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Theoretical Genetics (3.4/10.2)

Mendelian Genetics

Mendelian WebQuest:


Blood Typing Game

Dihybrid Crosses and Gene Linkage

YouTube Video

YouTube Video

Sickle Cell Explained at the protein level

Image result for Is this autosomal recessive or sex-linked flowchart?

YouTube Video

Linked Genes

YouTube Video

YouTube Video

3.4 Inheritance

Nature of science:

Making quantitative measurements with replicates to ensure reliability. Mendel’s genetic crosses with pea plants generated numerical data. (3.2)


Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.

Gametes are haploid so contain only one allele of each gene.

The two alleles of each gene separate into different haploid daughter nuclei during meiosis.

Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.

Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects.

Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles.

Some genetic diseases are sex-linked. The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes.

Many genetic diseases have been identified in humans but most are very rare.

Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer.

Applications and skills:

Application: Inheritance of ABO blood groups.

Application: Red-green colour blindness and hemophilia as examples of sex-linked inheritance.

Application: Inheritance of cystic fibrosis and Huntington’s disease.

Application: Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl.

Skill: Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.

Skill: Comparison of predicted and actual outcomes of genetic crosses using real data.

Skill: Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases.


Alleles carried on X chromosomes should be shown as superscript letters on an upper case X, such as Xh.

The expected notation for ABO blood group alleles is:








    |A| A or | Ai

    |B| B or | Bi

    |A| B

Theory of knowledge:

Mendel’s theories were not accepted by the scientific community for a long time. What factors would encourage the acceptance of new ideas by the scientific community?


Aim 8: Social implications of diagnosis of mutations, including the effects on the family and stigmatization.

Essential idea: Biologists have developed techniques for artificial manipulation of DNA, cells and organisms.

10.2 Inheritance

Nature of science:

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)


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.

Applications and skills:

Application: Morgan’s discovery of non-Mendelian ratios in Drosophila.

Application: Completion and analysis of Punnett squares for dihybrid traits.

Application: Polygenic traits such as human height may also be influenced by environmental factors.

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

Skill: Identification of recombinants in crosses involving two linked genes.

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


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

Theory of knowledge:

The law of independent assortment was soon found to have exceptions when looking at linked genes. What is the difference between a law and a theory in science?


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


Aim 4: Use analytical skills to solve genetic crosses.

Aim 8: Ethical issues arise in the prevention of the inheritance of genetic disorders.

Essential idea: Gene pools change over time.

Jessica Clark,
Apr 24, 2017, 10:14 AM