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Gregor Mendel (1822-1884)
Mendel developed a strategy of research before
beginning his experiments. His goals were:
• to determine the number of different forms of
hybrids produced
• to arrange the forms according to generations
(F1, F2 etc.)
• to attempt to evaluate the statistical relations
(i.e. the proportions) of the various forms
Seven characteristics of Peas studied by Mendel
• Round or wrinkled ripe seeds
• yellow or green seed interiors
• green or yellow unripe pods
• purple or white flowers
• inflated or pinched pods
• axial or terminal flowers
• long or short stems
Mendel also found that parental, F1, and F2 peas
behaved differently when crossed:
P1 (yellow)
X
P2 (green)
->
F1 (all yellow)
F1 (yellow)
X
P2 (green)
->
½ yellow
½ green
F2 (yellow)
X
P2 (green)
->
2/3 yellow
1/3 green
When he examined each F2 (yellow) family
separately, he found that:
• 2/3 of the F2 yellows gave ½ yellow and ½
green offspring
x
=
•1/3 of the F2 yellows gave all yellow offsping
x
=
Mendel’s explanation
P1 (yellow)
X
P2 (green)
YY
yy
YY
yy
y
y
Y
Y
Yy
Yy
Yy
Yy
All offspring (F1 generation) = Yy
Yy
When he crossed the F1 generation: (F1 x F1)
Yy
x
Yy
Yy
Yy
F2 generation
Y
y
Y
YY
Yy
y
Yy
yy
=
¼
YY
½
Yy
¼
yy
When he examined each F2 (yellow) family separately,
he found that:
• 2/3 of the F2 yellows gave ½ yellow and ½ green
offspring
x
=
•1/3 of the F2 yellows gave all yellow offsping
x
=
F2 yellow individuals are either YY or Yy
(two different punnet squares with the yellow genotypes)
We can use a conditional probability.
Pr(Yellow F2 is Yy) = Pr(Yy in F2)/Pr(F2 is yellow) = (1/2)/(3/4) = 2/3
Pr(Yellow F2 is YY) = Pr(YY in F2)/Pr(F2 is yellow) = (1/4)/(3/4) = 1/3
Mendel’s Law of Independent Segregation: the
genetic basis of any trait is determined by one
particle from each parent (one from the mother
and one from the father). These particles that
Mendel refers to are now known as genes on
chromosomes.
Mendel’s Law of Independent Assortment:
particles (chromosomes) are distributed
randomly into gametes during meiosis. (i.e. each
of these particles is equally likely to be
transmitted when gametes are formed)
If a trait is dominant, an individual has to carry only
one allele for that trait for it to be expressed in the
phenotype.
For a recessive trait to be expressed, both alleles
must be recessive.
Incomplete dominance: Expression of a
phenotype that is intermediate between
those of the parents
Codominance: full phenotypic expression of
both alleles in the heterozygous condition
Walter Sutton and Theodore
Boveri (1903)
separately proposed that
chromosomes were the cellular
components that physically
contained the genes.
locus: the position on a chromosome where a given
gene (or other structure) occurs.
A gene codes for a protein or a portion of a
protein
alleles: alternate forms of a gene
The Cell cycle:
Interphase: the portion of the cell cycle during which
metabolic processes and other cellular activities occur.
Mitosis is the second major part of the cell cycle.
Mitosis: A form of cell division that produces two
identical daughter cells, each with the same
complement of chromosomes as the parent cell.
At the same time as mitosis, the process of cytokinesis
occurs.
Cytokinesis is the division of the cytoplasm.
Mitosis takes place in four stages:
Prophase: the chromosomes condense (they are more
threadlike during interphase so that protein
transcription can occur) and become visible as
chromatids. Spindle fibers begin to form.
Mitosis (continued)
Metaphase: the nuclear membrane dissolves. The
chromosomes are pulled by the spindle fibers (which
attach at the centromere) and align at the equator of
the cell.
Mitosis (continued)
Anaphase: the centromeres divide, and each
chromatid is pulled toward opposite poles
Mitosis (continued)
Telophase: the chromosomes arrive at opposite
poles and begin to uncoil so that they are no
longer visible. The nuclear membrane begins to
form and the cytoplasm divides during
cytokinesis.
Meiosis: the process of cell division during which
one cycle of chromosome replication is followed
by two successive cell divisions to produce four
haploid cells.
Meiosis occurs in two division events:
Meiosis I: like mitosis, the chromosomes have
replicated during interphase.
Prophase I : the chromosomes coil and condense and then
the members of a chromosome pair physically associate
in a process known as synapsis (i.e. the sister chromatids
are joined by a single centromere). Each chromosome
pair associates with its homologous chromosome pair
forming a tetrad. At this point crossing-over occurs
Chromatid
(a chromosome)
Chromosome pair
(sister chromatids)
tetrad
Crossing-over: a process of recombination where nonsister (but
homologous) chromatids physically overlap (or cross-over) and
then exchange material.
Figure 02.13
Meiosis (continued)
Metaphase I: the nuclear membrane
disappears, the tetrads align at the center of
the cell
Meiosis (continued)

Anaphase I: the tetrad is pulled apart with one
pair of sister chromatids (still attached by one
common centromere) being pulled to one pole
and the other pair being pulled to the other. This
process is called disjunction.
Meiosis (continued)
Telophase I: the nuclear membrane may reform or the
cell may go directly into meiosis II. (after cytokinesis)
Meiosis II
Prophase II: Chromosome duplication does not occur again but
the chromosomes do condense again and spindle begins to form
Metaphase II: The chromosomes line up (sister chromatids are
still attached together at a single centromere) in the middle of the
cell.
Anaphase II: The centromere of each sister chromatid pair
divides, the sister chromatids separate and start to move toward
opposite poles (pulled by spindle fibers)
Telophase II: The chromosomes uncoil, the nuclear membrane
forms and the cytoplasm begins to divide (cytokinesis).
RESULT = four HAPLOID cells
At the start of
Meiosis II
At the end of
Meiosis
Chromosomal Theory of Inheritance: The
theory that genes are carried on chromosomes
and that the behavior of chromosomes during
meiosis is the physical explanation for Mendel’s
observations on the segregation and independent
assortment of genes.
The structure of
DNA of identified in
1953 by Francis
Crick and James
Watson.
Transcription: the gene is copied into messenger RNA
What is RNA?
• RNA = ribonucleic acid
• RNA is single stranded
• RNA has a different type of sugar
• instead of the base T (thymine), RNA uses U
(uracil) which is complementary to A.
mRNA = messenger RNA
The central dogma of biology
transcription
translation
DNA
RNA
Protein
Point mutation:
Hb
DNA
RNA
Amino
Acids
Hb S
DNA
RNA
Amino
Acids
CAC
GTG
GAC TGA
GGA CTC CTC TTC
GUG
CAC
CUG ACU
CCU GAG
val
his
CAC
GTG
GAC TGA
GGA CAC CTC
GUG
CAC
CUG ACU
CCU GUG
val
his
leu
leu
thr
thr
pro
pro
glu
val
TTC
Human Genome Project (HGP)
• aids in basic understanding of how the body
functions (this has medical applications)
• helps the understanding of human evolution
because we can compare the information with
that from other species to see how we differ
and we can look at the diversity (or extent of
variation) within humans.
The Four Forces of Evolution:
1.
2.
3.
4.
Mutation
Gene Flow (or migration)
Genetic Drift
Natural Selection
From a modern genetic perspective,
evolution is a a change in the
frequency of alleles from generation
to the next.
population: a community of
individuals where mates are usually
found
Microevolution: small changes in allele
frequencies
Macroevolution: change over many
generations
The Four Forces of Evolution:
1. Mutation: a change in the DNA sequence
2. Gene Flow (or migration): the exchange
of genes between populations
3. Genetic Drift: changes in allele frequency
that occur at random
4. Natural Selection: the process that
produces adaptation (provides directional
allele frequency change relative to
specific environmental factors)
Genotype: the combination of alleles that
characterizes an individual at some set of genetic
loci. For example: someone may have the genotype
AO at the ABO locus.
Phenotype: the observable characteristics of
organisms. For example, a person with the genotype
AO, would have the phenotype of type A blood.
Random mating: mating takes place at random with
respect to the gene or trait in question
Hardy-Weinberg Model Assumptions
• random mating
• nonoverlapping generations
• large population size (i.e. genetic drift isn’t an issue)
• no migration
• no mutation
• no natural selection