A2 · Practice questions · Monohybrid and dihybrid crosses

Gametes first, ratios second.

Six original Cambridge-style questions on genetic diagrams, from blood groups and haemophilia to linked genes and epistasis.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

One allele per gene per gamete.

Layout: phenotypes, genotypes, gametes, Punnett square, offspring, ratio.
Codominant and sex-linked alleles are written as superscripts.
Test cross: with the homozygous recessive; offspring mirror the gametes.
Linkage: few recombinants; epistasis: one gene masks another.

Recall

[3 marks]

Explain the difference between the terms gene and allele, and between genotype and phenotype.

Gene: a length of DNA at a particular locus that codes for a polypeptide. ✓
Allele: one of the alternative forms of a gene (differing in base sequence). ✓
Genotype: the alleles present in an organism; phenotype: its observable characteristics, resulting from the genotype and the environment. ✓

Analysis

[4 marks]

Haemophilia is caused by a recessive allele of the F8 gene on the X chromosome. A woman whose father had haemophilia has children with an unaffected man. Draw a genetic diagram and state the probability that a son will have haemophilia.

The woman must be a carrier, X^HX^h (she received X^h from her father); the man is X^HY. ✓
Gametes: X^H, X^h from the mother; X^H, Y from the father. ✓
Offspring: X^HX^H normal female, X^HX^h carrier female, X^HY normal male, X^hY haemophiliac male. ✓
Probability that a son has haemophilia = 1/2 (0.5). ✓

Analysis

[4 marks]

In a plant, tall (T) is dominant to short (t) and purple flowers (P) are dominant to white (p); the genes are on different chromosomes. Use a genetic diagram to predict the offspring of TtPp × ttpp.

Gametes of TtPp: TP, Tp, tP, tp (in equal proportions); gametes of ttpp: all tp. ✓
Offspring genotypes: TtPp, Ttpp, ttPp, ttpp. ✓
Phenotypes: tall purple, tall white, short purple, short white. ✓
Ratio 1 : 1 : 1 : 1 (this is a test cross). ✓

Explain

[3 marks]

Explain why a cross between two plants heterozygous for two linked genes (AB/ab) gives mostly two phenotypes instead of four.

The two genes are on the same chromosome, so alleles A and B (and a and b) are inherited together / do not assort independently. ✓
Each parent makes mainly AB and ab gametes, giving offspring mostly with the dominant phenotype for both genes or the recessive phenotype for both (about 3 : 1). ✓
Crossing over between the loci produces a few Ab and aB recombinant gametes, so a small number of the other two phenotypes appear. ✓

Analysis

[3 marks]

In a species of flower, gene E controls pigment production: ee plants cannot make any pigment and are white. Gene R controls pigment colour: R gives red, rr gives yellow. Predict the phenotype ratio from EeRr × EeRr.

Of 16 offspring, 9 have E_R_ (red) and 3 have E_rr (yellow). ✓
4 are ee (eeR_ and eerr), which are white whatever the R alleles, because gene E is epistatic to gene R. ✓
Ratio 9 red : 3 yellow : 4 white. ✓

Analysis

[3 marks]

Sickle cell anaemia: alleles Hb^A and Hb^S are codominant. Two parents with sickle cell trait have a child. Give the possible genotypes and phenotypes of the child, with their probabilities.

Parents Hb^AHb^S × Hb^AHb^S; gametes Hb^A and Hb^S from each. ✓
Hb^AHb^A unaffected 0.25; Hb^AHb^S sickle cell trait 0.5; Hb^SHb^S sickle cell anaemia 0.25. ✓
The heterozygote has its own phenotype (trait) because both alleles are expressed: codominance. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.