Transcript Ch 11 Introduction to Genetics

1 Review What is probability
Use Models How are Punnett squares used to predict
the outcomes of genetic crosses
2 Review What is independent assortment
Calculate An F1 plant that is homozygous for
shortness is crossed with a heterozygous F1 plan.
What is the probability that a seed from the cross will
be tall. Use a Punnett Square to explain your answer
and to compare the probable genetic variations in
the F2 plants
3 Review How did Gregor Mendel contribute to our
understanding of inherited traits
Apply Concepts Why is the fruit fly an ideal organism
for genetic research
CH 11 INTRODUCTION TO GENETICS
11.2 Applying Mendel’s Principles

Probability
 Math
prediction as to chances of something
happening
 Chance of passing on a particular gene
 Coin has two sides, a head and a tail, if you flip it, the
chance of getting a head is ½.



If you flip a coin three times in a row, what is the
probability that it will land heads up every time
Each flip is and independent event with a chance
of ½
1/2 × 1/2 × 1/2 = 1/8.


F1 plant had one tall
allele and one short
allele (Tt)
1/2 of the gametes
would carry the short
allele (t).

The only way to
produce a short (tt)
plant is for two gametes
carrying the t allele.



Chance of getting t
allele is ½
Probability of being
short (tt) is
½ x ½ = ¼.


For each of Mendel’s seven crosses, about ¾ of the
plants showed the trait controlled by the dominant
allele
¼ showed the recessive trait.


Probabilities predict the average outcome
The larger the sample the closer to the predicted.

Homozygous
 Two
of the same alleles
 TT or tt

Heterozygous
 One
 Tt.
of each version of the alleles, Hybrid

Genotype
 What
the actual genes are, you can’t physically see
 TT, Tt, or tt

Phenotype
 What
the trait looks like, what you can physically see
 Tall or short.
Punnett Squares


Genetic cross to predict the genotype and
phenotype using mathematical probability
Monohybrid
 One

trait cross
Dihybrid
 Two
trait cross.
How To Make a Punnett Square
1.
2.
3.
4.
5.
Start with the Parents
Figure out the Gametes
Line them up
Fill in the Punnett Square
Figure out the Results.
Start with the Parents

Write the genotypes of the two parents
Bb and Bb.
Figure out the Gametes

Determine what alleles would be found in all of the
possible gametes that each parent could produce.
Line them up


Draw a table with enough spaces for each pair of
gametes from each parent
Enter the genotypes of the gametes produced by
both parents on the top and left sides of the table.
Fill in the Punnett Square

Fill in the table by combining the gametes’
genotypes.
Figure out the Results


Determine the genotypes and phenotypes of each
offspring
Calculate the percentage for each.
Homozygous round x
Homozygous wrinkled
Homozygous round x
Homozygous wrinkled
R
r
r
R
Homozygous round x
Homozygous wrinkled
R
r
r
R
Rr
Rr
Rr
Rr
Homozygous round x
Homozygous wrinkled
R
r
r
R
Round
Round
Round
Round
Independent Assortment


Genes for different traits can segregate
independently during gamete formation
A past coin flip won’t influence a future coin flip.

Mendel crossed true-breeding round yellow peas
with wrinkled green peas.

Round yellow peas had
the genotype RRYY,
which is homozygous
dominant.

Wrinkled green peas
had the genotype rryy,
which is homozygous
recessive.


All of the F1 offspring
produced round yellow
peas
F1 offspring was RrYy,
heterozygous for both
seed shape and seed
color.

Mendel then crossed the F1 plants to produce F2
offspring.


Alleles for seed shape
segregated
independently of those
for seed color
Mendel’s experimental
results were very close
to the 9:3:3:1 ratio that
the Punnett square
shown predicts.
Start with Parents
Figure out the Gametes
Line them Up
Fill in the Punnett Square
Figure out the Results
Thomas Hunt Morgan

Repeated Mendel’s work using fruit flies.