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Genetics
Exam 1 September 16nd
Assignment 1 due Sept. 16nd
Sections of chapters 8, 9, 10
Chapters 8 and 9
• Gregor Mendel
– Was the first
person to analyze
patterns of
inheritance
– Deduced the
fundamental
principles of
genetics
Figure 9.4
Mendel’s Peas
The garden of the Augustinian Convent
in Brno, Moravia (Czech Republic)
Mendel’s Experiments with Peas
• He created what he called true-breeding
varieties of the plants:
-All purple plants, when self-fertilized, will
have purple offspring
-All white plants, when self-fertilized, will
have white offspring
• Mendel then cross-fertilized the two
different true-breeding varieties, creating
hybrids
Monohybrid Crosses
• A monohybrid
cross is a
cross between
parent plants
that differ in
only one
characteristic
(in this case,
flower color)
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
All plants have
purple flowers
F1 Generation
Fertilization
among F1 plants
(F1  F1)
F2 Generation
3/
of plants
have purple
flowers
4
(a) Mendel’s crosses tracking one
characteristic (flower color)
1/
of plants
have white
flowers
4
Figure 9.8a
Mendel’s Discoveries
• Define alleles.
• What are dominant and recessive alleles?
Alleles
• In Mendel’s peas and other examples:
– For each trait there is 1 gene
– Gene exists in 2 forms called alleles
– Each parent can only give one allele to their
offspring.
– Some alleles mask others
Genetic makeup (alleles)
pp
PP
P plants
• An
explanation
of Mendel’s
results,
including a
Punnett
square
Gametes
All
P
All p
F1 plants:
(hybrids)
Gametes
All Pp
1/
2
F2 plants:
Phenotypic ratio
3 purple : 1 white
P
Eggs
1/
P
P
PP
p
2
p
Sperm
p
Pp
Pp
pp
Genotypic ratio
1 PP : 2 Pp : 1 pp
(b) Explanation of the results in part (a)
Figure 9.8b
An Example
Gene: Seed shape
Dominant allele: R (round)
Recessive allele: r (wrinkled)
3 Genotypes:
RR, Rr, rr
2 Phenotypes:
Round & Wrinkled
Some definitions
• Define Homozygous.
• Define Heterozygous.
• Homozygous
– When an organism has identical alleles for a gene
• Heterozygous
– When an organism has different alleles for a gene
• Gamete: sexual reproduction cell carrying 1 of
each chromosome pair and therefore 1 allele
• What do gametes have to do with the first two
definitions?
• Mendel’s principle of segregation
– Pairs of alleles (one from mom, other from dad)
segregate (separate) during gamete formation
– The likelihood that a gamete (sperm or egg) will
receive one or the other allele from the allele
pair in a gene is 50:50 (like a coin toss)
– The fusion of gametes at fertilization creates
allele pairs again
You try it
r
r
r
R
Mother’s alleles
• Finish the following cross:
Rr x rr
Father’s alleles
• What are the father’s phenotype
and genotype?
• How many different genotypes could result from
this cross? How many different phenotypes?
Using a Testcross to Determine an
Unknown Genotype
• A testcross is a
mating between
– An individual
of unknown
genotype
and
–A
homozygous
recessive
individual
Testcross:
Genotypes
P_
pp
Two possibilities for the purple flower:
PP
Gametes
P
P
Offspring
Pp
Pp
All purple
P
p
Pp
p
pp
1 purple : 1 white
Figure 9.12
Now try this
Alleles for flower color: P (dominant, purple flower), p (recessive, white flower)
Genetics in humans
• Study humans using
pedigrees
pedigrees
Recessive
Recessive or ??
Dominant
What are genes?
• The search was on to find where in the cell
the alleles were located.
• With the invention of microscopes and
microscopic techniques scientists
discovered the chromosomes.
Chromosomes
• Found in every cell (a few exceptions)
• Come in pairs
• Chromosomes are visible during cell
division
Chromosomes are visible during
cell division
Onion root tip
Cell division in sex organs is different
Sex cells (gametes)
are made so they only
carry half the
chromosomes
Early observations about
Chromosomes
• How do chromosomes behave like
Mendel’s alleles?
Human Chromosomes
• 23 pairs = 46 total
• 1 pair is not like the others, sex chromosomes
determine your gender
– XX = female
– XY = male
• Pairs called homologous pairs, each member of
the pair carries one of the two alleles
• Chromosome numbers vary in other species
We can use cell division to
look at our chromosomes
Making a karyotype
obtain sample cells
culture cells
burst cells and place on slides
add stain
photograph
cut out and sort chromosomes
Genetics
Part 2
More from chapters 8 and 9
Not all traits are Mendelian
Exceptions to Mendelian Genetics
• What phenotypic ratio would you expect in
the offspring of the following cross?
Rr x Rr
(R = red flowers, r = white flowers)
• What if the offspring were ¼ red, ¼ white
and ½ pink, what does that indicate?
Incomplete Dominance in Plants
P Generation
Red
RR
• In incomplete
dominance, F1
hybrids have an
appearance in
between the
phenotypes of
the two parents
White
rr
Gametes
F1 Generation
Pink!
Rr
1/
Gametes
1/
Eggs
With INCOMPLETE dominanceF2 Phenotypic ratio:
1 purple : 2 pink : 1 white
r
R
1/
F2 Generation
2
2
2
R
1/
2
1/
R
r
r
2
Sperm
R
Red
RR
1/
2
r
Pink
rR
Pink
Rr
White
rr
Figure 9.18
Exceptions to Mendelian Genetics
Codominance
Example: human blood types
Phenotypes = A, B, AB and O blood types
There are 3 alleles: A and B are dominant
Codominance – Multiple alleles in blood type
• The ABO blood groups in humans are
examples of multiple dominant alleles
• Two of the human blood type alleles
exhibit codominance
• Both alleles are expressed in the
phenotype
Exceptions to Mendelian Genetics
• Polygenic traits: traits determined by multiple
genes.
– Eye color, height, blood type including Rh factor.
Ex: Human Blood types
Phenotypes = A+, A-, B+, B-, AB+, AB-, O+ and O2 genes,
1. Rh factor has 2 alleles, + and – (+ is dominant)
2. Type has 3 alleles, A, B and O (A and B are
dominant)
Polygenic Inheritance
P Generation
• Polygenic inheritance
is the additive effects of
two or more genes on
a single phenotype
aabbcc
(very light skin)
AABBCC
(very dark skin)
F1 Generation
AaBbCc
AaBbCc
F2 Generation
Possible Eggs
Possible Sperm
• In the example to the
right, what is the
dominant phenotype?
• How many genes are
responsible for skin
color in this example?
Figure 9.22
Pleiotropy
• Pleiotropy is the impact of a single gene
on more than one characteristic. These
characteristics are thus usually expressed
together.
Examples: Red hair and freckles
or sickle-cell disease
Human Chromosomes
• 23 pairs = 46 total
• 1 pair is not like the others, sex
chromosomes determine your gender
– XX = female
– XY = male
• Pairs called homologous pairs, each
member of the pair carries one of the two
alleles
• Chromosome numbers different in other species
– This is part of the reason why you can’t breed different species
Genes on Chromosomes
• There are many more traits than there are
chromosomes
• There must be more than one trait on each
chromosome
P
S
T
E
P
s
T
e
Chromosome pair 1
A
b
R
V
A
b
R
v
Chromosome pair 2
Homologous chromosomes:
Gene loci
Dominant
allele
a
P
P
Genotype:
PP
Homozygous
for the
dominant allele
a
aa
Homozygous
for the
recessive allele
B
b
Recessive
allele
Bb
Heterozygous
Figure 9.9
Sex Determination in Humans
Male
44
+
XY
• Sex chromosomes
– Are
designated X
and Y
– Determine an
individual’s
sex (boy or
girl)
22
+
X
Female
Somatic
cells
22
+
Y
44
+
XX
22
+
X
Sperm
44
+
XX
Female
Egg
44
+
XY
Male
Figure 9.27
SEX CHROMOSOMES AND
SEX-LINKED GENES
• Sex chromosomes
– Determine gender
– Influence the inheritance of certain traits
Sex-Linked Genes
• Sex-linked genes
– Are any genes located on a sex chromosome
– Were discovered during studies on fruit flies
(a)
(b)
Figure 9.28
Sex-Linked Genes
• Red-green
color
blindness
– Is
characterized
by a
malfunction of
light-sensitive
cells in the
eyes
Figure 9.30
Sex-Linked Genes
• Hemophilia
– Is a bloodclotting
disease
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.31
DNA, Replication, and Mitosis
Chapter 10
DNA Structure:
the double helix
C
A
G
T
DNA base pairs
A–T
C–G
DNA is “supercoiled” around
proteins called Histones to form Chromosomes
This is an actual microscopic
photograph of DNA supercoiled into a chromosome.
Chromosomes
• Chromosomes as
seen with a light
microscope
Onion root tip
Above is what is sometimes called
the “central dogma” of cell biology.
In the cell nucleus, DNA is involved with two major processes:
(1) DNA replicates itself, making more double stranded DNA
(2) One strand of DNA is:
A. transcribed to make RNA
B. that RNA copy leaves the nucleus, and is then
translated at the ribosome to make proteins
Cell Cycle
Chapter 8
• Life cycle of a cell.
–
–
–
–
–
Cell is “born” from another cell
Cell grows
Cell gets ready to reproduce
Cell reproduces (it splits)
Or, the cell dies
• When cells reproduce, they pass on their genes
• Cell reproduction is called mitosis
Cell Cycle
• Cell Cycle
– What happens at each
stage?
• Interphase
• Mitosis
Cell Cycle
• The most well understood parts of the cell
cycle are replication and mitosis
• Replication is the process of copying
genes before splitting the cell
• Mitosis is the process by which one cell
becomes two identical daughter cells
Cell Cycle
• The stages of mitosis
can be seen with a
microscope
Onion root tip
Genes
Chapter 10
• DNA carries genetic
information
• What does DNA look
like?
• What is it’s chemical
structure?
• How is that
represented?
DNA Replication
• Now that we know what DNA looks like
and how that can be represented,
• We can learn how it copies itself.
• Replication occurs before mitosis and
meiosis
DNA Replication
• Animation of replication
• www.youtube.com/watch?v=4jtmOZaIvS0
Genetics 3
How do genes determine
physical traits?
DNA to proteins
Genes and mutations
Chapter 10
The Genetics Story:
How does DNA act as instructions
for a trait?
DNA  RNA  Protein
Genotype  Phenotype
DNA
“Book of instructions”
Transcription
RNA
“Page of instructions”
Translation
Protein
Gene Expression (trait)
Overview
Ribosomes
RNA
Protein
DNA
Transcription
5`
Copy from
DNA to RNA
Messenger
RNA (mRNA)
3`
A
A
G
G
T
C
G
T C
C A
G
3`
T
C
C
A
G
C
3`
A
G
C
T
5`
U
5`
RNA Polymerase
Translation
mRNA goes out of the cell nucleus and to
the ribosomes.
Ribosomes are made of ribsomal RNA
(rRNA) and proteins.
Translation = RNA to protein
Ribosomes “read” the mRNA.
mRNA
RNA language
Three nucleotides of RNA correspond
to one amino acid = Codon
GCUGUCCCCAGCUUAGCG
Alanine
Leucine
Serine
Proline
Valine
Alanine
Table 7.2
Transcription
Translation
Ribosomes
mRNA
DNA
Protein
Summary
1. DNA is transcribed into mRNA.
- RNA is slightly different from DNA.
2. mRNA is translated into amino acids.
- ribosomes match the codons of the
mRNA to the anticodons of the tRNAs.
3. Certain codons are translated into
amino acids. Start…………Stop.
4. Amino acids linked together = protein.
Example
DNA:
TAC CCA GTG GAG GTA CCT GAT ACT
RNA:
AUG GGU CAC CUC CAU GGA CUA UGA
Amino Acids:
Start Gly His Leu His Gly Leu Stop
DNA to protein
•
Animation of gene expression
•
www.youtube.com/watch?v=NJxobgkPEAo
Mistakes can happen
DNA and RNA polymerase can make
a mistake in copying = mutation.
DNA:
TAC CCA GTG GAG GTA CCT GAT ACT
Should be A
RNA:
AUG GGU CAC CUC CAU GGG CUA UGA
Amino Acids:
Start Gly His Leu His Gly Leu Stop
DNA mutations can occur from
exposure to x-rays, ultraviolet
radiation, human papillomavirus
(HPV), and many other mutagens.
DNA mutations result in a change
in RNA, which then can result
in a change in the amino acid
sequence of a protein.
An effective or missense
mutation is one where the
mutation actually causes a change
in the amino acid sequence.
GCA codes for the amino acid
Alanine, instead of Glycine (GGA)
A non-effective, or silent
mutation is one where the
mutation caused no change in
the amino acid sequence, due to
redundancy in amino acid codons
(GGG and GGA both code
for Glycine)
Fig. 7.9
Sickle cell anemia:
This is the result of only ONE base pair mutation to DNA, which results in the
changing of only ONE amino acid in an amino acid sequence.