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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 pos... 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
To complete the Punnett square for two heterozygous parents (Dd), we can organize the alleles as follows:
D d
---------
D | DD | Dd |
---------
d | Dd | dd |
This shows the possible genotypes: 1 DD, 2 Dd, and 1 dd.
Step 2
State the percentage probability that their children could have sickle cell disease.
Answer
The percentage probability that their children could have sickle cell disease can be calculated from the genotype outcomes. The only genotype that expresses sickle cell disease is dd.
Thus, the probability is:
Step 3
Explain why none of their children will have sickle cell disease.
Answer
In this case, the father has the genotype dd, which means he expresses sickle cell disease. The mother has the genotype DD, indicating that she is homozygous dominant for the normal allele.
When they have children, all the offspring will inherit one allele from each parent. Therefore, the children will have the genotype Dd. Since sickle cell disease is recessive, and all children will be heterozygous (Dd), none will express the disease, as they must inherit two recessive alleles (dd) to be affected.