GENES AND VARIATION
Evolution: Section 4
I. Darwin’s Gaps
1. How do the ___________________ arise?
- Variation was one of the key “ingredients” of _______________________________
- Remember: variation, selection, time
- Some variations are better than others, the environment _________________ those.
2. How are ___________________variations (traits) passed on to offspring?
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What scientist’s work could have help Darwin with these questions? _______________________
(Remember, he described ___________________ of “factors” in pea plants).
II. Putting it all together!
By the 1930s, Mendel and Darwin’s work were combined. Molecular biologists had also discovered that:
1) DNA was the molecule of heredity. ___________determines phenotype.
2) Sources of variation include ________________and ______________
We can now describe evolution in _________________ terms. We can be more specific than “change”
over time.
Evolution (in genetic terms) is any _________________ in the relative _____________________ of
_______________________in a population.
AKA: biological evolution or microevolution
A change in genetic composition within a population over generations.
Gene Vocabulary Review: Gene, Trait, Genotype, Allele, Phenotype
Vocab:
1. A ______________________ is a group of individuals of the same species that interbreed and reproduce.
2. A ___________________ is all genes, including different alleles, that are in a population.
3. ________________________ is the # of times an allele appears in a population.
General Equation:
Relative frequency of an allele= # of the certain allele in the population
# of TOTAL alleles in the population
Main Idea:
Allele frequency
is about finding
Which mice genotypes contain the dominant allele?
________________________________
How many dominant alleles are in the homozygous dominant black mouse?_____ The heterozygous mouse?
***Relative frequency of a DOMINANT allele
= (# homozygous dominant x 2) + (# heterozygous x 1)
(# individuals in population x 2)
Why multiply the # of individuals by 2? ______________________________________________
Which mice genotypes contain the recessive allele? _____________________________________
How many recessive alleles are in heterozygous mice? ______ The homozygous recessive mice? ______
****Frequency of a RECESSIVE allele=
(# homozygous recessive x 2) + (# heterozygous x 1)
(# individuals in population x 2)
III. Example: Calculating the relative frequency
- Example 1: Let's consider a gene with only two alleles. In mice, Black fur color (BB or Bb) is
dominant to brown fur color (bb). In a population of 100 mice, 36 mice are homozygous dominant
(BB), 48 mice are heterozygous (Bb) and 16 are brown (bb).
-
Relative frequency of B= # of B alleles in the population
# of TOTAL alleles in the population
- Relative frequency of a dominant allele
= (# homozygous dominant x 2) + (# heterozygous x 1)
(# individuals in population x 2)
Relative frequency of B= (36 x 2) + (
How could we figure out the frequency of “b” without doing the big equation?
____________________
Note that the two allele frequencies add up to 1. *This is a law of population genetics:
The sum of all allele frequencies ______________________________. This is because 1 represents the
frequency of all possible alleles within the population.
Frequency of b =
# of b alleles in the population
# of TOTAL alleles in the population
Frequency of a recessive allele=
(# homozygous recessive x 2) + (# heterozygous x 1)
(# individuals in population x 2)
Frequency of b=
Note that the two allele frequencies add up to ONE.
Let’s say that these mice have several generations of offspring. We want to know if _____________________
has occurred. How would we know?
If the relative frequencies of the alleles _____________, then evolution has occurred!
IV. 2 Sources of Variation: A Review
1.Mutations are any change in the DNA sequence (AATAC  AATAT)
May be caused by:
o Mistakes during replication
o Radiation or chemicals in the environment
Types of mutations
Effects: __________________Fitness, __________________ Fitness, or _____ Effect
When would a mutation have no effect?
Ex: Silent mutation, no change in amino acid sequence
When would a mutation that caused a phenotypic change have no effect on fitness?
If it were something that did not effect ____________________and __________________________.
Ex. extra finger or extra teeth.
2. Sexual Reproduction (Gene Shuffling)
Q: Why don’t you look exactly like your parents? (what were the sources of genetic variation you
learned in meiosis?
-
Meiosis: random assortment of genes, random separation of chromosomes
).
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Crossing over: exchanging parts of homologous chromosomes
(
REMEMBER! Some variations are better than others, the environment __________________________those.
The source of variation is on the ________ level!
SUMMARY
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