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Lesson 13: Polygenic Inheritance
• The very first personality trait that was ascribed to a
known gene is “novelty seeking”. People can be
high novelty seekers – they like bungee jumping and
risky behaviors, or they can be low novelty seekers –
they like reading and solving Sodoku puzzles. While
as many as 10 genes are estimated to play a role in
this trait, the first one identified was the gene that
encodes a dopamine receptor. DRD2 (sometimes
referred to as D2DR) was the second dopamine
receptor identified and is found in the nucleus
accumbens among other places. The idea is that the
rewarding feeling of reading is enough for some
people, but others need more stimulation.
Lesson 13: Polygenic Inheritance
• appreciate how most behavioral traits, including
response to drugs, are attributable to many genes;
• learn how to make predictions about probabilities for
given polygenic phenotypes;
• understand the principles of independent assortment
that underlie these predictions; and
• appreciate that polygenic traits are “quantitative” and
bell-curve distributions of phenotypes are generally
observed in a population and even in a mating
What do we mean by polygenic traits?
“Brain Storm” by alexiuus.deviantart.com
What happens to traits controlled by
genes on different chromosomes?
Chromosomes move
to gametes
INDEPENDENTLY
How do gametes get random
combinations of chromosomes?
W
w
w
W
S
s
S
s
What do you
think?
For genes on DIFFERENT chromosomes, it is RANDOM which of each type
of chromosome an egg or sperm receives
ws
WS
Ws
wS
How do we predict for multiple genes?
Ww x Ww
W
w
WW
Ww
Can make a BIG
S=
Bent square
pinkiefors= straight pinkie
Punnett
w
Ww
independent
genes….
ww
WwSs x WwSs
W = widow’s peak w=none W
S
s
S
SS
Ss
s
Ss
ss
Ss x Ss
“Doc Linda’s Rule”
Aka Boolean logic: Activity 2
AND
OR
Versus
Using the Punnet Squares you’ve created, answer these Qs:
What proportion of kids will be WW AND ss?
¼ WW x ¼ ss = 1/16 MULTIPLY your fractions
What is the proportion of kids with the dominant trait for W?
¼ WW + ½ Ww = ¾ ADD your fractions
Led to second Thing Mendel Didn’t
Know
A trait can be controlled by MANY genes
example – height, skin color, novelty seeking
RR
Rr
rr
Most work by
incomplete dominance
Imagine several genes
influence a trait such as
height
Each gene has two type of
allele, one that adds inches
and one that doesn’t
Contributing allele
Non-contributing allele
Try it with alcohol response genes:
AADd x AaDd
AA x Aa
Dd x Dd
ADH
ALDH
Fraction of kids with ANY TWO (AA or DD)
contributing alleles (good metab. of alcohol) = ?
A
AA x Aa
A AA
A
Aa
d
D DD Dd
d
ADH
AA Aa
D
Dd x Dd
a
Dd dd
ALDH
Alcohol response gene problems:
A
AA x Aa
A AA
A
Dd x Dd
a
Aa
AA Aa
D
d
D DD Dd
d
Dd dd
Fraction of kids
with ALL
contributing alleles
(great metabolism of
alcohol) = ?
More Alcohol response genes problems
A
AA x Aa
A AA
A
Aa
d
D DD Dd
d
Fraction of kids with 2 and
only 2 contributing alleles
(medium metabolism of
alcohol) = ?
AA Aa
D
Dd x Dd
a
Dd dd
AAdd, OR AaDd, OR
aaDD – all equally
medium in alcohol
metabolism.
Imagine that two genes control hair color:
(mom)AaDd x AaDd (dad)
What are the probable ratios of their children?
1.
2.
3.
4.
5.
6.
1/8 are AAdd + 1/4 are AaDd
1/4 are AAdd + 1/4 are AaDd
1/4 are AAdd + 1/2 are AaDd
1/8 are AAdd + 1/16 are aaDD
1/4 are AaDd + 1/16 are aaDD
None of the kids has just two
contributing alleles
Bell Curves and Polygenic Inheritance
LOTS of
contributing
alleles
FEW
contributing
alleles
Why are there more people with an
intermediate phenotype (for polygenic traits?)
0
xxyyzz
1
2
3
4
5
6
XXYYZZ
XXyyzz = XxYyzz = XxyyZz = xxYYzz = xxYyZz = xxyyZZ
Doesn’t matter which two alleles are contributing (capitals)
The Enormous Pumpkin Problem
Y Y
X x
X XX Xx Y YYYY Z ZZ ZZ
x Xx xx y YyYy z Zz Zz
XX = ¼
YY = ½
ZZ = ½
What is the probability of getting a 6
pumpkin?
Chance of getting a 5? XXYYZz OR XXYyZZ OR XxYYZZ
1/16
1/16
1/8
Therefore:
• 1) drug response genes make proteins whose
“dosage” can be counted.
• 2) Use small/easy Punnett squares and Boolean
logic to compute probabilities.
• 3) Polygenic trait extremes are represented by
fewer genotypes and often least frequent
• Now you are ready for the LAB!
What happens to traits controlled by
genes on different chromosomes?
Chromosomes move
to gametes
INDEPENDENTLY
Part III of Lab:
• Example of branching diagram:
12-RrTt x RrTt
¼ TT
¼ RR
½ Tt
¼ tt
½ Rr
¼ rr
¼ TT
½ Tt
¼ tt
Etc.
What is probability of
having a tongue rolling,
non- taster? Check w/#4
Lab Questions:
• Why are phenotypes distributed in the manner
your graph shows for polygenic traits?
• How will you remember when to add and when
to multiply fractions?
• Pick up LAB HW when done
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