Genetics Review - Hartnell College
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Genetics Review
Chapter 5
Mendel’s Investigations
Gregor Mendel was
the first to closely
examine principles of
heredity.
Mendel chose peas to
study inheritance
because they possess
several contrasting
traits without
intermediates.
Mendel’s Investigations
Mendel was not aware of the existence of
chromosomes or genes.
It is easier to get the big picture of heredity by
combining Mendel’s results with what we know
about chromosomes.
Meiosis
Meiosis is the special type of cell division that
produces eggs and sperm.
In meiosis, a diploid cell with two sets of
homologous chromosomes will divide so that
the daughter cells are haploid and have one
set of chromosomes.
Meiosis
Chromosomes have replicated during
interphase just as in mitosis.
Meiosis actually consists of two separate
divisions.
Meiosis I – serves to separate the two versions of
the chromosome (homologues).
Meiosis II – serves to separate the two replicas of
each version (sister chromatids).
Meiosis
Because there is only one replication of DNA
but two cell divisions, each of the four daughter
cells is haploid – has only one set of
chromosomes.
Fertilization
Fertilization – reestablishes the diploid
chromosome number.
Union of egg and sperm produces a zygote (single
cell).
Contains chromosomes of egg and sperm – 2
sets of chromosomes (diploid).
Meiosis I
Prophase I –
Chromosomes
become visible.
The 2 versions of
each chromosome
pair up and exchange
segments. This is
called crossing over.
Late in prophase, the
nuclear envelope
disappears.
Meiosis I
Metaphase I – Spindle apparatus forms.
Chromosomes line up in the middle.
Which chromosome faces which pole is random.
This is called independent assortment.
Meiosis I
Anaphase I – The spindle is complete.
Homologues are pulled apart and move toward
opposite poles.
Sister chromatids NOT separated yet.
Each pole has half as many chromosomes (one set
rather than two) as the original cell.
Telophase I – the chromosomes gather at the
two poles and wait for the onset of meiosis II.
Meiosis II
After a brief interphase in which NO DNA
synthesis occurs, meiosis II begins.
Meiosis II is just like mitosis except that the
sister chromatids are no longer identical due to
crossing over.
Meiosis II
Prophase II – nuclear envelopes break down
as a new spindle forms.
Metaphase II – chromosomes line up in the
middle of the cell and spindle fibers bind to
both sides of the centromeres.
Meiosis II
Anaphase II – spindle fibers contract splitting
the centromeres and moving the sister
chromatids to opposite poles.
Telophase II – The nuclear envelope reforms
around the four sets of daughter chromosomes.
Meiosis II
The resulting 4 daughter cells are haploid.
No 2 cells are alike due to crossing over.
In animals, these cells develop directly into
gametes (eggs & sperm).
In plants, fungi & many protists they divide
mitotically to produce greater numbers of
gametes.
http://www.youtube.com/watch?v=D1_-mQS_FZ0&list=FL9N_Px072WuVorSwDfqf-9w&index=55&feature=plpp_video
Sex Determination
Sex chromosomes
vs. autosomes
Autosomes –
chromosomes
present in both
sexes, do not
influence sex.
In humans, females
have 2 X
chromosomes, while
males have and X
and a Y.
Sex Determination
Some species have
XX females and X
males.
Others have ZZ
males and ZW
females.
In others, sex is
determined
environmentally.
Mendel’s Laws
Mendel’s experiments with garden peas
resulted in his two laws of inheritance.
Law of segregation
Law of independent assortment
Mendel’s Peas
The peas can self-fertilize or outcross.
Mendel could control who the parents were.
Mendel always started with true-breeding
parents.
E.g. self-fertilized white flowered parents always
produced white flowered offspring.
Mendel’s Peas
He could cross true
breeding white with
true breeding purple
– this is the parental
generation.
Resulting in all
purple offspring –
this is the F1
generation.
Mendel’s Peas
Allowing the hybrid
F1 generation to self
pollinate gives the F2
generation with 3
purple: 1 white
offspring.
He kept careful
quantitative records
that allowed him to
find patterns.
Mendel’s Law of Segregation
Mendel’s explanation of the 3:1 ratio of purple
(dominant) to white (recessive) flowers
resulted in the Law of Segregation.
Mendel’s Law of Segregation
Alternative versions of genes account for
variations in inherited characters.
Two versions of the flower color gene are purple &
white.
We now call these versions alleles.
Mendel’s Law of Segregation
For each character, an organism inherits two
alleles, one from each parent.
Mendel deduced this without knowledge of
chromosomes!
If there are two different alleles present only
one of them – the dominant allele – determines
the appearance.
Mendel’s Law of Segregation
Mendel’s Law of
Segregation – the
two alleles for a
heritable character
separate during
gamete formation and
end up in different
gametes.
Each egg or sperm
will contain either one
of the two alleles, but
not both!
Genetic Terms
Homozygous – both alleles are the same.
PP homozygous dominant – purple flowers.
pp homozygous recessive – white flowers.
Heterozygous – two different alleles.
Pp heterozygous, shows dominant, purple color.
Genetic Terms
Genotype – the
alleles that are
actually present.
PP, Pp, pp
Phenotype – the
physical appearance
of the organism.
Purple or white
flowers.
Genetic Terms
Monohybrid cross – crossing two individuals
that are heterozygous for one particular trait.
Pp X Pp
Dihybrid cross – crossing two individuals that
are both heterozygous for two separate traits.
YyTt X YyTt
The Testcross
Given a purple
flowered pea plant
with unknown
parents, we will
cross it to a
homozygous
recessive (white)
individual to
determine its
genotype.
The Law of Independent Assortment
Following two traits at once:
Yellow (Y) vs. green (y)
Tall (T) vs. short (t)
Cross true-breeding yellow, tall (YYTT) with
true-breeding green, short (yytt) to get F1
individuals that are dihybrids (het for both traits
– YyTt).
The Law of Independent Assortment
Each pair of alleles
separates
independently of
other pairs during
gamete formation.
At least as long as
the pairs of alleles
are on separate
chromosomes.
Complexities
Mendel was fortunate to have chosen a simple
system for study.
In reality, there are a number of complicating
factors.
The Spectrum of Dominance
The traits that
Mendel examined
showed complete
dominance.
The heterozygotes
looked just like
homozygous
dominant
individuals.
The Spectrum of Dominance
Codominance occurs when both alleles affect
the phenotype in separate, distinguishable
ways.
Both phenotypes are expressed.
Not an intermediate.
AB blood types
The Spectrum of Dominance
In incomplete
dominance, the
phenotype of a
heterozygote
appears to be
intermediate to, or
distinct from, the
homozygous
dominant and
homozygous
recessive conditions.
Multiple Alleles
Most genes actually have more than two
different alleles.
Human Blood Type – 3 different alleles.
I A, I B, i
I AI A, I Ai
Type A blood
I BI B, I Bi
Type B blood
I AI B
Type AB blood
ii
Type O blood
Pleiotropy
Pleiotropy is a property where a gene has
more than one effect on the phenotype of an
organism.
The gene that causes sickle cell disease also
conveys some resistance to malaria.
Epistasis
Epistasis (from the
Greek word for
stopping) – one
gene can alter the
phenotypic
expression of
another gene.
Polygenetic Inheritance
Polygenetic
inheritance - Some
traits have more
than one gene
contributing to a
phenotype – like
skin color in
humans.
Alleles have a
cumulative effect.
Environmental Effects
Some traits can be affected by the
environment.
Exposure to sunlight affects skin color in
humans.
Nutrition affects height in humans.
Soil acidity affects color in hydrangea flowers.
Mendel & Modern View of Heredity
Mendel’s fundamental principles of heredity
can be expanded to understand the more
complex issues.
These principles can be applied to any living
organism.
The Chromosomal Theory of
Inheritance
Genes have specific positions (loci) on
chromosomes.
Chromosomes undergo independent
assortment and segregation.
Sex-Linked Genes
Genes located on the sex chromosomes (X or
Y – usually X) are called sex-linked genes.
Fathers pass on a sex-linked genes only to
daughters (sons only receive the Y).
Sex-Linked Genes
Color-blindness is a sex-linked trait in humans.
Much more common in males.
Males only need to inherit one recessive allele
Females need to inherit two – one from each
parent.
Experimental Evidence
T.H. Morgan’s
experiments
provided the first
evidence of the
association between
a specific gene &
chromosome.
Experimental Evidence
White eyes in fruit
flies are recessive to
red eyes.
White eyes found
only in males in F2
Eye color gene
located on X
chromosome.
Autosomal Linkage
Linked genes are located close together on
the same chromosome.
They are usually inherited together.
They do not follow the rule of independent
assortment!
Linked Gene Experiment
Two recessive
mutant traits:
Black rather than
grey bodies
Vestigial rather than
normal size wings
Parental
phenotypes:
Grey, normal wings
Black, vestigial
wings
Linked Gene Experiment
Testcross produced
mostly the parental
phenotypes.
Linked Gene Experiment
Some non-parental phenotypes were
produced.
Genetic recombination – Crossing over
explains how this happens.
Linkage Mapping
A genetic map of the
sequence of genes on
a chromosome can be
made using the
frequency of
recombination data for
a number of traits.
Crossing over more
likely to separate
genes that are further
apart.
Alterations in Chromosome Number
Errors in meiosis or mitosis may lead to one
extra or one less chromosome.
This is called aneuploidy.
Trisomic – 3 copies of a chromosome
Monosomic – only 1 copy
If the organism survives, it usually has symptoms
relating to the increase or decrease in proteins
coded for by the extra (or missing) chromosome.
Alterations in Chromosome Number
Euploidy – An addition or deletion of a whole
set of chromosomes.
Polyploidy is the condition where there are
more than 2 complete sets of chromosomes.
Triploid – 3 sets
Tetraploid – 4 sets
Common in plants
Having an entire extra set is not as detrimental as
having one extra chromosome.
Alterations of Chromosome
Structure
Genes
Gene
gene product
phenotypic expression
Gene products = proteins
Beadle & Tatum’s experiments using bread mold
led to the idea that one gene produces one
enzyme.
Today’s version: a nucleic acid sequence (usually
DNA) that encodes a functional polypeptide or
RNA sequence.
Nucleic Acids
DNA and RNA both built of nucleotides
containing
Sugar (deoxyribose or ribose)
Nitrogenous base (ATCG or AUCG)
Phosphate group
Nitrogenous Bases
Nitrogenous bases
can be double
ringed purines or
single ringed
pyrimidines.
Nitrogenous Bases
A purine will always pair with a pyrimidine.
DNA
The phosphate
group and sugar
make up the
backbone of the
DNA molecule.
DNA
The DNA backbone:
Phosphate groups and pentose sugars.
The 5' end of each strand has a free phosphate
group attached to the 5' carbon of the pentose
sugar.
The 3' end has a free hydroxyl group attached
to the 3' carbon of the pentose sugar.
DNA
DNA consists of two
complementary
chains connected by
hydrogen bonds.
A=T
C=G
DNA
DNA synthesis occurs in
the 5' to 3' direction in
both strands.
The DNA strands are
antiparallel
5' end of one is
associated with the 3'
end of the other.
The DNA ladder is
twisted into a double
helix
Ten base pairs occur
per turn.
RNA
RNA exists as a single polynucleotide chain.
Ribose
Uracil
DNA Replication
DNA must replicate itself
prior to cell division.
Enzymes are responsible
for each step of
replication, including
proofreading.
The helix unwinds,
separates, and each half
acts as a template for the
formation of a new
complementary strand.
Reaction catalyzed by
DNA polymerase.
Gene Expression
Gene expression – the use of information in
DNA to direct the production of particular
proteins.
Transcription – first stage of gene expression. A
messenger RNA (mRNA) is synthesized from a gene
within DNA.
Translation – second stage – mRNA is used to
direct production of a protein.
DNA Coding
DNA codes for the
sequence of amino
acids in a protein.
A codon is three
base-pairs long and
is a segment of
mRNA that codes for
an amino acid.
Transcription
Messenger RNA (mRNA) transcribes the DNA
and transports it out of the nucleus.
Transcription
Before leaving the nucleus, segments of mRNA
called introns are removed and the exons are
spliced together.
Exons contain the information coding for the
protein that will be synthesized.
Transcription Review
http://www.youtube.com/watch?v=OtYz_3rkvPk&list=FL9N_Px072WuVorSwDfqf-9w&index=47&feature=plpp_video
Translation
Translation occurs on ribosomes outside the
nucleus.
mRNA attaches to a ribosome and protein
synthesis begins.
Translation
Transfer RNA (tRNA)
collects free amino acids
from the cytoplasm and
delivers them to the
polysome (mRNA-ribosome
complex) where they are
assembled into a
polypeptide.
tRNA has a triplet – the
anticodon – that is
complementary to the codon of
mRNA.
Translation Review
http://www.youtube.com/watch?v=5bLEDd-PSTQ&feature=autoplay&list=FL9N_Px072WuVorSwDfqf-9w&playnext=1
Regulating Gene Expression
Cells control the expression of genes by
saying when they are transcribed, not how
fast.
Some regulatory proteins block the binding of
the polymerase, and others facilitate it.
Storage and Transfer of Genetic
Information
Regulation of Gene Expression in Eukaryotes
As tissues differentiate, they use only a part of the
genetic instruction present every cell.
In a particular cell or tissue most genes are inactive
at any given moment.
Gene Mutations
Gene mutations result in an alteration of the
sequence of bases in the
DNA.
Harmful
Neutral
Beneficial
The Importance of Genetic
Change
Evolution begins with alterations in the genetic
message.
Mutation creates new alleles
Gene transfer alters gene location
Recombination shuffles these changes
Chromosomal rearrangement alters the organization
of entire chromosomes.
The Importance of Genetic
Change
Changes that result in the organism leaving
more offspring are often preserved.
Other changes result in fewer offspring – these
changes are usually lost.
Genetic changes can only be inherited if they
occur in germ-line tissue!
The Importance of Genetic
Change
Genetic change through mutation and
recombination provides the raw material for
evolution.