Module 6 Lesson 1 - 2

Introduction to Mendelian Genetics

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Farmers have always known that keeping seeds from good crops or breeding prize cows or chickens helps to improve the chances of the next generation yielding better numbers and/or favourable traits or of being better in some other way. This is called selective breeding. However, exactly how favourable traits are passed on has been the interest of scientists for centuries. Early theories tended to give importance to the kind of life the parents led. Similar to Lamarkian's ideas of evolution in which body parts change in response to use, many early scientists believed life's experiences affected the traits of the next generation.

Gregor Mendel

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These theories were put to rest finally by a 19th century Austrian monk named Gregor Mendel who used garden peas as his experimental subjects. With breeding and observation, he laid the foundation of our current knowledge and understanding of heredity.

Mendel experimented with pea plants with seven distinguishable characteristics as shown in Figure 17.4. The pea plants were easy to fertilize and reproduced quickly. He allowed the selected pea plants to self-fertilize for several generations to ensure that they were true breeding plants that exhibited the same characteristics in every generation.

Then, Mendel crossed the true breeding pea plants that he wanted to study. He called this generation the parental or P generation. The offspring generation that resulted from crossing the true breeding plants was called the first filial or F1 generation. He allowed the F1 generation to self-fertilize and observed the offspring. This generation was called the second filial or F2 generation.

Read the sections on "Early Theories of Inheritance" and "Developing a Theory of Inheritance: Gregor Mendel's Experiments" in your textbook on pages 586 to 588.

Law of Dominance

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When Mendel crossed two true breeding plants with contrasting traits, he observed that all plants in the F1 generation exhibited only one of the two traits from the parental generation. For example, when he crossed true breeding purple flowering plants with true breeding white flowering plants, all F1 plants were purple flowering plants. The white flowering trait seemed to disappear. However, when the F1 generation plants were self-fertilized, their offspring, the F2 generation, were a mix of purple and white flowering plants.


Mendel proposed that each trait was controlled by factors. Now, we call these factors genes. You learned in the previous module that alternative forms of a single gene are called alleles. For this example, the true breeding parent with purple allele is crossed with true breeding parent with white allele for the flower colour gene.

Each plant has two alleles present for each characteristic, and it will pass along only one of these alleles when gamete cells are produced (sperm or egg).

Mendel referred to the expressed trait in the F1 generation as dominant. The unexpressed trait in the F1 generation was referred to as recessive. For example, the purple flowering trait is dominant and the white flowering trait is recessive.

Modern Genetic Terminology

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Mendel's observations on the pea plants is basis of the genetics of inheritance. To express Mendel's experiments using the correct terminology and conventions is important in the study of genetics.

Genes for specific traits or alleles are carried on chromosomes. Each somatic cell has two copies of the chromosomes (diploid) and each gamete resulting from meiosis contains only one copy of the chromosomes (haploid). Fertilization of the two haploid gametes occurs to form a diploid zygote. These gametes may carry different forms of the gene of certain traits called alleles. To differentiate between dominant and recessive alleles, modern geneticists use the following system:

• A letter is used to symbolize the characteristic. For example, the letter "P" can be used for flower colour.
  


• The dominant allele is written in upper case. Purple flower colour is dominant and is written as P.
  


• The recessive allele is written in lower case of the same letter. White flower colour is recessive and is written as p.
  

Usually, the alleles are written in pairs to reflect that each individual has two alleles for every gene, one from each parent, such as PP, Pp, or pp. These symbols are called the genotype. The physical expression of these alleles in an individual is called the phenotype.

For example, a pea plant with genotype PP has the phenotype of purple flowers. Because purple colour (P) is dominant to white colour (p), a pea plant with genotype Pp also has the phenotype of purple flowers. This means, white pea plants must have the genotype pp.

When the pea plant has two identical alleles (PP or pp), it is homozygous. When the alleles are different (Pp), the pea plant is heterozygous.

These terms and conventions are used throughout the remaining units. To understand and use these terms correctly is very important.

Watch and Listen

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Watch the following video to review the work of Mendel