Question

purple spots is dominant over green spots. two purple-spotted aliens mate and some of their offspring have green-spots. create a punnett square and explain how this could have happened.
purple spotted hetero x green spotted hetero
genotype | phenotype
aa= 1/4 25% | a=purple spots
aa=2/4 50% | a=green spots
aa=1/4 25%
parents genotype
mom aa dad aa

Explanation:

Step1: Identify parent genotypes

Since purple spots (A) are dominant, and the parents have purple spots but produced green-spotted offspring (aa), both parents must be heterozygous: $Aa \times Aa$.

Step2: Set up Punnett square

Place parent alleles on axes:

$$\begin{array}{c|cc} & A & a \\ \hline A & AA & Aa \\ a & Aa & aa \\ \end{array}$$

Step3: Calculate genotype ratios

Count each genotype:

• $AA$: $\frac{1}{4}$ (25%)
• $Aa$: $\frac{2}{4} = \frac{1}{2}$ (50%)
• $aa$: $\frac{1}{4}$ (25%)

Step4: Link genotypes to phenotypes

• $AA$ and $Aa$: purple spots (dominant trait expressed)
• $aa$: green spots (recessive trait only expressed when homozygous)

Answer:

Both purple-spotted parent aliens are heterozygous ($Aa$). When they mate, each can pass on either the dominant $A$ allele or recessive $a$ allele. The Punnett square shows there is a 25% ($\frac{1}{4}$) chance their offspring inherits two recessive $a$ alleles ($aa$), which results in the green-spotted phenotype. The Punnett square for the cross $Aa \times Aa$ is:

$$\begin{array}{c|cc} & A & a \\ \hline A & AA & Aa \\ a & Aa & aa \\ \end{array}$$

Genotype ratios: 25% $AA$, 50% $Aa$, 25% $aa$; Phenotype ratios: 75% purple spots, 25% green spots.