Shell banding in a species of snail is controlled by a gene with two alleles. The dominant allele B\text{B}B gives a banded shell, while the recessive allele b\text{b}b gives an unbanded shell.
A genetic test shows that a banded snail carries the allele b\text{b}b.
Which statement correctly gives the genotype and describes it as homozygous or heterozygous?
It is homozygous dominant because its genotype is BB\text{BB}BB.
It is homozygous recessive because its genotype is bb\text{bb}bb.
It is heterozygous because its genotype is Bb\text{Bb}Bb.
It could be homozygous or heterozygous because banding is dominant.
186 exam-style questions on OCR GCSE Biology 5.1 Inheritance, covering 5.1.1 The genome, 5.1.2 Environment and phenotype, 5.1.3 Variants and mutations, 5.1.4 How genetic variants influence phenotype, 5.1.5 Asexual and sexual reproduction, 5.1.6 Haploid and diploid, 5.1.7 Meiotic cell division, 5.1.8 Single gene inheritance, 5.1.9 Predicting single gene crosses, 5.1.10 Sex determination, 5.1.11 Multiple gene inheritance, and 5.1.12 Development of our understanding of genetics. Each one has a worked solution and a mark scheme showing where the marks go.