3.3.3 Monohybrid inheritance

Definition

Monohybrid cross

A genetic cross that follows the inheritance of a single gene.

A monohybrid cross follows the inheritance of a single gene.
A genetic diagram sets out the parents' genotypes, the alleles in their gametes, and the possible offspring.
Each parent passes just one allele of the gene into each gamete.
A Punnett square is a grid used to combine the parents' gametes and find the offspring genotypes.

Note

Each gamete carries only one allele of each gene, because meiosis halves the number.

Hint

Work in order: write the parents' genotypes, then their gametes, then combine them.

Genotype and phenotype ratios

Definition

Phenotype

The observable characteristics of an organism, produced by its genotype and its environment.

Combining the gametes gives the possible offspring genotypes.
From a cross of two heterozygotes ( $Bb \times Bb$ ), the offspring are $B B$ , $B b$ , $B b$ and $bb$ .
This is a genotype ratio of $1 : 2 : 1$ and a phenotype ratio of $3 : 1$ , dominant to recessive.
A cross of a heterozygote with a homozygous recessive ( $Bb \times bb$ ) gives a $1 : 1$ phenotype ratio.

A Punnett square diagram illustrating a monohybrid cross between two heterozygous parents (Rr x Rr). It shows the resulting genotype outcomes (RR, Rr, and rr) and the characteristic 3:1 phenotype ratio of dominant to recessive traits.

Common Mistake

Do not confuse the genotype ratio ( $1 : 2 : 1$ ) with the phenotype ratio ( $3 : 1$ ).

Example

Worked cross: two heterozygous pea plants

Parental genotypes: $Tt \times Tt$ . The allele $T$ is dominant for tall plants and $t$ is recessive for short plants.
Gametes: each parent produces gametes carrying either $T$ or $t$ .
Punnett outcomes: $T T$ , $T t$ , $T t$ and $tt$ .
Genotype ratio: $tt1\,TT : 2\,Tt : 1\,tt$ .
Phenotype ratio: $3$ tall : $1$ short because both $T T$ and $T t$ show the dominant phenotype.
Probability: each offspring has a $34\dfrac{3}{4}$ chance of being tall and a $14\dfrac{1}{4}$ chance of being short. These are probabilities for each fertilisation, not a guarantee that every four offspring will have exactly this pattern.

Using a Punnett square

Definition

Punnett square

A grid used to predict the possible genotypes and phenotypes of offspring from a genetic cross.

A Punnett square is a grid for combining the parents' gametes.
Write one parent's gametes along the top and the other's down the side.
Fill each box by combining the allele from its row with the allele from its column.
The filled boxes show all the possible offspring genotypes.
For $Bb \times Bb$ , the four boxes give $B B$ , $B b$ , $B b$ and $bb$ .

Hint

Put single alleles on the edges of the grid, not whole genotypes.

Common Mistake

Do not put both of a parent's alleles into one gamete: each gamete gets only one.

Self review

What is a monohybrid cross?
What is a Punnett square used for?
What phenotype ratio comes from crossing two heterozygotes?
What is the difference between a genotype ratio and a phenotype ratio?

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A monohybrid cross follows the inheritance of a single gene. Different forms of that gene are called alleles, such as $T$ for tall and $t$ for short.

An organism inherits one allele from each parent, so its genotype contains two alleles. During meiosis, these alleles separate so that each gamete carries only one allele of the gene.

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A monohybrid cross follows the inheritance of a [ ].

3.3.3 Monohybrid inheritance

Definition

Monohybrid cross

A genetic cross that follows the inheritance of a single gene.

A monohybrid cross follows the inheritance of a single gene.
A genetic diagram sets out the parents' genotypes, the alleles in their gametes, and the possible offspring.
Each parent passes just one allele of the gene into each gamete.
A Punnett square is a grid used to combine the parents' gametes and find the offspring genotypes.

Note

Each gamete carries only one allele of each gene, because meiosis halves the number.

Hint

Work in order: write the parents' genotypes, then their gametes, then combine them.

Genotype and phenotype ratios

Definition

Phenotype

The observable characteristics of an organism, produced by its genotype and its environment.

Combining the gametes gives the possible offspring genotypes.
From a cross of two heterozygotes ( $Bb \times Bb$ ), the offspring are $B B$ , $B b$ , $B b$ and $bb$ .
This is a genotype ratio of $1 : 2 : 1$ and a phenotype ratio of $3 : 1$ , dominant to recessive.
A cross of a heterozygote with a homozygous recessive ( $Bb \times bb$ ) gives a $1 : 1$ phenotype ratio.

Common Mistake

Do not confuse the genotype ratio ( $1 : 2 : 1$ ) with the phenotype ratio ( $3 : 1$ ).

Example

Worked cross: two heterozygous pea plants

Parental genotypes: $Tt \times Tt$ . The allele $T$ is dominant for tall plants and $t$ is recessive for short plants.
Gametes: each parent produces gametes carrying either $T$ or $t$ .
Punnett outcomes: $T T$ , $T t$ , $T t$ and $tt$ .
Genotype ratio: $tt1\,TT : 2\,Tt : 1\,tt$ .
Phenotype ratio: $3$ tall : $1$ short because both $T T$ and $T t$ show the dominant phenotype.
Probability: each offspring has a $34\dfrac{3}{4}$ chance of being tall and a $14\dfrac{1}{4}$ chance of being short. These are probabilities for each fertilisation, not a guarantee that every four offspring will have exactly this pattern.

Using a Punnett square

Definition

Punnett square

A grid used to predict the possible genotypes and phenotypes of offspring from a genetic cross.

A Punnett square is a grid for combining the parents' gametes.
Write one parent's gametes along the top and the other's down the side.
Fill each box by combining the allele from its row with the allele from its column.
The filled boxes show all the possible offspring genotypes.
For $Bb \times Bb$ , the four boxes give $B B$ , $B b$ , $B b$ and $bb$ .

Hint

Put single alleles on the edges of the grid, not whole genotypes.

Common Mistake

Do not put both of a parent's alleles into one gamete: each gamete gets only one.

Self review

What is a monohybrid cross?
What is a Punnett square used for?
What phenotype ratio comes from crossing two heterozygotes?
What is the difference between a genotype ratio and a phenotype ratio?

Previous Next

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Teach it back in your own words, spot gaps, and remember it better.

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A monohybrid cross follows the inheritance of a single gene. Different forms of that gene are called alleles, such as $T$ for tall and $t$ for short.

An organism inherits one allele from each parent, so its genotype contains two alleles. During meiosis, these alleles separate so that each gamete carries only one allele of the gene.

Flashcards

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20 flashcards

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A monohybrid cross follows the inheritance of a [ ].

3.3.3 Monohybrid inheritance

Genotype and phenotype ratios

Worked cross: two heterozygous pea plants

Using a Punnett square

Review 3.3.3 Monohybrid inheritance by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

3.3.3 Monohybrid inheritance Revision Guide

3.3.3 Monohybrid inheritance

Genotype and phenotype ratios

Worked cross: two heterozygous pea plants

Using a Punnett square

Review 3.3.3 Monohybrid inheritance by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

3.3.3 Monohybrid inheritance Revision Guide