Biology · Inheritance
Monohybrid Crosses and Punnett Squares
How likely is it that two parents will have a child with a particular characteristic? This lesson shows how to use a genetic diagram and a Punnett square to predict the outcomes of a monohybrid cross, and how to give them as ratios, fractions and percentages.
Learning Objectives
- 1Explain monohybrid inheritance using genetic diagrams and Punnett squares.
- 2Work out the genotypes and the phenotypes of the offspring from a cross.
- 3Calculate and analyse outcomes using probabilities, ratios and percentages.
- 4Explain why the actual results of a cross may differ from the predicted ones.
Retrieval practice
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1
What does heterozygous mean?
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Having two different alleles of a gene.
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2
Which alleles does a Tt parent put into its gametes?
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Half with T and half with t.
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3
What is a genotype?
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The alleles an organism has for a gene.
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4
What does the phenotype of Tt look like if T is dominant?
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The dominant characteristic, tall.
Predicting the offspring
A monohybrid cross is a cross that looks at the inheritance of one gene. Because the alleles in the gametes combine at random at fertilisation, we can work out the chances of each combination, and so predict the proportions of different offspring. This is done with a genetic diagram, and the most common version is a Punnett square, which is a grid that shows every possible combination of gametes.
Drawing a genetic cross
Always follow these steps in the same order, and show every one of them in an exam answer.
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1
Define the alleles
State the letters used, for example T for tall (dominant) and t for dwarf (recessive).
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2
Write the parents' genotypes
For example Tt and Tt.
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3
Write the gametes
Put each gamete in a circle. A Tt parent gives T and t.
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4
Draw the Punnett square
Put one parent's gametes along the top and the other's down the side, and fill each box with the two alleles.
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5
Read off the genotypes
Count the boxes with each genotype.
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6
State the phenotypes and the ratio
Say which characteristic each genotype gives, and write the phenotype ratio.
A cross between two heterozygous plants
Crossing two heterozygous parents gives a 3 to 1 ratio of dominant to recessive phenotypes.
Reading the result
The same result can be written in three ways, and you may be asked for any of them.
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Genotype ratio
1 TT : 2 Tt : 1 tt. Of the four boxes, one is TT, two are Tt and one is tt.
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Phenotype ratio
3 tall : 1 dwarf, because TT and Tt are both tall.
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Probability
The chance of a dwarf is 1 in 4, which is \(\frac{1}{4}\), 0.25 or 25%. The chance of a tall plant is 3 in 4, or 75%.
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Expected numbers
From 200 offspring we would expect about 150 tall and 50 dwarf.
Other crosses to know
Homozygous cross, TT x tt
- Gametes are all T from one parent and all t from the other
- All offspring are Tt
- All offspring are tall
- Ratio of phenotypes is all dominant
Test cross, Tt x tt
- Gametes are T and t from one parent, and all t from the other
- Half the offspring are Tt, half are tt
- Half are tall and half are dwarf
- Phenotype ratio 1 tall : 1 dwarf
A cross between two carriers
Cystic fibrosis is caused by a recessive allele, f. Two parents are both carriers, Ff. Draw a Punnett square, and work out the probability that their child has cystic fibrosis.
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- 1 Write the parents' genotypes Both parents are Ff.
- 2 Write the gametes Each makes F and f gametes.
- 3 Fill in the Punnett square The four boxes are FF, Ff, Ff and ff.
- 4 Find the genotype of a child with cystic fibrosis Only ff has the disorder, and that is 1 box out of 4.
- 5 State the probability \(\frac{1}{4} = 25\%\).
AnswerThe probability is 1 in 4, or 25%.
Using a ratio to predict numbers
Two heterozygous black guinea pigs (Bb, with B for black dominant to b for white) have 48 offspring in total. How many white offspring would you expect?
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- 1 Predict the ratio Bb x Bb gives 3 black : 1 white, so \(\frac{1}{4}\) are white.
- 2 Work out one quarter \(48 \div 4 = 12\).
- 3 Check the answer 36 black and 12 white add up to 48.
AnswerAbout 12 white offspring.
Why real results can differ
A genetic diagram gives probabilities, not guarantees.
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Fertilisation is random
Each fertilisation is a separate event, and any gamete can fuse with any other. The probabilities are the same each time.
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Small numbers
If there are only a few offspring, the results can be very different from the prediction. A family of four children from carrier parents could have none or two affected.
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Large numbers
The more offspring there are, the closer the results are to the predicted ratio.
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No memory
Having one affected child does not make the next any less likely to be affected. The chance is still 1 in 4.
Case study
Testing a plant of unknown genotype
A gardener has a tall pea plant but does not know if it is TT or Tt. To find out, the plant is crossed with a dwarf plant (tt). If the unknown plant is TT, all the offspring will be tall. If it is Tt, about half the offspring will be dwarf. This is called a test cross, and it shows how the results of a cross can be used to work out a genotype that cannot be seen.
Summary and exam focus
- A monohybrid cross follows one gene, and a Punnett square shows all the possible gamete combinations.
- Crossing two heterozygous parents gives 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive.
- The phenotype ratio is 3 dominant to 1 recessive.
- The chance of the recessive phenotype is 1 in 4, or 25%.
- Real results can differ from predictions, especially in small numbers.
Exam focus
Tall (T) is dominant to dwarf (t) in pea plants. A tall plant (Tt) is crossed with a dwarf plant (tt). Use a genetic diagram to show the possible offspring, and give the ratio of tall to dwarf plants. (4 marks) (4 marks)
Draw the Punnett square and show the gametes clearly, with a letter for each gamete. Always finish by stating the genotypes, the phenotypes and the ratio, because marks are given for each step.
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Monohybrid cross
- A genetic cross that looks at the inheritance of a single gene.
- Punnett square
- A grid used to show all the possible combinations of gametes in a cross.
- Genetic diagram
- A diagram that shows the genotypes of parents, their gametes and their possible offspring.
- Ratio
- A way of comparing amounts, such as 3 : 1.
- Probability
- The chance that something will happen, written as a fraction, a decimal or a percentage.
- Test cross
- A cross between an organism of unknown genotype and a homozygous recessive one.
- Offspring
- The young produced by parents.
- Parental generation
- The parents at the start of a cross.
- Expected ratio
- The ratio of offspring predicted from a genetic diagram.
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