14. The pedigree shown is of three generations and show which individuals suffer from
genetic disorder that is 100% penetrant and appears after 40 years of age:
individuals that show the disease are either shaded circles (female) or shaded males
(squares), individuals that do not suffer from the disease are unshaded, although it
should be noted that the grandchildren in generation C are still below 40 years of
age. Below this is a restriction fragment length polymorphism (RFLP) of each
individual’s DNA; individual a in generation A is labelled Aa and so on.
What is the chance that a child from a union between female Ch and the male, Ci
(from a family with no history of disease) will never suffer from this disease?
a. 0%
b. 25%
c. 50%
d. 100%
The correct answer is 50% (option C). A child from Ch × Ci has a 50% chance to never suffer from this late‑onset, 100% penetrant genetic disorder.
Understanding the pedigree and RFLP pattern
In the pedigree, shaded symbols are affected by a late‑onset autosomal dominant disorder, while unshaded individuals are either truly unaffected or too young (generation C is <40 years). The RFLP bands shown below (Aa, Ba, Ca, Ab, …, Ci, Ac, Bb, Ad) act as tightly linked markers that co‑segregate with either the disease allele or the normal allele in the family line. By comparing bands in affected versus unaffected individuals, one specific band pattern can be identified as “disease‑linked”, and its alternative pattern as “normal‑linked”. Ch carries the disease‑linked RFLP band, whereas Ci (from a disease‑free family) carries only the normal RFLP pattern, confirming that Ch is heterozygous (Dd) and Ci is homozygous normal (dd) at the disease locus.
Probability that the child never suffers disease
Because the disorder is autosomal dominant with full penetrance, any individual who inherits allele D will definitely develop the disease after age 40. Ch is Dd (one disease allele, one normal), and Ci is dd (two normal alleles). Their possible children are:
-
50% Dd (receive D from Ch, d from Ci) → affected in later life.
-
50% dd (receive d from both parents) → will never be affected.
Therefore, the probability that a child never suffers the disease (genotype dd) is 50%, which matches option C.
Detailed explanation of each option
-
Option A: 0%
This would mean every child must inherit the disease allele, which could only happen if both parents were affected or the affected parent was DD. Here, Ci is from an unaffected family and has only normal RFLP bands, and the RFLP analysis shows Ch is Dd, not DD. Thus, some children can be dd, so 0% is impossible. -
Option B: 25%
A 25% risk is typical for many autosomal recessive scenarios (carrier × carrier), but this trait is autosomal dominant. With a dominant heterozygous parent (Dd) crossed with a normal parent (dd), the Mendelian expectation is 50% affected and 50% unaffected, not 75% unaffected / 25% affected. Hence 25% “never affected” is incorrect here. -
Option C: 50% (Correct)
Ch (Dd) passes allele D or d with equal probability, and Ci (dd) always passes allele d. Thus, half the offspring (Dd) will eventually express the disease (after 40), while half (dd) will never show the disorder because the disease is fully penetrant and requires at least one D allele. This exactly gives a 50% chance the child never suffers from the disease. -
Option D: 100%
This would require that neither parent carry any disease allele, or that Ch is definitely dd. The RFLP pattern clearly shows Ch inherits the disease‑linked band, proving she carries the disease allele. Therefore, at least some children will inherit D and be affected, making 100% “never affected” impossible.
SEO‑optimized introduction
This pedigree RFLP genetic disorder question is a classic CSIR NET‑level problem that tests your ability to read three‑generation pedigrees, interpret linked RFLP markers, and calculate disease probability in offspring. By identifying the disease‑linked RFLP pattern and the genotypes of Ch and Ci, you can accurately determine the chance that their child will never develop this late‑onset, autosomal dominant disorder.
