CHAPTER 4: Extensions of Mendelian Genetic
Analysis
I. Multiple alleles
A. ABO system
1. A (IA) and B (IB) are codominant
2. O (i) is recessive
3. Diploid organisms have two alleles
a. May be the same (homozygous)
b. May be different (heterozygous)
B. Distinguishing multiple
alleles from multiple genes
1. Do all pairwise crosses
2. If F2 ONLY has parental phenotypes in expected Mendelian ratios, multiple
alleles
3. If additional phenotypes or non-Mendelian ratios, multiple gene loci
II. Incomplete dominance and codominance
A. Incomplete Dominance
1. Heterozygote has intermediate phenotype
2. Red x white produces pink offspring
B. Codominance "semidominance"
1. Heterozygote expresses both traits
2. AB blood type, red x white produces red and white striped flowers
C. Biochemical basis
for expression
1. Mutation results in no production of an enzyme
a. Heterozygote: only half the product is produced (for instance, a pigment)
- incomplete dom.
2. Mutation results in production of a different protein
a. Heterozygote will express two different proteins - codominance
III. Modified Mendelian ratios
A. Epistasis (see handout)
1. Interaction of two genes - seen in F2 of dihybrid cross
2. One gene may completely mask another gene
3. Often get F2 offspring of a 'new' phenotype
B. 9:7 ratio - both
genes must have dominant allele for expression
1. Peas have two genes for flower color
2. If either gene is homozygous for white, petals are white
3. Cross of two different pure breeding white lines
4. F1 is all purple
5. F2 is 9 purple: 7 white
6. 9:7 indicates BOTH genes must have at least one dominant allele for
expression
7. Known as complementation
a. Production of wild type phenotype when two genotypes determining recessive
phenotypes
come together in the same cell
b. Must cross two lines of similar phenotype to determine if mutation is
in same gene or if
complementary genes
C. 9:4:3
1. Expression of A gene is partial the expression of B gene.
A
B
2. White --------> yellow --------> green
3. If A makes yellow, B makes green
4. A-B- 9 green, aaB- and aabb 4 white; A-bb 3 yellow
D. Duplicate genes
15:1 ratio
1. Require one dominant allele in either of two genes
E. 9:6:1
1. Dosage affect; if A and B, more than A or B, more than aabb
F. Suppression: 13:3
(or sometimes 9 mutant:7 wild type)
1. Eliminates the expression of an allele of a completely different gene
2. Usually allows dominant expression of a homozygous recessive genotype
at another gene
3. Suppressor cancels the expression of an abnormal gene
4. Recognized because only TWO phenotypes are possible - normal and abnormal
G. Dominant epistasis
12:3:1
1. Two genes; dominant phenotype if EITHER is present
2. Example of foxglove
IV. Lethal genes
A. Classification of
mutations
1. Visible - alter some aspect of morphology
2. Sterile - limit reproduction
a. If dominant, eliminated in one generation. Why?
b. If recessive, hidden in heterozygotes
3. Lethal - interfere with necessary vital function
B. Recessive lethal
genes
1. Heterozygote may have no phenotypic expression
2. May be seen in heterozygote
a. Manx cat - spinal column development - no tail
b. Mouse coat color
3. Time of expression
a. May be early in development
b. Childhood: Duchenne muscular dystrophy, cystic fibrosis, PKU
C. Dominant lethal
genes
1. Huntington's disease
2. Definition - shortened lifespan, but MAY live to reproduce
D. Semilethal genes
1. Reduced viability, but class is not completely absent
2. Mendelian ratios are skewed, but no group is totally absent
3. e.g. Aa x aa: yield 55% Aa and 45% aa
V. Nature vs. nurture
A. Temperature sensitive
mutations
1. Heat shock - temp. sens. lethal
a. Drosophila
b. Lots of other organisms
c. Peptide that binds to DNA; affects transcription of genes
B. Gender
1. Pattern baldness - testosterone levels
2. PKU - buildup of phenylalanine
a. Missing enzyme in phenylalanine metabolism
b. Mental retardation
c. Eliminate phe from diet; reduce effects
C. Pleiotropic effects
1. Gene that has more than one phenotypic expression
2. May be connected, or may be unrelated
3. Waardenburg syndrome: deafness AND lock of white hair on forehead
VI. Phenotype = genotype + internal environment
+ external environment
A. Penetrance
1. Percentage of individuals of given genotype who express the phenotype
2. Affected by:
a. Modifiers
b. Suppressors
c. Epistasis
d. Environmental effects
e. May have gene and not express it
f. Some traits are 100% penetrant
g. May have to do with environment of other genes in body
3. Used to explain circumstances that have not yet been determined to influence
expression of a
gene
4. Complete or incomplete penetrance
5. Study of identical twins
B. Expressivity
1. Extent to which a genotype is expressed at phenotypic level
a. Environmental factors
b. Heterogeneity of other alleles in organism
2. Variable expressivity
a. Not all get the disease to same degree
b. Diabetes
(1) Type I or type II
(2) Age of onset
(3) Severity
3. Involves factors outside the gene causing the trait
a. Environment (external)
(1) diet
(2) lifestyle
b. Environment (internal)
(1) gene expression in context of all the other genes of an individual
