Genetics review; Intro to Hardy Weinberg
12.2, 19.1
1/23 and 1/25
Learning objectives
• Be familiar with genetics terms (genotype, phenotype,
allele, homozygous, heterozygous)
• Be able to explain how a genotype causes a phenotype
• Be able to describe evolution at the genetic level
• Know the 3 ways allele frequency could be determined
• Know the conditions for Hardy-Weinberg equilibrium
• Understand the H-W equation and be able to use it to
determine both allele frequencies and genotype
frequencies, and how to tell if a population is evolving
Office Hours reminder
• Dr. Thole: Mon 2-3, Tues 10-11; 114 MWH
• Shivika (section 02): Mon 10-11; lobby of CGC
• Mal (section 03): Thurs 11-12; lobby of MWH
SI schedule reminder
• Monday 7:00-8:00 pm in Ritter 231
• Tuesday 8:00-9:00 pm in Ritter 231
• Sunday 5:00-6:00 pm in Reinert Classroom
Genotype and phenotype
Genetic variation: genetic differences that exist
among individuals in a population at a particular
point in time
Genotype: the genetic makeup of a cell or organism
PP, Pp, pp for pea flower color
Phenotype: an individual’s observable
characteristics (i.e., height, eye color, weight, color
blindness, etc.)
Purple vs. white flowers
The different forms of any gene are
called alleles, and they correspond to
changes in the DNA sequence
• We all have the same genes, but different alleles
of them
• Since we are diploid, we have 2 copies of every
gene
• If an individual is homozygous, both alleles are
the same
– BB or bb
• If an individual is heterozygous, the two alleles
are different
– Bb
Yellow and green seeds are the result of
two different alleles of the same gene
True breeding means that these
lines are homozygous
Yellow: YY Green: yy
Same trait, same letter!
The dominant allele is a capital
letter, the recessive allele is a
lowercase letter
The principle of segregation – the
separation of alleles into different gametes
TRUE BREEDING
Aa plants produce
gametes A and a, 50%
each = segregation!
Punnett square
1:2:1 genotypic ratio
3:1 phenotypic ratio
Segregation of alleles happens during
meiosis
Phenotypes can also be referred to as
traits
• Simple traits = qualitative traits (Mendelian)
– Phenotype is either/or
– Often controlled by one gene
• Complex traits = quantitative traits
– Phenotype falls into a range
– Often controlled by multiple genes with a strong
environmental influence
Most traits are complex traits
Three common alleles of b-hemoglobin
Sickle cell anemia – how does the
genotype cause the phenotype?
Remember, mutations can be neutral,
beneficial, or harmful.
• In a human male at age 30, about 400 cycles
of cell division have taken place before
meiosis occurs. On average, there are about
30 mutations that arise per generation, so at
age 30 there have been about 1200 new
mutations!
• BUT, only about 2.5% of the human genome
codes for proteins
The sickle cell trait can be beneficial in parts
of the world where malaria is common
Plasmodium
(you don’t need to know these details…)
EVOLUTION
• = a change in the genetic make-up (allele or
genotype frequency) of a population over
time, where some changes will allow
adaptation to the environment and origin of
new species
• HOW do these changes occur, and how can we
measure the change?
Variation
Environmental
Genetic
What causes genetic variation?
Population genetics is the study of genetic variation
in natural populations; changes in genotype and allele
frequency over time
• 2 sources of variation:
• Mutation
– Somatic
– Germ-line
– Deleterious
– Neutral
– Advantageous
• Recombination
ADAPTATION!
Natural selection governs that variants
best suited for growth and reproduction
contribute more to future generations
Measuring Genetic Variation
Gene pool = sum of all alleles in a population
Allele Frequency =
# of copies of an allele
total # of alleles in population
Example 1: The genotypes for pea color in Mendel’s pea plants are
YY – yellow pea plant
Yy – yellow pea plant
yy – green pea plant
What are the frequencies of the Y and y alleles if every plant is green?
When a population exhibits only one allele, we say that population is
fixed for that allele.
Measuring Genetic Variation –
Example 2
Allele Frequency =
# of copies of an allele
total # of alleles in population
Example 2: A population of 100 pea plants has the following
genotype frequencies:
60% yy, 25% Yy, 15% YY
f(Y) =
f(y) =
How do we measure allele
frequencies?
• Phenotype (observable traits)
– When is this a problem?
• Gel electrophoresis
Gel Electrophoresis of DNA or protein
How do we measure allele
frequencies?
• Phenotype (observable traits)
– When is this a problem?
• Gel electrophoresis
• DNA sequencing
Populations evolve, not individuals
• Evolution is a change in genetic makeup of a
population over time
– Generation 1 – BB: 24, Bb: 16, bb: 10
– Generation 2 – BB: 24, Bb: 20, bb: 8
• ANY change in genetic makeup – allele
frequencies or genotype frequencies – is
evolution!
Hardy-Weinberg Equilibrium
p+q=1
p = dominant allele
q = recessive allele
Hardy-Weinberg Equilibrium
p2 + 2pq + q2 = 1
Helpful for predicting genotype frequencies, but can be used in reverse!
Hardy-Weinberg equilibrium describes
situations in which allele and genotype
frequencies do not change
• 5 conditions:
• There can be no differences in the survival and
reproductive success of individuals.
• Populations must not be added to or subtracted from
by migration.
• There can be no mutation.
• The population must be sufficiently large to prevent
sampling errors.
• Individuals must mate at random.
If a population is NOT in HardyWeinberg equilibrium…
• This tells us a population is evolving
• A starting point to determine if the population is
– Undergoing natural selection
– Migrating
– Experiencing genetic drift
– Experiencing new mutations
– Not mating randomly
Is the population in HWE?
• 25 plants have the genotype AA.
• 50 plants have the genotype Aa.
• 25 plants have the genotype aa.
What are the allele frequencies?
What are the genotype frequencies?
Is the population in HWE?
• 25 plants have the genotype AA.
• 50 plants have the genotype Aa.
• 25 plants have the genotype aa.
If there is random mating, what genotype
frequencies would you expect in the next
generation?
Is the population in HWE?
• 60 plants have the genotype AA.
• 20 plants have the genotype Aa.
• 20 plants have the genotype aa.
What are the allele frequencies?
What are the genotype frequencies?
Is the population in HWE?
• 60 plants have the genotype AA.
• 20 plants have the genotype Aa.
• 20 plants have the genotype aa.
If there is random mating, what genotype
frequencies would you expect in the next
generation?
In this population, what are the allele
frequencies?
In this population, what are the genotype
frequencies?
If the individuals in this generation mate
randomly, what would you expect the genotype
frequencies to be in the next generation?