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
F1 Cross
Parents – Father: Homozygous black, short hair: BBLL
Mother: Homozygous red, long hair: bbll
Possible Gametes –
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 –
The dihybrid cross would look like:
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.
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
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:
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.
