If both parents have a genetic defect, what is the likelihood that their child will inherit it?

When both parents carry a genetic defect (specifically if we are discussing a recessive trait, which is common in genetic inheritance discussions), there is a specific pattern of inheritance defined by Mendelian genetics. Each parent contributes one allele (gene variant) to the offspring. If both parents are carriers of a recessive gene defect (meaning they have one normal allele and one defective allele), their possible allele combinations can be illustrated using a Punnett square.

In this scenario, the potential combinations for the child's genotypes would be as follows:

1. Normal allele from both parents (homozygous normal).

2. Normal allele from one parent and defective allele from the other (heterozygous carrier).

3. Defective allele from one parent and normal allele from the other (heterozygous carrier).

4. Defective alleles from both parents (homozygous defective).

This results in four equally likely outcomes:

• 1 in 4 chance (25%) of inheriting two defective alleles (and thus expressing the defect).

• 2 in 4 chance (50%) of being a carrier with one defective allele (not expressing the defect but passing it on to their offspring).

• 1 in 4 chance (25%)