Q27 - Baldness, Rh Factor, and Genetic Counselling
Lily and Jacob visit a genetic counselling office where the doctor gathers the data summarized below.
| Person | Bald? | Rh |
|---|---|---|
| Lily | No | Rh+ |
| Lily’s mother | Yes | Rh+ |
| Lily’s father | No | Rh− |
| Jacob | Yes | Rh− |
| Jacob’s mother | Yes | Rh− |
| Jacob’s father | No | Rh+ |
Table 1.
Please note the following facts:
- Baldness is an example of a so-called sex-influenced trait. B/B women are bald, however B/b and b/b women are not. B/B and B/b men are bald, b/b men are not.
- Genotype of Rh− people is r/r, and that of Rh+ people is R/R or R/r.
- When the fetus of an Rh− woman is Rh+, the woman’s immune system may produce antibodies against the Rh+ fetus during a second pregnancy, which may result in the death of the fetus. This phenomenon is known as Rh incompatibility.
- The B (b) and the R (r) genes are autosome linked and are 20 cM apart.
Q27.1 (Lily). Baldness (B/b) is sex-influenced: B/B women are bald, B/b and b/b women are not; B/B and B/b men are bald, b/b men are not. Rh genotype is r/r (Rh−) or R/R, R/r (Rh+). The B and R genes are autosome-linked, 20 cM apart. Lily: not bald, Rh+. Lily's mother: bald, Rh+. Lily's father: not bald, Rh−. Determine Lily's combined genotype (written as one chromosome's BR/Br/bR/br haplotype over the other).
Q27.1 (Jacob). Jacob: bald, Rh−. Jacob's mother: bald, Rh−. Jacob's father: not bald, Rh+. Determine Jacob's combined genotype.
Q27.2. What percentage of Lily and Jacob's children will be bald?
Q27.3. What percentage of their bald children will be female?
Q27.4. What percentage of their daughters may face Rh incompatibility once pregnant (i.e. are Rh− themselves, r/r)?
Q27.5. What percentage of the daughters with the potential to develop Rh incompatibility would also be bald?
Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key