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.

The idea: sequence of graphics:  ModifiedRatios.html

 

Examples of modified ratios

agouti/non agouti...... hairs of wild and mutant

genotypes A and B

........AgoutiBlack.GIF

genotypes B and C..

........BlackAlbino.GIF

biochemical pathway for C... AlbinoPathway.GIF

Pepper color


       


lethal genes

example..Yellow x Yellow in mice ------------- 2/3 yellow; 1/3 wild

explanation. LethalYellow.GIF 

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