An individual who displays the disease sickle-cell anemia must have inherited the deleterious allele from both phenotypically normal parents. This individual is therefore:
A. Homozygous dominant
B. Homozygous recessive
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Let me walk you through this problem, step by step:
(And you can use the chart to help you understand.)
1. There are two parents.
2. NEITHER of them actually has sickle cell disease.
3. But BOTH of them carry the gene for sickle cell disease.
The gene just doesn't physically appear. That's why they don't actually experience the disease.
That means that the gene is RECESSIVE.
4. The genotypes of both parents is R r.
5. R is a DOMINANT allele. It's the NORMAL allele.
6. r is the RECESSIVE allele. It's the SICKLE CELL allele.
7. As you know, the DOMINANT allele is much more important than the RECESSIVE allele.
If the dominant allele is part of the child, then the child will NOT have sickle cell disease. (Just like the parents!)
8. If the child's alleles are both RECESSIVE (r r)
then the child WILL HAVE sickle cell disease.
9. HETEROZYGOUS means that the alleles are both DIFFERENT. (R r)
10. HOMOZYGOUS means that the alleles are both the SAME. (R R...or.....r r.)
(Look back at #8.)
Remember, the child will only have sickle cell disease if s/he gets both recessive alleles from the parent.
(Look back at #10.)
Remember, if the alleles are both the SAME, that means that they are HOMOZYGOUS.
So, a child with sickle cell disease has the genotype r r.
And that is a HOMOZYGOUS, RECESSIVE genotype.