Handout for Lecture 4 - C3032

  1. Terminology:

mutant vs. mutation
isolation vs. identification
phenotype vs. genotype
screen vs selection
wild type vs. wild-type
penetrance vs. expressivity
homozygous vs. heterozygous
autosomal vs. sex-linked
dominant vs. codominant vs. recessive
complementation vs. noncomplementation
phenocopy
Punnett Square
marker mutation
conditional mutation
complementation group
synthetic mutations (duplicate genes)
lethal mutation
self-fertilization

  1. Useful Approaches to Genetics Problems:

    1. Use the Punnett Square.

    2. Determine parental genotypes by progeny testing and test crosses.

    3. Use a marker mutation.

    4. Draw out chromosomes.

    5. Remember basic probability.

        1. If P is the probability of an event occurring, then 1-p is the probability that it does not occur

        2. for 2 independent events: and = a X b; or = a + b

 

  1. A Punnett Square for the double heterozygote a/+;b/+:

 

 

 

 

a;b

 

a;+

 

+;b

 

+;+

 

a;b

 

a/a;b/b

 

a/a;b/+

 

a/+;b/b

 

a/+;b/+

 

a;+

 

a/a;b/+

 

a/a;+/+

 

a/+;b/+

 

a/+;+/+

 

+;b

 

a/+;b/b

 

a/+;b/+

 

+/+;b/b

 

+/+;b/+

 

+;+

 

a/+;b/+

 

a/+;+/+

 

+/+;b/+

 

+/+;+/+

 

If a and b are recessive, then 9 WT: 3 A: 3 B: 1 AB.

What if a is recessive and b dominant?

What if both a and b are dominant?

What if a is recessive and b is dominant but shows 50% penetrance in heterozygotes (homozygous animals are express the phenotype at 100% penetrance)?

What if a and b are recessive and and mutations in both are needed for the mutant phenotype?

What if a and b are dominant and mutations in both are needed for the mutant phenotype?

  1. A Genetic Approach to Studying a Biological Problem: 

    1. Screen or select for mutants defective in the phenomenon being studied.

    2. Determine whether the phenotype breeds true and establish a mutant line.

    3. Backcross to wild type to remove extraneous mutations.

    4. Determine linkage.

    5. Determine map position.

    6. Test for complementation.

    7. Examine the phenotype.

    8. Obtain other mutations in the same gene and deduce the phenotype in the absence of the gene.

    9. Isolage the DNA for the gene.

    10. Characterize the gene product (including introducing alternative forms).

    11. Examine where and when the gene is expressed.

    12. Look for interacting genes.