Download Probability: A. Scientists use probability to predict the phenotypes

Probability:
A. Scientists use probability to predict the phenotypes and
genotypes of offspring in breeding experiments.
1. When you use fractions, percents, ratios, or decimals
to predict the outcome of an event, you’re measuring
probability.
2. In biology, Punnett squares are used to help predict
the results of breeding experiments
a. grid to organize genetic information
b. shows probabilities
B. A monohybrid cross (one trait, both parents are
heterozygous) will give a 3 dominant : 1 recessive ratio in
the offspring for Mendelian traits.
Example:
P
tall
Tt
F1
T
t
X
T
TT
Tt
3 tall : 1 short
t
Tt
tt
tall
Tt
Key:
T= allele for tall
t = allele for short
Genetics Problem Set-Up
KEY
(symbol here)
(symbol here)
P !
= allele for (dominant version of trait here)
= allele for (recessive version of trait here)
phenotype of 1st parent
genotype of 1st parent
F1
!
(if applicable) -
X !
phenotype of 2nd parent
genotype of 2nd parent
!
Answer to the question in a box
Also – If the sex of each parent is known (male, female) the
symbols for the sex must be included in the parental
generation and in the Punnett square. (see cloverleaf marks)
Male:
Female:
Answer Set-Up for Certain Genetics Problem Questions
If a genetics problem asks you to “list (give, name) the genotypes
and phenotypes of the offspring” there is a specific way to arrange
your answer. So that it is clear in your answer as to what is a
genotype and what is a phenotype, you must make a chart.
For example, if you are asked to list the genotypes and phenotypes
of the offspring for the following Punnett square:
T
t
T
TT
Tt
t
Tt
tt
then your answer would be:
genotypes
phenotypes
TT
tall
Tt
tall
tt
short
Also, if you are asked to give the “genotypic and phenotypic
ratios of the offspring” you must identify which ratio is
which. Using the same Punnett square from above, you
would write:
Genotypic ratio - 1 TT : 2 Tt : 1 tt
Phenotypic ratio - 3 tall : 1 short
Notice that each is labeled and colons separate each part.
Practice Single Trait Inheritance (STI) Problems
1. Cross a homozygous tall plant with a short plant. What
percentage of the offspring will be tall?
2. Cross a plant hybrid for flower position with a plant that
is purebred dominant for flower position. What fraction of
the offspring will be heterozygous?
3. Cross two heterozygous plants using the trait pea shape.
What percentage of the plants will be recessive?
A dihybrid cross (2 traits, both parents are completely
heterozygous for both traits) will result in a 9:3:3:1 ratio in
the offspring where:
9 offspring will have both dominant traits
3 have one dominant and one recessive trait
3 have one dominant and one recessive trait
1 has both recessive traits
(remember – works for Mendelian traits only)
Example:
P
purple tall
PpTt
X purple tall
PpTt
Key:
P = purple
p = white
T = tall
t = short
F1
PT
Pt
pT
pt
PT
PPTT
PPTt
PpTT
PpTt
Pt
PPTt
PPtt
PpTt
Pptt
pT
PpTT
PpTt
ppTT
ppTt
pt
PpTt
Pptt
ppTt
pptt
9 purple tall : 3 purple short : 3 white tall : 1 white short
How to Set Up a Double Trait Inheritance Problem
The hard part is getting the gametes from each parent to
make up the top and left side of your Punnett Square. Follow
the pattern:
The left side tells you
which letter to take
from the alleles for
the first trait.
1st 1st
1st 2nd
2nd 1st
2nd 2nd
The right side tells
you which letter to
take from the alleles
for the second trait.
So, following the pattern, here are the gametes you would get
from a parent with the genotype AaRr. Note that “Aa” are
the alleles from the first trait and that “Rr” are the alleles for
the second trait.
AaRr
1st 1st
1st 2nd
2nd 1st
2nd 2nd
AR
Ar
aR
ar
Now you take these combinations to
begin setting up your Punnett
square, as is shown below.
AR
To get the
gametes that go
down this side,
you do the same
procedure only
use the other
parent.
Ar
aR
ar
Shortcuts for Punnett Squares
You only need one of each different gamete from each parent for
your Punnett squares. If you have 2 or more copies of the exact
same gamete FROM THE SAME PARENT, eliminate the
duplicates.
For example, let’s say your crossing the parents RRGg and RrGG.
Use the 1st 1st, 1st 2nd, etc. pattern to get all the possible gametes.
1st 1st
1st 2nd
2nd 1st
2nd 2nd
RRGg
RrGG
RG
Rg
RG
Rg
RG
RG
rG
rG
Some of the gametes are the same for the same parent, so cross them
out.
1st 1st
1st 2nd
2nd 1st
2nd 2nd
RG
Rg
RG
Rg
RG
RG
rG
rG
Now, use the remaining gametes to make your Punnett
square. The blue shows the resulting offspring.
RG
Rg
RG
RRGG
RRGg
rG
RrGG
RrGg
Please note the following about double trait problems:
• You either have to cross out ALL of the duplicates or NONE of
them for a double trait problem to work out correctly.
• You will NEVER have 3 gametes from a parent in a double
trait problem. If you do, you either crossed out a gamete you
shouldn’t have, or you didn’t cross out ALL of the duplicates.
• Only a completely or fully heterozygous parent will get you 4
different gametes, which is the highest number of gametes you
can have for a double trait problem.