Practise › Questions › Monohybrid inheritance: genetic diagrams that score
Monohybrid inheritance: genetic diagrams that score questions
Genotype and phenotype, dominant and recessive alleles, the five-line layout of a genetic diagram, monohybrid ratios, test crosses, codominance, multiple alleles and the ABO blood groups, and sex linkage with the carrier mother cross worked through.
6 original questions · 23 marks · the monohybrid inheritance: genetic diagrams that score notes · Inheritance and population genetics
Every question here is written for this library rather than taken from a past paper. Write your answer out before opening a mark scheme: the schemes award marks point by point, and the points are much easier to see when you have something of your own to compare against.
In tomatoes, red fruit is dominant to yellow fruit. Complete a full genetic diagram for a cross between a heterozygous red-fruited plant and a yellow-fruited plant, showing the parental genotypes, the gametes, the offspring genotypes and the expected phenotype ratio.
Mark scheme
- B1 the symbols are defined, for example R for the allele for red fruit and r for the allele for yellow fruit, using one letter for both alleles
- B1 parental genotypes given as Rr for the red-fruited plant and rr for the yellow-fruited plant
- B1 gametes shown as circled single alleles rather than pairs: R and r from the red-fruited parent, r and r from the yellow-fruited parent
- B1 offspring genotypes obtained from the gametes as Rr and rr, in equal numbers
- A1 offspring phenotypes and ratio given as 1 red-fruited : 1 yellow-fruited
A breeder has a tall pea plant and needs to know whether it is homozygous or heterozygous. Explain how a test cross would answer the question, and explain why a result suggesting the plant is homozygous can never be certain.
Mark scheme
- B1 cross the tall plant with a homozygous recessive dwarf plant, which can only contribute a recessive allele to every offspring
- B1 if any offspring is dwarf, the tall parent must have supplied a recessive allele, so it is heterozygous
- B1 if a large number of offspring are all tall, the tall parent is very probably homozygous, because a heterozygote would be expected to give a 1 : 1 ratio
- A1 each fertilisation is independent, so a heterozygote can give a run of dominant offspring by chance: eight in a row happens about once in every 256 attempts
Cystic fibrosis is caused by a recessive allele. Two parents are both carriers and neither has the condition. Calculate the probability that their first child has cystic fibrosis, and calculate the probability that their first two children both have it.
Mark scheme
- M1 both parents are heterozygous, so the cross is Ff × Ff and the offspring genotypes are 1 FF : 2 Ff : 1 ff
- A1 one of the four equally likely outcomes is homozygous recessive, so the probability is 1 in 4, or 0.25
- M1 the two fertilisations are independent events, so multiply the separate probabilities: 0.25 × 0.25
- A1 1 in 16, or 0.0625
Haemophilia is caused by a recessive allele carried on the X chromosome. Explain why the condition is far more common in males than in females, and explain why a man with haemophilia can never pass it to his sons.
A woman of blood group AB has children with a man of blood group O. Explain why none of their children can be blood group AB, and explain why none of them can be blood group O.
State what is meant by the term allele, and state what is meant by describing someone as a carrier of a recessive condition.
Practise monohybrid inheritance: genetic diagrams that score one question at a time
The player marks nothing for you. It shows one question, waits, then shows the scheme so you can mark yourself, and brings a question back sooner when it went badly.