Classical Genetics Mendelian inheritance describes inheritance patterns that obey two laws Law of segregation Law of independent assortment Simple Mendelian inheritance involves A single gene with two different alleles Alleles display a simple dominant/recessive relationship
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Classical Genetics Mendelian inheritance describes inheritance patterns that obey two laws Law of segregation Law of independent assortment Simple Mendelian.
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Classical Genetics
Mendelian inheritance describes inheritance patterns that obey two laws Law of segregation Law of independent assortment
Simple Mendelian inheritance involves A single gene with two different alleles Alleles display a simple dominant/recessive
relationship
Prevalent alleles in a population are termed wild-type alleles
These typically encode proteins that Function normally Are made in the right amounts
Alleles that have been altered by mutation are termed mutant alleles
These tend to be less common in natural populations They are likely to cause a reduction in the amount or
function of the encoded protein Such mutant alleles are often inherited in a recessive fashion
A particular gene variant is not usually considered an allele of a given gene unless it is present in at least 1% of the population.
Rare gene variants (<1%) are termed polymorphisms rather than allelic variants
heterozygote exhibits a phenotype intermediate to the homozygotes
Also called intermediate dominance or dosage effect
Example: Flower color in the four o’clock plant governed
by 2 alleles CR = wild-type allele for red flower color CW = allele for white flower color
Incomplete Dominance
Figure 4.2
1:2:1 phenotypic ratio NOT the 3:1 ratio observed in simple Mendelian
inheritance
In this case, 50% of the CR protein is not sufficient to produce the red phenotype
complete or incomplete dominance can depend on level of examination
Incomplete Dominance
Alleles of white – X-linked eye color gene in Drosophila W – red (wildtype gene) w - white we - eosin
we allele was expressed with different intensity in the two sexes Homozygous females eosin Males light-eosin
Gene Dosage – A form of intermediate dominance
eosin ♀ and eosin ♂ phenotypes
Morgan & Bridges hypothesized that difference in intensity was due to the difference in number of X chromosomes Female has two copies of the “eosin color
producer” allele Eyes will contain more color
Males have only one copy of the allele Eyes will be paler
This is an example of gene dosage effect
Gene Dosage
The term multiple alleles is used to describe situations when three or more different alleles of a gene exist
Examples: ABO blood Coat color in many species Eye color in Drosophila
Multiple Alleles
ABO blood phenotype is determined by multiple alleles
ABO type result of antigen on surface of RBCs Antigen A, which is controlled by allele IA Antigen B, which is controlled by allele IB
Antigen O, which is controlled by allele i
Multiple Alleles
N-acetyl-galactosamine
Alleles IA and IB are codominant They both encode functional enzymes and
are simultaneously expressed in a heterozygous individual
Allele i is recessive to both IA and IB
Co-dominance
coat color in rabbits C (full coat color) cch (chinchilla pattern of coat color)
Partial defect in pigmentation ch (himalayan pattern of coat color)
Pigmentation in only certain parts of the body c (albino)
Lack of pigmentation INSERT Figure 4.4
Multiple Alleles
Multiple Alleles
Dominance hierarchy will exist for multiple alleles called an allelic series allelic series for ABO type
IA = IB > i allelic series for rabbit coat color alleles :
C > cch > ch > c allelic series for alleles of white gene
W+/_ > we/we > we/w > w/w = w/Y
The ch allele is a temperature-sensitive conditional mutant The enzyme is only functional at low
temperatures Therefore, dark fur will only occur in cooler areas
of the body
Conditional Mutations
Overdominance is the phenomenon in which a heterozygote is more vigorous than both of the corresponding homozygotes
Example: Sickle-cell heterozygotes are resistant to malaria increased disease resistance in plant hybrids
Overdominance
In some instances, a dominant allele is not expressed in a heterozygote individual
Example = Polydactyly Autosomal dominant trait Affected individuals have additional fingers
and/or toes A single copy of the polydactyly allele is usually
sufficient to cause this condition In some cases, however, individuals carry the
dominant allele but do not exhibit the trait
Incomplete Penetrance
Inherited the polydactyly allele from his mother and passed it
on to a daughter and son
Figure 4.11
Does not exhibit the trait himself even though he is a heterozygote
The term indicates that a dominant allele does not always “penetrate” into the phenotype of the individual
The measure of penetrance is described at the population level If 60% of heterozygotes carrying a dominant allele
exhibit the trait allele, the trait is 60% penetrant Note:
In any particular individual, the trait is either penetrant or not
Incomplete Penetrance
Expressivity is the degree to which a trait is expressed
In the case of polydactyly, the number of extra digits can vary A person with several extra digits has high expressivity
of this trait A person with a single extra digit has low expressivity
Expressivity
The molecular explanation of expressivity and incomplete penetrance may not always be understood
In most cases, the range of phenotypes is thought to be due to influences of the Environment