Jill is A positive and her mom is O negative. She marries Jack who is O negative. What's the chance in percent of having a child who is O positive?

Question

Jill is A positive and her mom is O negative. She marries Jack who is O negative.  What's the chance in percent of having a child who is O positive?

BlackTom AI · Accepted Answer

To approach this genetics problem, we must consider both ABO blood type inheritance and Rh factor inheritance, and then combine them to find the probability of a child being O positive.

Option 1: 50
- ABO: If Jill is A, her genotype could be AO or AA. Since Jill’s mother is O (OO), Jill cannot be AA (because she would need two A alleles, which is not possible from an O mother and an O father). Therefore Jill must be AO, giving a 50% chance of passing the O allele to a child when paired with an O partner. Jill’s partner Jack is O, so the cross is AO × OO and the possible ABO outcomes include O (from Jill passing O) about 50% of the time and A (from Jill passing A) about 50% of the time.
- Rh: Jill is Rh+, meaning she carries at least one D allele. Her parents include an O- mother and an O- father in this setup, so to be Rh+ she must have inherited D from her own other parent not specified here. Given Jack is Rh-, the child would be Rh+ only if Jill passes the D allele to the child, which occurs with 50% probability for each pregnancy. Therefore, the combined probability for an O child who is Rh+ would require both the ABO outcome to be O (50%) and the child to inherit D (50%), yielding 0.5 × 0.5 = 0.25 or 25%. If the ABO outcome were not O, the child would not be type O regardless of Rh.
- Since both conditions must be met to get O positive, the overall chance is 25%, not 50%.

Option 2: 0
- This would imply that an O positive child is impossible under the given parental genotypes, which is incorrect because the alleles described do allow for an O positive offspring under the right combination of alleles (O blood type and D allele inheritance).

Option 3: 75
- A 75% chance would require either a dominant likelihood of O type and D inheritance or an unrealistic scenario where almost all offspring become O positive. The combination here does not support such a high probability because each of the two necessary traits (O ABO type and presence of D) occurs with only about a 50% chance in the relevant transmissions, making 75% unattainable.

Option 4: 25
- This matches the calculated probability: there is a 50% chance to pass the O allele from Jill (AO × OO yields about half offspring as O), and a 50% chance that Jill passes the D allele to confer Rh+, with the father and mother contributing no D alleles (both Rh-). The event that both happen—the child being ABO type O and Rh+—occurs with probability 0.5 × 0.5 = 0.25, i.e., 25%.

In summary, the only option that aligns with the combination logic for ABO inheritance (AO × OO giving 50% O) and Rh inheritance (Dd × dd giving 50% Rh+) is 25%.

Jill is A positive and her mom is O negative. She marries Jack who is O negative. What's the chance in percent of having a child who is O positive?单项选择题

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