Download Weird Alleles, and Intro to Punnett Squares

Intro to Punnett
Squares
Homework
Objectives

Get familiar with more complicated allele formats

Be able to construct and interpret a monohybrid
cross (basic Punnett Square)


Use a Punnett Square to predict genotypes and
phenotypes
Know what each part of the Punnett Square
represents
Allele Formats

Most of the time in this class, we’ll use the
allele convention you’ve learned already,
where one letter = one allele.

But most of the time in real-world science,
scientists (naturally!) use a more complicated
route.
Allele Formats

There are SO MANY organisms, so many
genes, and only 26 letters in the alphabet. To
be specific, scientists usually use more than
one letter for an allele.

They typically use a normal capital letter to
stand for the gene, and then a superscript big
or little letter for the allele. Like this:
Allele Conventions



The gene for coat color may have the dominant
allele CB and the recessive allele Cb.
The gene for tail curling may have the dominant
allele TC and the recessive allele Tc.
The gene for coat consistency may be BCH or BCh.

I’ll be sticking to just the normal allele convention to help
you learn. But this more accurate convention is one that
will come back a bit next week, and MCAS may choose to
use it. So just be aware that these are alleles just like any
other, they still stand for different base sequences coding
for different versions of a protein, they’re just written by
scientists to be more specific.
Punnett Squares

A cool skill to have is the ability to predict the
base sequences of relatives and future
offspring. Knowing the alleles of living
organisms can tell you about the genes and
alleles of organisms that have long been
dead! For simplicity, we’ll stick with one or
two generations rather than thousands or
millions.

Here is how it works.
Punnett Squares


In The Dark Knight, Harvey Dent had a coin
with the same thing on both sides, heads and
heads (H and H).
When he flipped that coin, what were the
chances that it would come up heads (H)?
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Punnett Squares
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He acquired a coin that was heads (H) on
one side, and damaged (we’ll call it h) on the
other side. NOW what were the odds that, on
one flip, it would come up heads (H)?
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Punnett Squares
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There is a scene where he flips it twice. The
other two people in the scene are really
hoping that it comes up heads (H) both times.
What are the odds that it would?
Here’s how to set it up…
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Punnett Squares
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The first flip will be either heads (H) or
damaged heads (h):
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Punnett Squares
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The second flip will also be either heads (H)
or damaged heads (h):
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Punnett Squares
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These are the possible combinations that he
could have produced:
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Punnett Squares
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1 in 4 possible outcomes would be both
heads (HH). 1/4 = 25% = .25
h
H
H
h
H
H
H
H
h
h
h
h
Punnett Squares
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The actual outcome was one heads and one
damaged heads (Hh). What were the odds?
h
H
H
h
H
H
H
H
h
h
h
h
Punnett Squares
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2 of 4 possible outcomes = 1/2 = 50% = .5
h
H
H
h
H
H
H
H
h
h
h
h
Punnett Squares
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What if he’d had two coins somehow, one
with two normal heads (HH) and one with one
normal and one damaged side (Hh). Then,
he flipped both at the same time. What are
the odds that both coins would land with
normal heads up (HH)?
Punnett Squares
H
H
H H
H h
h
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H
H H
H h
This is called a Punnett Square. It’s a way of
mathematically determining the probability of an
outcome with some randomness (which side will
land up?) and some non-randomness (which sides
are possible?)

What could this have to do with gametes and genes?
Punnett Squares

Mom has two of every kind of chromosome, so, two
alleles for every gene.

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Dad has two of every kind of chromosome, so, two
alleles for every gene.

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Each egg has only of each kind of chromosome, so, one
allele. Half of her eggs have allele 1, half have allele 2.
Each sperm has only one of each kind of chromosome, so,
one allele. Half of his sperm have allele 1, half have allele
2.
And, it’s random which egg and which sperm fuse to
create the offspring.
Punnett Squares

If you know Mom’s genotype, and Dad’s
genotype, you can predict the likelihood that
their offspring would have a certain genotype
just like you could predict the likelihood of
some coin tosses!

Punnett Squares are simple once you’ve got
the idea, you just need to adhere to the
conventions.
Punnett Squares
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It doesn’t matter whether Mom or Dad is on
the side, just keep both eggs together and
both sperm together.
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Punnett Squares
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Just like before, if something is heterozygous,
write the dominant allele (capital letter) first.
A
A
A
a
a
A
a
a
Punnett Squares
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Take out your color-coded Meiosis Guides,
and with your partner, try to answer this
question: If Meiosis makes four gametes for
each parent, why do we make a Punnett
Square with only two gametes per parent?
Punnett Squares
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Results can be in two formats.

Genotype Fractions or Probabilities = The
likelihood of offspring with different genotypes.

You and your partner, make a Punnett Square for
this cross: Tt x tt
Punnett Squares
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Phenotype Fractions or Probabilities: Same
thing, but with phenotypes.

In the example you just tried, suppose that the T
allele in spiders codes for long legs and the t
allele codes for short legs.
Practice

Whiteboards out, one per table pair. Take
turns writing, be kind to the markers.

Construct a monohybrid cross for YY x YY.
Give genotype fractions and probabilities.
Practice

Construct a monohybrid cross for HH x Hh.
Give genotype fractions and probabilities.
Practice

Construct a monohybrid cross for Rr x rr.
Give genotype fractions and probabilities.
Practice

Construct a monohybrid cross for Dd x Dd.
Give genotype fractions and probabilities.
Practice

Construct a monohybrid cross for kk x kk.
Give genotype fractions and probabilities.
Practice

You cross a homozygous dominant tall pea
plant (TT) with a homozygous recessive short
pea plant (tt). What are the odds of an
offspring being a short plant?
Practice

You cross a homozygous white flower with a
homozygous red flower. Red is dominant to
white. What are the odds of the offspring
being a red flower? (note: when the problem doesn’t tell
you what letters to use for the genotype, feel free to use any
letter you want)
Practice

A male human is crossed with a female
human. Males are XY, females are XX.
What are the odds of a child being a girl?
Practice

You cross two heterozygous tall pea plants
(Tt x Tt). What will be the phenotype
probabilities for these offspring?
Practice

You cross two tall pea plants with each other,
a homozygous dominant (TT) with a
heterozygous (Tt). The pea plants produce
1000 offspring. How many of those offspring
do you predict will be heterozygous?
Punnett Squares
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Use the remaining time to work on your
homework.