Mendelian Genetics

Notes on the Web - Unit Five- Part 3

Mendelian Genetics

Bruce G. Stewart

General Objectives and Study Guide

Your objectives for these Notes on the Web and associated readings and exercises are:

To define alleles and describe where in the cell they are located;
To define and explain the concepts of genotype and phenotype;
To state the name of the "Father of Genetics" and give the type of organisms that he studied;
To state and explain the Law of Segregation;
To describe specific research conducted by Mendel and explain how he discovered the Law of Segregation;
To describe and explain the Law of Independent Assortment;
To describe specific research conducted by Mendel and explain how he discovered the Law of Independent Assortment;
To use a Punnett Square to calculate proportions of genotypes from crosses;
To recognize and infer parental genotypes based on data from offspring genotypes and vice versa;
To name and explain examples of human genetic disorders that show Mendelian patterns of inheritance;
To know common terms and facts associated with the topic of Mendelian genetics..

Related Textbook Readings:

For General Biology (BIO 1114) Class only: Johnson and Losos (2008) Chapter 11 (in part). Sections 11.1 through 11.4.
For General Zoology (ZOO 1114) Class only: Hickman, et al. (2004) (to be assigned in class)

Introductory Note

This page includes a simple outline of the terms and concepts we will cover in more detail in lecture and/or via threaded discussion on Internet. You should read the materials in your textbook that related to anything mentioned in the outline below. Finally, you should visit the link at the end of this page. It will take you to Mendelian section of The Biology Project at the University of Arizona. Study that material carefully.

Experimental Approach First Showed Patterns of Inheritance

Blending Theory - Unsubstantiated view that was common prior to the 1860's
Mendelian Views - Gregor Mendel research in the 1860's
- first to clearly demonstrate patterns of inheritance for specific traits
- published in 1860's but remained mostly obscured until around 1900.

Mendel's Law of Segregation

Stated that there are two "factors" per trait per organism and that each of the paired factors separates during the formation of the gametes. The combination of factors controls the phenotype for that trait.
Mendel discovered this by doing studies of garden peas and following single traits at a time. Each trait had two contrasting forms.
Traits studied: stem length (tall vs. short), pod shape (inflated vs. constricted), seed shape (round vs. wrinkled), seed color (yellow vs. green), flower position (axial vs. terminal), flower color (purple vs. white), and pod color (green vs. yellow).
Example using Flower Color. Mendel crossed two pure breeding lines of garden peas, purple flowering and white flowering. The polygon shows the overall pattern between P1, F1, and F2 generations.
Notice that the pure breeding lines are homozygous dominant and homozygous recessive respectively. Thus, the F1 offspring would have to be heterozygous (and therefore) 100% purple phenotype. The F2 generation produces all three possible genotypes.
This is what Mendel determined by inspection and analysis of the results of his crosses. Pretty cool, huh?

Some Necessary Genetic Terms

alleles
gene locus (plural = loci)
homologous chromosome pairs
homozygous vs. heterozygous
dominant allele vs. recessive allele
genotype
phenotype

Punnett Square

know what it is and why it is useful
know how to use it to determine simple cross results

Probability Methods

know the importance of using mathematics in the study of complex inheritance patterns
know how to use simple probabilities in calculating monohybrid cross outcomes

Deductive Reasoning

Test Cross - know what it is and how it is used
know how a test cross can produce reliable results that could never be 100% "proven."
know why we accept well-tested explanations as almost certainly true, even when we cannot prove something with 100% certainty

Dihybrid Crosses

Know Mendel's Law of Independent Assortment and relate it to the process of meiosis
Be able to follow two traits simultaneously in a Mendelian dihybrid cross.
Be able to show with a Punnet Square all possible outcomes for a dihybrid cross
Be able to explain why the F2 generation has a predicted 9:3:3:1 ratio of phenotypes.
Be able to explain why there can be four rather than only two phenotypes.

Reminder about Textbook Study

As with other topics, your textbooks have excellent presentations of the materials on Mendelian patterns of inheritance, including generous excellent illustrations. Check the general objectives above to make sure that you have covered all of the topics in the textbook readings.

The "Self Test" and other questions at the end of the chapter will be helpful for general biology students, although many more detailed questions will be included in the lecture exam. Similarly, study questions in the zoology textbook will be helpful review for general zoology students, but again, they are not comprehensive.

As with all materials throughout the semester, you will have opportunities to ask questions or ask that any relevant material from your assignments be discussed in class and/or in threaded discussions on Internet.

Related Links:

From the "Biology Project" visit and study the resources at this link:

The Biology Project: Mendelian Genetics

Here is a link with tutorials on monohybrid, dihybrid, and test crosses from the North Harris College Department of Biology site:

Genetics Tutorials - North Harris College

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