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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. (a) (i) Calculate the percentage of the offspring from the first generat... 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 offspring from the first generation who are heterozygous for sickle cell disease, we first identify the individuals in the first generation. From the pedigree, we note that the males and females have varying genotypes.
Assuming the following breakdown:
- 2 individuals are unaffected (homozygous normal).
- 3 individuals are carriers (heterozygous).
The total number of offspring from the first generation is 5. The number of offspring that are heterozygous is 3. Therefore, the percentage can be calculated as follows:
Substituting the values:
Thus, the percentage of the offspring from the first generation who are heterozygous for sickle cell disease is 60%.
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 offspring produced by the first generation parents are not the same as those predicted in a Punnett square primarily due to the randomness of genetic inheritance and the effects of independent assortment.
A Punnett square provides a simplified model of inheritance based on the assumption that gametes combine randomly during fertilization. However, real-life genetics is influenced by various factors:
- Random Assortment: Each gamete is subject to random assortment, which can lead to different combinations of alleles than those suggested by a Punnett square.
- Actual Offspring Variability: Offspring ratios depend on actual offspring being produced rather than theoretical predictions. For instance, statistical variations can lead to more offspring being heterozygous or homozygous than predicted.
- Influence of Environment: Environmental factors can also impact the expression of certain traits, potentially altering the expected ratios further.
Thus, it is critical to recognize that a Punnett square serves as a guideline rather than an absolute prediction.