Download Mendelian Genetics

Mendelian
Genetics
….aka some stuff from high school
science class you may have forgotten….
Gregor Mendel
• (1822-1884)
• Austrian priest
• Experimented with
pea plants.
• Mendel crossed pea plants
which had green seeds
with those with yellow
seeds.
• The result was not a greenyellow seed, but instead all
the seeds were yellow.
• This means there was one
trait (the color yellow)
which dominated the other
traits.
Why?
• Mendel showed that genetic information is
inherited in different patterns and is done so
in units --- genes.
– Genes: a section of DNA that is coded for a
particular function.
– Genotypes: combinations (1 of 3) of genes.
Why?
• The experiment was repeated with the new
yellow seeds. Mendel found that some of
their offspring had BOTH yellow and green
seeds, with a ratio of 3:1.
• Through these (and other) experiments
Mendel devised several principles of
inheritance. This is why today the study of
genetic inheritance is called Mendelian
Genetics.
Deoxyribonucleic Acid (DNA)
• Provides genetic code in
biological structures and
also works as a means to
“translate” this code.
• Structure:
– Bases: 4 kinds: A (adenine),
T (thymine), G (guanine)
and C (cytosine). These
bases carry out and specify
genetic instructions.
– Chromosomes: Long strands
of DNA sequences).
Ribonucleic Acid (RNA)
• Carries out the instructions for protein
synthesis specified by DNA.
• A major difference between RNA and DNA
is that in DNA “A” and “T” bond together
while in RNA “A” attracts “U” (uracil)
• Messenger RNA: transports genetic
instructions from the DNA molecule to the
site of protein synthesis.
Genes
• DNA sequences which have identifiable
functions.
• Homeobox genes: encode a sequence of 60
amino acids which regulate embryonic
development.
• Regulatory genes: act as a genetic switch
which turn protein coding genes on or off.
Mitosis and Meiosis
• Mitosis: process of the duplication of
chromosomes in body cells into diploid
cells. Creates two “daughter” cells.
– Diploid means two sets of chromosomes, one
from each parent.
• Meiosis: the creation of sex cells only, by
replication of chromosomes, followed by
cell division. Creates four “daughter” cells.
Mitosis
Meiosis
Dominant and Recessive Alleles
• Alleles: an alternative form of a gene or DNA
sequence that occurs at a given locus. Alleles occur
in pairs, one for each chromosome.
• Dominant Alleles: an allele which masks the effect
of the other allele.
• Recessive Alleles: an allele which is masked by the
effect of the other allele.
Ok, now back to Mendel…
• Mendel’s Law of
Segregation: states that sex
cells contain one of each
pair of alleles.
• Mendel’s Law of
Independent Assortment:
the segregation of any pair
of chromosomes does not
affect the probability of
segregation for other pairs
of chromosomes.
Microevolution
• Microevolution: the changes in the
frequencies of alleles and genotypes from
one generation to the next.
• Looks at the total pattern of an entire
biological population.
Population Genetics
• Population: A group of organisms, geographic or
political boundaries.
• Breeding population: smaller than the total
population. Tend to choose mates within that
group.
• Typically, “population” is determined by a specific
research question.
Allele Frequency
• In order to study microevolution, we must
look at changes in allele frequency and the
causes for those changes.
• We can make inferences about long-term
patterns of evolution based on allele
frequencies.
What causes changes in allele
frequencies?
• Mutation
• Natural Selection
• Gene flow
• Genetic Drift
Mutations
• Results in a random change in the
genetic code.
• Introduce new alleles into a population.
• The ultimate source for all genetic
variation.
• Important for evolutionary change.
Rates of mutations
• Specific mutations are pretty rare
events.
• Mutations are more apparent of they
involve dominant, rather than recessive,
alleles.
• Exact rate of mutations is difficult to
determine.
Natural Selection
• Will increase or decrease alleles over
generations.
• Is a mechanism for evolutionary change
which favors the survival and reproduction
of some organisms over others due to
biological traits.
Natural Selection
• Fitness:
– Is the probability that an organism will survive
and reproduce
– Measured through the genetic contribution
from one generation to the next.
– More fitness = more allele frequency
Natural Selection
• Animal and plant
breeders practice
controlled breeding,
natural selection
basically uses the same
principle.
Natural Selection
Gene Flow
• Movement of genes from
population to another.
• Sometimes referred to as
migration.
• The more two populations
mix, the more similar they
will become genetically.
• Introduces new genes into
populations (from
population A to population
B and visa versa).
Genetic Drift
• Random generational changes in allele
frequencies.
• Is the result of natural probability.
• Occurs in each generation.
• The effect of genetic drift on populations
depends on the overall size of the
population, the smaller the population, the
larger the effect of genetic drift.
Genetic Drift
• Often occurs when a small number of “founders”
create a new population. This is called the Founder
Effect or Bottleneck Effect.