Transcript Non-Mendelian Genetics

Non-Mendelian Genetics
What color offspring would you get
when you cross a pure (homozygous)
red snapdragon with a pure white
snapdragon?
Oddly Enough,
Neither Red Nor White
Snapdragon Flower
Color is controlled by
Incomplete
Dominance and a
new 3rd phenotype
is seen.
How does Incomplete Dominance
work?
Incomplete Dominance is a
Blending
Like Paint, the
RED Pigment
“MIXES” with the
WHITE to create
PINKFLOWERED
offspring
Incomplete Dominance can be seen in:
Horses
( Chestnut x Cremello 
Palomino)
 Snapdragons
 Japanese Four O’ Clocks
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Many flowers exhibit
incomplete dominance.
Allele Notation
HOW DO YOU WRITE OUT THE
GENOTYPE WHEN BOTH
ALLELES ARE DOMINANT?
You Must Use Different Letters
1.
2.
Choose different letters
to represent each form
(In this case we’ll use
“W” for the white allele
and “R” for the Red
allele).
Remember to always
use CAPITAL letters.
This is incomplete
DOMINANCE.
RR
WW
RW
So…The Cross Between Pure
Snapdragons Looks Like This
R
R
W
RW
RW
W
RW
RW
You’ve Seen A Cross between Pure
Snapdragons, now do the Punnet
Square for a cross between two
hybrids (heterozygotes).
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What are the alleles of a heterozygous snapdragon?
RW
What is the genotypic ratio?
1 RR : 2 RW : 1 WW
What is the phenotypic ratio?
1 Red : 2 Pink : 1 White
What kind of genetics do these
organisms exhibit?
Codominance
A cross between 2 tabbies (the
The same ratios as
Incomplete Dominance
occur:
A ratio of 1:2:1 for both
genotype and phenotypes
of a monohybrid cross

Different Phenotype:
The two original
phenotypes are
combined to give a
SPOTTED or
MULTICOLORED
phenotype.
heterozygotes) results in
1 black : 2 tabbies : 1 tan cat
Notation for Codominance
1.
2.
3.
4.
Choose a letter to represent the gene.
Choose different letters to represent each
of the alleles.
Take the letters representing the alleles
and turn them into superscripts.
Combine the two. Remember to use
capital letters.
Writing out the Genotype
1.
2.
3.
4.
5.
We’re going to use C to represent Coat Color of a
shorthorn cow.
Roan coat color is a combination of Red and White
hair so we’ll use “R” to represent Red and “W” to
represent white.
R  R and W  W
C + R  CR and C + W  CW
So a Red Cow would be CRCR , a white cow would be
CWCW, and a roan cow would be CRCW
Codominance occurs in:
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Shorthorn Cow (White +
Red)
Blue Roosters (White +
Black)
Human Blood Typing
(AB)
Tabby Cats (Black and
Tan Fur)
Let’s Practice
Tabby cats exhibit codominance between tan and
black fur.
1. Can Tan Cat be heterozygous? Why or Why
not?
2. If you mate a tabby with a black cat, what is
the phenotypic ratio of their offspring?
3. If the offspring of two cats are composed of
36 tabbies and 40 black cats, what are the
parental phenotypes and genotypes?
Blood Typing
Human blood
typing is an
example of both
Codominance and
Multiple Alleles
 What does Multiple
Alleles Mean?

The Alleles
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A and B blood types are
coded for by the alleles:
IA and IB respectively.
These two alleles are
CODOMINANT.
Blood type O is coded
for by the allele i and is
recessive to both IA and
IB (notice the lower case
letter).
Alleles and Their Blood Types
ii
IA IA
IA i
IB IB
IB i
IA IB
O
A
B
AB
Let’s Practice
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Could a man with AB blood have an AB child with a
woman with type O blood?
If a child has type B blood, what are all the possible
blood types for his/her parents?
A paternity test is being performed. The child has type
A blood. The mother is type A. Potential Dad #1 has
type B blood. Potential Dad #2 has type O blood.
Who is the father?
Challenge: What two parents could produce the most
diverse children as far as blood type is concerned?
Codominance
Sickle-cell Disease
-common in people
of African descent
-sickled allele
causes hemoglobin
to change and red
blood cell forms a
crescent shape –
can’t carry oxygen
well
Codominance
Genotypes:
BN BN: normal blood cells
BNBS: normal blood cells and
sickled cells
BSBS: sickled blood cells
HOMEWORK
Finish Bikini
Bottom Genetics
Packet
Sex-linked Traits
Traits (genes) located on the sex
chromosomes
Sex chromosomes are X and Y
XX genotype for females
XY genotype for males
Many sex-linked traits carried on
X chromosome
Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
Sex-linked Trait Problem
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
XRY
x
XrXr
Remember: the Y chromosome in males
does not carry traits.
r
r
X
X
RR = red eyed
Rr = red eyed
R
X
rr = white eyed
XY = male
XX = female
Y
Sex-linked Trait Problem
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
XRY
x
XrXr
Remember: the Y chromosome in males
does not carry traits.
r
r
X
X
RR = red eyed
Rr = red eyed
R
X
rr = white eyed
XY = male
XX = female
Y
Sex-linked Trait Solution:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyed
female
50% white eyed
male
Female Carriers
Question: Is it possible for
a female carrier of
hemophilia to have a
daughter with hemophilia?
XH
Xh
Xh XH Xh XhXh
Y
XH Y Xh Y
Epistasis
When one gene hides the effects
of another gene
2 genes determine coat color
“E” gene can mask “B” gene
Epistasis
Ee or EE=
Can have
dark pigment
ee= can’t have
dark pigment
B = darker
Dark pigment
b = lighter
Dark pigment
Dosage Compensation
Females have 2
“doses” of X;
Males only have 1
“dose” of X;
X chromosome:
To balance this
Genes necessary
difference one of the
For male/female
X chromosomes
Y chromosome:
is inactivated or
Genes necessary
turned off
For male only
Dosage Compensation
Chromosome inactivation: One X in each cell
is randomly inactivated.
Orange = cells where X chrom. with black
allele is inactivated
Black = cells where X chrom. With orange
allele is inactivated
Barr bodies:
Inactivated X
chromosome forms
a condensed dark spot
in nucleus
Polygenic Traits
A trait that is controlled by more than one
gene.
Polygenic Traits
So, instead of only 2
alleles contributing to
a trait, there can be 4
or 6 or 8, etc. alleles
contributing to a trait.
Skin color: the more AABbCc = darker skin
dominant alleles for aaBbCc = lighter skin
each gene, the darker
the skin
Gene Linkage
Mendel studied genes on different chromosomes,
so they assorted independently – many
combinations
Genes located close together on the same
chromosome are said to be linked – they get
passed down to gametes together – don’t assort
independently