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10.2 - Dihybrid Crosses and Gene Linkage

10.2.1 - Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid

crosses involving unlinked autosomal genes

A dihybrid cross is a cross involving two genes that control two different characteristics.

Unlinked genes are found on different chromosome, so they will be separated by random

assortment during meiosis. Autosomal genes are found on any chromosomes other than the

XY gender-determining chromosomes.

According to the law of independent assortment, each pair of alleles segregates into

gametes independently. This means that different types of gametes can form. Therefore, a

cross between two heterozygous parents always has the ratio 9 : 3 : 3 : 1

For example

F1 Cross

Parents - Father: Homozygous black, short hair: BBLL

Mother: Homozygous red, long hair: bbll

Possible Gametes -

Father BL BL BL BL

Mother bl bl bl bl

The only possible genotypes for the offspring are: BbLl.

The offspring are heterozygous for both traits, expressing only the dominant allele.

F2 Cross

Parents – Father: Heterozygous black, short hair: BbLl

Mother: Heterozygous black, short hair: BbLl

Possible Gametes –

Father BL Bl bL bl

Mother BL Bl bL bl

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The dihybrid cross would look like:

Female Gametes

BL Bl bL bl M

ale

Gam

etes

BL BBLL BBLl BbLL BbLl

Bl BBLl BBll BbLl Bbll

bL BbLL BbLl bbLL bbLl

bl BbLl Bbll bbLl bbll

Genotypes Phenotypes

BBLL

Short, black hair x 9 BBLl

BbLL

BbLl

BBll Long, black hair x 3

Bbll

bbLL Short, white hair x 3

bbLl

bbll Long, white hair x 1

9 : 3 : 3 : 1 is the F2 dihybrid ratio. It is a prediction of the offspring ratio, but may differ in

real life.

Test Cross

To test if an organism is heterozygous for particular traits, a test cross is done. The organism

is crossed with a homozygous recessive. If it is heterozygous, the ratio will be 1 : 1 : 1 : 1

This is called the back-cross ratio or dihybrid test ratio.

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10.2.2 - Distinguish between autosomes and sex chromosomes

Autosome - A chromosome that is not a sex-chromosome. They do not vary depending on

gender.

Sex Chromosome - A chromosome which determines sex rather than other body (soma)

characteristics.

10.2.3 - Explain how crossing over between non-sister chromatids of a homologous pair in

prophase I can result in an exchange of alleles

During prophase I, the homologous pairs of chromosomes pair up and are in close proximity

to each other. Breakages may occur along the chromatids, allowing fragments to be

exchanged between the non-sister chromatids. The rejoining of non-sister chromatids forms

chiasmata, which remain intact until the chromosomes are separated by the microtubules.

As a result of crossing over, recombination of linked genes can occur.

10.2.4 - Define linkage group

Linkage group - the genes carried on any one chromosome

These tend to be inherited together, which results in fewer genetic combinations

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10.2.5 - Explain an example of a cross between two linked genes

Sweet Peas: Lathyrus odoratus

The F2 cross is done between the heterozygous parents. In this case, the gametes formed

during meiosis can be groups into two categories:

The majority of the offspring in the F2 cross will have genotypes that match the parents,

such as:

The third genotype seen above is the result of random assortment of the chromosomes

during meiosis.

However, there may also be a low frequency of recombinant genotypes as a result of

crossing over:

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10.2.6 - Identify which of the offspring are recombinants in a dihybrid cross involving

linked genes

Recombinants can be recognised by unpredicted combinations of characteristics, low

frequency of new combinations of phenotype and statistically significant difference from

the ratio expected.