Download alleles - Mahtomedi Middle School

Probability
and Heredity
3:2
Just a little recap of
Mendel’s experiments with
pea plants
• Evidently the F1 “tall” offspring must have been
carrying the short trait, but it had been hidden.
• The short trait had been passed down to the offspring
and it reappeared in the F2 generation.
Why did the recessive allele seem to
disappear in the F1 generation and then
reappear in the F2 generation
• Mendel realized that organisms have two
alleles for every trait.
• These two alleles are inherited, one from
each parent.
• If the offspring receives a dominant allele
from one parent, that dominant trait will
appear in the offspring.
• Recessive traits show up in the offspring
only if the offspring receives recessive
alleles from each parent.
Gametes are: sex cells or egg and sperm cells.
Remember: the capital
letter, T, represents a
dominant allele.
The lower case letter, t,
represents a recessive
allele.
In this way, a parent passes
one allele for each gene to
their offspring.
During meiosis, the
DNA is _replicated
once and then goes
through two
separations to form
_4 gametes .
What Is Probability?
Probability - a number that describes how
likely it is that an event will occur.
The laws of probability predict what is likely to
occur, not what will occur.
# of Tosses
Probability of
Heads
Probability of
Tails
1
50%
50%
20
50%
50%
Probability
Probability - a number that
describes how likely it is that an
event will occur.
• The result of one toss will not
affect the results of the other.
• Each event occurs independently.
Probability and Genetics
Punnett square: a chart
showing all the ways
alleles can combine in a
genetic cross.
In a genetic cross, the combination of
alleles that parents can pass to an
offspring is based on probability.
1. The alleles of the male parent
(or 1st parent mentioned) will be
placed on top of the square.
2. The alleles of the female parent
(or 2nd parent mentioned) will be
placed along the left side of the
square.
3. The possible gene combinations
of the offspring are placed inside
the squares.
6. A capital letter represents
a dominant allele.
7. A lower case letter
represents a recessive
allele.
4. Letters represent the alleles.
Probability and Genetics
Copy the male parent’s alleles into the boxes beneath them.
A completed Punnett square shows all the possible allele
combinations the offspring can have.
R
R
p
p
r
P
P p
P p
R
P
P p
P p
Hybrid cross
Purebred cross
Phenotype:
The physical appearance
of an organism, or its
physical traits
Genotype:
The genetic combination of
an organism or its alleles
In Mendel’s pea plants, the
tall allele was dominant
over the short allele
Genotype
TT
Tt
tt
Phenotype
Tall
Tall
short
Homozygous:
An organism having two
identical alleles for a trait
Heterozygous:
An organism having two
different alleles for a trait
In Mendel’s pea plants, the
tall allele was dominant
over the short allele
Genotype
TT
Tt
tt
Phenotype
Tall
Tall
short
Phenotypes and Genotypes
SS
Homozygous
Ss
Heterozygous
ss
Homozygous
Mendel began his experiments
using purebred pea plants.
Cross a purebred tall pea plant
with a purebred short pea plant.
What is the genotype of the
first parent mentioned?
TT
What is the genotype of the
second parent mentioned?
tt
Place the alleles of the first
parent on the top of the
square. Place the alleles for
the second parent on the
left side of the square.
T
T
t
Tt
Tt
t
Tt
Tt
Fill in the Punnett square to show
the possible combinations.
What is the ratio of tall:short?
4:0
In the previous problem, none of the offspring will show the
short trait. As we learned earlier, Mendel wondered what had
happened to the short trait. He allowed the F1 generation to
self-pollinate.
Show this cross using the Punnett square below.
What is the genotype of each parent.
Tt x Tt
T
t
T
t
TT
Tt
Tt
tt
TT
Tall
Tt
Tall
Short
tt
N
N
NN
n
Nn
Normal skin(N) is dominant over albino skin
(n). A woman who has normal skin, but whose
father was albino, marries a heterozygous,
normal skinned man. What type of offspring
might they expect? Describe the probability of
each genotype and phenotype in the data
table.
What is the genotype of the woman?
Nn
What is the genotype of the man?
Nn
%
n
What
What
What
What
What
Nn
nn
Genotype
Phenotype
probability
25%
NN
Normal skin
50%
Nn
Normal skin
25%
nn
Albino skin
are the possible genotypes?
NN, Nn, nn
are the possible phenotypes? Norman skin, Albino skin
percentage of offspring are homozygous dominant? 25%
percentage of offspring are heterozygous?
50%
percentage of offspring are homozygous recessive? 25%
Having dimples is dominant over the
absence of dimples. Cross a heterozygous
dimpled man with a woman who does not
have dimples. Show all work in the
Punnett square. Describe the probability
of each genotype and phenotype in the
data table.
What is the genotype of the man?
Dd
D
d
d
Dd
Dd
d
What is the genotype of the woman?
dd
%
dd
dd
Genotype
Phenotype
probability
50%
Dd
dimples
50%
dd
No dimples
In dogs, the allele for short hair (B) is
dominant over the allele for long hair (b). Two
short haired dogs have a litter of puppies.
Some of the puppies have short hair and some
of the puppies have long hair. Describe the
probability of each genotype and phenotype in
the data table.
What are the genotypes of the parents? Bb and Bb
B
b
%
Genotype
Phenotype
probability
B
b
BB
Bb
Bb
bb
25%
BB
Short hair
50%
Bb
Short hair
25%
bb
Long hair
If the litter of puppies contained 12 pups, how many would you expect
to have short hair? 9 pups or 75%
How many would you expect to have long hair?
3 pups or 25%