Allele Variation (Lecture 10) Flashcards
Outside Mendel’s garden, things are not so simple …
- Genes may (and usually) have __________
- Different alleles may affect the ________ in different ways
- A single gene may control _______
- Multiple genes may control _________
- Genes may (and usually) have more than 2 alleles
- Different alleles may affect the phenotype in different ways
- A single gene may control several traits
- Multiple genes may control a single trait
Genes usually have more than 2 alleles
What is the most common allele? What is it designated with?
What are all other alleles considered? What is it designated with
Any allele found at appreciable frequencies (at least 1%) in the population is considered to be what?
The most common allele is considered the wild type
- designated with a superscript + (e.g. A+ or c+)
All other alleles are considered as mutants
- designated with (cch, ch or c)
Any allele found at appreciable frequencies (at least 1%) in the population is considered to be a polymorphism
Different mutations in a gene can cause the same…
Different mutations in a gene can cause different…
Different mutations in a gene can cause the same disorder Different mutations in a gene can cause different phenotypes
What makes an allele recessive?
Recessive mutations almost always involve a mutation in a gene that results in at least some loss of protein function
Recessive:
One good copy (the “R” in a “Rr” heterozygote) of a gene is sufficient to make what?
One good copy (the “R” in a “Rr” heterozygote) of a gene is sufficient to make the protein required for the biological process to occur (example make round seeds). “R” would be dominant, “r” would be recessive.
What makes an allele dominant or recessive?
Mutations can be of different types what are the 2?
Mutations can be of different types:
1) complete loss of function (null allele)
2) partial loss of function (hypomorphic allele)
(Hypo=beneath; Morphic=morphology or phenotype)
What are the four different alleles of the ‘c” gene?
Wild type: c+c+
Hypomorph: chch
Hypomorph: cchcch
Null: cc
Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses
- Each version of a gene at a particular locus is defined as…
- Not necessarily as simple as…
- What are the 3 types of dominance?
- Each version of a gene at a particular locus is defined as allele
- Not necessarily as simple as dominant vs. recessive
- Types of dominance
- complete dominance
- incomplete dominance
- codominance
Complete dominance
A1A1 encodes red flowers
A2A2 encodes white flowers
If the heterozygote is red which allele is dominant?
If the heterozygote is white which allele is dominant?
If the phenotype of the heterozygote falls between the phenotypes of the two homozygotes, dominance is…
If the heterozygote is red the A1 allele is dominant over the A2 allele
If the heterozygote is white the A2 allele is dominant over the A1 allele
If the phenotype of the heterozygote falls between the phenotypes of the two homozygotes, dominance is incomplete
Incomplete dominance
If this is the P generation and the F1 generation
What does the F2 generation look like?
What is the incomplete dominance phenotype?
What is classical dominance/recessive genotype?
Note how the phenotypic ratios with Incomplete Dominance differ from the classic Dominant/ Recessive ratios:
Incomplete Dominance Phenotype is 1:2:1
Classical Dominance/Recessive phenotype is 3:1
Codominance
Phenotype is ____ intermediate between the two homozygotes
Phenotype simultaneously expresses the _________ of both homozygotes.
Phenotype is NOT intermediate between the two homozygotes; Phenotype simultaneously expresses the phenotype of both homozygotes.
Codominance
Black chicken + White chicken = ?
BB x WW
What would the punnet square look like?
Black chicken + White chicken = Checkered chicken
What is the definition of complete dominance?
Phenotype of the heterozygote is the same as the phenotype of one of the homozygotes.
What is the definition of incomplete dominance?
Phenotype of the heterozygote is intermediate (falls within the range) between the phenotypes of the two homozygotes.
What is the definition of codominance?
Phenotype of the heterozygote includes the phenotypes of both homozygotes.
The ABO blood group in humans is another example of a multi-allelic system AND an example of ?
The ABO blood group in humans is another example of a multi-allelic system AND an example of Codominance.
Red blood cells that do not react with the recipient antibody remain evenly dispersed, therefore?
Red blood cells that do not react with the recipient antibody remain evenly dispersed. Donor blood and recipient blood are compatible.
Blood cells that react with the recipient antibody clump together, therefore…
Blood cells that react with the recipient antibody clump together. Donor blood and recipient blood are not compatible
Type O donors can donate to any recipient this means?
Type AB recipients can accept blood from any donor this means?
Type O donors can donate to any recipient; they are universal donors
Type AB recipients can accept blood from any donor; they are universal recipients
Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses
What is Penetrance?
What is expressivity?
Penetrance: the percentage of individuals having a particular genotype that express the expected phenotype. (complete= 100%; incomplete <100%)
ex. 20% of people have blue eyes
Expressivity: the degree to which a character is expressed. Everyone has it, just some more than others.
ex. super curly hair vs a little curly/wavy
Incomplete Penetrance - Polydactyly
Individuals do not express a trait even though they have the …
Individuals do not express a trait even though they have the appropriate genotype.
Variable Expressivity
Not all Individuals express a trait at the same level even though they have the same _______
Not all Individuals express a trait at the same level even though they have the same genotype.
Given this pedigree…
What is this an example of?
Note that this is also an example of incomplete penetrance because the parents (I-1 and I-2) don’t have it, but 2 of their kids do. So one of the parents must the non-penetrant individual
Penetrance vs. Expressivity
What does each section represent?
1st: Incomplete penetrance
2nd: Variable expressivity
3rd: Incomplete Penetrance and Variable Expressivity
Incomplete Penetrance and Variable Expressivity looks a lot like Incomplete Dominance! How can you tell the difference?
What phenotypic ratios would you expect with Incomplete Dominance?
What phenotypic ratios would you expect with incomplete penetrance and variable expressivity?
What phenotypic ratios would you expect with Incomplete Dominance?
1:2:1
What phenotypic ratios would you expect with incomplete penetrance and variable expressivity?
Unpredictable and no clear intermediate phenotype.
All of the individuals who have been examined and found to clinically normal are also found to carry the mutation. Assume all others who don’t have the phenotype don’t have the genotype.
What would the penetrance of this phenotype be?
Total number of individuals with genotype = 18
Total number of individuals with phenotype = 12
Penetrance would be 66.7% (12/18)
What causes Incomplete Penetrance and Variable Expressivity?
Answer: Not clear but some possibilities:
- Environmental (temp during development; diet)
- Epi genetic (changes in gene expression, but not due to mutations)
- Maternal age (Icabod mutation is an example. The older the female fish the milder the phenotype.
