April 2008

Mendelian GeneticsMendelian GeneticsGene Linkage & PolyploidyGene Linkage & Polyploidy

April 9, 2008April 9, 2008Mr. BromwellMr. Bromwell

Genetic RecombinationGenetic Recombination

• new combination of genes produced by crossing over and independent assortment– possible combinations of genes in a gamete due to

independent assortment is 2n

– In humans 2n equals 223

– When male and female gametes fertilize, the possible offspring combinations are 223 x 223, or more than 70 trillion

Gene LinkageGene Linkage

Question: What about genes on the same chromosome? Can they “assort independently?”

Answer: Usually not.

Genes that are linked together on the same chromosome usually travel together into the same gamete.

The Discovery of Gene LinkageThe Discovery of Gene Linkage• Hypothesized by William Bateson & R.C. Punnett in

the early 1990s.

• Studied the inheritance of flower color and pollen grain shape in the sweet pea.

• In the F2 generation, the phenotypes obtained were very different from Mendel’s predictions. (see below)

Bateson & Punnett’s Results

Phenotype Observed Expected based on Mendel’s 9:3:3:1 ratio

Purple flower, long pollen 4831 3911

Purple, round 390 1303

Red, long 393 1303

Red, round 1338 435

TOTAL 6952 6952

The Discovery of Gene LinkageThe Discovery of Gene Linkage• In 1915, Thomas Hunt Morgan put forth an explanation

of Bateson & Punnett’s observation.

• Worked with fruit flies, Drosophila melanogaster.

• Suggested that genes that do not assort independently are located on the same homologous chromosome and therefore travel together into gametes.

• Thus in the F2 generation, the phenotypes obtained should be more similar to the parent’s genotypes than to follow Mendel’s predictions.

However, as Bateson & Punnett and Morgan both observed, linked genes do not always travel together during meiosis. They concluded that linked genes can separate during prophase I by crossing over.

The Exception to the Exception! The Exception to the Exception!

Chromosome MappingChromosome Mapping• Alfred H. Sturtevant hypothesized that how often two linked genes

recombined (separated from one another during crossing over) could be used to calculate the distance between them on a chromosome.

– The more often they recombine, the farther apart they must be on the chromosome.

– In other words, if genes are far apart on a chromosome, crossing over is more likely to occur between them, then if the genes were close together.

– Permitted “maps” to be made of known genes in organisms. First done with fruit flies (Sturtevant was a student of Morgan’s – go figure!)

Chromosome Mapping Chromosome Mapping Problem:Construct a chromosome map for the following genes:

D and F crossover 15% of the time.G and D – 2%E and G – 8%E and D – 10%G and F – 17%E and F – 25%

Place the genes in the proper order.

Solution:

15% 2%8%

10%17%

Challenge ProblemChallenge ProblemGene pair Crossover

frequency Gene Pair Crossover frequency

AB 5.5 BF 4.3

AC 6.4 CD 10.9

AD 4.5 CE 2.6

AE 9.0 CF 5.2

AF 1.2 DE 13.5

BC 0.9 DF 5.7

BD 10.0 EF 7.8

BE 3.5

• occurrence of one or more extra sets of all chromosomes in an organism.

– Rarely occurs in animals (always lethal in humans)– Many flowering plants are polyploid – usually imparts

increased health and size

PolyploidyPolyploidy

Strawberries (8n) Coffee (4n)

ReferencesReferencesBiggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007.Griffiths, Anthony J.F., Miller, J.H., Suzuki, D.T., Lewontin, R. C., & Gelbart, W.M. "The discovery of linkage." An

Introduction to Genetic Analysis. 2000. W. H. Freeman and Company. 8 Apr 2008 <http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.882>.

Nobel Lectures, Physiology or Medicine 1922-1941, Elsevier Publishing Company, Amsterdam, 1965 ."Thomas H. Morgan: The Nobel prize in Physiology or Medicine 1933." Nobelprize.org. 2008. The Nobel

Foundation. 8 Apr 2008 <http://nobelprize.org/nobel_prizes/medicine/laureates/1933/morgan-bio.html>.