BIOC16 Lecture 8
Correlations among traits
When you notice that the larger individuals have lighter fur than the smaller individuals so why
are these two traits correlated?!
- to now we've been dealing with single phenotypic traits!
- method to understand genetics and evolution of a group of traits simultaneously!
Why or how are traits correlated?!
- pleiotropic effects: one locus affects more than one trait!
Eg: morning glory ( ipomoea purpurea) Simmons and Bucher 1996!
W- ﬂoral pigments!
Ww - light! 50% dead !
ww - white! 61% dead
W locus has pleiotropic effects on ﬂoral color and toxicity!
Consider 2 genes A and B - each has 2 alleles A1A2 and B1B2!
p1 f(A1). q (B1)!
p2 f(A2) q(B2)! When the random association isn't observed we call it linkage disequilibrium!
- physical linkage!
- but physical linkage isn't essential!
-mixing of maternal and paternal genes into new combinations in offsprings!
Two instances: 1) meiosis- prophase 1- chromosomes pairing up!
- crossing over!
2) random probability that the maternal or paternal chromosomes of each homologous pair
line up on either side of metaphase plate.!
If two loci are unlinked (ie. different chromosomes) the probability of recombination between
them is 0.5 ( highest amount of recombination and this number decreases as things become
If these loci are sufﬁciently close together ( same chromosome, same arm of chromosome)...
They are linked.
The probability of recombination between them is now proportional to the physical distance
No crossing over - Far apart or on
right next to each different
other ( or same
Rate determining how fast a population in linkage disequilibrium
(LD) reaches linkage equilibrium (LE) What causes LD?!
1) selection for particular combinations of alleles!
2) mixture of 2 populations with different allele frequencies!
3) random genetic drift!
Eg: heterostyly in English Primrose ( primula vulgaris)!
Two genes Gg - style length - shank is dominant!
Aa - anther position - high is dominant!
"Pin" - long style and anthers !
are well below stigma! Stigma!
"Thrum"- short style, stigma is !
midway, anthers high!
Genes for style length and
anther position are in LD!
- selection against selﬁng Quantitative Trait Locus Mapping
Case study - lab working on mice and they are interested in longevity - how do you ﬁnd
genes for longevity - you can use knock out however your mutations will result in drastic
changes. Looking for subtle evolutionary changes you take a different approach.!
First step is to ask - does there appear to be variability in the trait of interest?
So you see that there is