Download Incomplete Dominance, Codominance, and ABO Blood Types

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Review of Simple Mendelian
 Law of Segregation: each gene has two
different alleles that are separated when
gametes form
 One allele goes to one gamete and the other allele
to a different gamete
 Law of Independent Assortment: genes for
different traits are inherited independently
from each other
Review of Simple Mendelian
 Dominant vs. Recessive alleles for a gene
 The dominant allele masks the recessive one, so
you see the dominant trait (for RR or Rr)
 The only way to see a recessive trait is to have two
recessive alleles (rr)
 Dominant allele is represented as a capital letter
 Recessive allele is represented as a lowercase
letter (r)
Incomplete Dominance
 Sometimes neither allele is fully dominant
over the other
 Incomplete Dominance: neither allele is
dominant but combine and display a new
trait that is a mixing of the two alleles
Incomplete Dominance
Incomplete Dominance
 In incomplete dominance, the two alleles are
represented as
 Two capital letters, one with an apostrophe to
indicate the different allele that is involved
 Ex. R (for the red allele) and R’ (for the white
 When these two alleles come together, they
portray a mixing of the two phenotypes!
 Other times both alleles are fully dominant
 Codominance: both alleles of a gene are
dominant and the heterozygous phenotype
has both traits equally expressed
 In codominance the two alleles are
represented as
 Two capital letters: Use the first letter of one trait
(B for Brown) and the first letter of the other trait
(W for White)
 When they come together as a heterozygote,
both traits show as the phenotype
Winter xgames?
Let’s Stop and Think…
Let’s say there are two alleles for
the hair color trait- red and blue
What would be the resulting
phenotype of a heterozygous pair if
the alleles showed codominance?
Red and Blue patches
Multiple Alleles
 Sometimes there are more than two alleles
that govern the phenotype of a trait
 Multiple Alleles: the presence of more than
two alleles for a genetic trait
Multiple Alleles
 Blood Type
 Human blood type is governed by the presence of
3 different alleles:
 However, each person only has 2 of these 3 alleles
in their DNA
ABO Blood Type
 Blood types follow both Codominant and
simple Dominant inheritance
 The A allele and B allele are codominant with each
 The A allele and B allele are both purely dominant
over the O allele
 The O allele is recessive
Which blood type are you if
you have…
 AA
 Type A blood
 BB
 Type B blood
 AB
 Type AB blood
 AO
 Type A blood
 BO
 Type B blood
 OO
 Type O blood
ABO Blood Types
 The blood type gene and alleles are
represented differently than you have seen
 The blood type gene is I
 For this I gene you can have the following
 For A: IA
 For B: IB
 For O: i
Let’s stop and think…
 What are the two allele combinations you can
have for type A blood?
 IAIA and Iai
 What are the two allele combinations you can
have for type B blood?
 IBIB and IBi
Let’s Stop and Think…
 What is the only allele combination you can
have for type AB blood?
 What is the only allele combination you can
have for type O blood?
 ii
Sex-Linked Traits
 genes which are carried only on the sex
 ex: red green color blindness
 2:25 males
 1:150 females
Sex-Linked Traits
 Traits are usually not seen in women because
it can be masked by another dominant gene
on the other X-chromosome
 Female – XBXb
 Male – XbY
 But it is possible for it to happen in women!
Color Blindness Test
Can you see the letters and numbers in these squares?
Polygenic Inheritance
 Sometimes traits are carried on more than
one gene, making it difficult to tell if it’s
dominant or recessive
 Examples: Skin color, height, body mass
Polygenic Inheritance
 There’s often a wide range of phenotypes:
you can be anywhere from 5’ to 7’ tall, there is
no tall or short in humans!
Human Height
It’s often hard to tell
where a person might
get their height from
Multiple Genes
 In this pattern there is more than one gene
responsible for a single trait
 Makes it possible for many types of
genotypes and phenotypes
 Continuous variation
With simple patterns of inheritance like dom/rec,
and incomplete
dominance you
very easy to distinguish
each phenotype
may only get a few phenotypes and each is
distinct from one another.
Imagine people with only these skin colors
When there are multiple genes involved with a single trait
you can expect more of a continuum of variation. This is
more representative of human skin color
Sorry Smurfs!