Linked Genes and Polygenic Inheritance

1. Linked Genes and Polygenic Inheritance

2. Objectives for Linked Genes • Define linkage group • Explain an example of a cross between two linked genes • Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

3. Linked Genes • Linkage group – two or more genes inherited together because they are found on the same chromosome • Linked genes do not follow Mendel’s Law of Independent Assortment which would give a typical result of 9:3:3:1 (instead they give a wide variety of ratios)

4. Common Example of Linked Genes • For fruit fly Drosophila the gene for body color and wing length are linked b/c they are located on same chromosome • For fruit fly: G = grey body g = black body L = long wings l = short wings

5. In a lab we have two parent flies with alleles shown: (linkage shown) G Lg l G L g l • These two flies mate and their offspring gets one trait from each parent G L g l • Then, the offspring mates with another fly which is homozygous recessive (grey/long wings mates with black/short wings) G Lg l g l g l

6. Complete a Punnett grid to show the results of the mating 5. Notice the two offspring in bold are in combinations unlike their parents (grey/short wings and black/long wings) therefore they must have formed through crossing over and we call them recombinants

7. Objectives for Polygenic Inheritance • Define polygenic inheritance • Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin color

8. Polygenic Inheritance • It is when 2 or more genes influence the expression of one trait • Since there are 2 or more allelic pairs found at different loci, the number of possible genotypes is greatly increased *This is why interpreting the human genome is especially difficult since many genes can effect the same trait

9. Continuous Variation • When multiple genes produce a spectrum of resulting phenotypes (this can make it so the genotype is not clear) • Examples include skin color: multiple genes effect the intensity of pigment in the skin (at least 3 genes affect skin color)

10. Other examples of continuous variation include eye color, height, body shape and intelligence • Note: many of these traits are also affected by the person’s environment (nature vs. nurture) so they are called multifactorial • Traits that do not vary along a continuum are referred to as discontinuous variation; examples of these traits include your blood group, widow’s peak, attached vs. unattached earlobes