A. Gregor Mendel and his pea plants

1. started ca. 1857
2. true-breeding strains
3. easy to control fertilization
4. phenotypes he studied
a. yellow (Y) vs. green (y) seeds
b. round (R) vs. wrinkled (r) seeds
c. tall (T) vs. dwarf plants (t)
d. purple (P) vs. white (p) flowers
5. scientific method
6. blending hypothesis
7. monohybrid cross
8. P, F₁, and F₂ generations
9. example:
a. P - white X purple
b. F₁ - all purple
c. F₂ - 705 purple, 224 white (ca. 3:1)
10. did this with many plants
11. conclusions
a. white "factor" must be present in F₁ individuals
b. but purple is "dominant" over it
c. white gets expressed in F₂ when no purple "factor" is present

B. Terminology

1. phenotype
2. genotype
3. homozygous
4. heterozygous
5. dominant
6. recessive
7. gene
8. locus
9. allele
10. segregation of homologous chromosomes (and their respective alleles) during meiosis
11. independent assortment (2ⁿ)

C. Punnet squares

D. Dihybrid cross

E. Multiple alleles

1. some genes have more than two alleles
2. example: ABO blood group
a. allele: I^A, I^B
b. codominance
c. possible genotypes
d. possible phenotypes

F. Sex linkage

1. Human males have only one X chromosome
2. So, alleles on a male's X chromosomes are always expressed, regardless of whether the are dominant or recessive
3. Human females have two X chromosomes, and alleles function as they do for autosomes.
4. example: hemophilia
a. alleles: X^H, X^h
b. possible genotypes
c. possible phenotypes

G. Incomplete dominance

1. sometimes one allele is not completely dominant

H. Why are some alleles dominant and others recessive?

1. best answered by examining products of specific alleles
2. example Tay-Sachs disease
a. organismal level - complete dominance
b. biochemical level - incomplete dominance
c. molecular level - codominance
3. recessive does not equal rare
4. recessive does not equal bad

I. Alternative to Punnett squares -- Rules of Probability

1. rule of multiplication
a. example: Aa x Aa; what is the probability of having an aa offspring?
2. rule of addition
a. example: Aa x Aa; what is the probability of having an Aa offspring?

J. Pleiotropy - one gene affecting more than one characteristic of an organism

1. example - sickle cell anemia

K. Epistasis - one gene affecting the expression of another gene

1. example - fur color in mice
a. one gene - brown vs. black
b. another gene - pigment deposited in fur or not
c. possible phenotypes - black, brown, white

L. Polygenic inheritance - many genes affecting one trait

1. examples - height, skin color
2. quantitative characters

M. The genotype plus environmental conditions determine the phenotype.

1. examples - height, weight, skin color
2. multifactoral disorders
a. examples - heart disease, cancer

N. Pedigrees

1. useful in genetic counseling
2. useful in conjunction with molecular techniques for locating genes
a. example - Huntington's disease
- caused by a dominant allele
- located on chromosome 4
- individuals can now be tested for presence of this allele

O. Evaluating genetic problems

1. carrier recognition
a. examples - Tay-Sachs, sickle-cell
2. amniocentesis
a. not done until week 14-16
b. wait several weeks for results
c. karyotype
3. chronic villus sampling (CVS)
a. can be done as early as week 8-10
b. results within 24 hours
c. karyotype
4. examining fetus
a. ultrasound
b. fetoscopy
5. newborn screening
a. example - phenylketonvria (PKU)
6. ethics

P. Alternations of chromosome structure

1. deletion - segment of chromosome is lost
2. duplication - segment of chromosome is duplicated
3. inversion - segment of chromosome is inserted in reverse order
4. translocation - segment of a chromosome is added to a nonhomologous chromosome
a. reciprocal
b. nonreciprocal

Q. T. H. Morgan and his fruit flies (Drosophila)

1. early 1900's
2. first described sex linkage
3. gene linkage (example)
a. body color - grey (G), black (g)
b. wings - normal (W), vestigial (w)
c. GgWw x ggww
d. possible gametes
e. Punnett square
f. looked at 2300 offspring
g. expected ratio
- 575 GgWw
- 575 ggww
- 575 Ggww
- 575 ggWw
h. observed ratio
- 965 GgWw
- 944 ggww
- 206 Ggww
- 185 ggWw
i. most offspring were like the parents
j. explanation - the body color and wing genes are linked (on same chromosome)
k. why wasn't the observed ratio 1150 GgWw : 1150 ggww?

R. Gene linkage and mapping of a chromosome's genes

1. parental types vs. recombinants
2. recombination frequency

a. number of recombinants / total offspring
b. example - (206 + 185) / 2300 - 17%
3. recombination frequency for two genes indicates how close together they are on a chromosome
4. this information can be used to determine the relative positions of genes on a chromosome
5. 1 map unit was defined to equal 1% recombination frequency
6. recombination frequency of 50% is the maximum expected (genes are on separate chromosomes or very far apart on same chromosome)
7. mapping example - three genes (A, B, C)
a. A-B : 17 map units
b. B-C : 9 map units
c. A-C : 8 map units
d. sequence of genes : A-C-B