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Classical Genetics Lectures:
Chapter 11
Mendelian Genetics
Chapter 12
Chromosomal inheritance, sex linkage & determination
Chapter 13
Extensions of Mendelian Genetics
(multiple alleles, gene interactions, etc.)
Non-Mendelian Inheritance
(mtDNA, chloroplasts, maternal effects, etc.)
Epigenetic Inheritance
Numerous factors contribute to the phenotype:
Genotype
Activities of
genes & gene
products
Environment &
development
Phenotype
Genotype = collection of genes (and alleles) in an organism
Phenotype = observable properties of an organism
Is inheritance blending or particulate?
1.
In the mid 19th century,biologists believed that inheritance was
blending, that is, traits of offspring were the average of their
parents.
2.
Problematic because new genetic variations would quickly be
diluted and could not be accumulated and passed to subsequent
generations as theory of evolution predicted.
3.
Blending inheritance was quickly discredited by Gregor Mendel’s
experiments, which showed that inheritance is particulate.
F1
F2
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.
“Versuche über Pflanzen-Hybriden”
“Experiments in Plant Hybridization”

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. 11.4, Mendel’s 7 garden pea characters.
Some basic terminology:
Generations:
P = parental generation
F1 = 1st filial generation, progeny 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 (Fig. 11.7):
Results of Mendel’s monohybrid parental cross:
“Mendel’s Principle of Uniformity in F1”
F1 offspring of a monohybrid cross of true-breeding strains resemble
only one of the parents.
Fig. 11.5
Why?
Smooth seeds (allele S) are completely dominant to wrinkled seeds
(allele s).
Fig. 11.8
Smooth and wrinkled
parental seed strains
crossed.
Punnett square
F1 genotypes
4/4 Ss
F1 phenotypes
4/4 smooth
F1 x F1 crosses (Fig. 11.6):
Mendel also discovered that traits
that disappear in the F1 generation
reappear in the F2 generation in a
1:3 ratio.
“Mendel’s Principle of Segregation”
F1 x F1 Punnett square (Fig. 11.8):
F2 genotypes
1/4 SS
1/2 Ss
1/4 ss
F2 phenotypes
3/4 smooth
1/4 wrinkled
Fig. 11.9, 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
Fig. 11.11, 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.
Fig. 11.12a Dihybrid cross:
F1 generation
Fig. 11.12b Dihybrid cross:
F2 generation
Ratio:
9:3:3:1
Trihybrid crosses:
1.
Involve three independently assorting character pairs.
2.
Results:
1.
64 combinations of 8 different gametes
2.
27 different genotypes
3.
8 different phenotypes (2 x 2 x 2)
4.
Predicted ratio of phenotypes = 27:9:9:9:3:3:31
Summary of Mendel’s Three 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.
Rediscovery of Mendel’s Principles:

William Bateson (1902)-experiments with fowl first demonstrated
that Mendel’s principles applied to animals.

Bateson argued that mutation (not selection) was the most
important force shaping variation in plants and animals.

William Bateson also coined the terms:
Genetics
Zygote
F1
F2
Allelemorph ( allele)
1907 - Reginald Punnett and William Bateson
1905 - Letter from Bateson to Alan Sedgewick
Statistical analysis of genetic data:
1.
Mendelian ratios can be predicted mathematically  null hypothesis.
2.
Null hypothesis = difference is due to chance.
3.
Compare null hypothesis to observed data with goodness of fit test.
4.
Chi-square (2) test is one of the most common GF tests.
2 =  (# observed
- # expected)2 / # expected
1.
Requires a P-value (probability that the difference between
observed and expected values is due to chance).
2.
P-value is obtained from a table of probability values (0.05,
0.10. 0.30, etc.) and known degrees of freedom (df).
3.
P = 0.05 is typical cited as significant.
4.
df = # phenotypic classes - 1 (n - 1)
Test-cross: SsYy x ss yy  1/4 + 1/4 + 1/4 + 1/4 (see Table 10.2)
Phenotype
# obs.
# exp.
obs - exp
(O - E)2
(O - E)2/E
Smooth/
yellow
136
142
-6
36
0.25
Smooth/
green
138
142
-4
16
0.11
Wrinkled/
yellow
144
142
+2
4
0.03
Wrinkled/
green
146
142
+4
16
0.11
df = 4 -1 =3, Critical
2 for P = 0.05 and 3 df = 7.82
0.50