Extensions of Mendelian Genetics
of a gene are called
that occurs most frequently in nature is called the
A pea combed chicken crossed to a rose combed rooster produces F that are all walnut combed. The F phenotype ratios are 9 walnuts, 3 roses, 3 peas, and 1 single comb, indicating that two genes control chicken comb shape.
Each gene is found at a region on a chromosome called its locus.
(change in the DNA) that causes the reduction or loss of a wild-type function is called a loss of
mutation. If the loss is complete, the allele that results in a
Some loci exhibit
where neither allele is dominant.
Incomplete Dominance Neither allele in snapdragon flower color is dominant. The F generation exhibits an intermediate phenotype. The genotypic ratio (1:2:1) of the F generation is identical to that of Mendel's monohybrid cross, but the phenotypic ratio is identical to the genotypic ratio. These phenotypes are due to the production of a red pigment by the R enzyme; the R^2 allele produces no pigment; a heterozygote produces half as much pigments as the homozygous R^1 and appears pink.
generation, 3/4 of the plants exhibit the same (dominant) trait as the F generation, and 1/4 exhibit the contrasting trait that disappeared in the F generation. To explain these results, Mendel proposed the existence of particulate unit factors (genes) for each characteristic (phenotype). The two contrasting traits of the same gene are called alleles. An individual possessing two copies of the same allele is homozygous. An individual possessing different copies of an allele is heterozygous. The F genotype ratio is 1:2:1; while the phenotype ratio is 3:1.
Some genes exhibit
, where both alleles contribute to the phenotype of a
. An example is the
Some traits are controlled by
alleles. Examples include the human
The I^A and I^B alleles are codominant and affected by multiple alleles. Blood type is one tool for excluding parenthood of individuals. For example, a type O person should not have a parent who is type AB.
Eye color in Drosophila is controlled by over 100 alleles. In this locus, eye color ranges from complete absence of pigment in the white (w) allele to a buff color in the white-buff (W^bf) allele, in which the amount of pigment in the eyes is reduced to less than 20 percent of that found in the red wild-type eye.
A loss of
mutation may behave as a recessive
state, but exhibit a separate
in a heterozygote.
The yellow allele (A^Y) is a recessive lethal in the homozygous state sonce the A^Y/A^Y genotype does not survive. But it is dominant to the wild type agouti allele (A) in producing yellow coat color in a heterozygote.
Genes present on the
exhibit unique patterns of inheritance due to the presence of only one X chromosome in
In Drosophila, the wild-type red eye color is dominant to white eyes. Reciprocal crosses between white-eyed and red-eyed flies yields different results. The results indicate the white (w) locus is X-linked (present on the X chromosome). Males carry only one allele for X-linked genes and are called hemizygous.
X-linkage exhibits a crisscross pattern of inheritance. Recessive X-linked alleles such as color blindness are passed from homozygous mothers to all sons. Heterozygous females are carriers and pass the allele to half of their sons, who develop the disorder. Affected fathers pass the allele to his daughters, who are usually carriers.
Some phenotypes are affected by sex
and can be either
if the phenotype is restricted to one sex, or
if restriction is not absolute.
Phenotypic expression may also be influenced by the
as well as by genotype and result in
of the genotype.
Variable expressivity in Drosophila. Environmental factors may affect gene expression. Expressivity measures the range of expression of the mutation. Homozygous recessive eyeless flies show phenotypes that range from normal eyes to a partial reduction in size to the complete absence of eyes.
One component of the
, such as the position
In the Drosophila white locus, the w^+/w genotype normally results in a wild-type red eyes. However, if the wild-type w^+ allele is translocated to a heterochromatic region (where gene expression is often inhibited), the eyes are mottled with red and white patches (variegated), reflecting intermittent expression of the dominant w^+ allele due to a position effect.
Another component of the
and many phenotypes show temperature sensitivity.
Many chemical reactions are affected by the temperature, which can then influence phenotype. Siamese cats and Himalayan rabbits exhibit dark fur in regions where the body temperature is cooler; probably because the pigment-producing enzyme is temperature-sensitive, being more active at the lower temperatures in the extremities.
In cases of
, phenotypic expression may depend on the
origin of the chromosome.
Genomic Imprinting The mouse Igf2 gene produces a growth factor, and homozygous mutants are dwarf mice. Heterozygotes that receive the normal allele from their father are normal in size. Heterozygotes that receive the normal allele from their mother, which has been imprinted, are dwarf. The normal Igf2 gene is imprinted to function poorly during the course of egg production, but functions normally when it has passed through sperm-producing tissue in males. This process may involve DNA methylation.