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Mendel’s Peas Part II
Testcross and Backcross
x
Parents
WW
ww
F1
x
ww
w
w
Ww
ww
w
Ww
ww
Ww
Testcross
Progeny
ww
W
Ww
Mendel and two genes
Round
Yellow
Wrinkled
Green
x
All F1 Round, Yellow
Round
Yellow
315
Round
Green
108
Wrinkled
Yellow
101
Wrinkled
Green
32
Mendel and two genes
Round
Yellow
315
Round
Green
108
Wrinkled
Yellow
101
Round = 423
Wrinkled = 133
Each gene has a 3 : 1 ratio.
Wrinkled
Green
32
Yellow = 416
Green = 140
Punnett Square
Yellow
¾
Green
¼
Round
¾
Round, Yellow
¾ x ¾ = 9/16
Round, Green
¾ x ¼ = 3/16
Wrinkled
¼
Wrinkled, Yellow
¼ x ¾ = 3/16
Wrinkled, Green
¼ x ¼ = 1/16
Ratio for a cross with 2 genes

Crosses with two genes are called dihybrid.
 Dihybrid crosses have genetic ratios of
9:3:3:1.
Principle of Independent
Assortment
F1
Gametes & WG
Frequencies ¼
Ww Gg
Wg
¼
wG
¼
wg
¼
If a gamete contains W the probability that it contains G is
equal to the probability that it contains g.
¼ WG
¼ WG WW GG
1/16
¼ Wg WWGg
1/16
¼ wG WwGG
1/16
¼ wg WwGg
1/16
¼ Wg
¼ wG
¼ wg
WW Gg
1/16
WWgg
1/16
WwGg
1/16
Wwgg
1/16
WwGG
1/16
WwGg
1/16
wwGG
1/16
wwGg
1/16
WwGg
1/16
Wwgg
1/16
wwGg
1/16
wwgg
1/16
Phenotypes

W = Round
 w = Wrinkled
 W is dominant to w.
 G = Yellow
 g = Green
 G is dominant to g.
F2 Progeny
Genotype
Phenotype
1/16 WWGG + 2/16 WWGg + 2/16
WwGG + 4/16 WwGg
9/16 Round Yellow
1/16 wwGG + 2/16 wwGg
3/16 Wrinkled Yellow
1/16 WWgg + 2/16 Wwgg
3/16 Round Green
1/16 wwgg
1/16 Wrinkled Green
Principle of Independent
Assortment

Segregation of the members of any pair of
alleles is independent of the segregation of
other pairs in the formation of reproductive
cells.
Summary of Mendel

Inherited traits are controlled by the alleles
present in the reproductive cells that fuse to
form the embryo.
 In a diploid, progeny inherit one allele from
the mother and one from the father.
 Differences in the DNA sequence of two
alleles for a gene may result in different
phenotypes.
Summary

The phenotype is the same if the gene is
inherited from the mother or from the father.
 One allele from the diploid is inherited in
each reproductive cell.
DNA

Hereditary material.
 Contains all information to make proteins.
 Linear polymer of nucleotide.
 Each one has sugar, phosphate and a base.
Four Bases

A=Adenine
 T=Thymine
 C=Cytosine
 G=Guanine
How Does DNA Carry Information?
To answer this question we must take a closer look at
DNA.
DNA is a biopolymer
•Polymers are molecules made of repeating units or
building blocks
•DNA has four chemical building blocks symbolized
by the letters A,G,C,& T
•The letters of your DNA are in a specific order that
carries information about you!!
So, a DNA polymer can be represented as a string of
letters:
AG C T TAG G G TAAAC C CATATA
DNA Carries Information in the Sequence of DNA Letters
. . .A G C T T A G G G T A A A C C C A T A G . . .
A gene
• A gene is a length of DNA letters that contain
an instruction for a cell to follow.
• The cell uses specially designed protein machines
to read the information in genes.
The Order of DNA Letters Encodes the Genetic Information
The order or sequence of the A, G, C and T letters in the DNA
polymer encodes the actual genetic information
Example of the DNA letters in a gene:
AGCTTAGGGTAAACCATATAGGGCCATACCCTATCGGTAAGCTT
The specific order of the DNA letters carries
the information.
• Changing the order of the DNA letters will
change the information carried by the gene.
• We will talk about how this happens later!
AGCTTAGGGAAAACCCATATAGGGCCATACCCTATCGGTAAG
Genes Contain Instructions for Building Proteins
Genes contain instructions for making proteins, one of the major types
of the molecules of life, or “biomolecules”
Proteins, like DNA, are polymers
•
Protein building blocks are called amino acids
•
Amino acids are strung together into long, linear polymers by
following the instructions in genes
•
In general, a gene encodes the instructions for one protein
When a gene is “misspelled,” the protein made from it
•
may be made with an incorrect amino acid
•
may not work properly
Review of Gene Expression Pathway in Cells
GENE DNA
mRNA copy of gene
mRNA goes to cytoplasm
Focus on the Genetic Code!
Ribosomes translate genetic information encoded in the mRNA
into protein building blocks (chains of amino acids)
Protein folds into 3D active structure
Protein functions in cell
Genetic Code is Written in 3-Letter DNA Words (Codons)
-TACCTCATGATTATACA- DNA(DNA strands separated)
-AUGGAGUACUAAUAUGU mRNA (copied from DNA)
5’-AUGGAGUACUAAUAUGU mRNA
5’-AUG GAG UAC UAA UAU mRNA
mRNA code “read”
by ribosome in
TANDEM triplets
called codons.
Codon adaptors
convert RNA letters
CODON MEANINGS:
into the correct
•A “START PROTEIN” SIGNAL: AUG
amino acid building
•A “STOP PROTEIN” SIGNAL: UAA, UGA, UAG
blocks in the protein
•An amino acid building block of a protein
•Codons identified in the Genetic Code Table chain.
The Universal Genetic Code Table
Name of Building
Block Amino Acid:
Phe=Phenylalanine
Leu=Leucine
Ile=Isoleucine
AUG CODON:
Signal to start
making the protein.
http://anx12.bio.uci.edu/~hudel/bs99a/lecture20/lecture1_6.html
STOP
Codons:
UAA
UAG
UGA
Genetic Code is Written in 3-Letter DNA Words
-TACCTCATGATTATACA- DNA STRAND
AUGGAGUACUAAUAUGU mRNA copied from DNA
mRNA code is “read” in TANDEM CODONS
5’-AUGGAGUACUAAUAUGU mRNA
5’-AUG GAG UAC UAA UAU mRNA
Met-Glu-Tyr-STOP
N Met Glu Tyr C
A SHORT PROTEIN IS A PEPTIDE
CODON MEANINGS:
•“START PROTEIN HERE”: AUG (START) Methionine (Met)
•“STOP PROTEIN HERE”: UAA, UGA, UAG
•Amino acid building blocks: N-Met-Glu-Tyr-C
•Codons are identified in the Genetic Code Table
One Gene-One Protein

Archibald Garrod (1902) described
alkaptonuria, a hereditary disorder as an
“inborn error of metabolism”.
 Proposed that mutations cause specific
biochemical defects.
 Alkaptonuria defect is dark urine.
A DNA Spelling Mistake Can Alter the Protein Chain
START ADD
ADD
ADD
ADD
ADD
ADD
ADD
STOP
ATG TTC AGG CCA AAT TTT GTC GCG UAA GGA ATT
Spelling Mistake
The DNA “word” TTC is changed to TTT
ATG TTT AGG CCA AAT TTT GTC GCG
TTC to TTT spelling change causes a different protein building
block to be inserted in the second position. That is all it takes.
ADD = Codon specifies the amino acid specified by 3-letter “word”
ATG/AUG = Codon specifies start and methionine (met)
UAA = STOP adding amino acids to protein chain
A Mutation is a DNA “Spelling Mistake”
Mutant Genes Encode Defective Proteins:
(1) WILDTYPE
Example: AAA GCT ACC TAT
TTT CGA TGG ATA
Phe Arg Trp Ile
PROTEIN:
WT FUNCTION
(2) MUTANT
AAA GCT ATC TAT
TTT CGA TAG ATA
Phe Arg Stop
UAG
NO FUNCTION
(1) Normal DNA and amino acid sequence makes a wild-type protein.
(2) Mutation in DNA changes Trp to Stop to make a short, mutant protein.
Mutations in DNA can be Caused by:
• Mistakes made when the DNA is replicated (wrong base inserted)
• Ultra violet (UV) light and ionizing radiation (X-rays) damage DNA
• Environmental chemical carcinogens can damage DNA
• Other factors
DNA Technology: The Awesome Skill, I E Alcamo, Harcourt Academic Press, 2001
Misspelled Genes: 3 Possible Outcomes
DNA
A misspelled gene
Cell may not
be able to
follow
damaged
instruction
Cell does not
make the
protein
OR
X
X
Damaged
protein is
made
Damaged
protein may or
may not be able
to function in
the cell.
OR
Spelling
error may be
harmless
Functional
protein made
by the cell
Mutants across organisms

Sometimes mutations in the same gene in
different organisms have similar phenotype.
 This allows researchers to choose the
organism with the best genetic resources to
study the normal function of that gene.
Xeroderma pigmentosa

Autosomal recessive
 UV exposure damages DNA
 Defect in DNA damage repair
 Risks include caner, telangiectasia,
disfigurement
 Can be diagnosed before birth
 Treatment is total protection from
sun/flourescent light.
UV damages tissue that
contains molecules that can
absorb light.
Mechanisms of UV damage

Molecular fragmentation—proteins,
enzymes, and nucleic acids contain double
bonds that can be ruptured by UV.
 Free radical generation—molecules of
susceptible tissues absorb UV and eject an
electron which is taken up by oxygen, then
termed super oxide, a free-radical.
Free radicals

Are scavanged by superoxide dismutase,
vitamin C, vitamin E, glutathione
peroxidase, carotene
Xeroderma pigmentosa
Lesion mutant in maize