The risk of genetic diseases in consanguineous marriage between first cousins is?

Correct Answer: C. 4-8%

Consanguineous marriages, particularly between first cousins, significantly increase the risk of autosomal recessive genetic diseases. First cousins share approximately 6.25% of their DNA (coefficient of relationship = 1/16), making them the closest relatives who commonly marry in many Indian communities. The empirical risk of genetic disease in offspring of first-cousin marriages is 4–8%, which is approximately 2–3 times higher than the baseline population risk of 1–2%. This increased risk arises because both parents are more likely to carry the same recessive alleles inherited from a common ancestor. In India, consanguinity is prevalent in certain communities (South Indian, Muslim, and tribal populations), and this risk figure is well-established in epidemiological studies and genetic counseling guidelines. The risk encompasses all autosomal recessive conditions (thalassaemia, sickle cell disease, cystic fibrosis, spinal muscular atrophy) and some autosomal dominant conditions with reduced penetrance. This 4–8% figure is the standard counseling estimate used in Indian genetic services and is distinct from the baseline population risk (1–2%) and the risk in more distant relationships.

Why the other options are wrong

A. 1-2% — This represents the baseline population risk for genetic diseases in the general (non-consanguineous) population, not the increased risk in first-cousin marriages. Selecting this option reflects failure to account for the amplified risk from shared ancestry. This is the trap for students who confuse general population risk with consanguinity-specific risk. B. 12-14% — This risk is too high and approximates the risk seen in second-degree relatives (uncle–niece, aunt–nephew) or in populations with very high rates of consanguinity over multiple generations. First cousins (third-degree relatives) carry lower risk than this. This option traps students who overestimate the genetic proximity of first cousins. D. 8-10% — This represents an intermediate or slightly elevated estimate that may be cited in some older literature or for populations with cumulative consanguinity effects, but it exceeds the standard empirical risk of 4–8% for a single first-cousin marriage. This traps students who conflate different consanguinity scenarios or misremember the exact range.

High-Yield Facts

Mnemonics

COUSIN RISK Rule Cousins = 4–8% risk; Baseline = 1–2%; Uncle–niece = 12–14%. Use this to anchor the three key consanguinity tiers in Indian genetic counseling. 1/16 → 4–8% Memory Hook First cousins share 1/16 of DNA → roughly 6% shared ancestry → 4–8% disease risk. The risk range brackets the coefficient of relationship.

NBE Trap

NBE pairs consanguinity with population baseline risk (1–2%) to trap students who conflate general epidemiology with consanguinity-specific risk. The question tests whether students know the empirical multiplier effect of first-cousin marriages, not just the concept of recessive inheritance.

Clinical Pearl

In Indian genetic clinics, when a first-cousin couple presents for preconception counseling (common in South India and Muslim communities), the 4–8% risk figure is the cornerstone of informed consent discussions. Carrier screening for thalassaemia, SMA, and other recessive conditions prevalent in that community should follow this risk stratification.

_Reference: Park's Textbook of Preventive and Social Medicine (Ch. 5: Genetics & Consanguinity); Harrison's Principles of Internal Medicine (Ch. 63: Genetic Counseling)_