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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.