Download 10.2 - Dihybrid Crosses and Gene Linkage

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
Mother
BL
bl
BL
bl
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
Mother
BL
BL
Bl
Bl
bL
bL
bl
bl
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The dihybrid cross would look like:
Male Gametes
Female Gametes
BL
Bl
bL
bl
BL
BBLL
BBLl
BbLL
BbLl
Bl
BBLl
BBll
BbLl
Bbll
bL
BbLL
BbLl
bbLL
bbLl
bl
BbLl
Bbll
bbLl
bbll
Genotypes
Phenotypes
BBLL
BBLl
BbLL
Short, black hair x 9
BbLl
BBll
Bbll
bbLL
bbLl
bbll
Long, black hair x 3
Short, white hair x 3
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.
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