BIOL 2500 - Topic 6 (part 2)

Bateson and Punnett

They studied sweet peas, specifically the flower colour and pollen grain shape, which is what led them to the discovery of linkage

Bateson and Punnett looking at the two sweet pea traits separately

When studies separately, the two traits generated the usual 3:1 phenotypic ratio in F2

Bateson and Punnett looking at the two sweet pea traits together

1.) They crossed the homozygous dominant x homozygous recessive genotypes together, resulting in the homozygous phenotype in F1

2.) But in F2, the typical 9:3:3:1 ratio was not observed, as there were more parental phenotypes observed than recombinant phenotypes

Bateson and Punnett: coupling

The term they used when they noticed that certain traits were inherited together

Thomas Morgan

In 1911, he confirmed genetic linkage and constructed one of the first genetic maps

Thomas Morgan experiment (part 1)

1.) He looked at the autosomal genes for eye colour and wing length

2.) He crossed a double WT with a double mutant, resulting in an F1 with the dominant phenotypic traits

3.) He then did a testcross with the F1, to see what the phenotypic ratio would be

Testcross results from Morgan’s first F1 experiment

He saw more of the parental (homozygous genotypes) and less of the recombinants (heterozygous genotypes)

Why do we see more parental than recombinants?

Because males do not undergo recombination, only females do

Thomas Morgan experiment (part 2)

1.) He repeated the experiment, but with different gene combinations, such that he crossed a two true-breeding heterozygotes

2.) He then did a testcross with the result F1 progeny

Testcross results from Morgan’s second F1 experiment

1.) He got more of the parental (heterozygotes) and less of the recombinants (homozygous)

2.) Therefore, he concluded that these two genes were linked/inherited together

Final conclusion of Morgan’s experiment

Dihybrid testcrosses of linked genes will have…

1.) Equally frequent nonrecombinant classes, which occur more than 50%

2.) Equally frequent recombinant classes, which occur less than 50%

Two conformations of linked genes in a dihybrid testcross

1.) Cis formation

2.) Trans formation

Cis formation

Both the dominant alleles of the two genes are on the same homolog and the recessive alleles are on the other homolog (i.e. PRVG / prvg)

Trans formation

The dominant alleles of the two genes are on different homologs (i.e. PRvg / prVG)

/ meaning

Represents being on separate homologs

• meaning

Genes are on the same chromosome

Discovery of cytological evidence of recombination (i.e. crossing over)

It was first demonstrated simultaneously by Creighton, McClintock, and Stern

Creighton and McClintock experiment

1.) They observed recombination in corn, using chromosome 9 homologs, with two genes

2.) One homolog was normal and carried the two genes

3.) The other homolog was slightly altered, with a knob and translocated bit of chromosome 8 on the ends

Creighton and McClintock experiment results

1.) They observed an actual physical exchange of genetic information between the homologs, such that the translocated bit was transferred to the normal homolog

2.) Therefore, it has a different genotype than what the parents started with