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Welcome to Mendelian Genetics
Genotype
Activities of
genes & gene
products
Environment &
development
Phenotype
Genotype = collection of genes (and alleles) in an organism
Phenotype = observable properties of an organism
Mendelian Genetics:
Gregor Johann Mendel (1822-1884)

Augustinian monk, Czech Republic.

Foundation of modern genetics.

Studied segregation of traits in the
garden pea (Pisum sativum) beginning
in 1854.

Published his theory of inheritance in
1865.

Mendel was “rediscovered” in 1902.
Mendel’s Experiments:
1.
Began by self-fertilizing 34 different pea strains (phenotypes) so
that they bred true (selfing, the opposite of cross-fertilization).
2.
Focused on 7 well-defined garden pea traits by crossing different
phenotypes one at a time:
Flower/seed coat color:
Seed color:
Seed shape:
Pod color:
Pod shape:
Stem height:
Flower position:
3.
purple vs. white flowers
grey vs. white seed coats
(*controlled by single gene)
yellow vs. green
smooth vs. wrinkled
green vs. yellow
inflated vs. pinched
tall vs. short
axial vs. terminal
Counted offspring of each phenotype and analyzed the results
mathematically.
Fig. 10.4, Mendel’s 7 garden pea characters.
Some basic terminology:
Generations:
P = parental generation
F1 = 1st filial generation, progeny (offspring) of the P generation
F2 = 2nd filial generation, progeny of the F1 generation (F3 and so on)
Crosses:
Monohybrid cross = cross of two different true-breeding strains
(homozygotes) that differ in a single trait.
Reciprocal cross = sexes for the two strains are reversed (and if the
results are the same, trait is not sex-linked).
Dihybrid cross = cross of two different true-breeding strains
(homozygotes) that differ in two traits.
*Genetics etiquette - female conventionally is written first
Dominant & recessive alleles:
Results of Mendel’s monohybrid parental cross:
“Mendel’s Principle of Uniformity (the same) in F1”
F1 offspring of a monohybrid cross of true-breeding strains resemble
only one of the parents.
Fig. 10.5
Why?
Smooth seeds (allele S) are completely dominant to wrinkled seeds
(allele s).
Smooth and wrinkled
parental seed strains
crossed.
Punnett square
F1 genotypes
4/4 Ss
F1 phenotypes
4/4 smooth
F1 x F1 crosses:
Mendel also discovered that traits
that disappear in the F1 generation
reappear in the F2 generation in a
1:3 ratio.
F1 x F1 Punnett square:
F2 genotypes
1/4 SS
1/2 Ss
1/4 ss
F2 phenotypes
3/4 smooth
1/4 wrinkled
Crosses also can be represented with branching diagrams.
What about the six other phenotypic traits?
1.
Results of reciprocal crosses always were the same.
2.
F1 progeny always resemble the parental strain.
3.
In the F2 progeny, parental strain lost in the F1 generation always
reappeared at a ratio of 1:3.
“Mendel’s Principle of Segregation”:
Recessive characters masked in the F1 progeny of two true-breeding
strains, reappear in a specific proportion of the F2 progeny.
Modern formulation of Mendel’s Principle of Segregation:
Two members of a gene pair segregate (separate) from each other
during the formation of gametes.
Confirming the Principle of Segregation with test-crosses:

SS x SS

true breeding (100% homozygous dominant)

ss x ss

true breeding (100% homozygous recessive)
How do you determine whether an individual with the dominant
phenotype is homozygous or heterozygous?
Cross it with homozygous recessive:
SS x ss

4/4 dominant trait
Ss x ss

1/2 dominant trait + 1/2 recessive trait
Test Crosses
Mendel’s dihybrid crosses:
1.
Mendel also performed crosses involving two pairs of traits, e.g.,
seed shape (smooth vs. wrinkled) and color (yellow vs. green).
2.
If alleles sort independently, four possible phenotypes (2n) appear
in the F2 generation in a 9:3:3:1 ratio.
“Mendel’s Principle of Independent Assortment”:
Alleles for different traits assort independently of one another.
Modern formulation of independent assortment:
Genes on different chromosomes behave independently in gamete
production.
Dihybrid cross:
F1 generation
Dihybrid cross:
F2 generation
Ratio:
9:3:3:1
Summary of Mendel’s Principles:

Mendel’s Principle of Uniformity in F1:
F1 offspring of a monohybrid cross of true-breeding strains resemble
only one of the parents.
Why? Smooth seeds (allele S) are completely dominant to wrinkled
seeds (allele s).

Mendel’s Principle of Segregation:
Recessive characters masked in the F1 progeny of two true-breeding
strains, reappear in a specific proportion of the F2 progeny.
Two members of a gene pair segregate (separate) from each other
during the formation of gametes. Inheritance is particulate, not
blending as previously believed.

Mendel’s Principle of Independent Assortment:
Alleles for different traits assort independently of one another.
Genes on different chromosomes behave independently in gamete
production.