Lecture 3 Flashcards
What are complete dominance and complete recessiveness?
Extremes of a range
What is incomplete dominance?
BB, Bb and bb all differ phenotypically; Bb is intermediate between homozygous phenotypes
Example: flower colour (red - RR + white - rr = pink - Rr)
What is codominance?
BB, Bb and bb all differ phenotypically, but Bb exhibits phenotypes of both homozygotes
Example: flower colour (red - RR + white - rr = red and white - Rr)
What happens in a population?
Multiple alleles may exist.
How many alleles exist in an individual (haploid)?
Two Alleles
What is an example of multiple alleles?
Fur Coloration in Cats
C: full colour
Cb: burmese
Cs: siamese
c: white. blue eyes
What is a Dominance series or allelic series?
For fur colouration, C > Cb = Cs > c > Ca
Where > indicates the dominance and = indicates incomplete dominance or codominance
What does an allelic series describe?
The dominance hierarchy of multiple alleles.
What is a null allele?
Nonfunctional
What is a hypomorphic allele?
Has partial function
What is ABO blood phenotype an example of?
Codominance
For blood types, what does Gene I encode?
Transferase enzyme
What are the three alleles for blood type?
IA, IB and I
What does IA encode?
A Transferase which adds Acetylgalactosamine
What does IA encode?
A Transferase which adds Galactose
What does i encode?
A Non-Functional Transferase (amorphic allele)
Type A Genotype
IAIA or IAi
Type B Genotype
IBIB or IBi
Type O Blood
ii
What is Type AB Blood
IAIB
- It is not a new phenotype, but rather an example of codominance that has both galactose and acetylgalactosamine
- In this case two different transferases are made
What is a Wildtype Allele?
A functional enzyme or other protein is produced (normal version of the gene)
What is a Loss of Function Allele?
An enzyme or other protein is no longer being produced, is produced at lower levels or in non-functional.
What type of allele is dominant?
The wild type allele is often dominant over loss of function allele.
What is Haplosufficiency?
Half as much protein is synthesized yet this is often sufficient enough to achieve this wild type phenotype (half of two genes is enough to be ok)
What can dominant alleles be?
A gain of function mutation, in which the mutant allele produces a protein that has increased detrimental function
Example: Huntingtons Disease Is a Dominant recessive disease, where it never skips a generation.
How can dominant alleles also be loss of function alleles?
In a heterozygote, half as much protein is synthesized and this is not sufficient for a normal phenotype therefore haploinsufficent.
What is an example of haploinsufficent?
Tailess cats (Manx), one copy causes the cat to have no tail..
What is a recessive lethal allele?
It will cause death only if homozygous (need two copies of it)
Example: if a mice receives two big YY, it will die - but if it is heterozygous Yy, it will survive and be yellow - if yy white
Therefore the effect of the allele on colour is dominant.
What ratio does a recessive lethal allele produce?
2 yellow: 1 non-yellow
What is an example of a recessive lethal allele with cats?
Manx Cats
mm = normal tail
Mm = no tail (phenotypically)
MM - lethal (cat dies)
Why is it considered to be Dominant Recessive for Lethal Alles?
As the allele is dominant but considered recessive as you need two copies of it for the cat to do
What is Dominant Lethal Genes
Only need one copy of it of the allele for it to be lethal (can be expressed in both heterozygote and homozygote)
Example:
Bb lethal, BB lethal, bb not lethal (one copy of big B will kill the creature)
What happens to individuals who are homozygote for Tay Sachs?
Often die before the age of 3
tsts = lethal, TSTS not lethal, TSts = not lethal
What does a Wild-Type allele produce?
A functional protein
What does a recessive amorphic loss of function allele produce?
A non-functional polypeptide (severe mutant)
What does a recessive hylomorphic loss-of-function allele produce?
A partially functional polypeptide.
What does a Dominant-negative allele produce?
A polypeptide that interferes with the wild-type polypeptide (severe mutant)
If you have one allele that is wild type and the other allele is recessive, what will be the phenotype?
Wild-Type as you only need one good allele - if the other allele was dominant then it would be a mutant.
