And Yet more Inheritance 10.2

Essential Idea: Genes may be linked or unlinked and are inherited accordingly. 10.2 Inheritance Understandings: Gene loci are said to be linked if on the same chromosome   Unlinked genes segregate independently as a result of meiosis Variation can be discrete or continuous Chi-squared tests are used to determine whether the difference between an observed and expected Application: Morgan’s discovery of non-Mendelian ratios in Drosophila Completion and analysis of Punnett squares for dihybrid traits Polygenic traits such as human height may also be influenced by environmental factors Skill: Calculate the predicted genotypic and phenotypic ratio of offspring of dihybrid crossing involving unlinked autosomal genes Identify recombinants in crosses involving two linked genes Use chi-square test on data from dihybrid crosses

I. Linked Genes Genes that are located on the same chromosome Linkage group – group of genes that tend to be inherited together because they are located on the same chromosome

C. Thomas Hunt Morgan – worked with Drosophila, results did not follow Mendelian probabilities 1. Failed to produce the expected 9:3:3:1 ratio

D. Genetic Recombination - production of offspring with a different combination of genes than either parent 1. Only way linked genes are not inherited together is if crossing over occurs between the two loci 2. The farther apart two genes are on a chromosome, the more likely it is that crossing over will occur between them

Sample Problem In Drosophila, body color and wing length are linked. Gray body (G) is dominant over black (g) and long wings (L) are dominant over vestigial (l). A test cross is done between and heterozygous fruit fly and a homozygous recessive fruit fly. X

If Mendelian laws were followed, what would the expected results be?

If Mendelian laws were followed, what would the expected results be?

If Mendelian laws were followed, what would the expected results be? 25% gray, long; 25% gray, vestigial; 25% black, long; 25% black, vestigial

If no crossing over was to occur, what would the results be?

If no crossing over was to occur, what would the results be?

If no crossing over was to occur, what would the results be? 50% gray, long; 50% black, vestigial All of the offspring would be like one parent or the other

What really happens?

What really happens?

What really happens? Sample data:

How far apart two genes are on a chromosome can be determined by the percentage of offspring that are recombinants (1% recombinant = 1 Map unit

How far apart two genes are on a chromosome can be determined by the percentage of offspring that are recombinants (1% recombinant = 1 Map unit 9% + 9.5% = 18.5% These two genes are 18.5 map units apart

II. Polygenic Inheritance Two or more genes control the expression of a trait Also influenced by environmental factors Continuous vs discrete variation