Linkage and Recombination

What is a testcross?

• for dihybrid genes

• cross between parent and tester parent carrying only recessive alleles

  • offspring reveal alleles from dihybrid parent

What is a dihybrid parent?

heterozygous for two different genes

Which parent goes on top for chromosomal notation?

maternal, top is one chromosome

What is chromosomal notation?

notation for studying genes on the same homologous chromosome

In an X linked gene, what is the expected frequency of phenotypes for a testcross (AaBb x aabb) according to the law of independent assortment and according to chromosomal theory?

• independent assortment: 4 phenotypes, 25%

• chromosomal theory: 2 phenotypes, 50%

What is recombinant frequency (Rf)? How is it calculated?

• percent of recombinant individuals

• Rf = (R1 + R2)/total

What are parental and recombinant phenotypes?

• parental phentoypes: offspring with allele combinations that match the original parents

• recombinant: new allele combos on chromosomes from crossing over during meiosis

What is chiasmata?

site of touching between homologous chromosomes where recombination/crossing over occurs

How does recombinant frequency change along a chromosome?

• cross over is equally likely across the chromosomes

• % Recombinants reflects physical distance between linked genes

• genes closer together cross over less

• genes further apart (up to 50 cM) cross over more

• ex. 33% recombinants = 33cM apart

compare linked vs unliked genes

• linked: genes located close together on the same chromosomes tend to be inherited together; produce more parental phenotypes, fewer recombinants

• unlinked: genes on different chromosomes or greater than 50cM apart on the same chromosome; assort independently, produce 50% recombination/parental

In a dihybrid cross involving linked genes we refer to phenotypes as being either parental or recombinant. How do these phenotypes relate to each other and how do they relate to the parents used to generate the cross (the “P generation”)? (alleles, crossing-over)

• parental phenotypes are the same trait combos as the P generation (AB and ab), reflecting the original allele pairings (ex. AB and ab); no crossing over

• recombinant: new combos (ex. Ab and aB) that were not in parent

How does RF indicate the relative position of genes on a chromosome?

• Low RF → genes are close together (linked)

• High RF —> genes are further apart (unlinked)

• max observable RF is 50%/50cM