Mendelian Genetics – Lesson Summary

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Learning Objective

In this lesson we will learn how Gregor Mendel discovered fundamental laws of inheritance by conducting quantitative experiments with garden peas.

Learning Outcomes

By the end of this lesson you will be able to:

  • Define ‘pure breeding line’ and differentiate between cross-pollination and self-fertilisation.
  • Construct Punnett squares for a monohybrid cross and state the genotypes and phenotypes of the F1 and F2 generations, including the Mendelian ratio.
  • State Mendel’s conclusions and discuss how they contributed to our current understanding of heredity.
  • Describe how a test cross can be used to determine the genotype of an individual with the dominant phenotype.

 
mendelian genetics lesson contents

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Lesson Topics

1 | Gregor Mendel

2 | Pure Breeding Lines and Cross-Pollination

3 | Monohybrid Cross

4 | Explaining a Monohybrid Cross Using Punnett Squares

5 | Test Cross

6 | Summary

Lesson Summary

  • By conducting quantitative studies of inheritance of several traits in peas, Gregor Mendel developed laws which form the basis of many aspects of modern genetics, known as Mendelian genetics.
  • Mendel set up experiments involving crosses between pure breeding lines of peas which differed in particular traits.
  • By counting the numbers of each phenotype present in the offspring, Mendel concluded that inheritance occurred by discrete ‘factors’, which we now call alleles.
  • Mendel developed a type of cross called a monohybrid cross, which produces two generations of offspring – F1 and F2.
  • The first stage involves cross-pollination of two pure breeding lines that differ in one trait.
  • This results in all F1 having the dominant phenotype, as they are all heterozygous.
  • The second stage involves self-fertilisation of the F1 generation.
  • This results in F2 having the dominant and recessive phenotypes in a 3:1 ratio, as ¼ are homozygous dominant, ½ are heterozygous and ¼ are homozygous recessive.
  • This 3:1 phenotype ratio is known as the Mendelian ratio.
  • Mendel also developed a test cross, which is a simple method for determining the genotype of individuals with the dominant phenotype.
  • A test cross involves crossing the individual in question with an individual which has the recessive phenotype.
  • If no offspring have the recessive phenotype, the individual in question is homozygous dominant.
  • If half (or any) of the offspring have the recessive phenotype, the individual is heterozygous.

 
peas mendel genetics

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