Mendelian Genetics · Recessive - gene or trait that is hidden in a hybrid The recessive trait is...
Transcript of Mendelian Genetics · Recessive - gene or trait that is hidden in a hybrid The recessive trait is...
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Mendelian Genetics
Gregor Mendel is
responsible for developing
three laws which govern
the inheritance of many
traits.Image modifed from
https://image.slidesharecdn.co
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Was an Austrian monk living from 1822-1884
He published his findings in 1865.
His work was lost for many decades.
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He worked with pea plants in his garden.
He tested over 28,000 pea plants between
1856 and 1853.
He did a mathematical analysis of inherited
traits and found the offspring retained traits
from their parents.
Mendel is often called the “Father of
Genetics”
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Mendel believed that physical
traits were inherited as
particles.
He had no knowledge of
genes or chromosomes which
came along decades later.
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Important Vocabulary
Trait - any characteristic that can be
passed from parent to offspring
Heredity - passing of traits from parent to
offspring
Genetics - study of heredity
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Gene – subunit of a chromosome
having the code for a trait
alleles – alternative forms of the genes
for a trait
(ie. T – tall t – short)
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Monohybrid cross - cross involving a
single trait
ie. pea plant height
Dihybrid cross - cross involving two traits
ie. flower color & plant height
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Dominant – gene or trait expressed in the
hybrid
The dominant trait is represented by a
capital letter (T)
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Recessive - gene or trait that is hidden in a
hybrid
The recessive trait is represented by a
lowercase letter corresponding to the dominant
trait. (t)
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Genotype - gene combination for a trait
representing the genetic makeup of an
organism
ie. (TT, Tt, tt)
Phenotype - the physical appearance
resulting from a genotype
ie. (tall, short)
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Homozygous -- genotype in which the two alleles are same
(2 dominant or 2 recessive genes)
ie. TT or tt
Homozygous is also called pure.
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Heterozygous – genotype where the two alleles are different
ie. Tt
Heterozygous is also called hybrid
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Punnett Squares are
used to
mathematically
represent genetics
problems.
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Traits Mendel Analyzed in the Garden
Peas
Image modified from http://ib.bioninja.com.au/_Media/mendels-pea-plants_med.jpeg
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What were some reasons Mendel was successful in
analyzing the genetics of the traits of the garden
pea?
1. They grow in a small area.
2. They produce huge numbers of offspring
as every pea in a pod is an offspring.
3. They produce genetically pure plants when
allowed to self pollinate.
4. Easy to artificially cross pollinated.
This allowed Mendel to control exactly
what traits he analyzed.
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5. Many traits were known in the garden
pea. Mendel focused on seven of them.
6. Peas are cheap and very easy to grow.
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Image source http://www.newtonsapple.org.uk/wp-content/uploads/2014/02/
Mendel used a small paint brush to control which pea
traits he mated. He could cut the stamens to prevent
self-pollination.
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Mendel produced pure breeding varieties of
peas by controlling his breeding of them for
several generations.
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Parental generation (P) – the original
parents in a genetic cross
F1 (first filial generation)
-- the offspring of the parental generation)
F2 (second filial generation) --the
offspring of the F1
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Problem: Tall is dominant over short in pea
plants.
Use punnett squares to show F-1 and F-2
results of an original parental generation
cross between homozygous tall and
homozygous short pea plant.
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T – tall
t -- short
P homo tall X homo short
TT X tt
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T t T t
T t T t
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TT Tt
Tt tt
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Modified from
http://www.public.coe.edu/~gcross/forum/2nd%20Talk/evolution/mendel/mendelresults1.gif
Mendel’s Results crossing hybrids
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Test cross -- a cross of a pure or a hybrid
dominant with a recessive
Sometimes a test cross is called a
back cross.
There are two possible testcrosses:
Homozygous dominant x recessive
Heterozygous dominant X recessive
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Trait: Seed Shape
Alleles: R – Roundr – Wrinkled
Cross: Round seeds x Wrinkled seeds
RR x rr
R
R
rr
Rr
RrRr
Rr
Genotype: 100% Rr
Phenotype: 100%
Round
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Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Round seeds x Wrinkled seeds
Rr x rr
r
r
rR
Rr
rrRr
rr
Genotype: 50% Rr,
50% rr
Phenotype: Round &
Wrinkled
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The point of the test cross is that if recessive appear in the offspring, the original dominant parent must have been a hybrid.
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Mendel’s Laws
1.) Law of Dominance
In a cross of homozygous organisms for single
pair of contrasting alleles, only one allele makes
an appearance in F1 generation (the dominant
character)
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Drosophila
Man
Character
Body Color
Eye color
Color of hair
Form of hair
Color of eye
Lips
Blood group
Dominant
Gray
Red
Dark
Curly
Brown
Broad and thicken
A, B
Recessive
Black
White
Light
Straight
Blue
Thin
O
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2.) 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.
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3. Law of Independent Assortment
Alleles for different traits are distributed to sex cells
(& offspring) independently of one another.
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Mendel “lucked out” as the seven pea plant traits he
studied were on each of different seven
chromosome pairs, otherwise this law would not
apply.
This law can be illustrated using dihybrid crosses.
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Show the results of a cross between a hybrid round, hybrid yellow seeded pea plant and another hybrid round, hybrid seeded pea plant.
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Traits: Seed shape & Seed color
Alleles: R roundr wrinkled
Y yellowy green
RrYy x RrYy
RY Ry rY ry RY Ry rY ry
All possible gamete combinations
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RY Ry rY ry
RY
Ry
rY
ry
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RRYY
RY Ry rY ry
RY
Ry
rY
ry
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Dihybrid Cross
RRYY RRYy RrYY RrYy
RY Ry rY ry
RY
Ry
rY
ry
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RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
RY Ry rY ry
RY
Ry
rY
ry
Genotypic ratio
1/16 homo round/homo yellow
2/16 homo round/het yellow
2/16 het round/homo yellow
4/16 het round/het yellow
2/16 het round/homo green
1/16 homo round/homo green
2/16 homo wrinkled/het yellow
1/16 homo wrinked/homo yellow
1/16 homo wrinkled/homo green
Phenotypic ratio
Round/Yellow: 9/19
Round/green: 3/16
wrinkled/Yellow: 3/16
wrinkled/green: 1/16
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Summary of Mendel’s laws
LAWPARENT CROSS
OFFSPRING
DOMINANCE TT x tttall x short
100% Tt tall
SEGREGATIONTt x Tttall x tall
75% tall 25% short
INDEPENDENT ASSORTMENT
RrGg x RrGground & green
x round & green
9/16 round seeds & green pods
3/16 round seeds & yellow pods
3/16 wrinkled seeds & green pods
1/16 wrinkled seeds & yellow pods