Incomplete Dominance,
Codominance, Multiple Alleles,
and Sex-Linked Genes
Incomplete dominance
• This is when neither allele is dominant.
• Both alleles are expressed and contribute
equally to the phenotype.
• A heterozygous genotype has an intermediate
phenotype as there is partial influence from both
alleles.
• Example: Red + White = Pink
flower flower flower
Incomplete dominance
• Snapdragons can be
– red (alleles = RR)
– white (alleles = rr)
– pink (alleles = Rr)
• If you cross a two pink flowers, is it
possible to produce a white flower?
¼ Red
¼ White
½ Pink
Yes!
Codominance
• Both alleles are dominant.
• They are independent, so there is no
'blending' as in the snapdragons, instead
the phenotype is a result of the full
expression of both alleles.
• Example: black + White = B&W spotted
dog
dog
dog
Codominance
•
•
•
•
•
Dogs can be
Black (alleles = BB)
White (alleles = bb)
Spotted (alleles = Bb)
Is it possible for a black dog to have one
parent that is white?
No!
Multiple alleles (blood typing)
• In the case of the ABO blood grouping,
there are 3 alleles for one gene
• They are written a little differently:
• i : this allele is recessive
• IA : this allele is co-dominant
• IB : this allele is co-dominant
Multiple alleles (blood typing)
•
•
•
•
•
Blood group (phenotype):
A ……………………………..
B ……………………………..
AB ……………………………
O ……………………………..
Possible genotype:
IA IA or IAi
IBIB or IBi
IAIB
ii
Multiple alleles (blood typing)
• A father with homozygous type A blood
has a child with a mother that has
heterozygous type B blood.
• Is it possible for them to have a child with
type O blood?
Sex determination
• Gender is determined by sex
chromosomes in many animals.
• The XY System:
– Female are XX, males are XY
• Y makes the guy!!!
• What are the odds that a couple have a
boy?
50/50
Sex-linked genes
• Some genes are linked to the sex
chromosomes and are inherited with them.
• Hemophilia is carried on the X
chromosome.
• The normal allele is dominant (H)
• The allele for hemophilia is recessive (h)
Sex-linked genes
•
•
•
•
•
XHXH = normal female
XHXh = carrier female
XhXh = female w/hemophilia
XHY = normal male
XhY = male w/hemophilia
A carrier does not
have the disease,
but can pass it on to
their offspring.
Sex-linked genes
• If a female carrier has offspring with a
normal male, what is the chances that
their offspring will have hemophilia?
25%
Sex-linked genes
• In cats the allele for coat color is carried on the X
chromosome.
• The alleles are black and orange but they are
codominant.
• XBXB = black female
• XBY = black male
• XbXb = orange female
• XbY = orange male
• XBXb = tortoiseshell female
Sex-linked genes
• Can an orange male and a black female
produce a tortoiseshell male?
No!
Sex-linked genes
What is the genotype
of the father?
Sex-linked genes
• Mother can provide XB or XO
• Father can provide Y plus 1 other allele.
• Since 1 offspring is XO XO, the father must
provide 1 of these allele.
• Therefore, the father's genotype is XOY