (WW)

  Punnett square:

  	R	W
  W	RW	WW
  W	RW	WW

  • Genotypes: 50% RW, 50% WW
  • Phenotypes: 50% Purple, 50% White

• b) Percentage of purple roots: 50%

Problem 3:

In a certain species of plant, the allele for tallness (T) is incompletely dominant to the allele for dwarfism (t). Heterozygous plants (Tt) are of medium height. If two medium-height plants are crossed:

• a) What are the expected genotypes and phenotypes of the offspring?
• b) What is the probability of producing a dwarf plant?

Solution:

• a) Medium height plant (Tt) x Medium height plant (Tt)

  Punnett square:

  	T	t
  T	TT	Tt
  t	Tt	tt

  • Genotypes: 25% TT, 50% Tt, 25% tt
  • Phenotypes: 25% Tall, 50% Medium, 25% Dwarf

• b) Probability of producing a dwarf plant: 25%

Practice Problems: Codominance

Now let's tackle some problems involving codominance.

Problem 1:

A cow can be red (RR), white (WW), or roan (RW). Roan is a coat color pattern where both red and white hairs are present.

• a) What offspring would you expect from mating a roan bull and a red cow? Give the genotypes and phenotypes with percentages.
• b) What cross would produce offspring with the ratio of 1 red: 2 roan: 1 white?

Solution:

• a) Roan bull (RW) x Red cow (RR)

  Punnett square:

  	R	R
  R	RR	RR
  W	RW	RW

  • Genotypes: 50% RR, 50% RW
  • Phenotypes: 50% Red, 50% Roan

• b) Cross to produce 1 Red : 2 Roan : 1 White:

  This ratio is characteristic of a cross between two heterozygotes. Therefore, the cross would be: Roan (RW) x Roan (RW). We already worked out this cross in a previous problem, but let's show it again for clarity.

  Punnett square:

  	R	W
  R	RR	RW
  W	RW	WW

  • Genotypes: 25% RR, 50% RW, 25% WW
  • Phenotypes: 25% Red, 50% Roan, 25% White
  • Phenotypic Ratio: 1 Red : 2 Roan : 1 White

Problem 2:

In humans, blood type AB is an example of codominance. A woman with blood type AB marries a man with blood type A (genotype I^Ai).

• a) What are the possible blood types of their children?
• b) What is the probability that their child will have blood type B?

Solution:

• a) AB woman (I^AI^B) x A man (I^Ai)

  Punnett square:

  	I<sup>A</sup>	I<sup>B</sup>
  I<sup>A</sup>	I<sup>A</sup>I<sup>A</sup>	I<sup>A</sup>I<sup>B</sup>
  i	I<sup>A</sup>i	I<sup>B</sup>i

  • Genotypes: I^AI^A, I^AI^B, I^Ai, I^Bi
  • Phenotypes: Blood type A, Blood type AB, Blood type A, Blood type B
  • Possible Blood Types: A, AB, B

• b) Probability of blood type B: 25% (the I^Bi genotype)

Problem 3:

A breeder has a flock of chickens with two feather color alleles: black (F^B) and white (F^W). Heterozygous chickens (F^BF^W) have speckled black and white feathers. If the breeder crosses a speckled chicken with a white chicken:

• a) What are the expected genotypes and phenotypes of the offspring?
• b) What percentage of the offspring will be speckled?

Solution:

• a) Speckled chicken (F^BF^W) x White chicken (F^WF^W)

  Punnett square:

  	F<sup>B</sup>	F<sup>W</sup>
  F<sup>W</sup>	F<sup>B</sup>F<sup>W</sup>	F<sup>W</sup>F<sup>W</sup>
  F<sup>W</sup>	F<sup>B</sup>F<sup>W</sup>	F<sup>W</sup>F<sup>W</sup>

  • Genotypes: 50% F^BF^W, 50% F^WF^W
  • Phenotypes: 50% Speckled, 50% White

• b) Percentage of speckled offspring: 50%

Distinguishing Incomplete Dominance from Codominance

A common point of confusion is differentiating between incomplete dominance and codominance. Here’s a table summarizing the key differences:

Think of it this way:

• Incomplete dominance: Mixing paint – red and white create pink.
• Codominance: A salad – you can clearly see and taste both the lettuce and the tomatoes.

Beyond Simple Examples: Complexities in Genetics

While these practice problems provide a solid foundation, remember that genetics can be incredibly complex. Many traits are influenced by multiple genes (polygenic inheritance), environmental factors, and epigenetic modifications. Incomplete dominance and codominance are just two pieces of the puzzle.

Common Mistakes to Avoid

• Confusing dominance with frequency: A dominant allele is not necessarily the most common allele in a population.
• Assuming all traits follow Mendelian inheritance: Many traits exhibit more complex inheritance patterns.
• Incorrectly assigning genotypes and phenotypes: Always double-check your work to ensure you're using the correct notation and terminology.
• Not using Punnett squares: Punnett squares are essential for visualizing and predicting the outcomes of genetic crosses.

Further Exploration

To deepen your understanding of genetics, consider exploring these topics:

• Sex-linked inheritance: Genes located on the sex chromosomes (X and Y).
• Polygenic inheritance: Traits controlled by multiple genes.
• Epistasis: The interaction of genes where one gene masks or modifies the expression of another gene.
• Environmental influences on phenotype: How the environment can affect gene expression.

Conclusion

Incomplete dominance and codominance are fascinating examples of how alleles can interact to produce diverse phenotypes. By understanding the key principles and working through practice problems, you can confidently navigate these concepts and gain a deeper appreciation for the complexities of inheritance. Remember to carefully analyze each problem, identify the inheritance pattern, and use Punnett squares to predict the genotypes and phenotypes of the offspring. Keep practicing, and you'll become a genetics whiz in no time!