Inbreeding is highly discouraged in humans because it may

Analyzing the question

The question asks why inbreeding (mating between closely related individuals) is discouraged in humans. We need the most direct genetic consequence.

The concept

Closely related people share many of the same genes. This raises the chance that both partners carry the same harmful recessive alleles. When two carriers have a child, the child can inherit two copies of the recessive allele (homozygous recessive) and show the disease.

In unrelated people, harmful recessive alleles are usually "hidden" in heterozygous carriers. Inbreeding increases homozygosity, so these hidden alleles are expressed.

Step-by-step evaluation of the options

• A. Increase the death rate of newborns: This can happen as a result of genetic disorders, but it is a secondary effect, not the main reason. It is not the underlying cause.
• B. Lead to the birth of dwarfs: Dwarfism is only one specific condition and is not a general result of inbreeding. This is too narrow.
• C. High rate of competition among offspring: This is an ecological idea, not a genetic one, and has nothing to do with relatedness of parents.
• D. Lead to outbreak of hereditary diseases: This is correct. Inbreeding increases the chance of homozygous recessive combinations, which brings out genetic disorders such as sickle cell anaemia, albinism, haemophilia and cystic fibrosis.

Answer: D

Exam tip: When you see "inbreeding" in a question, think "recessive genes come together → hereditary diseases/reduced fitness (inbreeding depression)." Questions on the opposite idea, outbreeding, link to hybrid vigour (heterosis).