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Introduction to Genetics
• GENETICS – branch of biology that deals
with heredity and variation of organisms.
• Chromosomes carry the hereditary
information (genes)
• Arrangement of nucleotides in DNA
• DNA  RNA  Proteins
• Chromosomes (and genes) occur in pairs
Homologous Chromosomes
• New combinations of genes occur in sexual
reproduction
– Fertilization from two parents
Gregor Johann Mendel
• Austrian Monk, born in what is now Czech Republic in
1822
• Son of peasant farmer, studied
Theology and was ordained
priest Order St. Augustine.
• Went to the university of Vienna, where he
studied botany and learned the Scientific Method
• Worked with pure lines of peas for eight years
• Prior to Mendel, heredity was regarded as a "blending"
process and the offspring were essentially a "dilution"of
the different parental characteristics.
Mendel’s peas
• Mendel looked at seven traits or characteristics of
pea plants:
• Mendel was the first biologist to use
Mathematics – to explain his results
quantitatively.
• Mendel predicted
The concept of genes
That genes occur in pairs
That one gene of each pair is
present in the gametes
Genetics terms you need to know:
• Gene – a unit of heredity;
a section of DNA sequence
encoding a single protein
• Genome – the entire set
of genes in an organism
• Alleles – two genes that occupy the same position
on homologous chromosomes and that cover the
same trait (like ‘flavors’ of a trait).
• Locus – a fixed location on a strand of DNA
where a gene or one of its alleles is located.
• Homozygous – having identical genes (one from
each parent) for a particular characteristic.
• Heterozygous – having two different genes for a
particular characteristic.
• Dominant – the allele of a gene that masks or
suppresses the expression of an alternate allele;
the trait appears in the heterozygous condition.
• Recessive – an allele that is masked by a
dominant allele; does not appear in the
heterozygous condition, only in homozygous.
• Genotype – the genetic makeup of an organisms
• Phenotype – the physical appearance
of an organism (Genotype + environment)
• Monohybrid cross: a genetic cross involving a
single pair of genes (one trait); parents differ by a
single trait.
• P = Parental generation
• F1 = First filial generation; offspring from a
genetic cross.
• F2 = Second filial generation of a genetic cross
Monohybrid cross
• Parents differ by a single trait.
• Crossing two pea plants that differ in stem size,
one tall one short
T = allele for Tall
t = allele for dwarf
TT = homozygous tall plant
t t = homozygous dwarf plant
TT  tt
Monohybrid cross for stem length:
P = parentals
true breeding,
homozygous plants:
F1 generation
is heterozygous:
TT  tt
(tall)
(dwarf)
Tt
(all tall plants)
Punnett square
• A useful tool to do genetic crosses
• For a monohybrid cross, you need a square divided by
four….
• Looks like
a window
pane…
We use the
Punnett square
to predict the
genotypes and phenotypes of
the offspring.
Using a Punnett Square
STEPS:
1. determine the genotypes of the parent organisms
2. write down your "cross" (mating)
3. draw a p-square
Parent genotypes:
TT and t t
Cross
TT  tt
Punnett square
4. "split" the letters of the genotype for each parent & put
them "outside" the p-square
5. determine the possible genotypes of the offspring by filling
in the p-square
6. summarize results (genotypes & phenotypes of offspring)
T
TT  tt
t
t
Tt
Tt
T
Tt
Genotypes:
100% T t
Tt
Phenotypes:
100% Tall plants
Monohybrid cross: F2 generation
• If you let the F1 generation self-fertilize, the next
monohybrid cross would be:
Tt  Tt
(tall)
T
t
T
t
TT
Tt
Tt
tt
(tall)
Genotypes:
1 TT= Tall
2 Tt = Tall
1 tt = dwarf
Genotypic ratio= 1:2:1
Phenotype:
3 Tall
1 dwarf
Phenotypic ratio= 3:1
Secret of the Punnett Square
• Key to the Punnett Square:
• Determine the gametes of each parent…
• How? By “splitting” the genotypes of each
parent:
T T

t t
If this is your cross
The gametes are:
T
T
t
t
Once you have the gametes…
T
T

t
t
T
T
t
t
Tt
Tt
Tt
Tt
Another example: Flower color
For example, flower color:
P = purple (dominant)
p = white (recessive)
If you cross a homozygous Purple (PP) with a
homozygous white (pp):
PP

Pp
pp
ALL PURPLE (Pp)
Cross the F1 generation:

Pp
P
p
Pp
P
p
PP
Pp
Pp
pp
Genotypes:
1 PP
2 Pp
1 pp
Phenotypes:
3 Purple
1 White
Mendel’s Principles
• 1. Principle of Dominance:
One allele masked another, one allele was
dominant over the other in the F1 generation.
• 2. Principle of Segregation:
When gametes are formed, the pairs of
hereditary factors (genes) become separated,
so that each sex cell (egg/sperm) receives
only one kind of gene.
Human case: CF
• Mendel’s Principles of Heredity apply
universally to all organisms.
• Cystic Fibrosis: a lethal genetic disease affecting
Caucasians.
• Caused by mutant recessive gene carried by 1 in
20 people of European descent (12M)
• One in 400 Caucasian couples will be both
carriers of CF – 1 in 4 children will have it.
• CF disease affects transport
in tissues – mucus is accumulated
in lungs, causing infections.
Inheritance pattern of CF
IF two parents carry the recessive gene of
Cystic Fibrosis (c), that is, they are
heterozygous (C c), one in four of their
children is expected to be homozygous for
cf and have the disease:
C
C C = normal
C c = carrier, no symptoms
c c = has cystic fibrosis
c
C
CC
Cc
c
Cc
cc
Probabilities…
• Of course, the 1 in 4 probability of getting the
disease is just an expectation, and in reality,
any two carriers may have normal children.
• However, the greatest probability is for 1 in 4
children to be affected.
• Important factor when prospective parents are
concerned about their chances of having
affected children.
• Now, 1 in 29 Americans is a symptom-less
carrier (Cf cf) of the gene.
Gaucher Disease
• Gaucher Disease is a rare, genetic disease. It
causes lipid-storage disorder (lipids accumulate in
spleen, liver, bone marrow)
• It is the most common genetic disease affecting
Jewish people of Eastern European ancestry
(1 in 500 incidence; rest of pop. 1 in 100,000)
Principle of Independent Assortment
• Based on these results, Mendel postulated the
3. Principle of Independent Assortment:
“Members of one gene pair segregate
independently from other gene pairs during
gamete formation”
Genes get shuffled – these many combinations are
one of the advantages of sexual reproduction
Relation of gene segregation to
meiosis…
• There’s a correlation between the
movement of chromosomes in meiosis and
the segregation of alleles that occurs in
meiosis
Beyond Mendelian Genetics:
Incomplete Dominance
Mendel was lucky!
Traits he chose in the
pea plant showed up
very clearly…
One allele was dominant over another, so
phenotypes were easy to recognize.
But sometimes phenotypes are not very
obvious…
Summary of Genetics
• Chromosomes carry hereditary info (genes)
• Chromosomes (and genes) occur in pairs
• New combinations of genes occur in sexual
reproduction
• Monohybrid vs. Dihybrid crosses
• Mendel’s Principles:
– Dominance: one allele masks another
– Segregation: genes become separated in gamete formation
– Independent Assortment: Members of one gene pair
segregate independently from other gene pairs during gamete
formation
Meiosis
Genes
• Tens of thousands of genes
• Lined up on chromosomes
Chromosomes
• Occur in pairs (Male,
Female)
• DIPLOID—A cell with
two of each kind of
chromosome is said to
be diploid, or 2n,
number of
chromosomes
Gametes
• Male (sperm) and Female (egg)
• Contain one of each kind of
chromosomes.
• A cell with one of each kind of
chromosome is called a HAPLOID and
is said to contain a haploid, or n,
number of chromosomes.
Homologous Chromosomes
• Pair chromosomes are
called homologous
chromosomes—
determine phenotype.
• Gene for same trait
– same order,
– chromosomes in a
homologous pair are not
always identical.
• (Chromosome 4
contains 3 traits
Mendel Studied)
Meiosis
From the Greek word meioun,
meaning “to diminish”.
Cell division that results in a
gamete containing half the
number of chromosomes of its
parents.
Meiosis
• Divisions: Meiosis I and Meiosis II
• Begins with
one diploid (2n) cell
four haploid (n) cells.
• Sex cells (gametes) haploid.
• Sperm fertilizes an egg-results in zygote
(diploid)
• Zygote develops by MITOSIS into a multicellular organism.
• Reproduction —Production and subsequent
fusion of haploid sex cells.
Interphase
• Chromosomes replicate
• Chromosome
– two identical sister chromatids held together by a
centromere
Prophase I
• Chromosomes coil up and a
spindle forms.
• Homologous chromosomes
comes together, matched
gene by gene, to form a
four-part structure called a
tetrad.
• Chromatid pair so tight that
sometimes non-sister
chromatids from
homologous chromosomes
sometimes exchange
genetic material in a
process known as
crossing over.
Crossing Over
•
•
•
•
Exchange of genetic material
Any location
Several locations at once
Humans-Two to three crossovers for each pair of
homologous chromosomes.
Metaphase I
• Centromere attaches to a spindle fiber
• Spindle fibers pull the tetrads into the middle, or
equator, of the spindle.
• Chromosomes are lined up side by side as
tetrads.
Anaphase I
• Chromosomes
separate and move to
opposite ends of the
cell.
• Centromeres holding
the sister chromatids
together do not split
like they do in
anaphase of mitosis.
• Ensures that each new
cell have only one
chromosome from
Telophase I
• Spindle is broken down
• Chromosomes uncoil
• Cytoplasm divides  2
new cells.
• Half of genetic
information of original
cell (one chromosome
from each homologous
pair)
• Another cell division
needed
Meiosis II
• Newly formed cells go through short interphase
(chromosomes don’t replicate)
• Prophase II—Spindle forms in each of the two new
cells and the spindle fibers attach to the
chromosomes.
• Metaphase II—The chromosomes, still made up of
sister chromatids, are pulled to the center of the cell
and line up randomly at the equator.
• Anaphase II—Centromere of each chromosome
splits, allowing sister chromatids to separate and
move to opposite poles.
• Telophase II—Nuclei reform, spindles break down,
and cytoplasm divides.
Meiosis Results
• Four haploid sex cells have been
formed from one original diploid cell.
• Each haploid cell contains one
chromosome from each homologous
pair.
• Haploid cells become gametes,
transmitting the genes they contain to
offspring.
The History of DNA
Early Work
Friedrich Miescher, 1869, first isolates a
substance from the nucleus of cells that
he calls “nuclein.” His student, Richard
Altmann, calls the substance “nucleic
acid.”
Biochemists identify two types of
nucleic acids, later called RNA and DNA.
In 1929, Phoebus Levine at the
Rockefeller center identifies the four
bases of DNA.
What Does DNA Do?
Though early researchers knew that DNA
was found in chromosomes, they doubted
that it was the hereditary material. There
were only four bases. How could for bases
code for all sorts of proteins?
Some researchers, including Linus Pauling,
thought that the protein also found in
chromosomes was probably the hereditary
factor.
Frederick Griffith
In 1928, Frederick Griffith carried out
experiments on pneumonia bacteria in
mice.
Discovery: something in heat-killed
virulent bacteria could be transferred to
live, harmless bacteria and make them
virulent.
Griffith’s Experiment
Oswald Avery
Avery continued working with
Griffith’s findings in hope of
discovering what factor in bacteria
carried the trait of virulence.
Isolated proteins, carbohydrates,
nucleic acids and applied them to nonvirulent bacteria. Only nucleic acids
(DNA) caused a change.
Avery’s Work
Erwin Chargaff
Chargaff studied DNA itself, in
hopes of providing some clues
about its structure.
Discovered that there are always
equal amounts of the bases
Adenine and Thymine, and equal
amounts of Cytosine and Guanine.
Chargaff proposed that these
bases pair with one another in
some way.
Wilkins and Franklin
Rosalind Franklin and Maurice Wilkins
worked with X-ray crystalography to
find more clues about the structure of
DNA.
Franklin’s X-ray images suggested a
helical structure.
Franklin and Wilkins
Watson and Crick
James Watson and Francis Crick were
also working on discovering the
structure of DNA.
Applied Chargaff’s rule, assumed that A
always pairs with T, C with G.
Watson was not entirely convinced of
the helical structure that Franklin had
suggested, and his critique of her work
led her to doubt herself.
Watson and Crick
Wilkins consulted with Watson and
Crick. Without Franklin’s knowledge,
he handed them the data that he and
Franklin had worked on.
Watson immediately recognized the
significance. He and Crick went to work
on a model of DNA.
The First DNA Model
DNA structure
DNA is made up of four bases. RNA also has four bases, but has uracil
instead of thymine.
DNA structure
Across the DNA double-ladder, A always pairs with T, C always pairs
with G because of the number of hydrogen bonds the bases form.
DNA structure
The DNA ladder forms a spiral, or helical, structure, with the two sides
held together with hydrogen bonds.