Download Genetics - Glasgow Independent Schools

Gregor Johann Mendel
Austrian monk
Studied the
inheritance of traits in
pea plants
Developed the laws of
inheritance
Mendel's work was
not recognized until
the turn of the 20th
century
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Gregor Johann Mendel
Between 1856 and
1863, Mendel
cultivated and tested
some 28,000 pea plants
He found that the
plants' offspring
retained traits of the
parents
Called the “Father of
Genetics"
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Site of Gregor
Mendel’s
experimental
garden in the
Czech Republic
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Important Terminology
 Genotype - gene combination for a
trait (e.g. RR, Rr, rr)
 Phenotype - the physical feature
resulting from a genotype (e.g. red,
white)
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More Terminology
 Dominant allele- An allele that
always expresses its phenotypic
effect if present
 Recessive allele- The allele that is
masked by the dominant
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Genotype & Phenotype in Flowers
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes RR
Phenotypes RED
Rr
rr
RED
YELLOW
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Genotypes
 Homozygous genotype - gene
combination involving 2 dominant or 2
recessive genes (e.g. RR or rr); also
called pure
 Heterozygous genotype - gene
combination of one dominant & one
recessive allele (e.g. Rr); also called
hybrid
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•
Homozygous Dominant
– Home = same Dominant = Uppercase
– Example Homozygous Dominant for Green Color
GG
– Homozygous Recessive
– Home = same Recessive = Lowercase
– Example Homozygous Recessive for albinism
aa
– Heterozygous
– Hetero = different
– Example Heterozygous for Tongue Rolling
Rr
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Odds and Ends
• Dominant will mask the recessive in all normal
situations
– Example Heterozygous for Tongue Rolling
–
Dominant mask the recessive
– Genotype = Genetic make up
Rr
– Phenotype = Physical make up Tongue Roller
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Practice
Write the following:
1. Heterozygous for Widows Peak
2. Homozygous Dominant for Tongue Rolling
3. Homozygous Recessive for Widows Peak
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Mendel’s Laws
copyright cmassengale
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Law of Segregation
• During the formation of gametes (eggs or
sperm), the two alleles responsible for a
trait separate from each other.
• Alleles for a trait are then "recombined"
at fertilization, producing the genotype for
the traits of the offspring.
copyright cmassengale
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Applying the Law of Segregation
copyright cmassengale
13
Law of Independent Assortment
• Alleles for different traits are
distributed to sex cells (& offspring)
independently of one another.
• This law can be illustrated using
dihybrid crosses.
copyright cmassengale
14
Generation “Gap”
• Parental P1 Generation = the parental generation
in a breeding experiment.
• F1 generation = the first-generation offspring in a
breeding experiment. (1st filial generation)
– From breeding individuals from the P1
generation
• F2 generation = the second-generation offspring
in a breeding experiment.
(2nd filial generation)
– From breeding individuals from the F1
generation
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Following the Generations
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
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Breed the P1 generation
• tall (TT) x dwarf (tt) pea plants
t
t
T
T
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Solution:
tall (TT) vs. dwarf (tt) pea plants
t
t
T
Tt
Tt
produces the
F1 generation
T
Tt
Tt
All Tt = tall
(heterozygous tall)
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Breed the F1 generation
• tall (Tt) vs. tall (Tt) pea plants
T
t
T
t
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Solution:
tall (Tt) x tall (Tt) pea plants
T
t
T
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:1 phenotype
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Monohybrid Crosses
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Results of Monohybrid Crosses
• Inheritable factors or genes are
responsible for all heritable
characteristics
• Phenotype is based on Genotype
• Each trait is based on two genes, one
from the mother and the other from the
father
• True-breeding individuals are
homozygous ( both alleles) are the same
copyright cmassengale
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P1 Monohybrid Cross
• Trait: Seed Shape
• Alleles: R – Round r – Wrinkled
• Cross: Round seeds x Wrinkled seeds
•
RR x rr
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Probability: 100%
Phenotypic
Probability: 100%
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P1 Monohybrid Cross Review
 Homozygous dominant x Homozygous
recessive
 Offspring all Heterozygous (hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is ALL ALIKE
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F1 Monohybrid Cross
• Trait: Seed Shape
• Alleles: R – Round r – Wrinkled
• Cross: Round seeds x Round seeds
•
Rr x Rr
R
r
R
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
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F1 Monohybrid Cross Review
 Heterozygous x heterozygous
 Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
 Offspring called F2 generation
 Genotypic ratio is 1:2:1
 Phenotypic Ratio is 3:1
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Monohybrid Problems
•A heterozygous man with Widows peak is crossed with a
female without widows peak.
•Cross the individuals
•Give Genotype, Phenotype and Probability for each
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Practice Problems
1. In pigs, white color is dominant & black color is
recessive. Show the results of the following crosses.
Provide genotype & phenotype.
a. A homozygous male white pig is mated with a
homozygous black pig.
b. A male heterozygous white pig is crossed with a
female heterozygous white pig.
c. A male heterozygous white pig is mated with a
homozygous black pig.
2. The gene for brown eye color is dominant to the allele
for blue eyes. A man with blue eyes marries a brown eyed
woman whose mother had blue eyes. What are the
genotypes & phenotypes of their children?
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Monohybrid Quiz
1. In leghorn chickens, colored feathers are due to a
dominant allele, white feathers are recessive. Cross a
homozygous white chicken with a heterozygous
chicken. Provide the ratio of possible genotypes &
phenotypes.
2. The hornless trait in cattle is dominant & the horned
trait is recessive. A hornless bull is mated to 3 cows.
Cow A is horned & gives birth to a hornless calf. Cow B
is horned & gives birth to a horned calf. Cow C is
hornless & gives birth to a horned calf. What are the
genotypes of the bull, the 3 cows, & the 3 calves? This
does not require a Punnett square only the genotypes
of your final answer.
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