WEEK 2
Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses
Types of Dominance
• Dominance—the idea that an individual organism
possesses two different alleles for a characteristic
but the trait encoded by only one of the alleles is
observed in the phenotype.
• With dominance, the heterozygote possesses the
same phenotype as one of the homozygotes.
• Dominance can be understood in regard to how
the phenotype of the heterzygote relates to the
phenotypes of the homozygotes.
• Incomplete dominance ~ is exhibited when the
heterozygote has a phenotype intermediate
between the phenotypes of the two
homozygotes.
→ When a trait exhibits incomplete dominance, a
cross between two heterozygotes produces a
1 : 2 : 1 phenotypic ratio in the progeny
→ As long as the heterozygote’s phenotype can
be differentiated and falls within the range of
the two homozygotes, dominance is
incomplete
→ When a trait displays incomplete dominance,
the genotypic ratios and phenotypic ratios of
Some students might ask why the pink flowers illustrated above
the offspring are the same, because each exhibit incomplete dominance—that is, why is this outcome not an
genotype has its own phenotype example of codominance?
• Dominance affects the phenotype that genes The flowers would exhibit codominance only if the heterozygote
produced both red and white pigments, which then combined to
produce but not the way in which genes are produce a pink phenotype. In the example, the heterozygote
inherited. produces only red pigment. The pink phenotype comes about
• Codominance ~in which the phenotype of the because the amount of pigment produced by the heterozygote is
less than the amount produced by the A1; A1 homozyogote. So,
heterozygote is not intermediate between the here, the alleles clearly exhibit incomplete dominance, not
phenotypes of the homozygotes; rather, the codominance.
heterozygote simultaneously expresses the
phenotypes of both homozygotes.
→ An example of codominance is seen in the
MN blood types.
→ The MN locus encodes one of the types of
antigens on red blood cells.
→ Unlike antigens foreign to the ABO and Rh
blood groups (which also encode red-blood-
cell antigens), foreign MN antigens do not
elicit a strong immunological reaction;
therefore, the MN blood types are not
routinely considered in blood transfusions
, Dependency of type of dominance on level of
phenotype observed
→ The type of dominance exhibited by a character
depends on the level of the phenotype examined.
→ This dependency is seen in cystic fibrosis, a
common genetic disorder found in Caucasians
and usually considered to be a recessive disease.
→ People who have cystic fibrosis produce large
quantities of thick, sticky mucus, which plugs up
the airways of the lungs and clogs the ducts
leading from the pancreas to the intestine,
causing frequent respiratory infections and
digestive problems.
→ The gene responsible for cystic fibrosis resides on
the long arm of chromosome 7.
→ It encodes a protein termed cystic fibrosis
transmembrane conductance regulator (CFTR)
which acts as a gate in the cell membrane and
regulates the movement of chloride ions into and
out of the cell.
→ Patients with cystic fibrosis have a mutated,
dysfunctional form of CFTR that causes the
channel to stay closed, and so chloride ions build
up in the cell.
→ This buildup causes the formation of thick mucus
and produces the symptoms of the disease.
→ Most people have two copies of the normal allele
for CFTR and produce only functional CFTR
protein.
→ Those with cystic fibrosis possess two copies of
the mutated CFTR allele and produce only the
defective CFTR protein.
→ Heterozygotes, having one normal and one
defective CFTR allele, produce both functional and
defective CFTR protein.
→ Thus, at the molecular level, the alleles for normal
and defective CFTR are codominant, because both
alleles are expressed in the heterozygote.
Codominance versus Incomplete dominance
→ However, because one functional allele produces
Co-dominance Incomplete dominance enough functional CFTR protein to allow normal
• The phenotype of the • The phenotype of the
heterozygote is intermediate
chloride ion transport, the heterozygote exhibits
heterozygote is a mixture
between the phenotype of the no adverse effects, and the mutated CFTR allele
of the phenotypes of the
homozygotes (i.e. both of two homozygotes appears to be recessive at the physiological level.
the homozygote
• Example: white-haired mouse X → The type of dominance expressed by an allele is a
black mouse = mouse with function of the phenotypic aspect of the allele
phenotypes are present) grey hair
• Example: green lizard X • Red snapdragon X white
that is observed.
red lizard = lizard with snapdragon = pink snapdragon Dominance entails interactions between genes at the
redand green spots • In the heterozygote, one of the same locus (allelic genes) and is an aspect of the
• Both alleles functional in alleles functional and the other
phenotype; dominance does not affect the way in which
heterozygote → two one not → one allele produces
genes are inherited.
different functional proteins a protein and the other one
makes a faulty/truncated The type of dominance exhibited by a characteristic
with slightly different
protein frequently depends on the level of the phenotype
properties are produced
examined
, Penetrance & Expressivity
• For some characters the genotype does not always produce the expected phenotype, a phenomenon
termed incomplete penetrance.
• Incomplete penetrance is seen in human polydactyly, the condition of having extra fingers and toes.
• There are several different forms of human polydactyly, but the trait is usually caused by a dominant
allele.
• Occasionally, people possess the allele for polydactyly (as evidenced by the fact that their children inherit
the polydactyly) but nevertheless have a normal number of fingers and toes.
• In these cases the gene for polydactyly is not fully penetrant.
• Penetrance ~ is the percentage of individual organisms having a particular genotype that express the
expected phenotype.
• Expressivity ~ the degree to which a character is expressed.
• In addition to incomplete penetrance, polydactyly exhibits variable expressivity.
• Some polydactylous persons possess extra fingers and toes that are fully functional, whereas others
possess only a small tag of extra skin.
• Incomplete penetrance and variable expressivity are due to the effects of other genes and to
environmental factors that can alter or completely suppress the effect of a particular gene.
• For example, a gene may encode an enzyme that produces a particular phenotype only within a limited
temperature range.
• At higher or lower temperatures, the enzyme does not function and the phenotype is not expressed; the
allele encoding such an enzyme is therefore penetrant only within a particular temperature range.
• Many characters exhibit incomplete penetrance and variable expressivity; thus the mere presence of a
gene does not guarantee its expression
Lethal Alleles
→ A lethal allele causes death, frequently at an early
developmental stage, and so one or more
genotypes are missing from the progeny of a cross.
→ Lethal alleles modify the ratio of progeny resulting
from a cross.
→ Many lethal alleles in nature are recessive, but
lethal alleles can also be dominant; in this case,
homozygotes and heterozygotes for the allele die.
→ Truly dominant lethal alleles cannot be transmitted
unless they are expressed after the onset of
reproduction, as in Huntington disease
→ Example of mice: Extensive deletion: regulatory
region of 𝐴𝑌 gene (yellow colour) plus coding
region of upstream Merc gene (maternally
expressed RNA binding protein); effectively
removes Merc and puts 𝐴𝑌 under control of Merc
promoter
→ Lethal alleles are mutant allele that eliminates an
essential function –kills the organism
→ Many are developmental mutations
→ Most are recessive because:
1. They would be immediately eliminated from the
population if they were dominant
2. Because they encode abnormal (non-
functional) versions of the proteins
