Question

A scientist studying a species of algae, Gracilaria domingensis, discovered that its color is controlled by a single gene with two alleles. The individuals with a red or green phenotype are homozygous for the red of green alleles and the individuals with brown phenotype are heterozygous

Answer 1

Call the two alleles R (red) and G (green). The genotype–phenotype mapping you described is:

- RR → red (homozygous red)
- GG → green (homozygous green)
- RG (or GR) → brown (heterozygote)

This is a case of incomplete dominance (the heterozygote shows a distinct phenotype rather than one allele being completely dominant).

Common crosses and their expected genotype/phenotype outcomes:

1. RG × RG (brown × brown)
- Genotypes: 1 RR : 2 RG : 1 GG
- Phenotypes: 1 red : 2 brown : 1 green

2. RR × GG (red × green)
- All offspring RG (100% brown)

3. RR × RG (red × brown)
- Genotypes: 1 RR : 1 RG
- Phenotypes: 50% red, 50% brown

4. GG × RG (green × brown)
- Genotypes: 1 GG : 1 RG
- Phenotypes: 50% green, 50% brown

5. RR × RR or GG × GG
- All offspring are the parental homozygote phenotype (all red or all green, respectively).

Using crosses you can infer unknown genotypes: for example, if a red individual crossed with a brown produces any brown offspring, the red parent must be RR (if all red, the red parent would still be RR — to distinguish RR from something else you would cross differently). A classical test: cross the unknown with a known homozygote (RR or GG) and look at offspring ratios to determine the unknown genotype.