Chapter 4; Extensions of Mendelian analysis
"exceptions" to Mendelian genetics, but really not, just extensions
For example, recently there was some new work on the phenomenon of meiotic
drive or segregation distortion. See: Pennisi, E. (2003). "Meiotic
drive. Bickering genes shape evolution." Science 301(5641): 1837-9.
Multiple alleles (more than two alleles for a gene)
ABO blood type (three alleles of ABO gene)
ABO gene ..(Table 4.1, Fig 4.2).....ABOtypes.gif
alleles of the white locus ....Table 4.2. ...X-linked, males
with one allele, females with two alleles
Dominance relationships
(Remember, we determine dominance/recessiveness by the phenotype
of the F-1)
Complete dominance
F-1 phenotype same as one P-generation individual
examples: the seven genes of the garden pea
Incomplete dominance
F-1 phenotype between two P-generation individuals
example: flower color in the snapdragon plant
Codominance
F-1: both alleles expressed in the heterozygote
examples: Sickle cell anemia ...HbA/HbS
ABO blood group Table 4.1, Fig 4.2
modified Mendelian ratios.......(non-allelic interactions)
Most of the names for the different kinds of interactions are not
important for us to learn; know epistasis and complementation.
Examples of modified ratios
genotypes A and B
genotypes B and C..
Pepper color
lethal genes
example..Yellow x Yellow in mice ------------- 2/3 yellow;
1/3 wild
In humans .. Huntington Disease ..Dominant autosomal ..
lethal age 40-50's .. Woody Guthrie
and hemophilia, if untreated
Here is a table which gives an overview of several types of gene
interaction; this table is sililar to the one on page 86 of our
text, but easier to understand.....ModifiedRatiosTable.GIF
Of great importance for later in the course is the discussion of
complementation (page 85). We have the cross of two mutants giving
a wild phenotype
Another complication is pleiotrophy - sickle cell example... Pleiotrophy.gif
pleitrophy: single mutant gene with multiple
phenotypic effects
The environment and gene expression
penetrance and variable expressivity
complete penetrance: seven Mendelian genes, ABO blood group
incomplete penetrance: neurofibromatosis 50-80% penetrant
variable expressivity: also neurofibromatosis from cafe au
lait spots ..see Figure 4.10...also story of Harvard Medical School
...
Porter Colley and Harvard Medical Students story also see text page
89:
examples of environmental influences on gene expression
Age of onset (Huntington disease)
sex
sex limited traits (testicular cancer)
sex influenced traits (baldness)
chemicals ... phenylketonuria
conversion of phenylalanine to tyrosine
Nature versus nurture
Norm of reaction (page 92) ... range of expression of a phenotype
Other exceptions in Chapter 5....Chapter05.html