Click HERE for your print-friendly copy. (If you have already printed lecture 7, then you already have this.)

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MULTIPLE ALLELES

Example: coat color in mammals as shown above for gene "C"

In this case...
agouti (C) > chinchilla (silver) c^ch > Himalayan (c^h) > albino (c^a)


Another Example: white chevron pattern in Clover (Trifolium sp.).

LETHAL ALLELES

• dwarfing gene in rabbits
• yellow coat color in mice
• manx allele in cats
• Duchenne Muscular Dystrophy
• Tay Sachs disease
• cystic fibrosis

GENE INTERACTIONS THAT PRODUCE POLYMORPHISMS IN WILD POPULATIONS

Let's have a look at skin color in Corn snakes (Elaphe guttata). Two pigments are involved, each manufactured by a different set of enzymes.

• Orange (carotenoid) pigment is produced by the o+ allele. The recessive mutation is o, and it encodes a non-functional enzyme in the orange pigment pathway. Hence, no orange pigment is made by an oo snake.
• Black/brown (melanin) pigment is encoded by the b+ allele. Recessive mutation b encodes a non-functional enzyme in the melanin pathway. Hence, no brown pigment is made by a bb snake.

  But you get unexpected phenotypes, even though the typical dihybrid genotype ratios are found in the offpring of a dihybrid cross:

  • o+_ b+_ - wild type
  • oo b+_ - grey
  • o+_ bb - orange
  • oo bb - albino

PENETRANCE AND EXPRESSIVITY

• if dominant, in either the homozygous or heterozygous condition
  
• if recessive, only if present in homozygous condition

THIS IS NOT ALWAYS THE CASE

PENETRANCE

is the proportion of individuals with a specific genotype who manifest that genotype at the phenotypic level. (Some individuals may not express a gene if modifiers, epistatic genes or suppressors are also present in the genome, which thwart expression.

Penetrance = 1.0 (100%) when all homozygous recessive individuals express the recessive form of the allele, and all homozygous dominant individuals and heterozygous individuals express the alternate form of the allele.

Penetrance < 1.0 if *not* all homozygous recessive individuals express the recessive allele.

EXAMPLES:

• Brachydactyly (short digits) in humans
  
• Neurofibromatosis in humans (disease causes tumorlike growths to appear all over the surface of the body, if it is fully expressed.

EXPRESSIVITY

is the degree to which a particular genotype is expressed in the phenotype of a particular individual. (That is, phenotype may be altered by heterogeneity of other genes which affect the expression of the particular locus in question, or by environmental influence.)

EXAMPLES:
Neurofibromatosis (as described above): This can vary in expressivity from a few "cafe au lait" spots on the skin, to actual tumorlike growths known as neurofibromas which may cover the skin, to severe cases with major skeletal abnormalities.

(NOTE: Although the made famous by the David Lynch movie, "The Elephant Man," Joseph Merrick, the protagonist of that story, did not have neurofibromatosis. In fact, medical scientists today are not exactly certain what caused his bizarre phenotype.

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Another example of variable expressivity:
Piebald spotting in beagles (and some other mammals): dogs who have the same genotype for the piebald locus often express various patterns and total coverage of white fur patches.


Thus, PENETRANCE describes what happens in a population, and EXPRESSIVITY describes what happens in a particular individual.

AGE DEPENDENT EXPRESSION

Examples:

Huntington's disease (age usu. 30 or over)
Male pattern baldness (age usu. 20 or over)
Duchenne muscular dystrophy (age 2-5)

SEX-DEPENDENT EXPRESSION

In sex-influenced traits, expression varies depending on the sex of the individual carrying the alleles.

e.g. - finger length (we already did this one)

In other cases, autosomal genes cause the expression of a trait ONLY in one gender or the other (sex-limited traits).

Examples of sex-limited traits:


• horns in some mammals
  
• lactation in female mammals
  
• ovaries in females; testes in males
  
• various secondary sex characteristics

PHENOCOPIES

Presence of rubella virus during the first 12 weeks of pregnancy in humans can mimic the phenotypic effects of certain rare, recessive alleles (at several different loci) which can cause deafness, cataracts and defects in cardiac development.

Thalidomide (a drug administered in the 1950's to woman at risk of miscarriage) mimicked a rare mutation causing a disorder known as phocomelia--failure of the long bones of the limbs to develop. (Note that thalidomide did NOT produce a phenocopy effect in the test mammal, Rattus norvegicus!)

The formation of birth defects due to environmental agents (viruses, chemicals, etc.) is known as TERATOGENESIS.

EPIGENESIS

Still largely unexplained, but possibly due to parental imprinting of the DNA. This phenomenon occurs when phenotypic alterations occur without change to the DNA sequence, and yet these phenotypic changes are heritable!
EXAMPLE: Pigmentation in Biston betularia.
If you missed class, you missed the campfire story told by Dr. Krempels about why you just can't always trust dogma.

Which is more important in the formation of the organism, Nature (genotype) or Nurture (environment)?

It seems (at least so far) that the genotype "sets the limits" for a particular organism's phenotypic potential. The environment works on the plasticity of expression to produce different phenotypes from similar genotypes. This is evident even in identical twins.