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Sickle cell disease is a recessive genetic disorder in humans - Edexcel - GCSE Biology - Question 3 - 2021 - Paper 1
Question 3
Sickle cell disease is a recessive genetic disorder in humans. (i) Two parents are heterozygous for sickle cell disease. Complete the Punnett square to show the po... show full transcript
Worked Solution & Example Answer:Sickle cell disease is a recessive genetic disorder in humans - Edexcel - GCSE Biology - Question 3 - 2021 - Paper 1
Step 1
Complete the Punnett square to show the possible genotypes of their children.
Answer
The completed Punnett square is shown below:
D | d
----------------
D | DD | Dd
----------------
d | Dd | dd
The possible genotypes of the children are DD, Dd, and dd, with D representing the normal allele and d representing the sickle cell allele.
Step 2
State the percentage probability that their children could have sickle cell disease.
Answer
Since sickle cell disease is caused by the homozygous recessive genotype (dd), we can determine the probability by examining the completed Punnett square. The genotype dd appears once in the four squares:
- DD: 1
- Dd: 2
- dd: 1
Calculating the probability:
or 25%.
Thus, the percentage probability that their children could have sickle cell disease is 25%.
Step 3
Explain why none of their children will have sickle cell disease.
Answer
In this case, the mother has the genotype DD, which means she can only pass on the dominant allele (D) to her children. The father, with genotype dd, can only pass on the recessive allele (d). Therefore, all children will inherit one D from their mother and one d from their father, resulting in the heterozygous genotype Dd for all offspring.
As a result, all children will be carriers of sickle cell disease but will not express the disease phenotype, as they do not inherit two recessive alleles (dd).