A2 · A2 2 Biochemistry, Genetics and Evolutionary Trends
Dihybrid inheritance: how to set out the cross and get the ratio right
The short answer
Dihybrid inheritance follows two genes at once. Each parent that is heterozygous for both genes (e.g. RrYy) makes four gamete types — RY, Ry, rY, ry — so a cross between two such parents gives sixteen combinations and the classic 9:3:3:1 phenotypic ratio.
The layout that scores every mark
Each of those five lines is separately creditworthy. A correct final ratio with no working rarely gets full marks, so never do a dihybrid cross in your head.
Write the parental phenotypes in words, then their genotypes underneath.
Circle the gametes — all four types per heterozygous parent. Missing a gamete type is the single most common lost mark.
Draw a 4 × 4 Punnett square and fill in every box.
List offspring genotypes with their phenotypes.
Finish with the ratio on its own line, in the order the question named the phenotypes.
Where 9:3:3:1 comes from
Of the sixteen boxes, nine show at least one dominant allele of both genes, three show the first dominant with the second recessive, three show the reverse, and one is doubly recessive. It only appears when both parents are heterozygous for both genes, the genes are on different chromosomes, and there is no gene interaction.
Cross
Expected ratio
RrYy × RrYy
9 : 3 : 3 : 1
RrYy × rryy (test cross)
1 : 1 : 1 : 1
RrYy × Rryy
3 : 3 : 1 : 1
When the ratio is not what you expect
Autosomal linkage: the genes are on the same chromosome, so parental combinations dominate and only a few recombinants appear.
Sex linkage: write genotypes on the X chromosome (X^H X^h, X^H Y) — a bare 'Hh' loses the mark.
Epistasis: one gene masks another, collapsing 9:3:3:1 into ratios such as 9:7 or 12:3:1.
Small sample size: real data rarely land on the exact ratio, which is why you then run a chi-squared test.
Phrases that earn the marks
parental phenotypes and genotypes stated
all four gamete types shown for each heterozygote
Punnett square completed correctly
offspring phenotypes identified
ratio stated in the order asked
genes are on different chromosomes / assort independently
Where marks get lost
Writing only two gamete types for a double heterozygote.
Using the same letter for both genes, so R and r collide with the second locus.
Giving genotypes but never converting them to phenotypes.
Claiming a deviation from 9:3:3:1 proves linkage without testing it statistically.
Try these dihybrid inheritance questions yourself
Write your answer, then see it marked against every mark scheme point before the model answer is revealed.
1/2
A2 2 genetics question — appears in almost every series · 5 marks
In a species of plant, tall (T) is dominant to dwarf (t) and red flowers (R) are dominant to white (r). Two plants heterozygous for both genes were crossed. Using a genetic diagram, determine the expected phenotypic ratio of the offspring.
Marked instantly, no account needed
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Exam questions on dihybrid inheritance, marked
Two A2-standard questions in CCEA paper style, each with the full mark scheme, a full-mark answer written the way you should write it, and the point where most candidates drop marks.
A2 2 genetics question — appears in almost every series · 5 marks
In a species of plant, tall (T) is dominant to dwarf (t) and red flowers (R) are dominant to white (r). Two plants heterozygous for both genes were crossed. Using a genetic diagram, determine the expected phenotypic ratio of the offspring.
Mark scheme — 5 creditworthy points
Parental phenotypes and genotypes stated: tall red TtRr × tall red TtRr
Gametes correctly identified and circled: TR, Tr, tR, tr from each parent
Punnett square completed correctly with 16 offspring genotypes
Offspring phenotypes identified from the genotypes
Ratio stated as 9 tall red : 3 tall white : 3 dwarf red : 1 dwarf white
Full-mark answer
Parents: tall red (TtRr) × tall red (TtRr). Gametes from each parent: TR, Tr, tR, tr. A 4 × 4 Punnett square gives 16 offspring: 9 with at least one T and one R (tall red), 3 with at least one T and rr (tall white), 3 with tt and at least one R (dwarf red), and 1 ttrr (dwarf white). Expected phenotypic ratio = 9 tall red : 3 tall white : 3 dwarf red : 1 dwarf white.
Examiner insight: Marks here are for the layout, not just the answer. Writing '9:3:3:1' alone scores one mark out of five. Always label parental phenotypes, circle the gametes and name the four offspring phenotypes.
A2 2 'suggest' question — epistasis application · 3 marks
A dihybrid cross produced offspring in a ratio of 9 : 3 : 4 rather than the expected 9 : 3 : 3 : 1. Suggest an explanation for this result.
The two genes interact / one gene affects the expression of the other — epistasis
The recessive (or dominant) allele of one gene masks the expression of the second gene
Two of the expected phenotype classes therefore appear identical and are counted together (3 + 1 = 4)
Full-mark answer
The two genes are interacting, a phenomenon called epistasis. When the epistatic gene is homozygous recessive it masks the expression of the second gene, so the double recessive class and one of the single recessive classes have the same phenotype. Those two classes (3 and 1) are counted as one group of 4, giving 9 : 3 : 4 instead of 9 : 3 : 3 : 1.
Examiner insight: The arithmetic point — 3 + 1 = 4 because two classes look alike — is a separate mark and is the one most often missed. Naming 'epistasis' alone is one mark.
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
What is the difference between monohybrid and dihybrid inheritance?
Monohybrid inheritance follows one gene and gives a 3:1 ratio from two heterozygotes; dihybrid inheritance follows two genes at once and gives 9:3:3:1.
How many gametes does a RrYy parent produce?
Four genetically different types: RY, Ry, rY and ry, produced in equal proportions by independent assortment in meiosis.
Do I need a chi-squared test for a dihybrid cross?
Whenever you are given observed offspring numbers and asked whether they fit an expected ratio, yes — compare the calculated value to the critical value at p = 0.05 with 3 degrees of freedom.
Test yourself on this topic
Generate 10 spec-worded questions on dihybrid inheritance, or photograph your written answer and see it marked line by line against the mark scheme.
