10.2 Inheritance

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.
The phenotypes of polygenic characteristics tend to show continuous variation.

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

Looking for patterns, trends and discrepancies—Mendel used observations of the natural world to find and explain patterns and trends. Since then, scientists have looked for discrepancies and asked questions based on further observations to show exceptions to the rules. For example, Morgan discovered non-Mendelian ratios in his experiments with Drosophila. (3.1)

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.

Alleles are usually shown side by side in dihybrid crosses, for example, TtBb.

In representing crosses involving linkage, it is more common to show them as vertical pairs, for example:
T B
……..
t b
This format will be used in examination papers, or students will be given sufficient information to allow them to deduce which alleles are linked.

Syllabus and cross-curricular links: Biology
Topic 3.4 Inheritance

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

Identification of recombinants in crosses involving two linked genes.

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

An understanding of inheritance allowed farmers to selectively breed their livestock for specific characteristics.