Transcript Origin of the Science of genetics

Chapter 15
Genetics 2
H.L.
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Lesson Objectives
At the end of this sub section you should be able to:
1. State the Law of Segregation
2.State the Law of Independent Assortment
3. Describe the experiments used to formulate these
2 laws.
4.Complete dihybrid crosses using punnett square
5.Define linkage
6.Explain outcome in results with linked genes
7. Explain sex linked traits
8.Discuss non nuclear inheritance
9.Describe DNA structure & protein synthesis
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Gene
• A segment of DNA that contains coding for a
polypeptide or protein
• A unit of hereditary information.
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Heredity
• The transmission of characteristics from parents
to offspring.
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Gregor Mendel
• Father of genetics – Gregor Mendel, an
Austrian monk
• 1857 – began collecting pure lines of peas
• Chose self-fertilizing peas so that all
offspring look exactly like their parent
• Mendel chose 7 traits for study
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Pea Traits used by Mendel
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Mendel’s Peas – Why Peas?
1.Pure lines with easily identifiable traits were
available
2.Peas are self-fertilizing with a flower structure that
minimizes accidental pollination
3.Peas can be artificially fertilized which allows
specific crosses to be made
4.Peas have a short growth period
5.Peas produce large numbers of offspring
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Mendel’s Experiments
1. He crossed pure plants with alternative
phenotypes for a single trait
2. He recorded how many offspring were of each
type – 1st generation results (F1 generation)
3. He allowed these offspring to self-fertilize
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Mendel’s Findings
4. He again recorded the nature of the offspring – F2
generation
5. He did a mathematical analysis
6. He deduced several principles
7. He published paper in good scientific journal –
1866
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Example of Results – Seed Coat
(Smooth seeds vs Wrinkled seeds)
Parents:
• one parent had smooth seeds
• the other wrinkled seeds
Result: F1 all smooth
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2nd Generation
2nd generation offspring – F2 Results
Parents – both heterozygous smooth
Offspring:
¾ of offspring were smooth
¼ of offspring were wrinkled
3:1 Ratio
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Results
For all seven traits he got a 3:1 ratio
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Summary of Mendel’s Results
• F1 showed only one of two parental traits
• All crosses were the same; it did not matter which
plant the pollen came from
• Trait not shown in F1 reappeared in 25% of the
plants in the F2 generation
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Mendel’s Results
• Traits are not blended as they are passed from
parent to offspring
• Each parent makes equal contribution
• Genes can be carried but not expressed
• Appearance may be similar but genetic make up
may differ
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Why was Mendel successful?
1) Good luck – he chose peas!
2) Naturally self-fertilizing – easy to have pure lines
but it can be forced to cross with a different line
and experimenter can completely control crosses
3) Analyzed his results quantitatively, large
numbers yielded good statistical ratios
4) Started simply – worked from simple to complex
one trait at a time then two at a time, etc
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Learning Check
1.
2.
3.
4.
5.
6.
7.
Who was Gregor Mendel?
Why did he choose peas?
Give examples of the pea traits used by Mendel
What types of peas did he cross?
What is meant by F1 generation
What results did he get in the F2 generation?
What conclusions can be drawn form his results?
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Mendel’s Laws
• The Law of Segregation
• The Law of Independent Assortment
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Mendel’s First Experiment
Crossed Pure Tall x Pure Short (Dwarf)
Predictions: The offspring would be either:
1. Some would be tall and some short
2. All intermediate
3. All short
4. All tall
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Mendel’s Experiments




1st Experiment: C
Pure Tall x Pure Short
Results:
All offspring (F1) tall




2nd Experiment:
Bred F1 Plants
Results:
Ratio of 787 tall to 277
short (3:1)
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2
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Mendel’s First Principle
• Mendel assumed that the two “Factors” for each
trait must exist in the parental cells producing the
gametes
• These “Factors” came from the parents’ parents
and were united in fertilization
• In forming pollen and egg, the two”factors” for any
trait must separate and go into different gametes
• This became known as Mendel’s “Principle of
Segregation”
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The Law of Segregation
“states that organisms contain 2 factors for
each trait. These factors separate in gamete
formation producing gametes with only one
copy of each factor.”
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F1 Generation
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F2 generation
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How the results are explained
• Chromosome movement in meiosis explained
Mendel’s results
• Chromosomes occur in pairs – genes occur in
pairs
• Paired chromosomes separate in meiosis just
as paired alleles separate in gamete
formation
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Law of Independent Assortment
States that alleles of any one gene are
transmitted independently of any other pair of
alleles
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Animation Explaining Independent Assortment of
Genes
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THE DIHYBRID CROSS
Studying the inheritance of two characters
simultaneously
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Mendel’s peas
Character
Seed shape
Seed colour
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Trait
Allele
Round
R
Wrinkled
r
Yellow
Y
Green
y
Possible Combinations
Phenotype
Genotype
Round Yellow
Round Yellow
Round Yellow
Round Yellow
Round Green
Round Green
Wrinkled Yellow
Wrinkled Yellow
Wrinkled Green
RRYY
RRYy
RrYY
RrYy
RRyy
Rryy
rrYY
rrYy
rryy
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Probability
• Probability: the likelihood that a
specific event will occur.
• May be expressed as a decimal, %,
fraction, or ratio.
• Probability= # of actual times
# of opportunities
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The expected probability of each type of seed can
be calculated:
Probability
of an F2 seed being round = 75% or ¾
Probability
of an F2 seed being wrinkled = 25% or ¼
Probability
of an F2 seed being yellow = 75% or ¾
Probability
of an F2 seed being green = 25% or ¼
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THE LAW OF INDEPENDENT ASSORTMENT
• It appears that the inheritance of seed shape has no
influence over the inheritance of seed colour
• The two characters are inherited
INDEPENDENTLY
• The pairs of alleles that control these two
characters assort themselves independently
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Learning Check
1. What are Mendel’s two Laws?
2. State the Law of Segregation
3. State the Law of Independent Assortment
4. What is the difference between a monohybrid and
a dihybrid cross?
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Homozygous x Homozygous
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Homozygous x Heterozygous
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Heterozygous x Heterozygous
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Learning Check
1. If R is dominant to r, what the offspring of
the cross of RR with rr be?
2. According to Mendel, what kind of genes
"disappear" in F1 pea plants?
3. Assuming complete dominance, the F2
generation following the cross Aa x Aa will
show a phenotypic ratio of _____ .
4. In meiosis what happens to chromosome
from each homologous pair?
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Dihybrid test cross
In a dihybrid cross the test cross is made with
an individual which is homozygous recessive
for both characters RRYY and rryy
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Parents Phenotypes:
X
Parents Genotypes:
X
Gametes:
F1 Genotypes:
F1 Phenotypes:
Ratio
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Phenotype
Round, yellow
Genotype
RRYY
Gametes
all
F1
X
rryy
RY
all
Wrinkled, green
all
ry
RrYy
All of the F1 generation would be heterozygous for
both characteristics, meaning that they would all be
round and yellow. Mendel then crossed two of the
F1 generation together…
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Phenotype
Round, yellow
Genotype
RrYy
Gametes
RY, Ry, rY, ry
X
Round, yellow
RrYy
RY, Ry, rY, ry
F2
Use a punnett square
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RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
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F2
9 : 3 : 3 : 1
Round
Round Wrinkled Wrinkled
Yellow
Green Yellow
Green
This is the typical ratio
expected in a dihybrid cross.
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Parents Phenotypes:
Parents Genotypes:
X
X
Gametes
Genotypes
Phenotypes
Ratio
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Dihybrid test cross
Phenotypes
Round Yellow
Genotypes
RrYy
rryy
RY, Ry, rY, ry
ry
Gametes
Genotypes
ry
X
Wrinkled Green
RY
Ry
rY
ry
RrYy
Rryy
rrYy
rryy
Phenotypes
Round
Yellow
Round
Green
Wrinkled
Yellow
Wrinkled
Green
Proportions
25%
25%
25%
25%
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Learning Check
1. What is a dihybrid test cross?
2. In pea plants, tall plant (T) is dominant over a
short plant(t). Purple flower (P) is dominant over
Short white(p) plant.
A homozygous Tall purple flower (P) plant is
crossed with a Short white(p) plant.
State the genotypes of the parent plants and of
the F1 generation by doing out a cross.
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Parents Phenotypes: Tall purple X Short white
Parents Genotypes:
Gametes:
TTPP
X
TP
ttpp
tp
F1 Genotype:
TtPp
F1 Phenotype:
Tall purple
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4. If the F1 plants are self fertilised what will
the genotypes of the F2 generation be?
• Use a punnett square
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Parents Phenotypes: Tall purple X Tall purple
Parents Genotype:
Gametes:
TtPp
X TtPp
TP Tp tP tp X TP Tp tP tp
TP
TP
TTPP
Tp
TTPp
tP
TtPP
tp
TtPp
Tp
TTPp
TTpp
TtPp
Ttpp
tP
TtPP
TtPp
ttPP
ttPp
tp
TtPp
Ttpp
ttPp
ttpp
Ratio
9:3:3:1
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Linkage
• Linked Genes are genes are contained in the same
chromosome which tend to be inherited together.
• In a dihybrid heterozygote, if the genes are not
linked, 4 gamete types are produced in equal ratio.
• If the genes are linked only 2 types of gametes are
formed in equal ratio.
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Linkage
The genes A and B are linked as are the genes
a and b
A
a
B
b
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Locus
The locus of a gene is its position on a chromosome
Cell
Locus of
Gene
Chromosome
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Activity sheet 3 -Linkage
In Drosophila Straight wing (S) is dominant to
curled wing (s) and grey body (G) is dominant
to ebony body (g). S and G are linked.
Parents:
Ss Gg
x
ssgg
Gametes:
SG, sg
x
sg
F1 genotypes: SsGg
and
ssgg
F1 phenotypes: Straight wing
curled wing
Grey body
ebony body
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Sex Linked Genes
Are genes found on the X chromosome without a
corresponding gene on theY chromosome.
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Human Chromosomes
• We have 46 chromosomes, or 23 pairs.
• 44 of them are called autosomes and are
numbered 1 through 22. Chromosome 1 is the
longest, 22 is the shortest.
• The other 2 chromosomes are the sex
chromosomes: the X chromosome and the Y
chromosome.
• Males have and X and a Y;
• females have 2 X’s
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Male Karyotype
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Female Karyotype
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Sex-linked Genes
• Genes on the X chromosome are called “sexlinked”, because they expressed more often in
males than in females
• There are very few genes on the Y chromosome.
• Since males only have one X chromosome, all
genes on it, whether dominant or recessive, are
expressed.
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Mutations on the X Chromosome
• In contrast, a mutant gene on an X chromosome
in a female is usually covered up by the normal
allele on the other X.
• Most mutations are recessive. So, most people
with sex-linked genetic conditions are male.
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Boy or Girl?
Males produce sperm with X chromosome, and
sperm with their Y chromosome.
 The X-bearing sperm lead to daughters
 The Y-bearing sperm lead to sons.
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Sons or Daughters?
• Sons get their only X chromosome from
their mothers
• The father’s X chromosome goes only to
daughters.
• The Y chromosome is passed from father to
son.
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• Historically, women have been
held responsible (blamed!) for not
producing the male heir.
• King Henry VIII blamed his wife
and subsequently had her
beheaded for not producing a
male heir.
• Are women responsible for the
sex of the child?
• Which parent actually determines
the gender of the child?
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Learning Check
1. What is the difference between linkage and sex
linkage?
2. What is a locus of a gene?
3. What is the sex of an individual determined by?
4. What can we predict about the phenotype of
offspring when genes are linked?
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Color blindness
• We have 3 color receptors in the retinas of our eyes.
• They respond best to red, green, and blue light.
• Each receptor is made by a gene.
• The blue receptor is on an autosome
• The red and green receptors are on the X
chromosome (sex-linked).
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Inheritance of Colourblindness
• A heterozygous female has normal color vision.
• Sons get their only X from their mother.
• So, ½ of the sons of a heterozygous mother are
colorblind, and ½ are normal.
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Colour-blindness
• A colourblind male will give his X chromosome to
his daughters only.
• If the mother is homozygous normal, all of the
children will be normal.
• However, the daughters will be heterozygous
carriers of the trait, and ½ of their sons will be
colorblind.
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Haemophilia
• blood does not clot when exposed to air.
• People with hemophilia can easily bleed to death
from very minor wounds.
• Hemophilia is another sex-linked trait.
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Treatment of Haemophilia
• Hemophilia is treated by injecting the proper
clotting proteins
• These are isolated from the blood of normal
people.
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Non Nuclear Inheritance
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Non Nuclear Inheritance
• Non nuclear DNA found in mitochondria and
chloroplasts.
• When these organelles replicate during cell
division, they pass on their DNA.
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Learning Check
1. What is the difference between linkage and sex
linkage?
2. Give an example of a sex linked condition
3. What is meant by non nuclear inheritance?
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Tutorials
•
•
•
•
•
•
The Biology Project – Monohybrid Crosses
The Biology Project – Dihybrid Crosses
Slide Show
Drag and Drop Genetics
Java Genetics problems
BBC Bitesize tutorial
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This is a molecule of messenger RNA.
It was made in the nucleus by
transcription from a DNA molecule.
codon
A U G G G C U U AAA G C A G U G C A C G U U
mRNA molecule
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A ribosome attaches to the mRNA molecule.
ribosome
A U G G G C U U AAA G C A G U G C A C G U U
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Amino acid
tRNA molecule
A transfer RNA molecule arrives.
It brings an amino acid to the first three bases
(codon) on the mRNA.
anticodon
The three unpaired bases (anticodon) on the
tRNA link up with the codon.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
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Another tRNA molecule comes into place,
bringing a second amino acid.
Its anticodon links up with the second codon on
the mRNA.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
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Another tRNA molecule brings the
next amino acid into place.
A U G G G C U U AAA G C A G U G C A C G U U
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A peptide bond joins the second and third
amino acids to form a polypeptide chain.
A U G G G C U U AAA G C A G U G C A C G U U
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The process continues.
The polypeptide chain gets longer.
This continues until a termination (stop)
codon is reached.
The polypeptide is then complete.
A U G G G C U U AAA G C A G U G C A C G U U
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End
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