**Core Concept**
Incest type relations refer to familial incestuous relationships, which are genetically significant due to the close familial bond. These relationships can lead to increased genetic similarity between the individuals involved, affecting their offspring.

**Why the Correct Answer is Right**
The incest type relation between a mother and son is an example of **consanguinity**. This is because they share a common ancestor (the father), making them biologically related. Consanguinity can increase the chances of genetic disorders and birth defects in offspring due to the increased risk of homozygosity for recessive alleles. This is because both parents can pass on the same recessive allele to their offspring, increasing the likelihood of a genetic disorder.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because consanguinity specifically refers to the biological relationship between individuals who share a common ancestor, not just any familial relationship.

**Option B:** This is incorrect because though it is a type of familial relationship, it is not an example of consanguinity.

**Option C:** This is incorrect because though it is a familial relationship, it is not biologically significant in the context of consanguinity.

**Clinical Pearl / High-Yield Fact**
Consanguinity is a significant risk factor for genetic disorders and birth defects, particularly in offspring. It is essential to note that not all familial relationships are examples of consanguinity, and the distinction between familial relationships and consanguinity is crucial in genetics and medicine.

**Correct Answer: D. Consanguinity.**