Download - Origin of the Science of genetics
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ORIGIN OF THE SCIENCE OF GENETICS
Gregor Mendel
and Mendel’s
Laws
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Learning Outcomes
At the end of this sub section students should be able to
1. Discuss the work of Gregor Mendel2. State the Law of Segregation 3.State the Law of Independent Assortment 4.Complete Dihybrid crosses
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5. Define linkage 6. Explain the change in a 1:1:1:1
probability 7. Sex linkage in genes8. Give xamples of common sex-
linked traits9. Discuss non nuclear inheritance
Learning Outcomes
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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
Mariana Ruiz
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Mendel’s Peas – Why Peas?
Pure lines with easily identifiable traits were available
Peas are self-fertilizing with a flower structure that minimizes accidental pollination
Peas can be artificially fertilized which allows specific crosses to be made
Peas have a short growth period 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
1st generation offspring – F1 all smooth
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2nd Generation
2nd generation offspring – F2 Results
Parents – both heterozygous smooth
Offspring¾ 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! naturally self-fertilizing – easy to have pure
lines but it can be forced to cross with a different line experimenter can completely control crosses
2) Analyzed his results quantitatively, large
numbers yielded good statistical ratios 3) 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. Who was Gregor Mendel?2. Why did he choose peas?3. Give examples of the pea traits used
by Mendel4. What types of peas did he cross?5. What is meant by F1 generation6. What results did he get in the F2
generation?7. 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:
All tall All short All intermediate Some would be tall and some
short
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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)
1
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
“Characteristics are controlled by pairs of factors; but only one
factor can be carried in a gamete”
During gamete formation these factors segregate from each other with only one member of the pair being found in each gamete
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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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Learning Check1. 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.
What is the F2 phenotypic ratio of a monohybrid cross ?
4. Assuming complete dominance, the F2 generation following the cross Aa x Aa will show a phenotypic ratio of _____ .
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Law of Independent Assortment
“ Where two or more characteristics are concerned, the “factors” for
these enter the gametes independently of each other”
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Law of Independent Assortment
States when gametes are formed either pair of factors is equally likely
to combine with another pair of factors
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THE DIHYBRID CROSS
Studying the inheritance of two characters simultaneously
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Mendel’s peas
Character Trait Allele
Seed shape Round R
Wrinkled r
Seed colour Yellow Y
Green y
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Possible Combinations
Genotype PhenotypeRRYY Round YellowRRYy Round YellowRrYY Round YellowRrYy Round YellowRRyy Round GreenRryy Round GreenrrYY Wrinkled YellowrrYy Wrinkled Yellowrryy Wrinkled Green
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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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Predicting how many seeds we could expect to get in a sample
We could expect
What Mendel observed
556 x 9/16 round yellow 313 315
556 x 3/16 round green 104 108
556 x 3/16 wrinkled yellow
104 101
556 x 1/16 wrinkled green 35 32
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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 Laws2. State the Law of Segregation3. State the Law of Independent
Assortment4. How did Mendel explain his results?5. What is the difference between a
dihybrid and a monohybrd cross?
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Mendel & Meiosis
The pairs of chromosomes could orientate in various ways at Anaphase 1
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Phenotypes
Round Yellow seed
X Wrinkled Green seed
(Pure bred)
Genotypes
RRYY rryy
meiosis meiosis
Gametes RY ryfertilisatio
nF1
Phenotypes
RrYy (Selfed)
Genotypes
Round Yellow
Proportions
100%
Dihybrid cross genetic diagram
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F1
Phenotypes
RrYy
Genotypes
Round Yellow
Gametes Y yR RY Ryr rY ry
Dihybrid cross
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Gametes Y y
R RY Ryr rY ry
fertilisation
F2 Genotypes RY Ry rY ryRY RRYY RRYy RrYY RrYyRy RRYy RRyy RrYy RryyrY RrYY RrYy rrYY rrYyry RrYy Rryy rrYy rryy
Dihybrid cross
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Dihybrid cross proportions
Phenotypes ProportionsRound Yellow 9/16 or 56.25%
Round Green 3/16 or 18.75%
Wrinkled Yellow
3/16 or 18.75%
Wrinkled Green
1/16 or 6.25%
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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)
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Dihybrid test cross
Phenotypes
Round Yellow X Wrinkled Green
Genotypes RrYy rryy
Gametes RY, Ry, rY, ry ryGenotypes RY 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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Phenotype Round, yellow Wrinkled, green
Genotype RRYY rryy
Gametes all RY all ry
F1 all 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…
X
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Phenotype Round, yellow
Genotype RrYy
Gametes RY, Ry, rY, ry
F2
X Round, yellow
RrYy
RY, Ry, rY, ry
Use a punnett square
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RY¼
Ry¼
rY¼
ry¼
RY¼
RRYY RRYy RrYY RrYy
Ry¼
RRYy RRyy RrYy Rryy
rY¼
RrYY RrYy rrYY rrYy
ry¼
RrYy Rryy rrYy rryy
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F2
9 3 3 1: : :Round
Yellow
Round
Green
Wrinkled
Yellow
Wrinkled
Green
This is the typical ratio expected in a dihybrid cross.
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Learning Check
1. How is a dihybrid test cross made?2. In pea plants, round seed (R) is
dominant over wrinkled seed (r). A wrinkled seed plant is crossed with a
homozygous round seed plant.State the genotypes of the parent
plants and of the F1 generationIf the F1 plants are self fertilised what
will the genotypes of the F2 generation be?
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LINKAGE
When genes are located on the same chromosome
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Locus The locus of a gene is its position on
a chromosome
Locus ofGene
Cell
Chromosome
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Locus The locus of a gene is its position on
a chromosome
A pair of Alleles
A 2nd pair of Alleles
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LinkageThe genes A and B are linked as are
the genes a and b
a
b
A
B
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SEX LINKAGE
When a characteristic is controlled by a gene on an X chromosome
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Genes on the X chromosome are called “sex-linked”, 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.
Sex-linked Genes
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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.
Mutations on the X Chromosome
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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.
Mutant Genes in Females
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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.
Boy or Girl?
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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.
Sons or Daughters?
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Colorblindness
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 Colorblindness 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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A colorblind male will give his X to his daughters only.
If the mother is homozygous normal, all of the children will be normal.
However, the daughters will heterozygous carriers of the trait, and ½ of their sons will be colorblind.
Colour-blindness
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Colour-blindness
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Haemophilia
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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Hemophilia is treated by injecting the proper clotting proteins
These are isolated from the blood of normal people.
Treatment of Haemophilia
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NON NUCLEAR INHERITANCE
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Non Nuclear Inheritance
Non nuclear genes
Present in small circles of DNA
In mitochondria and chloroplasts
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Mitochondria and chloroplasts are normally passed on to the next generation in the cytoplasm of the egg
They follow a maternal line of inheritance because male gametes contribute only their nucleus to the zygote at fertilization.
Non Nuclear Inheritance
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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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What have you learned?Can you …………………
1. Discuss the work of Gregor Mendel and state the Law of Segregation and the Law of Independent Assortment
2.Complete monohybrid crosses3.Complete dihybrid crosses and know the
expected ratios from Mendel’s Laws4.Define, understand and identify linkage and
sex linkage in genes5.Discuss non nuclear inheritance
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End