Dihybrid cross
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of monohybrid crosses and meiosis. You can test your knowledge on these below.
What is a monohybrid cross?
A genetic cross between individuals examining the inheritance of one gene.
What is meant by an allele?
Different forms of the same gene, often represented by letters (e.g., A or a).
What is the term for the random alignment of homologous chromosomes in meiosis?
Independent Assortment.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains dihybrid cross or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What is a Dihybrid Cross?
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A dihybrid cross tracks the inheritance of two different characteristics controlled by two different genes.
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In each individual each gene has two alleles, which may be dominant or recessive.
Example of Dihybrid Traits
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Seed shape: Round (R) dominant to wrinkled (r)
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Seed color: Yellow (Y) dominant to green (y)
Step-by-Step: How to Complete a Dihybrid Cross
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Identify the parent genotypes.
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Example: Heterozygous for both traits → RrYy.
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Determine all possible gametes.
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R from Rr, Y from Yy → RY
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R from Rr, y from Yy → Ry
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r from Rr, Y from Yy → rY
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r from Rr, y from Yy → ry
Remember, gametes are produced via meiosis. Meiosis produces haploid gametes with single chromosomes. Therefore there will only be one allele of each gene in the gametes.
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Draw a Punnett Square (4x4 grid).
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Write one parent's gametes across the top, the other down the left hand side.
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Fill in the grid.
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Combine each pair of gametes to find possible offspring genotypes.
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Interpret the Results:
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Group the offspring by phenotype.
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Calculate ratios.
Example Punnett Square
|
RY |
Ry |
rY |
ry |
|
|
RY |
RRYY |
RRYy |
RrYY |
RrYy |
|
Ry |
RRYy |
RRyy |
RrYy |
Rryy |
|
rY |
RrYY |
RrYy |
rrYY |
rrYy |
|
ry |
RrYy |
Rryy |
rrYy |
rryy |
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RRYY, RRYy, RrYY, RrYy will all be Round and Yellow
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RRyy or Rryy will be Round and Green
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rrYY or rrYy will be Wrinkled and Yellow
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rryy will be Wrinkled and Green
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Phenotypic Ratio (if both parents are RrYy):
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9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green
If parents are heterozygous for both genes, we expect a 9:3:3:1 ratio
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Key Terms
- Dihybrid cross: A genetic cross involving the inheritance of two different genes.
- Gametes: Sex cells carrying one allele for each gene.
- Phenotypic ratio: The ratio of different observable traits in the offspring.
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Exam Tips
Answer phenotype ratio in simplest whole numbers, e.g., 9:3:3:1.
Clearly identify the parental gamete combinations, offspring genotypes and offspring phenotypes.
No answer provided.
In peas, the allele for tall plants (T) is dominant to dwarf (t), and the allele for purple flowers (P) is dominant to white flowers (p).
Two plants heterozygous for both traits are crossed. Predict the phenotypic ratio of their offspring.
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Parent genotypes: TtPp × TtPp
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Gametes: TP, Tp, tP, tp (for both)
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Punnett Square setup:
|
TP |
Tp |
tP |
tp |
|
|
TP |
TT PP |
TT Pp |
Tt PP |
Tt Pp |
|
Tp |
TT Pp |
TT pp |
Tt Pp |
Tt pp |
|
tP |
Tt PP |
Tt Pp |
tt PP |
tt Pp |
|
tp |
Tt Pp |
Tt pp |
tt Pp |
tt pp |
Phenotypic ratio:
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9 Tall, Purple
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3 Tall, White
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3 Dwarf, Purple
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1 Dwarf, White (ttpp)
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!