Solving Genetic Crosses with Red and White Traits

What You're Actually Working With: Red and White Genetics

Most textbook problems involving red and white traits come down to two scenarios: complete dominance (one color wins) or incomplete dominance (colors blend). Your first job is figuring out which one you're dealing with.

Incomplete dominance is common with flower colors. Red snapdragon crossed with white snapdragon gives you pink. That's the blend. Complete dominance is what you see in Mendel's peas—one allele masks the other completely.

Read the problem. If it mentions "intermediate phenotype" or "blended," you're in incomplete dominance territory. If it just says red and white with no mention of blending, assume complete dominance.

The Allele System You'll Actually Use

For red and white traits, you're typically working with two alleles:

• R = allele for red
• r = allele for white

With incomplete dominance, the heterozygous genotype (Rr) shows a third phenotype—usually pink, rose, or whatever color results from the blend. With complete dominance, Rr shows red because red is dominant.

Dominance vs. Incomplete Dominance: The Difference

That's the core difference. Memorize it.

How to Solve Any Genetic Cross: Step by Step

Here's the method that works every time, no exceptions.

Step 1: Identify Parent Genotypes

The problem will give you this. Look for phrases like "homozygous red" or "heterozygous red." Homozygous means both alleles are the same (RR or rr). Heterozygous means they're different (Rr).

If the problem says "purebred red," that's homozygous dominant (RR). "Purebred white" is homozygous recessive (rr). Heterozygous is the hybrid (Rr).

Step 2: Set Up the Punnett Square

Place one parent's alleles across the top. Place the other parent's alleles down the side. Each box gets one allele from each parent.

For a cross between RR and rr:

Every offspring is Rr. With incomplete dominance, that's pink. With complete dominance, that's red.

Step 3: Determine Offspring Ratios

Count your genotypic and phenotypic outcomes. Write them as ratios.

From the RR × rr cross above: 4 Rr offspring. Genotypic ratio is 4:0:0 (all Rr). Phenotypic ratio depends on dominance type.

A more useful cross: Rr × Rr (heterozygous × heterozygous).

Genotypic ratio: 1 RR : 2 Rr : 1 rr

Phenotypic ratio (complete dominance): 3 red : 1 white

Phenotypic ratio (incomplete dominance): 1 red : 2 pink : 1 white

Working Through a Real Example

Problem: A red-flowered plant is crossed with a white-flowered plant. All F1 offspring are pink. What happens when you cross two F1 plants?

Step 1: The fact that F1 offspring are pink tells you this is incomplete dominance. Red parent is RR. White parent is rr. F1 plants are all Rr.

Step 2: Cross Rr × Rr.

Step 3: You get 1 RR (red) : 2 Rr (pink) : 1 rr (white). Phenotype ratio is 1:2:1.

Step 4: If the question asks for probability, each offspring has a 25% chance of being red, 50% chance of being pink, and 25% chance of being white.

Quick Reference: Common Crosses

Mistakes That Will Cost You Points

• Forgetting which trait is dominant. If the problem doesn't specify incomplete dominance, assume complete dominance. Red is usually dominant over white.
• Mixing up genotypes and phenotypes. Genotype is the genetic makeup (RR, Rr, rr). Phenotype is what you actually see (red, pink, white).
• Writing the wrong allele letters. Dominant alleles get uppercase. Recessive get lowercase. Don't reverse this.
• Skipping the Punnett square. Some students try to do this mentally. Don't. Draw the square every time until it's automatic.

The Bottom Line

Solving genetic crosses with red and white traits comes down to three things: knowing whether you're dealing with complete or incomplete dominance, setting up your Punnett square correctly, and reading what the question actually asks for.

Practice the Rr × Rr cross until you can do it in your sleep. That's the cross that shows up most often, and it generates the ratios you'll need for almost any follow-up question.