18.1: Gene linkage and crossing-over

Independent assortment of different alleles at 2 loci

observed when the genes are located on different chromosomes, resulting in 4 possible meiotic products with equal likelihood of production.

Dihybrid cross ratio for independent assortment

9:3:3:1 in F2 or in a test cross ratio 1:1:1:1

Linked genes

A pair of alleles for each locus that segregate as a unit with the alternative combination of alleles observed at a lower frequency; found close together on the same chromosome.

Crossing over

during pro1 non-sister chromatids in homologous pairs break and rejoin to exchange information; can form new combinations of alleles.

Chiasma

plural = chiasmata

X-shaped connection between non-sister chromatids in homologous pairs = reflects exchange of genetic info during crossing over

Recombinant chromatids

2/4 chromatids in pairs show new combination of genes after crossing over - other two are non-recombinant.

50% Recombination Frequency

When recombination frequency is 50%, genes are considered genetically unlinked.

Synaptonemal complex

Protein framework that manifests between maternal and paternal versions of the same chromosome, linking them during crossing over in meiosis.

Genetic recombination

New combinations of genetic information resulting from crossing over. Chromatids end with new unique blends of parental information.

purpose of crossing over

To increase genetic diversity by exchanging genetic material between homologous chromosomes during meiosis.