Chapter 13: Genetics

In this chapter you will learn about the story of Karpechenko's crossing of a radish and a cabbage which introduces the topic of genetics. This is followed by a brief history of Mendel and his classic experiments with pea plants. Each of Mendel's principles or laws is discussed, and examples of monohybrid and dihybrid crosses are given. Next, the backcross, linkage, chromosomal mapping, the Hardy-Weinberg law, and interactions between genes are introduced. The chapter concludes with a discussion of DNA structure and protein synthesis.

At the end of this chapter the successful student will be able to

Recognize the significance of Mendel's experiment with peas
Using Mendel's experiment with peas and flowers be able to apply the following terms:
- monohybrid cross, dihybrid cross, F1 generation, F2 generation, Dominant, Recessive
Using Punnet Squares, compare (and contrast)
- genotype and phenotype
- Hetrozygous and Homozygous
Identify the characteristics of offspring using Punnet Squares recognizing complete dominance and incomplete dominance for monohybrid crosses
Use a Punnet Square to recognize how gametes combine in a dihybrid cross
Assignments
Glossary

Return to Schedule

Recognize the significance of Mendel's experiment with peas

Mendel crossed self-pollinated pea plants and recorded his observations. The results were astonishing. He noted that when he crossed tall with short plants, there were no short or intermediate plants. What was happening? He allowed the offspring plants to pollinate themselves. Again there was a surprise. They produced offspring in a ratio of approximately three tall plants to one short. What was happening? These observations lead to the start of serious study of genetics. He had succeeded in developing a model for inheritance when countless others had failed because he performed carefully planned and executed crosses from genetically stable plants. He carefully kept records of the results by counting the offspring and comparing the results with his predictions. He chose traits under simple genetic control so that the data was not so complex that patterns were difficult to tract and he applied statistical analysis to the data to get mathematical models of the results. His breakthroughs not only lead to a scientific approach to genetics, but also validated the importance of using scientific methods involving careful planning, careful collection of data, and careful analysis in experiments that are repeatable.

Return to top

Mendel's experiment with peas and flowers

Mendel crossed self-pollinated pea plants and recorded his observations:

Crossed a tall plant with a short plant. (see figure 13.10)
Parental Generation (P)
- All offspring were tall.
- First Filial Generation (F1)
  - Cross offspring yielded a ratio of three tall individuals to one short individual.
  - Second Filial Generation (F2)
Law of Unit Characters
- Factors (Alleles), which always occur in pairs, control the inheritance of various characteristics.
  - Genes are always at the same position (locus) on homologous chromosomes.
Law of Dominance
- For any given pair of alleles, one (dominant) may mask the expression of the other (recessive).

Return to top

Punnet Squares

Phenotype refers to an organism’s physical appearance.
Genotype refers to the genetic information responsible for contributing to the phenotype.
- Homozygous - Both alleles identical.
- Heterozygous - Alleles are contrasting.

Return to top

Characteristics of offspring

Monohybrid Cross (see figure 13.11)
- A cross is made between two true-breeding parents differing for a single trait, producing an F1 generation.
  - These plants are intercrossed to produce an F2 generation.
Backcross
- A cross between a hybrid and one of its parents.
  - Can be used to test inheritance theory.
Incomplete Dominance (see figure 13.13)
- Some genes exhibit traits in which a heterozygote is intermediate in phenotype to the two homozygotes.
Click here to interact with a monohybrid cross

Return to top

How gametes combine in a dihybrid cross

Dihybrid Cross (see figure 13.12)
- Cross made for parents differing in two traits.
  - Mendel’s Law of Independent Assortment
    - Factors (genes) controlling two or more traits segregate independently of each other.
      - Linked genes do not segregate independently.
    - Punnett Square used to determine genotypes of the zygotes.
Click here to interact with a dihybrid cross

Return to top