Genetic Relatedness Ranking

The question requires arranging specific genetic relationships in descending order based on the proportion of genes inherited from a common ancestor. This order is determined by the degree of genetic relatedness between the individuals involved.

Proportion of Shared Genes Analysis

The standard proportion of genes shared (relatedness coefficient, denoted as $\textit{r}$) for each relationship type is as follows:

• Monozygotic twin (C): Share approximately 100% of their genes ($\textit{r} = 1.0$). They originate from a single fertilized egg.
• Dizygotic twin (D): Share approximately 50% of their genes on average ($\textit{r} = 0.5$). They develop from two separate fertilized eggs, similar to fraternal siblings.
• Half Sib (B): Share approximately 25% of their genes on average ($\textit{r} = 0.25$). They have only one biological parent in common.
• Second Cousin (A): Share approximately 6.25% of their genes on average ($\textit{r} = 0.0625$). Their common ancestors are typically great-grandparents.

Determining Descending Order

To find the descending order, we compare the relatedness coefficients:

• C: $\textit{r} = 1.0$
• D: $\textit{r} = 0.5$
• B: $\textit{r} = 0.25$
• A: $\textit{r} = 0.0625$

Arranging these from the highest proportion to the lowest gives the order: $1.0 > 0.5 > 0.25 > 0.0625$.

Mapping this back to the relationship labels, the correct descending order is C, D, B, A.