Photo AI
The family pedigree shows the inheritance of sickle cell disease through three generations - Edexcel - GCSE Biology - Question 6 - 2013 - Paper 1
Question 6
The family pedigree shows the inheritance of sickle cell disease through three generations. Calculate the percentage of the offspring from the first generation who ... show full transcript
Worked Solution & Example Answer:The family pedigree shows the inheritance of sickle cell disease through three generations - Edexcel - GCSE Biology - Question 6 - 2013 - Paper 1
Step 1
(i) Calculate the percentage of the offspring from the first generation who are heterozygous for sickle cell disease.
Answer
To determine the percentage of heterozygous offspring from the first generation, we can analyze the pedigree. The first generation shows two parents: one is affected (likely homozygous recessive, represented by a filled circle) and the other is a carrier (heterozygous, represented by a half-filled circle).
The potential genotypes of their offspring can be mapped out:
- Parent 1 (affected): ss (homozygous recessive)
- Parent 2 (carrier): Ss (heterozygous)
The possible combinations for their offspring are:
- From Ss x ss, the possible offspring genotypes are:
- Ss (carrier)
- ss (affected)
Calculating the ratios, we find:
- 1 Ss (heterozygous) : 1 ss (affected)
Thus, out of these two offspring, 50% are heterozygous. If we assume there are a total of 4 offspring as seen in the pedigree, this means:
- 2 out of 4 offspring are heterozygous, resulting in a percentage of:
Step 2
(ii) Explain why the offspring produced by the first generation parents are not the same as those predicted in a Punnett square.
Answer
The Punnett square provides a theoretical probability distribution of offspring genotypes based on parental alleles. However, the actual offspring produced by the first generation parents may not strictly follow this predicted ratio due to several factors.
For instance:
- The number of offspring can lead to variation from predicted ratios due to random assortment and chance.
- Environmental influences and genetic linkage are also potential factors that can affect the phenotypic expression of the offspring. Thus, while the Punnett square predicts a 50% heterozygous offspring proportion, the actual observed proportions can differ.