Humboldt State University ® Department of Chemistry

Richard A. Paselk

	VUB Biology	Fall 2001
Lecture Notes:: 31 October	© R. Paselk 2001

Genetics

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)

Terms

• Character: a heritable feature, such as eye color
  • trait: a character variant
• True-breeding: all offspring are the same variety as parents
• Hybridization: a mating or crossing of two different varieties
  • monohybrid: a crossing involving a single character
    • P generation = parental generation
    • F₁ generation = hybrid offspring of P generation
    • F₂ generation = offspring of interbred F₁ generation

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:

• When an organism has two copies of the same allele it is said to be homozygous.
• When an organism has two different alleles of the same gene it is said to be heterozygous.

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  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.

• chromosomes and genes
• dominance
  • complete dominance
  • codominance (other extreme)

For example:

• Incomplete dominance: phenotype of offspring is between phenotypes of parents
• Multiple alleles: can have more than two versions of the same gene (Blood groups. Get four groups (A, B, O) from three alleles: I^A, I^B, and i.

Pleiotropy: genes with multiple phenotypic effects.

Epistasis: second gene determines expression of a first gene.

Polygenic inheritance: multiple genes determines color - quantitative charecters.

• Ex. skin color has multiple contributors. Simplified text model shows three contributiong genes with additive contributions giving light to very dark.

"Nature vs. Nurture"

Discussion of genetic vs environmental effects:

• note wide range of environmental effects, including hormonal etc. environment of womb for mammals.

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Notes

Last modified 31 October 2001

© R Paselk