Transcript GenesPunnettSquare

Genes & Punnett
Squares
Objectives 3-4
Genes
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Genes are the
characteristics/traits coded in
the DNA
Each gene is made up of 2
alleles (one from each parent)
Principle of Dominance
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Dominant Allele
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Always determines the physical feature of
the individual.
Upper case Letter (i.e. B or T).
Recessive Allele
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Only determines the physical feature of
the individual when there is no dominant
allele.
Lower case letter (i.e. b or t).
Genotype and Phenotype
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The genotype of an individual is
the description of the genetic
information (DNA, Alleles)
The phenotype of an individual
is the physical features made by
the genotype (what they look
like, a picture)
Allele Combinations
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If both alleles are the same, the
gene is called homozygous
BB = homozygous dominant
 bb = homozygous recessive
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If the alleles are different, the
gene is called heterozygous. (Bb)
Is heterozygous and homozygous
an example of genotype or
phenotype?
Label the following as homozygous dominant,
homozygous recessive, or heterozygous.
1.
2.
3.
4.
5.
6.
7.
8.
BB
Aa
nn
Kk
GG
ff
qq
Hh
Probability and Punnett
Squares
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Meiosis separates the alleles of a parent into
different sex cells (gametes).
Example: Dad is heterozygous for Brown
hair over blonde hair = Bb (B = brown / b =
blonde)
 Through meiosis, these alleles are copied
and separated into 4 sperm.
B
B
b
b
Single Trait Crossing
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A Punnett Square can help you
find out the trait of a child
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Example: Dad is heterozygous
(Bb) for Brown hair over blonde
hair. Mom is also heterozygous
(Bb) for brown hair over blonde
hair.
Setup the Punnett Square
B
B
Mom’s
alleles
b
b
Dad’s
Alleles
Set up the Punnett Square
B
B
b
BB
Bb
Mom’s
alleles
b
Bb
bb
Dad’s
Alleles
4
possible
offspring
Interpreting Results
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¼ or 25% chance of an offspring
with homozygous dominant
genes for brown hair.
½ or 50% chance of an offspring
with heterozygous genes for
brown hair.
¼ or 25% chance of an offspring
with homozygous recessive
genes for blond hair.
You try it!
1. Mom is heterozygous for
Brown eyes over blue eyes. (Ee)
Dad is homozygous recessive
with blue eyes. (ee) What are
the probabilities for the
children’s genotypes? What are
the probabilities for the
children’s phenotypes?
Setup the Punnett Square
e
E
Mom’s
alleles
e
e
Dad’s
Alleles
Setup the Punnett Square
E
e
e
Ee
Ee
Mom’s
alleles
e
ee
ee
Dad’s
Alleles
4
possible
offspring
Results:
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½ or 50% of the children would
inherit heterozygous genes for
Brown eyes (Ee)
½ or 50% of the children would
inherit homozygous recessive
genes for Blue eyes (ee)
Hmmm…
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Is it possible for two Brown
haired parents to produce a
blond child? Explain.
Is it possible for 2 blue eyed
parents to produce a brown
eyed child? Explain.
Can you figure out your
genotype for hair color? How do
you know?
Review Questions
1.
2.
3.
How is meiosis related to
heredity?
What is the difference between
heterozygous, homozygous
dominant, and homozygous
recessive?
Is it possible for two brown
haired parents to produce a
blond child? Explain.
Review cont…
4.
5.
What is the difference between
genotype and phenotype?
What are the chances that two
heterozygous parents with
Brown hair(Bb) produce a child
that is also heterozygous?
Review cont…
6.
7.
In pea plants, round peas are
dominant over wrinkled peas.
(R and r). What would the
offspring of a homozygous
dominant, and a homozygous
recessive cross look like?
If the offspring from question
#6 were to pollinate each other
(as plants often do), what
would be the genotype
probabilities for the offspring?
Two Trait Crosses
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When looking at two different
genes in the same cross, 4
alleles are used
For instance, a heterozygous
haired(Bb), and homozygous
recessive eyed(ee) parent would
be Bbee.
 Through meiosis, there would be 4
possible allele combinations in the
sex cells.
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FOIL!
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Just like in math class, the first,
outside, inside, and last rule can
help you get the answer
(B b) (e e)
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These alleles would produce these
four combinations.
Be
Be
be
be