Codominance and multiple alleles Flashcards
What is Codominance
. Both alleles are expressed in the phenotype
. So instead of one allele being dominant and one being recessive, both alleles are equally dominant
What are multiple alleles
When there are more than two alleles, of which only two may be present at the loci of an individuals homologous chromosomes
Give the example of it in shorthorn cattle
Where one allele codes for an enzyme that catalyses formation of red pigment in hairs
And the other allele codes for an altered enzyme that lacks this catalytic activity so doesn’t produce the pigment so hairs are white
They are both Codominant so predict the phenotype of the cattle homozygous for the first allele
What about homozygous for the other allele
What about heterozygous
So two of the first alleles will both code for the enzyme that leads to red hairs so the phenotype will be red
C^RC^R
So two of the second allele means no pigment will be produced so the cattle will have white hair
C^WC^W
In heterozygous, both the red pigment allele and the lack of pigment allele will be expressed so a mix of these will lead to light red (roan)
C^RC^W
What would happen if a shorthorn with a red coat was crossed with one with a white coat
How do you write co-dominant alleles
The offspring would all have roan coat
You have to use different letters for the different colours, because if you used the same where one is capital and one is lowercase, it would seem like there is a dominant and recessive allele.
So use C which codes for colour
So allele that codes for red pigment will be C^R and the allele for no pigment is C^W
So produce of red coat and white coat:
Gametes are C^R and C^R for red
and C^W and C^W for white
So crossed in punnet square:
There will be 4 C^RC^W
What will be produced as offspring if two Roan cows were crossed together
Both will alleles C^RC^W
So the gametes will be C^R and C^W so put them in punnet square
Products will be C^RC^R, C^WC^W,
and two C^RC^W
So there is 50% roan coat
25% red coat
25% white coat
When a gene has more than two alleles, only two of these will be present on the loci of the two homologous chromosomes.
What is the example for multiple alleles
Blood groups
ABO
There are 3 alleles associated with the gene 1 (immunoglobulin gene)
Which leads to presence of different antigens on the cell surface membrane of red blood cells
What are the 3 alleles that produce the different antigens leading to different blood groups
There are different antigens on the cell surface membrane of red blood cells
And these are from the 3 alleles ABO
Allele I^A leads to production of antigen A
Allele I^B leads to production of antigen B
Allele I^O doesn’t lead to production of either antigen
Why can only two alleles be expressed at once
Although there are 3 alleles, only two can be present in an individual at one because there are only two homologous chromosomes so only two gene loci
Which alleles for blood group are recessive / dominant
The alleles I^A and I^B are Co-dominant whereas the allele I^O is recessive to both
What blood group would offspring have made with parent with AB and parent with O
So gametes are
I^A and I^B
And I^O and I^O
So offspring phenotypes will be
I^AI^O and I^BI^O
So 50% of offspring will be blood group A and 50% will be blood group B
Because O is recessive so won’t be expressed in the presence of a dominant allele
What blood group offspring would be made from a parent with heterozygous group A and heterozygous group B
The gametes would be I^A and I^O crossed with I^B and I^O
So produced would be
I^AI^B, I^OI^A, I^BI^O, I^OI^O
25% group AB
25% group A
25% group B
25% group O
