A simple poker game will be used as an analogy for the polygenic trait with a single locus of large effect in an effort to simplify the picture. Let's say that the minimum ante into a poker game is $10.00. This $10.00 will be analogous to a threshold.

A man has money in three different pockets. These pockets will represent three different genes. These three different genes will be represented by different colors (red does pass the threshold, purple does not pass the threshhold, teal represents a single gene of large effect).

The man's wife puts different denominations of money, representing the genotype, in each pocket:

  • Pocket one has either a $5.00 bill or a single penny
  • Pocket two has either a $10.00 bill or a $5.00 bill or a $1.00 bill
  • Pocket three has either a $5.00 bill or a $2.00 bill

So there are many different combinations the man might have. Some combinations would allow him entrance into a poker game and other combinations would not.

A combination that does let the man into a poker game, is if he has $20.00 in the following denominations in his pockets:

  • pocket one, a $5.00 bill
  • pocket two, a $10.00 bill
  • pocket three, another $5.00 bill

This combination exceeds the minimum ante (threshold) of $10.00.

On the other hand, a combination that does not let the man into a poker game, is the combination of these denominations:

This combination gives the man only a total of $3.01. This combination does not reach the minimum ante (threshold) of $10.00, therefore excluding him from a poker game.

ALL possibilities can be listed:

Combination Pocket
Total
# 1 # 2 # 3

A

$20.00 $5.00 $10.00 $5.00

B

$17.00 $5.00 $10.00 $2.00

C

$15.00 $5.00 $ 5.00 $5.00

D

$12.00 $5.00 $5.00 $2.00

E

$11.00 $5.00 $ 1.00 $5.00

F

$ 8.00 $5.00 $ 1.00 $2.00

G

$15.01 $0.01 $10.00 $5.00

H

$12.01 $0.01 $10.00 $2.00

I

$10.01 $0.01 $ 5.00 $5.00

J

$ 7.01 $0.01 $ 5.00 $2.00

K

$ 6.01 $0.01 $ 1.00 $5.00

L

$ 3.01 $0.01 $ 1.00 $2.00

 

  • pocket one, one penny
  • pocket two, a $1.00 bill
  • pocket three, a $2.00 bill

Lets say that pocket one has a large effect on the man's financial situation. Notice that if the man has a $5.00 bill in pocket one, in all instances except one (combination F), the man can get into a poker game. BUT if the man has only a penny in pocket one, then in three out of the six combinations (combinations J,K,L) the man cannot get into the game.

This is analogous to whatever alleles are at a single gene of large effect influencing the outcome of a trait governed by many genes.

The combinations that did pass the threshold are A, B, C, D, E, G, H, I. These are shown in red.

The combinations that did not surpass the threshold are F, J, K, L. These are shown in purple.

NOTE: The combination in F does not surpass the threshold, even though the man has the $5.00 bill (allele) in pocket one. Because of the combination of the other pockets, he still does not have enough money (does not surpass the threshold) for the minimum ante into a poker game. It is the total combination that determines the threshold, even though pocket one has a large effect on the man's financial situation.

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