Mendel's Laws and Their Extensions: Quick Revision

A brief note on Mendel's Laws and Their Extensions. Read it before a test. Try each question at the end before you open its answer.

Mendel's crosses

• Worked on: the garden pea, Pisum sativum, from 1856 to 1863. Seven pairs of contrasting traits.
• Monohybrid F2: genotypes 1 : 2 : 1, phenotypes 3 : 1 (complete dominance).
• Test cross: the unknown plant × a homozygous recessive. All dominant means a homozygote. A 1 : 1 ratio means a heterozygote.
• Dihybrid F2 (RrYy × RrYy): 9 : 3 : 3 : 1, from 16 boxes. A dihybrid test cross gives 1 : 1 : 1 : 1.
• Laws: dominance, segregation (the alleles separate in the gametes) and independent assortment (genes on different chromosomes).
• Counting: n heterozygous genes give $2^n$ gametes, $4^n$ boxes and $3^n$ genotypes.

Beyond simple dominance

• Incomplete dominance: the F1 is in between (red × white snapdragon gives pink). The F2 is 1 : 2 : 1 for the phenotype too.
• Codominance: both alleles show in full (blood group AB).
• Multiple alleles: the ABO gene has $I^A$, $I^B$ and i. Six genotypes, four phenotypes. A person has only two alleles.
• Pleiotropy: one gene, many effects (phenylketonuria).
• Polygenic inheritance: many genes, one trait, a bell-shaped spread (skin colour, height).

A worked example

Two people of the genotype AaBbCc have children. What share of the children have exactly 3 capital alleles?

1. There are 6 alleles in all, so there are 64 equally likely combinations.
2. The ways to choose 3 capital alleles out of 6 are 20, so the share is $\tfrac{20}{64} = \tfrac{5}{16}$.

Practice with new numbers

A program to try

The program counts three standard crosses exactly.

Mistakes to avoid

• Writing 3 : 1 as a genotypic ratio. The genotypes are 1 : 2 : 1.
• Mixing up codominance and incomplete dominance. Codominance shows both alleles. Incomplete dominance gives a middle trait.
• Saying that one person has three ABO alleles. A person has two.
• Mixing up pleiotropy and polygenic inheritance. One gene with many effects, or many genes with one effect.

Check yourself

1. How many kinds of gamete does a plant with the genotype AaBbCcDd make?

2. A red snapdragon is crossed with a pink one. What is the ratio of the offspring?

3. A man of group A and a woman of group B have a child of group O. What are their genotypes?

4. What cross reveals whether a dominant plant is homozygous?

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 4.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 12.