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Genetics
Chapter 13
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Outline
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Genetics--> Definition
Structure of DNA
DNA Functions
Transcription
Translation
Mutation
Cytogenetics
Mendelian Genetics
Quantitative Traits
Linkage
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Genetics can be defined as
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The biological discipline involving the study
of heredity
 the study of genes.
 the inheritance of physical traits.
 the study and inheritance of DNA.
 the study and inheritance of chromosomes
 all of the above.
Focus on the MOLECULAR-->DNA
(deoxyribonucleic acid)
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Structure of DNA
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DNA molecule organized into chain of
nucleotides composed of three parts:
 Nitrogenous Base
 5-Carbon Sugar (Deoxyribose)
 Phosphate Group
Nucleotides bonded to each other forming a
helix.
 Hydrogen bonds hold each base on one
side of the helix to another base on the
other side.
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Three Parts of DNA
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Structure of DNA-->nucleotides
Four types of DNA nucleotides:
 Purines--Adenine (A) and Guanine (G)
 Pyrimidines--Cytosine (C) and Thymine (T)
Nitrogenous Base Part
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Helix structure:
Two chains
binding together
using hydrogen
bonds
(base pairs)
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DNA Functions (ACGT)
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Storage of Genetic Information
 Gene - Segment of DNA that directs
protein synthesis.
- Protein may act as an enzyme
influencing cell activities.
 Genome - Sum total of DNA in an
organism’s chromosomes.
- Codon - Groups of three nucleotides.
(in a row, ex. AAT; AAA; GCT; etc…)
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DNA Functions
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Replication of Information
 Occurs during S phase of cell cycle.
- Strands of double helix unzip allowing
DNA polymerase (enzyme) to pair
individual nucleotides with the template
strands.
 Semi-Conservative Replication
 Occurs simultaneously in both
directions, and begins at several
points simultaneously.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
DNA Replication
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
DNA Functions
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Expression of Information
 Different subset of the genetic information
is read in each cell type.
 Expression requires two processes.
- Transcription - Copy of gene message
using RNA (intermediate nucleic acid).
- Translation
 Messenger RNA (mRNA) translated to
produce proteins.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Outline
•
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•
•
•
•
•
•
•
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Genetics--> Definition
Structure of DNA
DNA Functions
Transcription
Translation
Mutation
Cytogenetics
Mendelian Genetics
Quantitative Traits
Linkage
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Central Dogma of Molecular Biology
Language metaphor
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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Transcription
Three different types of RNA produced:
 Messenger RNA (mRNA)-->proteins
 Transfer RNA (tRNA)-->translation
 Ribosomal RNA (rRNA)
RNA Synthesis (ACGU nucleotide parts)
 Nucleotides added to single stranded DNA
molecule by RNA polymerase.
- Only portions of the genome replicated.
 Remainder is noncoding DNA
(includes directions for expression!).
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Translation
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mRNA transcripts are decoded into proteins.
 Genetic code based on codons.
- 64 possible combinations of ACGT/U)
(43)
 Code for 20 amino acids.
tRNA acts as a translator.
 Anticodon binds to mRNA codon.
 Ribosomal RNA (rRNA) binds to mRNA
and facilitates translation
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Genetic Code/ codons encode for
20 amino acids = proteins
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Mutation
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Mutations - Changes in DNA sequence.
 Mutagens - Agents that alter DNA.
- Ultraviolet light
- Ionizing radiation (Pasta wheat/Ruby red)
 Somatic Mutation - Occurs in body cell.
 Germ-Line Mutation - Occurs in tissues that
will produce sex cells.
- Passed on to future generations.
All genetic variability due to mutations.
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Cytogenetics
Cytogenetics is the study of chromosome
behavior and structure, from a genetic point
of view.
Changes in Chromosome Structure
 Inversion - Chromosomal piece is broken
and reinserted in the opposite orientation.
- ABCDE
ABDCE
 Translocation - Chromosomal piece breaks
off and attaches to another chromosome.
- Important in speciation (rye)
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Cytogenetics
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Changes in Chromosome Number
 Mistakes during chromosome pairing and
separation can result in gametes carrying
extra or missing chromosomes.
- Aneuploid - Carries, or is missing, an
extra chromosome(s).
- Polyploid - Has at least one complete
extra set of chromosomes.
- Many cultivated plants are polyploid
because they are bigger and produce
more.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
S. tuberosum
Tetraploid
Diploid
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Genetics from Mendel to Today
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Gregor Mendel (Austrian Monk)
inheritance of traits (18501865); rediscovered in 1900s
Used peas
Watson & Crick model of DNA
(1954)
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Mendelian Genetics
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Mendel crossed self-pollinated pea plants
and recorded his observations.
 Crossed a tall plant with a short plant.
 Parental Generation (P)
- All offspring were tall.
- First Filial Generation (F1)
 Cross offspring yielded a ratio of three
tall individuals to one short individual.
 Second Filial Generation (F2)
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Mendelian Genetics
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Law of Unit Characters
 Factors (Alleles), which always occur in
pairs, control the inheritance of various
characteristics.
- Genes are always at the same position
(locus) on homologous chromosomes.
Law of Dominance
 For any given pair of alleles, one
(Dominant) may mask the expression of
the other (recessive).
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Mendelian Genetics
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Phenotype refers to an organism’s physical
appearance.
Genotype refers to the genetic information
responsible for contributing to the phenotype.
 Homozygous - Both alleles identical.
 Heterozygous - Alleles are contrasting.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
start
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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Monohybrid Cross (one trait)
A cross is made between two true-breeding
parents differing for a single trait, producing
an F1 generation. RR X rr
 These plants are intercrossed to produce
an F2 generation.
R
r
R
R
r
Rr
Rr
R
RR
Rr
r
Rr
Rr
r
Rr
rr
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Mendelian Inheritance Pattern
•The phenotype of the F1
resembled one parental
form.
•All F1s looked the same.
•The “hidden” parental
form reappeared in the F2
generation.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Dihybrid Cross
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Cross made for parents differing in two traits.
 Mendel’s Law of Independent Assortment
- Factors (genes) controlling two or more
traits segregate independently of each
other.
 Linked genes do not segregate
independently.
- Punnett Square used to determine
genotypes of the zygotes.
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Dihybrid Cross
Parents differing
in 2 unlinked genetic
traits
Green pod, Tall plant
(GGTT)
X
yellow pod, dwarf
plant
(ggtt)
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Mendelian Genetics
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Backcross
 A cross between a hybrid and one of its
parents.
- Can be used to test inheritance theory.
Incomplete Dominance
 Some genes exhibit traits in which a
heterozygote is intermediate in phenotype
to the two homozygotes.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Incomplete Dominance
Camellias
Snapdragons
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Quantitative Traits
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Quantitative traits exhibit a range of
phenotypes rather than discrete phenotypes
studied by Mendel.
 Molecular geneticists are able to identify
chromosomal fragments, quantitative trait
loci, associated with quantitative traits.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Inheritance of Corn height (Quantitative)
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Cross Tall corn (6 m) x short corn (2 m)
F2 are not 3:1 tall to short but all heights
between 1.5 m and 8 m
# of
plants
P
1
P
2
Plant height
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Linkage
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Occurs with 2 traits - often located close
together on chromosome
Often segregate like a single trait
No recombination occurs
Get a shortage of unique gamete
genotypes in F1 generation
Get fewer genotypes in F2 generation
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Linkage and Mapping
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Genes that are together on a chromosome
are said to be linked.
 Each gene has a specific location (locus)
on a chromosome.
- Crossing-over should be more likely
between two genes located far apart on
a chromosome that between two genes
located closer together.
 Crossing over frequency can be used
to construct a genetic map.
 1 map unit = 1% crossing over.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Review
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Structure of DNA
DNA Functions
Transcription
Translation
Mutation
Cytogenetics
Mendelian Genetics
Quantitative Traits
Linkage
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Extranuclear DNA
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Entranuclear DNA is found in both
mitochondria and chloroplasts.
 Endosymbiont Hypothesis theorized
mitochondria and chloroplasts were freeliving, and at some point became
incorporated into cells of organisms that
evolved into plants and established a
symbiotic relationship.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Transcription
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Promoter Region signals transcription
enzymes to attach to the DNA.
 Single-stranded RNA transcript produced.
Chromosomes contain genes for building
tRNA.
 Each form of tRNA has a specific
anticodon loop.
- Base pairs with codon.
Genes for rRNA also transcribed in the
nucleus - Used to construct ribosomes.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Hardy-Weinberg Law
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Hardy-Weinberg states the proportions of
dominant alleles to recessive alleles in a
normally interbreeding population will remain
the same from generation to generation in
the absence of external forces.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Central Dogma of Molecular Dogma
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