Photo AI
This question is about inheritance - AQA - GCSE Biology Combined Science - Question 5 - 2020 - Paper 2
Question 5
This question is about inheritance. Humans have 23 pairs of chromosomes in each body cell. One pair of chromosomes determines sex. Complete Figure 5. In some fami... show full transcript
Worked Solution & Example Answer:This question is about inheritance - AQA - GCSE Biology Combined Science - Question 5 - 2020 - Paper 2
Step 1
Complete Figure 5
Answer
The completed Punnett square would show the male gametes (X and Y) on one side and the female gametes (X and X) on the other:
Female
| X | X |
---------------------
X | XX | XX |
---------------------
Y | XY | XY |
This illustrates the possible offspring combinations.
Step 2
In some families all children are the same sex. Give the reason why.
Answer
The reason could be that there is a 50% chance for each child to inherit an X or Y chromosome from the male parent. If the sperm that fertilizes the egg consistently carries the same type of chromosome (either X or Y), all offspring will be of the same sex.
Step 3
Draw a Punnett square diagram to find the probability that their next child will have CF.
Answer
Both parents must be shown as heterozygous (Tt):
Woman
| T | t |
---------------------
T | TT | Tt |
---------------------
t | Tt | tt |
The phenotype probabilities are: 1 TT (normal), 2 Tt (carriers, no CF), and 1 tt (with CF).
The probability of having a child with CF is therefore ( \frac{1}{4} ) or 25%.
Additionally, the genotype of any child with CF is tt.
Step 4
Suggest three reasons for choosing Method 1 instead of Method 2.
Answer
- Method 1 does not require In Vitro Fertilisation (IVF), which is a stressful and invasive procedure.
- There is a higher chance of successful pregnancy as the risk of miscarriage is lower compared to IVF (around 30%).
- The procedures of Method 1 are more widely available and do not require advanced planning or going through IVF protocols.
Step 5