Dihybrid crosses and gene linkage. Topic 10.2. Assessment Statements. 10.2.1 Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes. 10.2.2 Distinguish between autosomes and sex chromosomes .
10.2.1Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
10.2.2Distinguish between autosomes and sex chromosomes.
10.2.3Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.
10.2.4Define linkage group.
10.2.5 Explain an example of a cross between two linked genes.
10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.
Allowing heterozygous offspring to self-pollinate yields following proportions:
Two non-sister chromatids can swap segments of their DNA
A maternal chromosome can end up with a segment of a paternal chromosome and vice versa
Any two genes which are found on the same chromosome are said to be linked to each other.
Linked genes are usually passed on to the next generation together.
Linkage group - groups of genes on the same chromosome inherited together
Linked genes are the exception to Mendel’s law of independent assortment
A cross between homozygous dominant true-breeding fruit fly (GGLL) and a homozygous recessive true-breeding fruit fly (ggll) would result in flies which were all heterozygous for both of the traits (GgLl)
If heterozygotes for these traits are mixed with homozygous dominant flies, then one could not determine the genotype by sight.
What to do?
10.3.1 Define polygenic inheritance.
10.3.2 Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour.
Involves two or more genes influencing the expression of one trait
Increased number of possible genotypes
Believed that most human traits are too complex to be determined by one gene
UVB radiation helps the skin to make vitamin D
Vitamin D is necessary for proper growth and bone formation
It is good to have a MODERATE amount of sunlight
In regions of low Sun exposure people need light-colored skin