Monohybrid Crosses & Law of Segregation

Monohybrid Crosses

When two parents that differ in only one characteristic breed, the process is called a monohybrid cross. Monohybrid crosses allow the genotype of offspring to be predicted.

Parental genotype

The first step in constructing a monohybrid cross involves identifying the parental genotypes.
- E.g. Two true-breeding pea plants have yellow or green peas.
  - The dominant seed color is yellow so the parental genotype is YY for yellow pea plants and yy for green pea plants.

Gamete alleles

Gametes are haploid, so only one allele from each parent is found in the gametes.
All possible combinations of the parental alleles should be identified. This represents the meiotic segregation into haploid gametes. In our pea plant example:
- 100% of the gametes of yellow pea plants will have Y alleles.
- 100% of the gametes of green pea plants will have y alleles.

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Illustrative background for F<sub>1</sub> offspring

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F₁ offspring

F₁ offspring are the first generation of offspring.
A monohybrid cross produces four different combinations of possible offspring.
- For the pea plants, both parents are homozygous. This means all the F₁ offspring produced have a Yy genotype (all heterozygous).

Gamete alleles

The F₁ pea plants have two different alleles. They are heterozygous.
The gametes for an individual F₁ offspring may contain either the Y allele or the y allele.
- 50% of an organism's gametes will contain the Y allele.
- 50% of an organism's the gametes will contain the y allele.

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F₂ offspring

F₂ offspring are the second generation of offspring.
When the F₁ pea plants breed, there are three possible genotypic combinations:
- YY
- Yy
- yy

Predicting genotypic ratios

Monohybrid crosses allow predictions to be made about the genotypic and phenotypic ratios of offspring.
- In the pea plant example, the ratio of yellow peas to green peas is 3:1. A monohybrid cross between two heterozygotes will always produce this ratio.
Monohybrid crosses can be drawn in two ways:
- Genetic diagrams.
- Punnett squares.

Support for law of segregation

Heterozygotes could arise from two different pathways (receiving one dominant and one recessive allele from either parent).
Heterozygotes and homozygous dominant individuals are phenotypically identical.
- So, the law supports Mendel’s observed 3:1 phenotypic ratio.
The equal segregation of alleles is the reason we can apply the Punnett square to accurately predict the offspring of parents with known genotypes.

1Cell Biology

2Molecular Biology

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2.1.2IB Multiple Choice - Water & Hydrogen Bonding

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2.2.4Cellulose

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2.3Proteins

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2.4Enzymes

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2.4.4End of Topic Test - Enzymes

2.5Structure of DNA & RNA

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