Lecture 2 – Mendelian genetics
I. Blending?
A. predominant theory during 19th century
1. germ cells contained sample of essences from each somatic cell
2. problem: clearly not all traits blended
eg: Blue eyes X dark eyes
dark eyes (rather than dark blue eyes)
3. Simple experiment (Weissman, 1883)
Cut off mouse tails; tail essence should be missing, so offspring should
have short/no tails. Result:
B. Mendel’s experiments lead to particulate theory of inheritance
II. Introduction to Mendel
A. Who he was:
1. monk in what is now Czech Republic
2. experiments in ~1865 looking at heredity in peas
3. work was rediscovered in 1900
4. Why successful:
B. why peas?
1. several varieties available (seed shape, seed color, flower color, pod shape, pod
color, flower and pod position, plant size)
2. easy to cross or self-fertilize
3. easy to grow
4. relatively short generation time
B. importance of true breeding strains
1. what is a true breeding strain?
2. why is this important?
III. monohybrid cross:
A. simple cross
1
B. summary of results
1. F1 hybrids express “dominant” trait
2. F2 express either “dominant” or “recessive” trait
3. In F2, dominant:recessive is 3:1
4. Same results for all 7 traits examined
C. reciprocal cross
pollen from round X stigma from wrinkled vs pollen from wrinkled X stigma
from round
D. conclusions
1. existence of genes
2. genes are in pairs
3. principle of segregation
4. gametic content
5. random fertilization
IV. Mendel’s genetic hypothesis
A. Hereditary determinants for the traits in the parental
line were inherited as 2 different elements that retain their
purity in the hybrid.
B. Punnett square
Ww X Ww
C. How to test Mendel’s genetic hypothesis?
1. Allow the F2’s to self-fertilize
of F2, round:
2
and
wrinkled:
2. Test cross
X
X
X
3. so what did Mendel see with F1’s?
V. Important terms
A. hybrid
B. F1
C. F2
D. phenotype
E. genotype
F. gene
3
G. alleles
H. homozygous
I. heterozygous
J. test cross
4