Topic 10.2

Essential idea: Genes may be linked or unlinked and are inherited accordingly.


10.2.U1 Gene loci are said to be linked if on the same chromosome

Comparing geni loci in linked and unlinked genes

10.2.U2 Unlinked genes segregate independently as a result meiosis

Segregation of unlinked genes diagram

10.2.U3 Variation can be discrete or continuous

Defining continuous and discrete variation

10.2.U4 The phenotype of polygenic characteristics tend to show continuous variation

Polygenic inheritance is the additive effects of two or more genes on a single phenotypic characteristic. Shown below is a hypothetical example using skin color with the assumption that it is controlled by three genes that are inherited separately. The effects of each gene are small where uppercase and lowercase letters did not imply dominance or recessiveness but rather the additive effects of each allele to skin color through melanin production. That is, A, B and C each make equal contributions to melanin production while a, b and c contribute nothing to melanin production. The phenotypic expression of each polygenic trait is also influenced by environmental variation. The graph illustrates that the variation in skin colour is continuous and individuals do not fall into discrete (e.g., "light" or "dark") groups and the phenotypes form a spectrum. Another example of of polygenic inheritance is grain colour in wheat where each gene promotes pigment production.

Example of polygenic inheritance: skin colour

10.2.U5 Chi-squared tests are used to determine whether the difference between and observed and expected frequency distribution is statistically significant

Click on Go to Chi-squared Test to see how to compare predicted and actual outcomes in a monohybrid cross.

You can use the same step procedures when doing the chi-squared test on a dihybrid cross.

Go to Chi-squared Test


10.2.A1 Morgan's discovery of non-Mendelian ratios in Drosophila

Morgan's 1st experiment

After completing the punnett squares click on the figure click on the figure to view the punnett squares. Click on figure to return back to original figur Click on Show explanation to view an explanation of Morgan's results.

Morgan's expected results for white-eyed drosophila experiment
Show explanation Morgan's Experiment White-eyed Drosophila | Hide explanation Morgan's Experiment White-eyed Drosophila
Morgan experiment and sex-linked genes explained

Morgan's 2nd experiment

Place your mouse on the figure to view explanation

Dihybrid punnett squares

10.2.A2 Completion and analysis of Punnett squares for dihybrid traits

Punnett square dihybrid cross

10.2.A3 Polygenic traits such as human height may also be influenced by environmental factors

Polygenic inheritance and height


10.2.S1 Calculation of predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes

Punnett square dihybrid cross

10.2.S2 Identification of recombinants in crosses involving to linked genes

Diagram identifying recombinants

10.2.S3 Use of a chi-squared test on data from dihybrid crosses

Read the dihybrid question shown below and then carry out the steps necessary in a chi-squared test (see 10.2.U5 to review the steps).

Place your mouse on the figure to view chi-squared test completed steps

Dihybrid punnett squares
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