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WEB TUTORIAL 4.1
Extensions of
Mendelian Inheritance
OVERVIEW
Mendel's pea plant studies used single gene traits involving two alleles inherited in a
dominant or recessive manner. However, when gene expression does not adhere to this
simple mode of inheritance or when multiple genes influence the expression of a single
trait, the observed phenotypic ratios can differ from those predicted by Mendel. This tutorial examines some alternative modes of inheritance and shows how they can modify classic Mendelian ratios.
TEXTBOOK REFERENCES
In Incomplete Dominance, Neither Allele Is Dominant (pp. 68-69)
In Codominance, the Influence of Both Alleles in a Heterozygote Is Clearly Evident (p. 69)
Incomplete Dominance
Illustrated at left are two snapdragons from two strains true breeding for red and white
flowers. What phenotype do you expect when the two are crossed? If the relationship
between the R1 and R2 alleles is one of dominance and recessiveness, you would expect
the F1 progeny to have a single flower color phenotype, either red or white, depending on
which allele is dominant and which is recessive.
As expected, all of the progeny have one flower color phenotype. However, unlike what
one would predict from Mendel's second postulate, the flowers are neither red nor white,
but pink. This is an example of incomplete dominance.
In incomplete dominance, heterozygotes display a novel phenotype that is not clearly that
of either homozygous phenotype. In this example, the phenotype is a result of the "blending" of gene products from both alleles.
What ratio of phenotypes would you expect in the F2 progeny?
The F2 progeny confirm that snapdragon flower color is determined by a single pair of
alleles. The F2 genotypic ratio is that expected from a monohybrid cross, 1:2:1. However,
because the alleles for flower color are incompletely dominant, the heterozygous phenotype differs from both homozygous phenotypes. Thus, the phenotypic ratio is the same as
the genotypic ratio, rather than the 3:1 ratio one would expect in a monohybrid cross
involving dominant and recessive alleles.
Codominance
If a heterozygous individual expresses both alleles in its phenotype, the mode of inheritance is called codominant.
The MN blood group system of humans is an example of this mode of inheritance. A glycoprotein on the surface of red blood cells functions as an antigen, providing a simple level
of immunological identity. In humans, the two common forms of this glycoprotein, M and
N, are expressed by the autosomal alleles LM and LN. An individual with both alleles will
have the combined phenotype of MN.
What ratios of genotypes and phenotypes would you expect from this cross of codominant
heterozygotes?
The genotypic ratio is that expected from a monohybrid cross, 1:2:1. In codominance, the
phenotypic ratio is the same as the genotypic ratio, rather than the 3:1 ratio one would
expect in a monohybrid cross involving dominant and recessive alleles.
The reason both phenotypes are expressed in a codominant heterozygote can be found at
the level of gene expression. In heterozygotes, both alleles are transcribed and translated,
resulting in both glycoproteins being present on the cell surface and producing the MN
phenotype.
ABO Blood Type and the Bombay Phenotype
ABO Blood Type
The pedigree at left depicts the ABO blood types of three generations of a family. Note the
female in the second generation with type O blood. Why is her phenotype unexpected?
Here we will examine the genetic and biochemical basis of the ABO blood types and of this
anomalous phenotype, known as the Bombay phenotype.
The ABO blood types are based on the presence or absence of specific sugar residues on
the surface of red blood cells.
Almost all individuals possess molecules called the H substance on their red blood cells.
Individuals with the IA allele express an enzyme that attaches the sugar N-acetylgalactosamine (AcGalNH) to the H substance.
The addition of the terminal AcGalNH on the H substance produces the A antigen. This
antigen on the surface of red blood cells is the basis of the type A phenotype.
Alternatively, individuals with the IB allele express an enzyme that attaches the sugar
galactose (Gal) to the H substance. The presence of a terminal Gal results in the B antigen
on red blood cells and the type B phenotype.
Individuals with the IO allele have a mutation in the I gene that results in a nonfunctional
protein. Therefore, individuals homozygous for the IO allele lack an enzyme to modify the
H substance. These individuals have only the H substance (without a terminal sugar
added) on their red blood cells and test positive for neither A nor B antigens. They are
identified as blood type O.
The Bombay Phenotype
The formation of the H substance itself requires another gene called H. The enzyme produced by the H gene adds a sugar residue (fucose) to the H substance precursor, forming
the complete H substance.
Most people have the dominant H allele of this gene, which codes for a functional enzyme.
However, some people have the recessive h allele, which does not code for a functional
enzyme. Therefore, people who are homozygous hh cannot produce the H substance.
Consider a scenario in which an individual carries an IA allele or IB allele of the I gene. If
this individual is genotypically hh, then he or she will not be able to produce the H substance and therefore will not be able to produce either the A or B antigen. The formation
of A and B antigens requires the H substance as a precursor.
The woman with type O blood in the pedigree has the rare Bombay phenotype as a result
of being homozygous for the h allele. Study of the pedigree reveals that she is either type
AB or type B with respect to alleles for catalyzing the formation of A and B antigens.
However, because she lacks the enzyme to synthesize the H substance from its precursor,
the A and B alleles are not able to function. As a result, she is type O in phenotype but IAIB
or IBIO in genotype.
Gene Interaction
Phenotypes are often the result of more than one gene, as is the case for fruit shape and
color in the summer squash Cucurbita pepo.
The fruit phenotype is the result of alleles at two unlinked genes. If the dominant alleles
(one or two copies) of both loci are present, the fruit is disc shaped. The presence of a dominant allele for only one of the loci results in the spherical phenotype. The long phenotype
results from all recessive alleles at both loci.
The grid depicts a Mendelian F1 cross of two squash plants with disc fruit phenotype.
What phenotypes and phenotypic ratios do you expect? The ratios are a variation of the
expected 9:3:3:1 ratio.
Because the two genes for squash fruit shape are not linked, the alleles assort independently, producing the expected dihybrid genotypic ratios. However, interaction among alleles produces a modified dihybrid phenotypic ratio of 9:6:1.
Other types of modified dihybrid ratios occur for other traits. For example, fruit color
involves alleles at two different loci. The presence of a dominant allele at one locus pro-
duces white fruit, regardless of the genotype at the other locus. That is, the second locus
is epistatic to the first locus. If the first locus is homozygous recessive and the second has
the dominant allele, the fruits are yellow. Green fruits occur only when both loci are
homozygous recessive. The modified dihybrid ratio involving squash fruit color is 12:3:1.
CONCLUSION
The study of transmission genetics has expanded to include many alternative modes of
inheritance that may not conform to classic Mendelian ratios. For traits that exhibit incomplete dominance or codominance, heterozygotes display a phenotype that is distinct from
either homozygous phenotype. In many cases, phenotypes may be influenced by multiple
genes in a variety of ways. A population can also have multiple alleles of the same gene
that influence the phenotype in various ways. An allele of one gene may mask the expression of alleles at another locus, a phenomenon known as epistasis. Gene epistasis or gene
interactions can result in novel phenotypes and modified dihybrid phenotypic ratios.
YOU
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SHOULD NOW BE ABLE TO
Contrast the F2 phenotypic ratios of traits that exhibit incomplete dominance and
codominance with the expected Mendelian ratios.
Explain the basis for the Bombay phenotype.
Describe the modes of inheritance of the ABO blood group system.
Explain how gene interactions can influence a particular characteristic.
Give examples of the different variations of Mendelian inheritance.
KEY TERMS
Bombay phenotype
codominance
epistasis
gene interaction
incomplete dominance