chapter 8 mendel & heredity · summary of mendel’s principles a. inheritance of...
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Gregor Mendel
What is Genetics? the study
of heredity
Gregor Mendel’s Peas
Pollen: plant’s sperm
Egg Cells: plants
reproductive cells
Fertilization: joining of pollen
+ egg cells develops into
embryo in a seed
Born in 1822. His work with pea
plants laid the foundation for
Genetics.
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Working with pea plants…
Self-pollinating: pollen fertilizes egg cells in the
SAME flower (single parent reproduction)
True-breeding: offspring genetically identical to
parents due to self-pollination
Cross-pollination: combining reproductive cells
from 2 DIFFERENT parent plants
Mendel could cross- breed a purple flower with a white flower…
What do you think is the color of the offspring?
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Genes and Dominance
TRAIT: specific characteristic (seed color, plant height, etc)
What did Mendel do in his pea plant experiments?
Studied 7 different traits each with contrasting characteristics (ex) Height, short or tall
He crossed the plants (with contrasting characters) and looked at their offspring Website upload 2014
P = parental generation = original pair of plants
F1 = first filial generation= first generation
Hybrids: offspring from parents with different
traits Website upload 2014
Tracking Generations
Parental generation P
mates to produce
First-generation offspring F1
mate to produce
Second-generation offspring F2
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CROSS-POLLINATION: Mendel cut the male parts of one
flower and dusted the female parts
with pollen from another flower.
P GENERATION: purple x
white flowers
F1 GENERATION: all
purple flowers
HYBRID
PLANTS
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What happened in Mendel’s crosses?
All the offspring only had one of the parent’s
characters…the other parent’s character
disappeared!
Mendel’s Conclusions:
1. Inheritance is determined by factors that are
passed down
2. GENES: the factors that determine traits
3. Contrasting characters are different forms of a
gene called ALLELES
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some alleles are dominant, some are recessive
DOMINANT ALLELE: form of trait that will
always be exhibited; usually expressed in
capitals
RECESSIVE ALLELE: form of trait is only
exhibited when the dominant allele is NOT
present
(ex) Allele for tall is dominant for and the allele
for short is recessive
Mendel’s Principle of Dominance
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What happened to the
recessive allele?
Mendel wanted to
know if the
recessive allele
disappeared from
the F1 plants.
F1 CROSS: He
self-crossed the F1
generations to
make F2 offspring Website upload 2014
THE F2 GENERATION…
The recessive traits reappeared!!
~¼ plants had white flowers, the recessive trait
Summary of Crosses:
tall plants X short plants tall plants
P P F1
tall plants self-pollinating ¼ short, ¾ tall
F1 Cross F2 Website upload 2014
Explaining the F1 Cross
Why did the recessive allele reappear? At some point, the recessive allele had to separate from the dominant allele. This is called…
SEGREGATION: separation of alleles occurs during formation of gametes (eggs & sperm) in anaphase II of meiosis.
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Explaining the F1 Cross
F1 plants inherited 1 tall allele & 1 short allele from parents
When gametes are formed, the two alleles segregate from each other each gamete has 1 copy of each gene
So, 2 different types of gametes are formed (one w/ tall allele, one w/ short allele)
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SEGREGATION
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Punnett Squares
What is a Punnett Square?
A diagram showing the possible genetic combinations from a particular cross
Can be used to predict and compare the genetic variations that will result from a cross
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Punnett Squares
What do the letters represent in a Punnett square?
Letters represent alleles: capital = dominant lowercase = recessive
Homozygous: has two identical alleles for a trait (ex) TT or tt
Heterozygous: has two different alleles for the same trait (ex) Tt
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Punnett Square
for TT x Tt
Punnett
Square
for YY x yy
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Genotype vs Phenotype
GENOTYPE: the genetic
makeup of an organism
(ex) TT
PHENOTYPE: the
physical characteristics
exhibited (ex) tall plant
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Probability and Segregation
F2 generation from Tall F1 plants
¾ tall, ¼ short
3:1 ratio of tall to short plants
Punnet squares work to predict outcomes, so Mendel’s ideas about segregation are accurate!
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Exploring Mendelian Genetics
Mendel performed TWO-FACTOR CROSSES:
crossing 2 different genes and following traits
as they pass from one generation to the next
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Two-Factor Cross: F1
Two Genes: shape of pea & color of pea
The Cross:
Round yellow peas x wrinkled green peas
RRYY x rryy
What are the possible alleles parent 1 can pass? RY
What are the possible alleles parent 2 can pass? ry
All F1 were RrYy (round and yellow) or HYBRIDS
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Two-Factor Cross: F2
F1 Generation = RrYy
How would these alleles segregate when F1 self-
pollinated?
RrYy x RrYy
Do the two dominant alleles stay together?
What are the possible alleles each parent can pass
on?
There are 4 possible combinations: Ry, RY, rY, ry
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The F1 Hybrid cross produces a 9:3:3:1 phenotype ratio
Mendel found that the 2 alleles (seed shape & seed color) don’t influence each other’s inheritance
This is called the principle of Independent Assortment: genes for different traits can segregate independently during the formation of gametes
INDEPENDENT
ASSORTMENT
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Independent Assortment
Metaphase I:
Metaphase II:
Gametes:
1/4 AB 1/4 ab 1/4 Ab 1/4 aB
A A A A
A A A A
A A A A
B B
B B
B B
B B
B B B B
a a a a
a a a a
a a a a
b b b b
b b b b
b b b b
OR
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Summary of Mendel’s Principles
A. Inheritance of characteristics is determined by genes which are passed to offspring
B. If 2+ alleles of a trait exist, some alleles may be dominant, others may be recessive
C. Sexually reproducing organisms have 2 copies of each gene which segregate during gamete formation
D. Alleles for different genes segregate independently
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Beyond Dominant & Recessive Alleles
Genetics is more complicated
Some alleles are neither dominant nor
recessive
Many traits are controlled by multiple alleles
or multiple genes
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Other Inheritance Patterns…
1. Incomplete Dominance
2. Codominance
3. Multiple Alleles
4. Polygenic Traits
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X Homozygous
parent (RR)
Homozygous
Parent (rr)
All F1 are
heterozygous
X
F2 shows three phenotypes in 1:2:1 ratio
Incomplete
Dominance
When one allele is not completely dominant; recessive allele is not totally masked
Heterozygous phenotype is in between the two homozygous phenotypes Website upload 2014
X Homozygous
parent (RR)
Homozygous
Parent (rr)
All F1 are
heterozygous
X
F2 shows three phenotypes in 1:2:1 ratio
Incomplete
Dominance
(ex) Red snapdragon flowers (RR) X snapdragon white (rr) flowers pink hybrid flowers (Rr)
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X homozygous parent homozygous parent
All F1 offspring
heterozygous for
flower color:
Cross two of the F1
plants and the F2
offspring will show
three phenotypes in
a 1:2:1 ratio:
Incomplete Dominance
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Codominance
Both alleles contribute to the phenotype
Heterozygous genotype expresses both phenotypes
(ex) Feather colors in chickens: white feathers & black feathers speckled chicken
(ex) Horse coats: red & white roan coat
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Codominance:ABO Blood Types
Alleles that controls blood type are codominant
Two alleles A & B are both exhibited when paired, a third allele (i) is recessive to others
AA or Ai = Type A Blood
BB or Bi = Type B Blood
AB = Type AB Blood
ii = Type O Blood Website upload 2014
Multiple Alleles
> 2 possible alleles for a gene Individuals can still only have 2 alleles each
but more than 2 alleles exist in a population
(ex) coat color in rabbits lots of options due
to 4 different alleles
(ex) blood type is determined by multiple
alleles
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Polygenic Traits
Traits controlled by the interaction of 2+ genes
(ex) Fruit fly eye color (3+ different genes)
(ex) Skin color in humans (4+ different genes), eye color, height, weight
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