BIOL 1090 Lecture Notes - Lecture 7: Null Allele, Wild Type, Antennapedia

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Represented by + superscript!
Wild type allele = most common allele
Polymorphism allele = any allele found in appreciable frequencies (more than 1%) in population
May appear and disappear!
If a mutant is slightly advantageous, frequency may increase and then allele may become a
polymorphism or even wild type in future!
All influenced by chance events
Null allele = complete loss of funciton
i.
Hypomorphic allele = partial loss of function
ii.
Transcription factor protein (helps DNA binding)
Mitf +/+ --> complete set of Mitf protein (wild type)
BUT - can still function normally with half!
Mitf +/- --> half the set of Mitf protein!! (null allele)
Ex: Mitf protein
Why? --> because for most genes, one functional copy is enough for a normal (wild
type) phenotype!
Recessive mutations almost always involve a loss of gene function!
a.
For genes where one functional copy is NOT enough!
State = haploinsufficienty
Loss of function mutation
i.
Loss of function mutation that interferes with normal function of wild type
allele!!
So sometimes half the proteins is WORSE than none! ^^"
Ex: in Mitf +/M --> the proteins are deformed! Can't bind to DNA!
Dominant negative mutation
ii.
Enhances normal function of gene!
Ex: new function! (but very rare)
Mutation in regulatory region of Antp gene
Legs on top of head! Instead of thorax
Antennapedia mutation in Drosophila!
Ex: improve gene regulation (where/when transcription occurs)
Gain of function mutation
iii.
Dominant mutations can involve a loss of function OR a gain of gene function!
b.
What makes an allele dominant or recessive??
Genes usually have more than two alleles
1.
Phenotype of heterozygote = MIDWAY between the phenotypes of the 2 homozygotes!
One allele is partially dominant over the other!
Ex: WW (red) X ww (white) --> Ww (pink!)
Pink flower has half the pigment as red! (b/c W allele codes for pigmentation!)
How does it work?
Incomplete Dominance
A.
Heterozygote expresses phenotypes of both homozygotes!
Neither allele is dominant!
Ex: blood types! (AB blood type)
Codominance
B.
Ex: hair colour in rabbits have a hierarchy!
C+ (wild type) > Cch (chinchilla) > Ch (white + black extremities) > C (albino)
Multiple alleles of one gene will have a dominance hierarchy described by allelic series!
Different genes/alleles may affect the phenotype in many ways
2.
BOTH dominant visible allele (yellow fur) AND recessive lethal! (if AyAy ==> dead!)
Fur colour and embryonic development!
Therefore, involved in at least 2 different developmental processes!
Ex: A^y allele!
A single gene may affect/control more than one trait
3.
Outside Mendel's Garden, things are not so simple....
BIOL*1090-01
Lecture 7 - Allelic Variation & Gene Function
February 1, 2016
8:30 AM
BIOL 1090 Page 1
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Document Summary

Lecture 7 - allelic variation & gene function. Outside mendel"s garden, things are not so simple Mutant alleles = any other alleles (less than 1% frequency) -> because for most genes, one functional copy is enough for a normal (wild type) phenotype! Recessive mutations almost always involve a loss of gene function! What makes an allele dominant or recessive? polymorphism or even wild type in future! i. ii. a. Transcription factor protein (helps dna binding: mitf +/+ --> complete set of mitf protein (wild type) But - can still function normally with half! Mitf +/- --> half the set of mitf protein!! (null allele) For genes where one functional copy is not enough! Dominant mutations can involve a loss of function or a gain of gene function! i. ii. iii. Loss of function mutation that interferes with normal function of wild type allele! So sometimes half the proteins is worse than none!

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