Download 16-1 Genes and Variation - Lincoln Park High School

Evolution of Populations
Chapter 16
I. Genes & Variation
1. Genetic Variation is common
Most organisms have many different alleles
(forms of a gene) for one trait
2. Genetic variation seems invisible b/c it can
be small differences in biochemical processes
like cell respiration or metabolism
3. Individuals can be heterozygous for a trait
which results in more variation.
1.
II. Variation & Gene Pools
1. Populations are used
to study genetic
variation
2. Gene Pool- A
common group of
genes shared by a
population.
 Ex: the gene pool of a
population of maple
trees in a park
This population of frogs has genes
from the same gene pool
3. Relative Frequency-
the number of times an
allele occurs in a gene
pool in comparison to
the number of times the
other alleles occur for
the same gene.
a) Is expressed as a
percentage
b) Relative frequency has
nothing to do with the
gene being dominant
or recessive.
a)
Ex: In a population
of cats the relative
frequency of the
dominate allele (B)
is 65% & the
relative frequency
of the recessive
allele (b) is 35%
Fig. 1: Imagine that you go to the mountaintop this
year, sample these beetles, and determine that 80%
of the genes in the population are for green
coloration and 20% of them are for brown
coloration.You go back the next year, repeat the
procedure, and find a new ratio: 60% green genes
to 40% brown genes.
 Gene Pool for Fur Color in Mice
5. Evolution is any
change in the
relative frequency
of alleles in a
population
Distribution of the O type blood in
native populations of the world
Distribution of the B type blood
allele in native populations of the
world
Distribution of the A type blood allele
in native populations of the world
III. Sources of Genetic Variation
1. There are two main
sources of genetic
variation:
a) Mutation
b) Gene Shuffling
Americans are a genetically varied population
2. Mutations (a change in the sequence of DNA)
a) Can happen b/c of mistakes during DNA Replication
b) Exposure to radiation
c) Exposure to chemicals in the environment
3. Sometimes mutations affect the phenotype of an
organism; sometimes they do not
4. Mutations can affect the “fitness” of an organism
5. Gene Shuffling
a) Occurs during
when gametes are
made
b) Also occurs
crossing over
during meiosis
c) most genetic
variation results
from gene
shuffling.
IV. Single-Gene & Polygenic Traits
What Determines How Many
Phenotypes A Population Has?
1. Depends on how many genes control the trait
2. Traits can be single-gene or polygenic
3. Single-gene trait- a trait that is controlled by one
gene w/ two alleles
 Ex: a widow’s peak in humans
 In this example population having a widows peak is more
common than not having a widows peak.
4. Many traits are polygenic
5. Polygenic trait- a trait that controlled by two or
more genes
6. b/c each gene of a polygenic trait has two or more
alleles there are many different phenotypes that can
result
 Human hair color is a
polygenic trait
 Variations of hair color in humans.
What is your hair color?
A bell shaped curve
represents the
distribution of polygenic
traits in humans.
8. This is called “normal
distribution”
9. Most people are in the
middle
10. Very people are on either
extreme very light brown
or very dark brown. Very
tall or very short
7.
 Most humans
skin color is in
the middle of
this bell curve