Question

test cross in corn
table 10.3
observed: purple, smooth 24; purple, wrinkled 34; yellow, smooth 37; yellow, wrinkled 41; total 136
unknown parent

Explanation:

Step1: Recall test cross rules

A test cross involves crossing an individual with an unknown genotype to a homozygous recessive individual (for both traits: yellow, wrinkled, genotype $yyrr$). The offspring phenotypes directly reflect the gamete types produced by the unknown parent.

Step2: Analyze offspring phenotype ratios

First, calculate the proportion of each offspring phenotype:

• Purple, smooth: $\frac{24}{136} \approx 0.176$
• Purple, wrinkled: $\frac{34}{136} = 0.25$
• Yellow, smooth: $\frac{37}{136} \approx 0.272$
• Yellow, wrinkled: $\frac{41}{136} \approx 0.301$

The ratios are approximately 1:1.4:1.5:1.7, which is close to a 1:1:1:1 ratio (the expected ratio when the unknown parent is heterozygous for both traits, producing 4 types of gametes in equal proportion).

Step3: Deduce unknown parent genotype

Since the test cross offspring show four distinct phenotypes in near-equal proportions, the unknown parent must be able to produce four different gamete types. This means it is heterozygous for both the color (purple $Y$ dominant over yellow $y$) and texture (smooth $R$ dominant over wrinkled $r$) traits.

Answer:

The unknown parent has a genotype of $YyRr$ (heterozygous purple smooth).