Mendelian Genetics
Reading: Chap. 14
I. IntroA. Motivating question
B. Mendel
II. Mendel’s findingsA. Law of segregation
B. Law of independent assortment
III. Complications
IV. Examples from human genetics
Terms and Concepts- character, trait, alleles
- P, F1, F2
- dominant/recessive
- law of segregation
- law of independent assortment
- homozygous/heterozygous
- phenotype/genotype
- testcross
Rules of probabilityComplications:
- complete, incomplete and co- dominance- multiple alleles- pleiotropy- Epistasis- quantitative characters: polygenic inheritance
Motivating question: Radiation of the Galápagos finches
Beak sizes
Food availability
Range overlap
Probable ancestors
Galápagos Islands
What Darwin knew (and inferred):
Patterns of distribution
Mechanism of natural selectionheritable traits
“struggle for existence”
higher fitness --> more offspring
Shift in average traits in population
What he didn’t know:
How did heritability work?
What exactly was passed down from parents to offspring?
Blending vs. particulate?
No idea about: Genes, chromosomes, DNA, mitosis and meiosis
Fig 22.1Fig 22.1
Gregor Mendel
Austrian contemporary of DarwinPublished shortly after Darwin - but work was “buried”
Who was Mendel?
- Austrian monk
- Background in agriculture (grew up on a farm)
- Failed his teacher’s exam
- University of Vienna: math, causes of variation in plants
- Teaching at the Brünn Modern School
What did he do?
Pea breeding
Testing mechanisms of inheritance
Used many different characters
Published results in 1865
Why did his experiments succeed?
Control over fertilization
Multiple generations: P, F1, F2
True breeding parents
“Either/or”characters
II. What did Mendel find?
A. Law of segregation (of alleles)
B. Law of independent assortment (of traits)
A1. Mendel’s experiments: Simple cross
P - true breeding parents withdifferent traits for same character.
F1 - Cross two of same generation
F2 - evaluate resulting traits: 3 to 1
3 to 1!!!
Did Mendel fudge?
Mendel tested many traits
- one factor from each parent
- dominant vs. recessive
- particulate inheritance: can get pure traits back
A2. Mendel’s interpretation
homozygous vs. heterozygous
Genotype vs. phenotype
When hybrid plants produce gametes, the two parental factors segregate: half the gametes get one type, half get the other type.
3. Law of segregation
All possible combinations, random combinations
4. Rules of probability
- multiplicity
- additivity
OK, prove it! The testcross
Dominant phenotype: what genotype?
Predictions follow from particulate inheritance
5. What do we know now?
Chromosomes, genes, and alleles
P
p
Alleles segregate onthe homologouschromosomes
How does the law of segregation relate to
meiosis?
Fig. 13.6
Homologous chromosomes separate after doubling
Sister chromatids separate
B. Law of independent assortmentWhat about two or more characters? Are they
inherited together or independently?
1. Two traits: an exampleTogether Independent
Law of independent assortment (of characters)
“Independent segregation of each pair of alleles (i.e., genes coding for each character) during gamete formation.”
Rules of probability
Yellow round: YYRR YYRr YyRR YyRr(1/4*1/4) + (1/2*1/4)+(1/2*1/4)+(1/2*1/2)= 9/16
From YyRr x YyRr
Green round:yyRR yyRr(1/4*1/4) + (1/4*1/2) = 3/16
Yellow wrinkled:YYrr Yyrr(1/4*1/4) + (1/2*1/4) = 3/16
Green wrinkled:yyrr (1/4*1/4) = 1/16
2. What we know now:
Mendel’s independent assortment referred to characters.
fig. 13.9
How does this relate to independent assortment of chromosomes in meiosis?
What if genes for two traits are on the same chromosome?
Independent or linked?
Linked, except for…?Crossing over
Depends how close they are: genes further apart are more likely to behave as indpendent.
Mendel got lucky…twice
(not that way - he was a monk!)
1. Genes for traits he studied were either on separate chromosomes, or
2. Far enough apart on the same chromosome that they assorted independently
III. Complications
A. Dominance, Incomplete dominance and Codominance
A1. Incomplete dominance in snapdragon
- Phenotype is intermediate
- NOT blending
Fig. 14.9
A2. Codominance - M, N, MN blood groups
MM
NN
MN
BOTH traits expressed
B. Complications: Multiple alleles
ABO blood groups
fig. 14.10
Dominant
Dominant
Codominant
Recessive
C. Complications: Pleiotropy
- One gene affects many characters
- Sickling allele of hemoglobin
fig. 14.15
D. Complications: Polygenic Inheritance and Quantitative Characters
- One trait determined by multiple genes
- Converse of pleiotropy
- e.g., skin color: at least 3 genes
fig. 14.12
E. Complications: Epistasis
- Expression of one gene depends on another
- Mouse coat color:
B - black coat
b - brown coat
C - pigment
c - no pigment
fig. 14.11
IV. Examples from human genetics
Several excellent examples in the book.
- Simple traits, geneologies
- Genetic disorders (Tay-Sach’s disease, Huntington’s disease, cystic fibrosis, etc.)
Understand how they work, but don’t need to memorize the details of each.
Why might mating between close offspring lead to increased incidence of genetic disorders?
Where do we go from here?Have:
Mechanism for natural selection
Mechanism for heritability
Not yet:
Understanding of meiosis, maintenance of genetic variability
“Molecular carrier” of heritable information
Fig 22.1Fig 22.1
The modern synthesisDarwinMendelPopulation geneticsDNA