9-5-05

FUNDAMENTAL MENDELIAN GENETICS

1. What did Mendel actually DO in his seven years of pea experiments nearly 150 years ago?

* Established true-breeding parental strains for seven visible traits (Fig. 3.1).
* Crossing these strains gave F1 progeny that all showed the "dominant" phenotype.
* Self-fertilization of these F1s gave F2 progeny in ratio 3 dominant : 1 "recessive".
* Crosses involving two traits typically gave a 9:3:3:1 probability ratio in F2 progeny.

 

2. How do the results of a "monohybrid cross" lead to the "Principle of Segregation"?

Figure 3.5 shows a monohybrid cross giving the typical 3:1 phenotypic ratio in the F2 generation.

Principle of Segregation: "In the formation of gametes, the paired hereditary determinants (genes) separate (segregate) in such a way that each gamete is equally likely to contain either member of the pair." (textbook, page 97)

Five crucial words you must have in your active vocabulary now:
alleles, genotype, homozygous, heterozygous, phenotype




3. How do the results of a "dihybrid cross" lead to the "Principle of Independent Assortment"?

Figure 3.10 shows a dihybrid cross giving the typical 9:3:3:1 phenotypic ratio in the F2 generation.

Principle of Independent Assortment: "Segregation of the members of any pair of alleles is independent of the segregation of other pairs in the formation of reproductive cells." (textbook, page 102)

This statement means that the four possible genotypes of the gametes produced by the F1s are all equally likely (1/4 each of WG, Wg, wG, and wg). This statement is true for any two genes (two pairs of alleles) that are on different chromosomes.

 

4. What are some example "pedigrees" for human dominant and recessive diseases?

Figure 3.16 shows an example family pedigree for the case of Huntington disease, which exhibits DOMINANT inheritance. Note that someone with the disease will always have a parent who also has the disease. (I prefer to use the single letter designations, H and h, for the alleles,rather than using double letters as your textbook does.)

Figure 3.17 shows a pedigree for human albinism, an example of RECESSIVE inheritance (i.e., to show the phenotype, a person must be homozygous for the recessive allele). The most common way to inherit two copies of this (rare) allele is to have parents who are both heterozygous (i.e., "carriers" of the recessive allele).

 

5. How do we know whether to add or multiply to figure out overall probabilities of a combination of events or traits?

For "mutually exclusive" events or traits, we get the overall probability by ADDING the individual probabilities: (textbook, page 111)
Prob [A or B]= Prob [A] + Prob [B].
Example: Prob that the next person walking into a room is a male or a female = 1/2 + 1/2 = 1
(50% + 50% = 100%).

For "independent" events or traits, we get the overall probability by MULTIPLYING the individual probabilities: (textbook, page 112)
Prob [A and B] = Prob [A] x Prob [B].
Example: Prob that the next two people walking into a room are both females = 1/2 x 1/2 = 1/4.

A very simple example that combines these is to ask "What is the probability that a two-child family will be one girl and one boy?" The birth order "boy, then girl" has probability 1/2 x 1/2 = 1/4. The birth order "girl, then boy" has probability 1/2 x 1/2 = 1/4. The overall result "one girl, one boy" has probability 1/4 + 1/4 = 1/2.