Humboldt State University ® Department of Chemistry

Richard A. Paselk

VUB Biology

Fall 2001

Lecture Notes:: 31 October

© R. Paselk 2001


Mendel's Law of Segregation

Working with peas, Mendel observed that many genetic characters follow an all or nothing kind of expression, as opposed to a continuum.

Example of pea flower color as in text (purple vs white, with purple dominant)


To explain inheritance in peas Mendel postulated:

  1. Variations in inherited characters are due to alternative versions of the same genes (alleles = alternative gene versions)
  2. Each organism (diploids) inherits two alleles, or copies of each gene. (This genetic makeup is known as the organisms genotype.)
  3. If the two alleles are different, then only one has an effect on the organisms appearance, or phenotype. The allele responsible for the appearance is the dominant allele the other is the recessive allele.
  4. The alleles segregate, during the production of gametes. That is each sperm or egg gets only a single allele.

Let's look at how this works with a Punnett square:

   P  p
 P  PP  Pp
 p  Pp  pp

In order to determine the genotype of an organism expressing the dominant phenotype we can use a testcross. In this case one crosses the unknown with an individual homozygous for the recessive allele. Two possible results as seen below:

 HOMO x homo    Hetero x homo
   p  p      p  p
 P  Pp  Pp    P  Pp  Pp
P  Pp  Pp    p  pp  pp


Mendel's Law of Independent Assortment

Using dihybrid crosses (parents differ by two characters) Mendel was able to determine that alleles often sort separately, as shown below for green (y) vs. yellow (Y) peas with round (R) vs. wrinkled (r) peas:

   YR  Yr  yR  yr
 Yr  YYRr  YYrr  YyRr  Yyrr
 yR  YyRR  YyRr  yyRR  yyRr
 yr  YyRr  Yyrr  yyRr  yyrr

Non-Mendelian Inheritance

Turns out that most inheritance isn't as simple as described in Mendel's laws. (Mendel was very clever as a scientist - he picked traits that did follow simple patterns that were relatively easy to describe, understand, and model. In many respects the ideal situation!)

Let's think a bit about what we know regarding genes and chromosomes, about what dominance is, etc.

For example:

Pleiotropy: genes with multiple phenotypic effects.

Epistasis: second gene determines expression of a first gene.

Polygenic inheritance: multiple genes determines color - quantitative charecters.


"Nature vs. Nurture"

Discussion of genetic vs environmental effects:


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© R Paselk