Two Carriers, One Affected Child: Explaining Risk to a Family
At a community clinic in Trinidad, 500 adults are screened for haemoglobin genotype: 380 are AA, 100 are AS (carriers) and 20 are SS (affected). In the same clinic, a couple, both AS, have one child with sickle cell anaemia and a second child who is AA. The father's brother is AA and has four children, all AA. The mother's grandmother tells the family that the illness came from the mother's side and that the father's family is 'clean', and she also says the next baby is certain to be affected because 'the bad one has already come out'. The parents want to know the risk for a third pregnancy and whether anything can be done before the baby is born. The clinic has no genetic counsellor; you are the biology teacher helping the health worker prepare an explanation.
1. Using a genetic diagram, explain the parents' genotypes and the risk to a third pregnancy. Address the grandmother's claim that the next baby is certain to be affected.
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Each parent is heterozygous, AS. Meiosis separates the two alleles, so each parent produces two kinds of gamete in equal numbers, A and S.
| A (father) | S (father) | |
|---|---|---|
| A (mother) | AA | AS |
| S (mother) | AS | SS |
Offspring ratio: 1 AA : 2 AS : 1 SS. So for each pregnancy there is a 25% chance of AA (unaffected, not a carrier), a 50% chance of AS (unaffected carrier) and a 25% chance of SS (sickle cell anaemia).
The existing children do not change this. Each pregnancy is an independent event, like tossing a coin: having already had one SS child and one AA child does not make the next child certain to be affected, nor does it make an unaffected child more likely. The risk stays 1 in 4 every time. The grandmother is confusing the family's overall pattern with the next independent event.
2. Evaluate the grandmother's claim that the illness came from the mother's side and that the father's family is 'clean'. Explain what the father's brother's genotype does and does not tell you.
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The claim is wrong on both counts. A child can be SS only if it inherits an S allele from each parent, so both the mother and the father must be carriers - the father's AS genotype is demonstrated by the affected child, to whom he contributed one S allele. The illness did not come from one side alone.
The father's brother being AA says nothing about the father's genotype. A child of two carriers has a 1 in 4 chance of being AA, so an AA sibling is entirely expected and does not mean the father's family is 'clean'. Because carriers are healthy, the S allele can be passed silently through either side of a family for generations, and in many Caribbean populations the allele is common enough that two carriers meeting is not unusual.
Speaking of 'bad blood' or blaming one parent is scientifically meaningless and may damage the family. The health worker should stress that neither parent did anything wrong and that both are healthy carriers.
3. The health worker plans to tell the community that '1 in 4 people in Trinidad will have sickle cell anaemia', using the 25% figure. Using the screening data, evaluate this planned message and state what the 25% actually describes.
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The 25% figure is being misused. It describes the chance for each child of two carrier parents; it is not the proportion of people in a population who are affected. The screening found 20 out of 500 affected - 4% - and 100 out of 500 carriers - 20%. Those are population figures from one clinic sample and cannot be predicted from a single family's 1 : 2 : 1 ratio. A message that '1 in 4 people will have sickle cell anaemia' would alarm the community and, in this sample, overstate the affected proportion about six times.
A defensible message is: about 1 in 5 adults screened carried the trait and about 1 in 25 had the disease; two carriers have a 1 in 4 chance in each pregnancy of an affected child; carrier status can be detected by a blood test. The sample may also be biased, because people who attend a clinic are not a random sample of the island, so the figures should be presented as local screening results rather than national prevalence. The health worker should also explain that prenatal testing is available by referral, since the parents asked about it.