Chapter 14

Basics of Mendelian Genetics

I am going to move through the basics fairly quickly. If you have trouble with the material see me ASAP- I have some good websites you can go to to better learn the material.

One Character- Two Traits

Character: Flower Color

Traits: •Purple flowers

•White flowers

Refer to figure 14.2, page 241.

Mendel began by producing pure breed plants.

•Pure purple flowers

•Pure white flowers

Crossed them and got an interesting result in the F1 generation: All purple.

Question: Where have all the white flowers gone?

Crossed these purples and got another interesting result in the F2 generation:

3/4 purple; 1/4 white

Question: What is going on?

What he anticipated. . .(see figure 14.3, page 241)

•That there was a “gene” for flower color, and this gene came in two versions-what we know as alleles- purple and white. These alleles reside on homologous chromosomes.

•Each organism inherits two alleles, one from each parent.

•One allele is dominant and one is recessive. The dominant is fully expressed in the offspring. The recessive is expressed as long as there is no dominant.

•The two alleles for each character segregate during the production of gametes:

Mendel’s Law of Segregation.

Examine figure 14.4 page 242:

Key ideas:

1. Parental generation produce only one kind of gamete each.

2. F1 generation has their gametes “segregate” - therefore they produce 50% P and 50% p.

3. Punnett Square: Used to find result of crossing individuals.

Phenotype: Organism’s appearance

Genotype: Organism’s genetic make-up

•Different ratios for each. We usually talk about the phenotype ratio.

Genetic Terms:

Homologous: Identical alleles of a gene in an individual (PP or pp)

Heterozygous: Different alleles of a gene in an individual (Pp)

Testcross: To find out the true genetic makeup (genotype) of an individual. You cross the individual with a homozygous recessive individual. e.g. Purple can be PP or Pp; a testcross will tell you which you have!

See figure 14.6, page 244