If we are all the same species (Homo sapien), why don’t we all look the same?

Population Genetics

If we are all the same species, why don’t we look the same? Alleles: different versions of genes For example, we all have the gene for hair color. Some of us have the blonde version, while some of us have the brown version.

Genetic Variation Within Populations Variation in a population increases the chance that some individuals will survive. Genetic variation leads to phenotypic variation. Phenotypic variation is necessary for natural selection.

Which mouse probably has the highest fitness?

What causes variation in a population? 1.Mutations – a change in the DNA sequence makes a new version of a gene How would a mutation affect the resulting protein?

Normal Red Blood Cells vs. Sickle Cells

Balding is caused by a mutation on the X chromosome.

What causes variation in a population? 2. Gene Shuffling – independent assortment of chromosomes & crossing over during meiosis gives us different combinations of genes

Gene Shuffling During meiosis, the chromosomes can line up several different ways before separating into different cells.

Genes & Variation We can figure out what the frequency of a particular allele is by calculating the number of times that the allele appears in a population compared to others in the entire gene pool. Basically, we are trying to answer this question: What percentage of the time does the brown allele occur in a population compared the black allele?

There are 25 mice in this population. How many alleles are in the gene pool? 50 What is the frequency of the black allele? 20 out of 50, or 40% What is the frequency of the brown allele? 30 out of 50, or 60%

In this sample population, is the most common allele the dominant one? NO! The most common allele does NOT have to be the dominant allele!

Gg GG gg G codes for green. 7 Gs in the gene pool. g codes for brown. 5 gs in the gene pool. There are 12 total alleles for skin color trait in the gene pool. Frequency of G = 7/12 = 58.3% Frequency of g = 5/12 = 41.7%

Why do we care about allele frequencies in a population? By calculating the allele frequencies in a population, we can keep track of how the population is evolving!

How do we know if evolution has occurred? When the frequency of an allele changes, “change over time” has occurred. This is evolution on a small scale!

When we did the dot activity with the fabric, all of the colors of dots started off equally. By the end of the activity, some colors increased & some decreased. Was evolution taking place?

Consider the alleles for polydactyly in a gene pool. The allele coding for extra digits, the polydactyly allele (P), occurs in 1% of the population (the frequency is.01). The allele for 5 fingers & toes (p) occurs in 99% of the population (the frequency is.99).

Polydactyly What would happen if over time, having extra fingers was an advantage? If natural selection selected FOR individuals with extra digits, a shift in the allele frequency might happen, & evolution would occur!