CHAPTER
14
If
one gene influences more than one phenotype it is referred to as pleiotropy.
Pleiotropy can make symbolizing alleles difficult
because the allele may act as a dominant for some phenotypic traits and a
recessive for others.
There
are other situations where more than one gene influence
a single phenotype.
When
two separate recessive mutant forms are found affecting the same phenotype
geneticists attempt to determine whether these mutations represent two alleles
for the same gene or two different genes. To establish this they perform a
complementation test. This involves crossing true breeding two parents, one for
each mutant form, thereby creating an F1 possessing both mutant messages. If the
F1 has the wild type phenotype, the mutant messages are said to complement each
other and they are considered messages on two different genes. If they do not
complement each other they are alleles for the same gene.
The
cell structure of many fungi is multinucleated haploids. If two different nuclei
are contained in the cells of these organisms (these are created by fusion of
fungal cells), they are referred to as heterokaryons.
Heterokaryons can be used for complementation tests.
Interaction
between alleles of one gene
Incomplete
dominance- the phenotype of a heterozygote is intermediate between two homozygotes
on some quantitative measurement scale. An example of incomplete dominance is
flower color in snapdragons. F2 genotypic ratios are 1:2:1,
and phenotypic ratios are 1:2:1
Codominance-
The heterozygote expresses aspects of the phenotypes of each of the homozygotes.An
example of codominance is the ABO blood group system
in humans. Genotypic and phenotypic ratios identical to
incomplete dominance.
Lethal
Alleles- Some alleles act as recessive lethals. In
such cases homozygous recessive individuals may be conceived, but typically are
not born. In mice, yellow coat color is controlled by an allele that acts as a
dominant on coat color, but a recessive on viability (pleiotropy).
Crossing 2 yellow heterozygotes produces offspring
in a 2:1 ratio of yellow to agouti. In some other examples the recessive
phenotype is often less viable than the dominant phenotype, this is referred to
as semi-lethal.
Interacting
genes
If
an allele of one gene masks the expression of another gene it is referred to as epistasis.
Epistasis is inferred when an allele of one gene
eliminates expression of alleles of another gene and expresses its own phenotype
instead. It points to interaction in genes in some biochemical or developmental
sequence. E.g., coat color in
A
suppressor is an allele that reverses the effect of a mutation of another gene,
resulting in the wild type phenotype. Because a suppressor cancels the
expression of a mutant allele and restores the corresponding wild type
phenotype, the modified dihybrid ratio involves only
two phenotypes whereas an epistatic allele
introduces a third phenotype into the ratio. Note, we
are referring to multiple genes affecting a single character.
Nonsense
suppressors revert nonsense mutations by making
compensatory mutations to anticodon coding regions
which prevent premature termination of translation.
Suppression
may also result from post-translational interactions.
The
expression of alleles may be influenced by environmental conditions. E.g.,
temperature sensitive coloration of Himalayan rabbits and Siamese cats.
Not
all individuals who have a genotype that should translate into a mutant
phenotype express that phenotype. This may be in part due to their overall
genotypic makeup or environmental effects. The percentage of the population
having the appropriate genotype that expresses the mutant phenotype is referred
to as its penetrance.
The
actual phenotypic expression of the mutant phenotype may be variable. This is
referred to as expressivity.