13.1 Ecologists Study Relationships

7.3 Gene Linkage and Mapping

Incomplete dominance

7.3 Gene Linkage and Mapping

Polygenetic

• Ex. Human skin color

7.3 Gene Linkage and Mapping

KEY CONCEPT Genes can be mapped to specific locations on chromosomes.

7.3 Gene Linkage and Mapping

Objectives

• Students will be able to describe the discovery of gene linkage.

• Students will be able to explain how linkage maps can be used to estimate distances between genes.

7.3 Gene Linkage and Mapping

Gene linkage was explained through fruit flies.

• Mendel’s Work– Crossed pea plants; determined 9:3:3:1 ratio in

dihybrid crosses– Conclusions: two alleles for each trait; alleles assort

independently• Punnett and Bateson’s Work

– Crossed pea plants; dihybrid cross ratios differed from 9:3:3:1

– Conclusions: Suggested that some traits were linked

7.3 Gene Linkage and Mapping

Gene linkage was explained through fruit flies.

• Morgan found that linked traits are on the same chromosome.

• Chromosomes, not genes, assort independently during meiosis.

Wild type Mutant

7.3 Gene Linkage and Mapping

• Linked genes are not inherited together every time.

• Chromosomes exchange homologous genes during meiosis.

7.3 Gene Linkage and Mapping

Linkage maps estimate distances between genes.

• Sturtevant’s Hypothesis: – The closer together two genes are, the more likely

they will be inherited together.– Cross-over frequencies are related to distances

between genes.• Linkage maps show the relative locations of genes.

7.3 Gene Linkage and Mapping

Sturtevant’s Experiments

• Studied linked traits in fruit flies; • recorded the percentage of times crossing over occurred; • used cross-over frequencies to make linkage maps

Wild type Mutant

7.3 Gene Linkage and Mapping

• Cross-over frequencies can be converted into map units.

– gene A and gene B cross over 6.0 percent of the time

– gene B and gene C cross over 12.5 percent of the time

– gene A and gene C cross over 18.5 percent of the time

7.3 Gene Linkage and Mapping

• Calculate at your table the order of these genes.• The crossover frequencies between linked genes A and B is

40%; between B and C, 20%; between C and D, 10%; between C and A, 20% and between D and B, 10%.