17.1 Genes and VariationMRS. MACWILLIAMSACADEMIC BIOLOGY

I. Genetics Joins Evolutionary Theory*1859- Darwin developed his “Theory of Evolution” without knowing how heredity worked

*1865- Mendel published his genetic experiments on inheritance in pea plants

a. 1900’s- genetic theory sky rocketed

A. Genotype and Phenotype in Evolution

1. Genetic variation if the raw material for natural selection

2. Molecular genetics helps us understand how evolution works

3. Gene- a sequence of DNA that codes for a protein and determines a trait, a specific characteristic of an individual

*Allele- specific forms of a gene

Ex. GENES= eye color

ALLELES= genes for blue eyes, brown eyes, etc.

4. Heritable traits are controlled by genes

5. Changes in a gene or chromosome, a structure of DNA + protein that contains genetic information, generate variation

6. You contain two sets of genes, one set from mom and one set from dad

7. genotype- genetic makeup of an organism

ex. Bb (B and b are genes for eye color)

8. phenotype- physical characteristics of an organism

ex. Blue eyes

9. genotype + environment creates the phenotype

10. Natural selection acts directly on PHENOTYPE

*It is the ORGANISM that survives or dies, not a single gene

11.Some individuals have phenotypes that are better suited to their environment than others = produce more offspring, pass on more copies of their genes to next generation.

B. Population and Gene Pools

1. population- group of individuals of the same species that mate and produce offspring

2. gene pool- all the genes that are present in a population3. allele frequency- number of times an allele occurs in a

gene pool, compared to the total number of ALLELES in the pool for the same GENE

ALLELE FREQUENCY???

# students with blue eyes# students with brown/green/hazel eyesTotal number of students

WHAT IS THE ALLELE FREQUENCY FOR BLUE EYES?

blue eyes/total # of students = (___%)

WHAT IS THE ALLELE FREQUENCY FOR NON-BLUE EYES?

non-blue eyes/total # of students = (___%)

LET’S TRY A CLASSROOM EXAMPLE!

4. Evolution, in genetic terms, involves the change in frequency of alleles in a population over time!

5. Natural selection operates on individuals, but resulting changes in allele frequencies show up in populations

Populations evolve, individuals do not!

II Sources of Genetic VariationA. Mutations- change in genetic material of a cell

1. Changes in phenotype may or may not affect fitness2. Some may be lethal or may lower fitness; others may be

beneficial3. Mutations matter in evolution only if they can be passed from

generation to generation mutation must occur in either eggs or sperm

4. Research suggests each of us is born with roughly 300 mutations that make some of our DNA different from our parents – most are neutralNOTE: Mutations matter ONLY if they can be passed to the next generation

B. Genetic recombination in Sexual Reproduction1. Mutations are not primarily why you look different than other

people (even parents and siblings)2. During creation of eggs and sperm, genes recombine increasing

new genotypes in each generation*You will learn more about when we talk about MEIOSIS

C. Lateral gene transfer1. a. organisms pass genes from one individual to another that is

NOT its offspringEx. bacteria can swap genes with other bacteria

2. can occur between organisms of the same or different species3. increases genetic variation in a species that picks up the “new”

genes

Lateralgene transfer toanotherspecies

III. Single Gene and Polygenic Traits A. single-gene trait- trait controlled by only ONE gene

1. may have just two or three distinct phenotypes Ex. red or white flower genes

2. The most common form of the allele can be dominant or recessivea. a dominant allele produces a dominant

phenotype in individuals who have ONE copy of the allele, which can come from just one parent (Red flowers; RR or Rr)

b. a recessive allele to produce a recessive phenotype, the individual must have TWO copies, one from each parent (white flowers; rr)

NOTE: Dominance of an allele does not necessarily mean that the dominant phenotype will always appear with greater frequency in a given population

B. Polygenic trait- trait controlled by TWO OR MORE genes1. Each gene of a polygenic trait often has two or more

alleles2. A single polygenic trait often has many possible

genotypes and even more different phenotypes

Example is HUMAN HEIGHT!

In the US, the average heights are:

Males ~5’10.5”

Females ~5’5”

But we have a huge variation!

IV. Alleles and FitnessA. Lethal Alleles- cause an organism to die only when

present in homozygous condition

1. Examples (2 recessive alleles) cystic fibrosis (cardiorespiratory disorder) and sickle cell anemia (blood disorder)

2. Ex. (2 dominant alleles) Huntington’s disease a degenerative brain disorder

B. Negative Impact of Low Allele Frequency

1. loss of genetic diversity

2. an increase in inbreeding

3. lack of adaptation to changing environmental conditions

4. population extinction