Question

in humans, broad lips (l) is dominant to thin lips (l), and the presence of dimples (d) is dominant to the absence of dimples (d). in a cross between an lldd parent and an lldd parent, what is the probability of an offspring inheriting the lldd genotype?
a. $\frac{0}{8}$
b. $\frac{2}{8}$
c. $\frac{3}{8}$
d. $\frac{5}{8}$

Explanation:

Step1: Analyze lip - gene inheritance

The cross for the lip - related gene is $Ll\times ll$. The possible genotypes of the offspring for this cross are $Ll$ and $ll$ with probabilities of $\frac{1}{2}$ and $\frac{1}{2}$ respectively. The probability of getting $Ll$ is $\frac{1}{2}$.

Step2: Analyze dimple - gene inheritance

The cross for the dimple - related gene is $Dd\times Dd$. Using the Punnett - square method, the genotypes and their ratios are $DD:Dd:dd = 1:2:1$. The probability of getting $Dd$ is $\frac{1}{2}$.

Step3: Calculate combined probability

Since the inheritance of the lip gene and the dimple gene are independent events, we use the multiplication rule of probability. The probability of getting $LlDd$ is the product of the probability of getting $Ll$ and the probability of getting $Dd$. So $P(LlDd)=\frac{1}{2}\times\frac{1}{2}=\frac{1}{4}=\frac{2}{8}$.

Answer:

B. $\frac{2}{8}$