biol1110_gene_trait_determination 6/26/07 Page 1 of 2
BIOL 1110 REEDER
GENE TRAIT DETERMINATION
I. Gene Trait Determination
A. Categories of Gene-Trait Relationships
1. Simple: a single gene pair may regulate the appearance of a single trait in a direct, precise,
and exact way
2. Complex: one gene pair may participate in the control of several traits, or many gene pairs
cooperate to regulate the appearance of a single trait
* a. Most genes probably have many multiple effects: pleiotropy
1) Evident in many genetic diseases, as in sickle-cell anemia, which may show a
combination of symptoms:
a) heart failure: due to having to pump harder than normal the sickle-shaped RBC's
b) anemia: due to the sickled cells breaking as they're caught in narrow blood
vessels
c) thickening of the blood due to the sickled cells with potential clumping resulting
in:
(1) Paralysis if clumps in the brain
(2) Rheumatism if clumps in the muscles
(3) Pneumonia if clumps in the lungs
2) Albinism have multiple effects:
a) lack of pigment (melanin) in the skin, hair, and eyes, plus crossed eyes
b. When one gene pair can mask or cover up the effect of a different gene pair: epistasis:
several pairs of alleles may interact to affect a single trait, or one pair may inhibit or
reverse the effect of another pair
1) Fur or skin color in mammals:
a) coat color of Labrador retrievers: dogs homozygous for the dominant B allele of
the black-coat gene have pure black color, while dog homozygous for the
recessive b allele have a soft chocolate colored coat. Bb animals are black
(1) Extension genes E and C: homozygous ee dogs are yellow no matter which
alleles they have at the B locus; E allele allows the B to show through.
b) Albinism involves the extension C gene locus: another allele pair affecting normal
melanin production besides the major ones
(1) Alleles at this gene locus code for tyrosinase, the first enzyme required in the
series of reactions by which melanin is produced.
(2) The C allele is dominant to c: melanin can be produced in a CC or Cc
individual, but is blocked in cc.
c) 12 pairs of alleles interact to produce coat color in rabbits
2) 100 pairs are concerned with eye color and shape in fruit flies
3) Bombay phenotype: result of two interacting genes (I gene and H gene)
a) I gene directly controls the ABO blood type
b) As long as at least one H allele is present, the ABO blood type associated with the
person's ABO gene type is expressed
c) A person with genotype hh always has blood type O, although the ABO genotype
can be any of the six possibilities
c. When two or more independent pairs of genes have similar and additive effects on the
same characteristic: polygenic (height, body form, skin color, milk and egg production,
ability to see distances, limb length, facial features,intelligence, clear or acne skin,
susceptibility to cancers, diabetes, allergies, heart attacks, and Alzheimer disease);
