Genetic basis of inheritance

Genetics andGenetics and
HeredityHeredity

HistoryHistory
• GeneticsGenetics is the study of genes.is the study of genes.
• InheritanceInheritance is how traits, oris how traits, or
characteristics, are passed on fromcharacteristics, are passed on from
generation to generation.generation to generation.
• Chromosomes are made up of genes,Chromosomes are made up of genes,
which are made up of DNA.which are made up of DNA.
• Genetic material (genes,chromosomes,Genetic material (genes,chromosomes,
DNA) is found inside the nucleus of a cell.DNA) is found inside the nucleus of a cell.
• Gregor MendelGregor Mendel is considered “The Fatheris considered “The Father
of Genetics"of Genetics"

Gregor MendelGregor Mendel
• Austrian Monk.Austrian Monk.
• Experimented with “pea plants”.Experimented with “pea plants”.
• Used pea plants because:Used pea plants because:
– They were availableThey were available
– They reproduced quicklyThey reproduced quickly
– They showed obvious differences in the traitsThey showed obvious differences in the traits
Understood that there was something thatUnderstood that there was something that
carried traits from one generation to thecarried traits from one generation to the
next- “next- “FACTORFACTOR”.”.

Mendel cont……
In the mid-1800s, the rules underlying
patterns of inheritance were uncovered in
a series of experiments performed by an
Austrian monk named Gregor Mendel.

Mendel's Plant Breeding Experiments
Gregor Mendel was one of the first to apply
an experimental approach to the question
of inheritance.
For seven years, Mendel bred pea plants
and recorded inheritance patterns in the
offspring.
Particulate Hypothesis of Inheritance
Parents pass on to their offspring separate
and distinct factors (today called genes)
that are responsible for inherited traits.

Mendelian GeneticsMendelian Genetics
• DominantDominant traits-traits- traits that are expressed.traits that are expressed.
• RecessiveRecessive traits-traits- traits that are covered up.traits that are covered up.
• Alleles-Alleles- the different forms of a characteristic.the different forms of a characteristic.
• Punnett Squares-Punnett Squares- show how crosses areshow how crosses are
made.made.
• Probability-Probability- the chances/ percentages thatthe chances/ percentages that
something will occur.something will occur.
• Genotype-Genotype- the types of genes (Alleles)the types of genes (Alleles)
present.present.
• Phenotype-Phenotype- what it looks like.what it looks like.
• Homozygous-Homozygous- two of the same alleles.two of the same alleles.
• Heterozygous-Heterozygous- two different alleles.two different alleles.

Mendel was fortunate he chose the Garden Pea
•Mendel probably chose to work
with peas because they are
available in many varieties.
•The use of peas also gave Mendel
strict control over which plants
mated.
•Fortunately, the pea traits are
distinct and were clearly
contrasting.

To test the particulate hypothesis, Mendel crossed true-
breeding plants that had two distinct and contrasting traits—for
example, purple or white flowers.
What is meant by “true breeding?”
Mendel cross-fertilized his plants by hand. Why is it important to control
which plants would serve as the parents?

For each monohybrid cross, Mendel cross-fertilized true-breeding plants that
were different in just one character—in this case, flower color. He then allowed
the hybrids (the F1 generation) to self-fertilize.

Typical breeding experiment
P generation (parental
generation)
F1 generation (first filial
generation, the word filial
from the Latin word for
"son") are the hybrid
offspring.
Allowing these F1
hybrids to self-pollinate
produces:
F2 generation (second
filial generation).
It is the analysis of this
that lead to an
understanding of genetic
crosses.

Mendel studies seven characteristics in the garden pea

:
Statistics indicated
a pattern.

How is it possible to maintain such
genetic continuity?
Martin Sheen
Charlie Sheen
Emilio EstevezKirk Douglas
Michael
Kirk

Chromosomes
Homologous chromosome: one of a matching pair of
chromosomes, one inherited from each parent.
Sister chromatids are identical

What genetic principles account for the transmission of such traits from
parents to offspring?
The Blending Hypothesis of Inheritance
In the early 1800’s the blending hypothesis was proposed. Genetic material
contributed by the two parents mixes in a manner analogous to the way blue
and yellow paints blend to make green.
What would happen if this was the case?

Law of Dominance
In the monohybrid cross (mating of two organisms that differ in only one
character), one version disappeared.
What happens when the F1’s are crossed?

The F1 crossed
produced the F2
generation and the
lost trait appeared
with predictable
ratios.
This led to the
formulation of the
current model of
inheritance.

Alleles: alternative versions of a gene.
The gene for a particular inherited character resides at a specific locus
(position) on homologous chromosome.
For each character, an organism
inherits two alleles, one from each
parent

How do alleles differ?
Dominant - a term applied to the trait (allele) that is expressed irregardless of
the second allele.
Recessive - a term applied to a trait that is only expressed when the second
allele is the same (e.g. short plants are homozygous for the recessive allele).
Dominant allele
Recessive
allele
Recessive allele
Recessive allele

Probability and Punnett Squares
Punnett square: diagram showing the probabilities of the
possible outcomes of a genetic cross

Genotype versus phenotype.
How does a
genotype ratio differ
from the phenotype
ratio?

Punnett squares - probability diagram illustrating the possible
offspring of a mating.
Ss X Ss
gametes

Testcross
A testcross is designed to reveal whether an organism that displays the
dominant phenotype is homozygous or heterozygous.

Variation in Patterns of Inheritance
Intermediate Inheritance (blending): inheritance in which
heterozygotes have a phenotype intermediate between the phenotypes of
the two homozygotes

How Does it Work?How Does it Work?

The Importance of the Environment
The environmental influences the expression of the genotype so the
phenotype is altered.
Hydrangea flowers of the same genetic variety range in color from blue-
violet to pink, depending on the acidity of the soil.
Multifactorial; many factors, both
genetic and environmental,
collectively influence phenotype in
examples such as skin tanning

Chromosome Theory of Inheritance
Improved microscopy techniques, understand cell processes and genetic
studies converged during the late 1800’s and early 1900’s.
It was discovered that Mendelian inheritance has its physical basis in the
behavior of chromosomes during sexual life cycles.
Walter S. Sutton Theodor Boveri Hugo de Vries

Pedigree analysis reveals Mendelian patterns in human inheritance
In these family trees, squares symbolize males and circles represent
females. A horizontal line connecting a male and female (--) indicates a
mating, with offspring listed below in their order of birth, from left to right.
Shaded symbols stand for individuals with the trait being traced.

Disorders Inherited as Recessive Traits
Over a thousand human genetic disorders are known to have Mendelian
inheritance patterns. Each of these disorders is inherited as a dominant or
recessive trait controlled by a single gene. Most human genetic disorders are
recessive.
A particular form of deafness is
inherited as a recessive trait.

Many human disorders follow
Mendelian patterns of inheritance
Cystic fibrosis, which strikes one
out of every 2,500 whites of
European descent but is much rarer
in other groups. One out of 25
whites (4% ) is a carrier.
The normal allele for this gene
codes for a membrane protein that
functions in chloride ion transport
between certain cells and the
extracellular fluid. These chloride
channels are defective or absent.
The result is an abnormally high
concentration of extracellular
chloride, which causes the mucus
that coats certain cells to become
thicker and stickier than normal.

Tay-Sachs disease is caused by a dysfunctional enzyme that fails to break
down brain lipids of a certain class. Is proportionately high incidence of
Tay-Sachs disease among Ashkenazic Jews, Jewish people whose
ancestors lived in central Europe
Sickle-cell disease, which affects one out of 400 African Americans.
Sickle-cell disease is caused by the substitution of a single amino acid in
the hemoglobin protein of red blood cells

Dominantly Inherited Disorders
Achondroplasia, a form of dwarfism with an incidence of one case among
every 10,000 people. Heterozygous individuals have the dwarf phenotype.
Huntington’s disease, a degenerative disease of the nervous system, is
caused by a lethal dominant allele that has no obvious phenotypic effect
until the individual is about 35 to 45 years old.

Sex-Linked Disorders in Humans
Duchenne muscular dystrophy, affects about one out of every 3,500
males born in the United States. People with Duchenne muscular dystrophy
rarely live past their early 20s. The disease is characterized by a progressive
weakening of the muscles and loss of coordination. Researchers have
traced the disorder to the absence of a key muscle protein called dystrophin
and have tracked the gene for this protein to a specific locus on the X
chromosome.
Posture changes during
progression of Duchenne
muscular dystrophy.

Hemophilia is a sex-linked recessive trait defined by the absence of one or
more of the proteins required for blood clotting.

Color Blindness In Humans: An X-Linked Trait
Numbers That You Should See If You Are In One Of The Following
Four Categories: [Some Letter Choices Show No Visible Numbers]
Sex-Linked Traits:
1. Normal Color Vision:
A: 29, B: 45, C: --, D: 26
2. Red-Green Color-Blind:
A: 70, B: --, C: 5, D: --
3. Red Color-blind:
A: 70, B: --, C: 5, D: 6
4. Green Color-Blind:
A: 70, B: --, C: 5, D: 2

Pattern Baldness In Humans: A Sex Influenced Trait
Baldness is an autosomal trait and is apparently influenced by sex hormones
after people reach 30 years of age or older.
In men the gene is dominant, while in women it is recessive. A man needs
only one allele (B) for the baldness trait to be expressed, while a bald
woman must be homozygous for the trait (BB).
What are the probabilities for the children for a bald man and
woman with no history of baldness in the family?

DNADNA
• DNA is often
called the
blueprint of life.
• In simple terms,
DNA contains
the instructions
for making
proteins within
the cell.

Why do we studyWhy do we study DNADNA??
We study DNA forWe study DNA for
many reasons:many reasons:
• its centralits central
importance toimportance to
all life on Earthall life on Earth
• medical benefitsmedical benefits
such as curessuch as cures
for diseasesfor diseases
• better foodbetter food
crops.crops.

Chromosomes andChromosomes and
DNADNA
• ChromosomesChromosomes
are made up ofare made up of
genes.genes.
• Genes are madeGenes are made
up of a chemicalup of a chemical
called DNA.called DNA.

The Shape of theThe Shape of the
MoleculeMolecule
• DNA is a veryDNA is a very
long molecule.long molecule.
• The basic shapeThe basic shape
is like a twistedis like a twisted
ladder orladder or
zipper.zipper.
• This is called aThis is called a
double helix.double helix.

OneOne Strand of DNAStrand of DNA
• The backbone ofThe backbone of
the molecule isthe molecule is
alternatingalternating
phosphatephosphate andand
deoxyribosedeoxyribose, a, a
sugar, parts.sugar, parts.
• The teeth areThe teeth are
nitrogenousnitrogenous
basesbases..
phosphate
deoxyribose
bases

The Double HelixThe Double Helix
MoleculeMolecule
• The DNA doubleThe DNA double
helix has twohelix has two
strands twistedstrands twisted
together.together.
• (In the rest of this(In the rest of this
unit we will lookunit we will look
at the structureat the structure
of one strand.)of one strand.)

The Nucleus
• DNA is located in
the nucleus

DNA
deoxyribonucleic acid
• The code of life

NucleotidesNucleotides
C C
C
O
Phosphate
O
C
C
O -P O
O
O
O -P O
O
O
O -P O
O
O
One deoxyribose together with
its phosphate and base make
a nucleotide.
Nitrogenous
base
Deoxyribose

The Basics
• Each side of the
ladder is made up of
nucleic acids.
• The backbone is a
phosphate and a
sugar
• The rung of the
ladder is the
nitrogen base.

Hydrogen BondsHydrogen Bonds
• When makingWhen making
hydrogen bonds,hydrogen bonds,
cytosine alwayscytosine always
pairs up withpairs up with
guanine,guanine,
• And adenineAnd adenine
always pairs upalways pairs up
with thymine.with thymine.
• (Adenine and(Adenine and
thymine are shownthymine are shown
here.)here.)
C
C
C
C
N
N
N
N
N
C
C
C
C
C
N
N
OO
OO
C

FourFour nitrogenous basesnitrogenous bases
• CytosineCytosine CC
• ThymineThymine TT
• AdenineAdenine AA
• GuanineGuanine GG
DNA has four different bases:DNA has four different bases:

Two Stranded DNATwo Stranded DNA
• Remember, DNARemember, DNA
has two strandshas two strands
that fit togetherthat fit together
something like asomething like a
zipper.zipper.
• The teeth are theThe teeth are the
nitrogenousnitrogenous
bases but whybases but why
do they stickdo they stick
together?together?

ImportantImportant
• Adenine andAdenine and
Thymine always joinThymine always join
togethertogether
A -- TA -- T
• Cytosine andCytosine and
Guanine always joinGuanine always join
togethertogether

Types of nitrogen bases
• A= adenine
• G= guanine
• C= cytosine
• T= thymine

Do Now!
• Where is DNA
located?
• What does it look
like?
• What are its bases?
• Why do you think
DNA is located
there?

Copying DNA
• Step 1- DNA unwinds and
unzips
• Step 2- Once the molecule
is separated it copies itself.
• The new strand of DNA has
bases identical to the original

DNA by theDNA by the
numbersnumbers• Each cell hasEach cell has
about 2 m of DNA.about 2 m of DNA.
• The averageThe average
human has 75human has 75
trillion cells.trillion cells.
• The averageThe average
human has enoughhuman has enough
DNA to go from theDNA to go from the
earth to the sunearth to the sun
more than 400more than 400
times.times.
• DNA has aDNA has a
The earth is 150 billion mThe earth is 150 billion m
or 93 million miles fromor 93 million miles from
the sun.the sun.

What’s the main difference
between DNA and RNA

RNA
• In RNA Thymine is
replaced by Uracil
• A-U (RNA)
• not
• A-T (DNA)

• IF the DNA strand is
GTACCAGATTAGC
• What would the RNA strand be?

Transcription
• When a secretary
transcribes a
speech, the
language remains
the same. However,
the form of the
message changes
from spoken to
written

Transcription
• Transcription- RNA
is made from a DNA
template in the
nucleus.
• This type of RNA is
called messenger
RNA or mRNA

Transcription
• DNA is protected
inside the nucleus.
• mRNA carries the
message of DNA
into the cytoplasm to
the ribosome's

Translation
• To translate English
into Chinese
requires an
interpreter.
• Some person must
recognize the worlds
of one language and
covert them into the
other.

tRNA Transfer RNA
• The cells interpreter
• tRNA translated the
three-letter codons
of mRNA to the
amino acids that
make up protein.

Translation
• Genetic translation
converts nucleic
acid language into
amino acid
language.

Codon
• The flow of
information from
gene to protein is
based on codons.
• A codon is a three-
base word that
codes for one amino
acid

• The flow of
information from
gene to protein is
based on codons.

Information Flow: DNA to RNA to
Protein

Let’s Go to the Video!Let’s Go to the Video!
DNA to RNADNA to RNA
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.

Let’s Go to the Video!Let’s Go to the Video!
DNA TRANSLATIONDNA TRANSLATION
QuickTime™ and a

Comparing DNA and RNAComparing DNA and RNA
QuickTime™ and a

Transcription/Translation ReviewTranscription/Translation Review
QuickTime™ and a

Genetic basis of inheritance

In this document