In a species of fish, body pattern is controlled by one gene. Use S S\,S for the striped-body allele and s s\,s for the plain-body allele. Striping is the dominant characteristic.
The same striped fish, A, was used in two crosses.
| Cross | Parents | Striped offspring | Plain offspring |
|---|---|---|---|
| 1 | striped fish A × plain fish B | 81 | 79 |
| 2 | striped fish A × striped fish C | 123 | 37 |
Explain how the results can be used to determine the genotypes of fish A, B and C.
Fish A and fish C are crossed again to produce 240 offspring.
Calculate the expected number of plain offspring.
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.