LECTURE CONNECTIONS

7 | Linkage, Recombination, and Eukaryotic

© 2009 W. H. Freeman and CompanyGene Mapping

7.1 Linked Genes Do Not Assort Independently

• Mendel’s work:

- Segregation: each individual diploid organism possess two alleles at a locus that separate in meiosis.

- Independent: the alleles at a locus act independently of at other loci.

• Genes located close together on the same chromosome are called linked gens and belong to the same linkage group.

• Linked genes travel together during meiosis and are not expected to assort independently.

7.2 Linked Genes Segregate Together and Crossing Over Produces Recombination between Them

Occasionally, genes switch from one homologous chromosome to the other through the process of crossing-over.

• Notation for Crosses with Linkage

• Complete Linkage Leads to Nonrecombinant Gametes and Nonrecombinant Progeny

• Crossing Over with Linked Genes Lead to Recombiant Gametes and Recombinant Progeny

7.2 Linked Genes Segregate Together and Crossing Over Produces Recombination between Them

Non-recombinant gametes: gametes that contain only original combinations of alleles present in the parents.

Recombinant gametes: gametes with new combinations of alleles.

No crossing-over = Non-recombinant gametes

Single Crossover = ½ of gametes are recombinants + ½ of gametes are non-recombinants

Concept Check 1

For single crossovers, the frequency of recombinant gametes is half the frequency of crossing over because:

a. a test cross between a homozygote and heterozygote produces ½ heterozygous and ½ homozygous progeny.

b. the frequency of recombination is always 50%.

c. each crossover takes place between only two of the four chromatids of a homologous pair.

d. crossovers occur in about 50% of meiosis.

Application about linkage and recombination

Calculating Recombination Frequency

• Recombination frequency =

Number of recombinant progeny

Total number of progenyX 100%

Coupling and Repulsion Configuration of Linked Genes

• Coupling (cis configuration): Wild type alleles are found on one chromosome; mutant alleles are found on the other chromosome.

p+ b+

p b

• Repulsion (trans configuration): Wild-type allele and mutant allele are found on the same chromosome.

p+ bp b+

Predicting the Outcome of crosses with Linked genes

Determining the proportions of the types of offspring requires an additional piece of information: the recombination frequency.

Example:

Cucumbers smooth fruit (t) is recessive to warty fruit (T) and glossy fruit (d) is recessive to dull fruit (D)

Page 170: Suppose we cross a plant HETEROZYGOUS for warty and dull fruit with a plant homozygous for smooth and glossy fruit…