Question 1
Which genotype is heterozygous?
- TT
- Tt
- tt
- T
Show mark scheme for question 1
Answer: B (Tt) (1)
Question 2
(a) Define the term genotype. [1] (b) Define the term phenotype. [1] (c) Two tall pea plants, both with genotype TT, are crossed. Explain why all their offspring will also be tall. [1]
Show mark scheme for question 2
- (a) the genetic make-up of an organism / the alleles present (1)
- (b) the observable features of an organism (1)
- (c) both parents are homozygous / can only make T gametes, so all offspring are TT / pure-breeding (1)
Question 3
The table gives information from a pedigree diagram for one family. The condition is controlled by one gene. The allele for the condition is d and the other allele is D. Persons 1 and 2 are the parents of persons 3 and 4. (a) State whether the allele for the condition is dominant or recessive. Explain your answer. [2] (b) State the genotypes of persons 1 and 2. [1] (c) State the genotype of person 3. [1] (d) Give the probability that a third child of persons 1 and 2 will have the condition. [1]
| Person | Sex | Relationship | Has the condition |
|---|---|---|---|
| 1 | male | father | no |
| 2 | female | mother | no |
| 3 | female | daughter of 1 and 2 | yes |
| 4 | male | son of 1 and 2 | no |
Show mark scheme for question 3
- (a) recessive (1) reject dominant
- parents 1 and 2 do not have the condition but their daughter / person 3 does (1) allow the parents carry the allele without showing it
- (b) Dd and Dd (1) both needed
- (c) dd (1)
- (d) 1 in 4 / 25% / 0.25 (1) allow 1/4
Question 4
In guinea pigs, black fur (B) is dominant to brown fur (b). A heterozygous black guinea pig is crossed with a brown guinea pig. (a) State the gametes produced by each parent. [1] (b) Complete a Punnett square to show the genotypes of the offspring. [1] (c) State the phenotypic ratio of the offspring. [1] (d) Give the percentage of the offspring expected to be brown. [1]
Show mark scheme for question 4
- (a) heterozygous parent: B and b; brown parent: b (only) (1)
- (b) Bb, Bb, bb, bb (1)
- (c) 1 black : 1 brown (1) allow 1 : 1
- (d) 50% (1)
Question 5
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanA farmer has a tall pea plant. Tall (T) is dominant to short (t). The farmer does a test cross to find the genotype of the tall plant. (a) State the genotype of the plant that should be crossed with the tall plant. [1] (b) Describe the offspring expected if the tall plant is TT. [1] (c) Describe the offspring expected if the tall plant is Tt. [1] (d) Suggest why the farmer should produce many offspring. [1]
Show mark scheme for question 5
- (a) tt (1)
- (b) all tall (1)
- (c) about half tall and about half short (1) allow 1 : 1
- (d) a small number could all be tall by chance even if the plant is Tt / a larger number is more reliable (1)
Question 6
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanA mother has blood group A and genotype IA Io. A father has blood group B and genotype IB Io. (a) Define the term codominance. [1] (b) Draw a Punnett square to show the possible genotypes of their children. [2] (c) State the probability that a child will have blood group O. [1] (d) State the blood group of a person with genotype IA IB. [1]
Show mark scheme for question 6
- (a) both alleles in a heterozygous organism contribute to the phenotype (1) allow both alleles are expressed in the heterozygote
- (b) gametes IA, Io and IB, Io correctly placed (1)
- offspring IA IB, IA Io, IB Io, Io Io (1)
- (c) 1 in 4 / 25% / 0.25 (1)
- (d) AB (1)
Question 7
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanRed-green colour blindness is caused by a recessive allele on the X chromosome. XB is the normal allele and Xb is the colour blindness allele. A woman who is a carrier has children with a man who has normal colour vision. (a) State what is meant by a sex-linked characteristic. [1] (b) Explain why colour blindness is more common in males than in females. [2] (c) State the genotypes of the offspring of these parents. [1] (d) Give the probability that a son of these parents is colour blind. [1]
Show mark scheme for question 7
- (a) the gene (responsible) is located on a sex chromosome (1) allow on the X chromosome
- (b) males have only one X chromosome / the Y has no matching allele, so one recessive allele causes colour blindness (1)
- females need two recessive alleles / would be carriers with one (1)
- (c) XB XB, XB Xb, XB Y, Xb Y (1) all four needed
- (d) 1 in 2 / 50% (1)