Biology 212 General Genetics
Lecture 4: "Mendelian Genetics I"
Spring 2007
Reading: Chap. 2 pp. 34-45
Lecture Outline:
1. Mendel's experiments
2. Molecular basis for round vs. wrinkled
3. Principle of segregation
Lecture:
Mendelian Genetics=Classical Genetics=Transmission Genetics=Study of how genes
are transmitted from generation to generation.
1. Mendel's experiments
Gregor Mendel


1856-1863
Monk, teacher of physics and natural history.
Experiments on garden peas.
Traits in peas: 7 in all, including
 Round vs. wrinkled seeds
 Green vs. yellow seeds
 Purple vs. white flowers
 Tall vs. short stems
Mendel's crosses
 Chose very distinct traits
 Selected stocks that were true-breeding for these traits
 Carried out controlled breeding experiments
 Crossed pollen of one plant with the ova of another
 Can self-cross plants: allow to self pollinate
Mendel found
 Each parent contributes distinct "factors" (=genes) to offspring
 Factors remain unchanged as they are passed on
Example of a monohybrid cross (=cross between organisms that differ in one trait)
P1 generation
F1 generation
True-breeding wrinkled x true-breeding round
All round
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Therefore round is dominant trait, wrinkled is recessive trait. See Fig. 2.5
2. Molecular Basis for Mendel's traits: round vs. wrinkled
round seed gene
wrinkled seed gene: has insertion mutation
SB enzyme I
defective SB enzyme I
Round have enzyme to make
starch; shrink evenly.
Wrinkled have defective
enzyme for making starch; shrink unevenly.
Nomenclature:
W=round
w=wrinkled
Capital letter for
dominant trait
Lower case letter for recessive trait
Round is wild type=
commonly found type
Wrinkled is mutant,
rarely found
Ways of distinguishing variants (=alleles of traits):
 Observe physical difference
o Example shape, color

Measure molecular difference
o Example: electrophoresis of DNA
Small version of gene=round
Larger version of gene=wrinkled
Electrophoresis diagrams of

Wrinkled

Round

F1 hybrid


See Figure 2.6
Visible traits are frequently dominant or recessive.
Often have codominant molecular forms. Neither form is dominant at the
molecular level.
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3. Principle of segregation
Before Mendel
 Blending inheritance
 Traits of parents become blended in the hybrid
 Mendel believe traits behaved as discrete units; don't blend
 To observe Mendelian inheritance must observe a couple generations of crosses
P1
round X
wrinkled
F1
round X
F2
5474 round
1850 wrinkled
F1 round (self-cross)
F2 ratio 2.96:1
Results: In F2 generation, the recessive wrinkled trait reappears. The ratio of dominant
to recessive is about 3:1.
Mendel predicted
a. Genes come in pairs (=alleles)
b. Paired genes separate (segregate) in gamete formation
c. gametes unite at random in fertilization
Fig. 2.6 Illustration of Mendel's concepts in monohybrid cross of round and wrinkled
peas.
WW
Round
x
ww
wrinkled
Gametes W
Fertilization
w
One copy of each gene in gamete
Ww
Intercrossing of hybrid: Ww x Ww
"Punnett square"
W
w
W
WW
Ww
w
Ww
ww
3/4 round (WW and Ww) to 1/4 wrinkled (ww)
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Principle of segregation: In the formation of gametes, the paired genes separate and
are equally likely to be distributed to the mature gametes.
Summary of Mendel's observations for monohybrid crosses
 One trait is dominant, one is recessive.
 In the F1 generation, the recessive trait does not appear in a cross of a truebreeding dominant individual and a true breeding recessive individual.
 The recessive trait will reappear in 1/4 of the progeny in an F1 x F1 monohybrid
cross
 the dominant trait will occur in 3/4 of the individuals
Other terms used in classical genetics:

Gene: the hereditary determinant of a trait

Allele: different forms of a particular gene

Genotype: genetic make-up of an individual

Heterozygote: individual with two different alleles for a gene

Homozygote: individual with two of the same types of alleles for a gene

Phenotype: the observable traits of an organism

Recessive trait: a trait that is only expressed when genotype is heterozygous

Dominant trait: a trait that is expressed when the genotype is either
homozygous or heterozygous
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