Download Mendel and his Peas

Mendel and his Peas
Chapter 9
State Objectives

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CLE 3210.4.5 Recognize how meiosis and sexual
reproduction contribute to genetic variation in a
population.
CLE 3210.4.3 Predict the outcome of monohybrid
and dihybrid crosses.
 SPI 3210.4.4 Determine the probability of a
particular trait in an offspring based on the
genotype of the parents and the particular mode
of inheritance.
CLE 3210.4.4 Compare different modes of
inheritance: sex linkage, codominance, incomplete
dominance, multiple alleles, and polygenic traits.
Sub-Objectives
Explain the experiments of
Gregor Mendel
 Explain how genes and alleles
are related to genotypes and
phenotypes
 Use a Punnett square to predict
genotypes and phenotypes

Gregor Mendel
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•
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•
A monk
Worked in the garden at the
monestary
Wrote Experiments in Plant
Hybridization in 1866.
Experiments unnoticed until
1900.
Gregor Mendel
Wanted to know how traits are
passed from one generation to
the next
 How some traits seem to skip a
generation and show up in the
next
 Chose the pea plant to study

Why Peas?
Grow quickly
 Self pollinating
 Each flower contained both
male and female parts

Image from: http://www.jic.bbsrc.ac.uk/germplas/pisum/zgs4f.htm
Sexual Reproduction in Peas

Pollen from the anther of one
plant is transferred to the
stigma of another. Pollen
travels down to to egg cell
Image from: http://anthro.palomar.edu/mendel/mendel_1.htm
Mendel’s Peas
Mendel began by studying one
trait at a time.
 That way, he could understand
the results
 Some of the traits he observed
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Plant height, seed shape, flower
color
How did he start?

He crossed pure-bred tall plants
with pure bred tall plants
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Results: All tall plants
He crossed pure-bred short
plants with pure bred short
plants
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Results: All short plants
Cross-pollination
Anthers of one plant are
removed so it can not self
pollinate
 Pollen from another plant is
used to pollinate the flower

Results from cross-pollination
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•
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When the peas were cross
pollinated, they produced
offspring.
These offspring are the first
generation
In the case of tall and short plants,
all the offspring came out tall
Dominant and recessive traits
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The trait that appeared in that first
generation was called the
dominant trait
 Dominant trait: masks the
presence of other traits
The trait that did not show up he
called the recessive trait
Mendel’s second experiment
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Mendel crossed the individuals
from the first generation with each
other
The next offspring are called the
2nd generation
In the second generation, the
recessive trait reappeared
Counting the offspring

Mendel counted the offspring
in the second generation with
each trait to determine the ratio
of individuals with the
dominant trait to those with the
recessive trait.
Mendel’s peas
Results of Mendel’s experiment
Results of Mendel’s experiment
Of Genes and Alleles


Mendel looked at the math and
decided for each trait offspring
had to have two “factors” one
from their mother and one from
their father
These factors that coded for the
same trait are called genes
So what’s a trait?

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A specific characteristic that varies
from one individual to another
Mendel looked at seven

Seed shape, seed color, seed coat
color, pod shape, pod color, flower
position and plant height
Of Genes and Alleles
For each gene, there may be
more than one form
 These different forms of genes
are called alleles

Terms
Gene: a unit of heredity on a
chromosome.
• Allele: alternate state of a gene.
•
Dominant: an allele that masks
the expression of other alleles.
•
Recessive: an allele whose
expression is masked by dominant
alleles.
•
Linking with Meiosis
How does the information of
two alleles for each gene
compare with what we know
from meiosis?
 What does each zygote get
when sperm and egg fertilize?

What we know from
meiosis
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Principle of segregation:
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Alleles on homologous
chromosomes separate during
the process of meiosis
Only one allele from each
parent is passed to the offspring
Segregation
Image from:
http://anthro.palomar.ed
u/mendel/mendel_1.htm
Probability
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The mathematical chance that an
event will occur
If you flip a coin, what’s the
chance it will come up heads?
What the chance of three tails in a
row?
Biologists use probability to
predict the outcome of genetic
crosses
Punnett Square
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To understand Mendel’s
conclusions, we use a diagram
called a Punnett square
Dominant alleles are symbolized
with capital letters
Recessive alleles are symbolized
with lower case letters
Remember for each trait
there are two alleles
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So a cross from a true breeding tall
plant will produce a tall offspring
whose alleles are written, TT
A true breeding short plant would
be tt
Genotype TT, tt, or Tt
The actual letters represent the
alleles this combination of
alleles is called the genotype
 Genotype: alleles present in the
organism
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Back to our pea plants
Genotype
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Two possibilities
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Homozygous: contains identical
alleles (TT or tt)
Heterozygous: contains different
alleles (Tt)
Phenotype
An organisms appearance, what
the gene looks like is the
phenotype
 Phenotype: the physical
appearance of the trait
 what it looks like

Making a Punnett Square
Baby Steps to a Punnett square
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1. determine the genotypes of
the parent organisms
2. write down your "cross"
(mating)
3. draw a p-square
4. "split" the letters of the
genotype for each parent & put
them "outside" the p-square
Baby Steps to a Punnett square
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5. determine the possible
genotypes of the offspring by
filling in the p-square
6. summarize results (genotypes
& phenotypes of offspring)
7. bask in the glow of your
accomplishment !
Making gametes
 Remember
the principle of
segregation, especially with
dihybrid crosses, Each
gamete only gets one allele
for each trait!!!!
Image from:
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/r
eprod/fert/gametes.html
Are all wrinkled peas
yellow??
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Once Mendel found how traits are
passed, he wanted to know if the
segregation of one pair of alleles
had anything to do with the
segregation of another pair
In other words, are all wrinkled
peas yellow???
Dihybrid cross
Use a Punnett square to track
two traits at once
 Works like a monohybrid cross,
but you have to take care in
forming your gametes

Principle of Independent
Assortment
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Genes for different traits do not
affect each other in segregation
Works for most traits unless they
are linked: close together on the
same chromosome
Other exceptions to
Mendel
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Incomplete dominance
Codominance
Multiple Alleles
Polygenic traits
Incomplete Dominance
 With
incomplete dominance, a
cross between organisms with
two different phenotypes
produces offspring with a third
phenotype that is a blending of
the parental traits.
Incomplete dominance
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Neither allele is completely
dominant over the other
Example: White and red four
o’clocks
White:W
Red: R
A Classic Example:
Snapdragons
R
= allele for red
flowers
W = allele for
white flowers
 red x white --->
pink
RR x WW --->
100% RW
Recognizing Incomplete
Dominance
 Two steps:
1) Notice that the
offspring is showing a 3rd
phenotype. Not shown in
the parents
2) Notice that the trait in
the offspring is a blend
(mixing) of the parental
traits.
Codominance
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In Codominance, traits appear
together in the phenotype of
hybrid organisms.
 red
x white ---> red &
white spotted
Another classic example:
Cows
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In cows if you cross a pure bred
red cow with a pure bred white
cow, the offspring are roan
The color difference in their coats
is because they have both red and
white hairs together
Practice problems
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Try problems three through
five on your practice problems
sheet
Multiple alleles
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Genes that have more than two alleles for a
trait
Each individual can only have two, but in
the population more than two exist
Another classic example: Blood Type
 Humans have three alleles for
blood type
IA:
Type A
IB: Type B
I: Type O
A and B are both dominant over
O, but are codominant with each
other
Polygenic traits
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Some traits are determined by the
interaction of many traits
Examples:
 Height
Hair color
 Skin color
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Studying genetics
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Thomas Hunt Morgan
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Uses fruit flies Drosophila
melanogaster
Breed a new generation every 14
days
Used because short generation time
allows production of many
generations
Image from: http://www.ceolas.org/fly/intro.html
Environmental Influences
 Our
genes aren’t all of what we
are
 Our environment has influences
as well
 Example: Heart disease
People
with poor diets have
higher incidences of heart
disease