10.2 Inheritance

2 Exercises

Read this article https://www.ndsu.edu/pubweb/~mcclean/plsc431/linkage/linkage1.htm

Morgan crossed Drosophila (fruit flies) red eye, normal wing flies (pr+pr+ vg+vg+) with purple eye, vestigal wing (prpr vgvg) flies.

pr+pr+ vg+vg+ pr+pr+ vgvg prpr

image image image

Adapted from https://commons.wikimedia.org/wiki/File:Drosophila_Gene_Linkage_Map.svg

Then he test crossed the offspring that were heterozygous for both alleles

pr+pr vg+vg crossed with pr+pr vg+vg

Using this data do a chi-squared calculation and show how significant the results are.

F1 Gamete

Testcross Distribution

Gamete Type

pr+ vg+



pr+ vg



pr vg+



pr vg



2. Calculate the genotype and the phenotype ratios for different dihybrid crosses involving unlinked autosomal genes.

a) A gene for the height of peas has two alleles T dominant for tall t for short

A gene for the shape of the peas has two alleles R for round and r for wrinkled

Investigate a variety of crosses over at least two generations (using 4 by 4 Punnet squares).

List the parental phenotypes and genotypes, gametes and the resulting genotypes and phenotypes.

3. Explain using annotated diagrams how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.

4. Define linkage group

5. Explain an example of a cross between two linked genes.

6. Use all the genetic vocabulary you know to complete this diagram.


Adapted from https://en.wikipedia.org/wiki/Dihybrid_cross#/media/File:Grasak.JPG

7. Dihybrid cross between Heterozygous for both alleles

Complete the probabilities answers at https://commons.wikimedia.org/wiki/File:Dihybrid_Cross_Tree_Method.png


Convert to 9:3:3:1 fractions ( /16)

Extra practice


HL Topic 10 Genetics Copyright © by Alex Black. All Rights Reserved.

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