In a species of snail, allele S S\,S gives a striped shell and is dominant to allele sss, which gives an unstriped shell. A breeder has a group of offspring from one cross, but does not know which of these two crosses produced them.
| Cross | Parent genotypes |
|---|---|
| A | Ss×ssSs \times ssSs×ss |
| B | Ss×SsSs \times SsSs×Ss |
The offspring have these phenotypes:
| Shell phenotype | Number of offspring |
|---|---|
| Striped | 152 |
| Unstriped | 48 |
Calculate the expected number of offspring with striped shells for each possible cross. Use the results to justify which cross is more consistent with the offspring data.
71 exam-style questions on Edexcel GCSE Biology 3.3 Inheritance, covering 3.3.1 Gregor Mendel and the basis of genetics, 3.3.2 Alleles and inherited characteristics, 3.3.3 Monohybrid inheritance, 3.3.4 Determination of sex at fertilisation, 3.3.5 Outcomes from monohybrid crosses, 3.3.6 Inheritance of ABO blood groups, 3.3.7 Inheritance of sex-linked disorders, and 3.3.8 Multiple genes and phenotypic features. Each one has a worked solution and a mark scheme showing where the marks go.