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

Genotype

Phenotype

Activities of genes & gene products

Environment & development

Genotype = collection of genes (and alleles) in an organism

Phenotype = observable properties of an organism

Numerous factors contribute to the phenotype:

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: purple vs. white flowersgrey vs. white seed

coats(*controlled by single

gene)Seed color: yellow vs. greenSeed shape: smooth vs. wrinkledPod color: green vs. yellowPod shape: inflated vs. pinchedStem height: tall vs. shortFlower position: axial vs. terminal

3. 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.

Why?Smooth seeds (allele S) are completely dominant to wrinkled seeds (allele s).

Fig. 11.5

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 SS1/2 Ss1/4 ss

F2 phenotypes

3/4 smooth1/4 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:

GeneticsZygoteF1

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)

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

Test-cross: SsYy x ss yy 1/4 + 1/4 + 1/4 + 1/4 (see Table 10.2)