Inheritance

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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 colour 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 true-breeding pea plant example:
- 100% of the gametes of green pea plants will have y alleles.
- 100% of the gametes of yellow pea plants will have Y alleles.

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 heterozygous. This means 50% of the offspring are homozygous (25% yy and 25% YY), and that 50% of the F₁ offspring produced have a Yy genotype.

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 gametes will contain the y allele.

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.

Dihybrid Crosses

When two parents that differ in two characteristics breed, the process is called a dihybrid cross.

Independent assortment

Mendel proved that genes do not influence each other with regard to the sorting of alleles into gametes. This is called the law of independent assortment.
The law of independent assortment means that genes separate independently of each other when gametes are made.
The combination of alleles can be shown in a dihybrid cross.

Dihybrid gamete alleles

In a dihybrid cross between two homozygotes, there is one possible gamete allele combination for each homozygote.
E.g. two pea plants differ in two characteristics: seed colour and seed texture. One plant has green, wrinkled seeds (yyrr) and one plant has yellow, round seeds (YYRR).
- 100% of the gametes of the green/wrinkled plant are yr.
- 100% of the gametes of the yellow/round plant are YR.

F₁ offspring

When two homozygotes breed, all the F₁ offspring have the same genotype.
- E.g. The offspring of the pea plants all have a YyRr genotype.

F₁ gamete alleles

The law of segregation predicts that each gamete in F₁ generation has an equal probability of receiving any allele (e.g. R, r, Y or y).
This means there are four possible combinations of gametes produced by the F₁ offspring. For example:
- YR.
- Yr.
- yR.
- yr.

F₂ offspring

When the F₁ offspring breed, the four possible gametes from one individual can combine with any of the four possible gametes from the other individual.
The total possible combinations in the F₂ generation is 16.

Predicting phenotypic ratios

Dihybrid crosses can be used in this way to predict genotypic ratios of the F₂ offspring.
In the pea plant example, the ratio of offspring is:
- Nine round/yellow.
- Three round/green.
- Three wrinkled/yellow.
- One wrinkled/green.
When two dihybrid heterozygotes breed, the ratio is always expected to be 9:3:3:1.

Codominance

Codominance is where both alleles for the same characteristic are simultaneously expressed in the heterozygote. This can influence the outcome of monohybrid and dihybrid crosses.

Codominance

Codominant alleles are both expressed in a heterozygote.
Neither of the alleles are recessive.
Codominance influences the phenotypic ratios of monohybrid and dihybrid crosses.

E.g. sickle-cell anaemia

An example of codominance is sickle-cell anaemia.
There are two alleles for sickle-cell anaemia:
- H^N - normal haemoglobin.
- H^S - sickle haemoglobin.

Sickle-cell phenotypes

People who have two copies of the H^N allele (homozygotes) do not have sickle-cell anaemia.
People who have two copies of the H^S alleles (homozygotes) do have sickle-cell anaemia.
People who have one H^N allele and H^S allele (heterozygotes) have both normal haemoglobin and sickled haemoglobin.
- H^N and H^S are codominant.

Phenotypic ratios

Codominance affects the phenotypic ratios of monohybrid and dihybrid crosses.
E.g:
- If two heterozygous (H^NH^S) breed, the ratio becomes 1:2:1 instead of the normal 3:1 that is expected in a monohybrid cross.