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Genetics
TEKS 6F: Predict possible outcomes of various
genetic combinations such as monohybrid
crosses, dihybrid crosses, and non-Mendelian
inheritance.
Gregor Mendel
Father of Modern Genetics
• Performed experiments with
pea plants
• Helped determined how traits
are passed from one
generation to the next.
How are offspring influenced by
genetic combinations?
• Parents pass on units of
information called genes.
• These genes produce traits,
which are characteristics that
make up an organism. Ex: height,
hair color, blood type,
intelligence.
• For each trait, an individual has
two alleles (different forms of a
gene). Alleles are represented as
letters (ex: TT can stand for the
trait of tallness). How are these
passed along?
Where she get that booty from?
She get it from her momma…and
her daddy
• Traits are passed on
from parents.
• You inherit ½ of your
genes from your mom
and the other ½ from
your dad.
• These come from the
sperm and the egg
when an egg is
fertilized.
Mendel’s principle of dominance
• Mendel’s principle of
dominance states that some
alleles are dominant and
others are recessive.
• Dominant alleles are written
with capital letters (ex: TT, Tt)
• Recessive alleles are always
written in lower case letters
(ex: tt)
• The recessive allele is exhibited
only when the dominant allele
is not present.
1.
2.
3.
How do I know if it’s dominant?
• If there is even one big letter,
it is dominant. (TT and Tt)
Why is Tt dominant if it has a
small letter?
• Because there is a big letter
next to it. Dominant will
always overpower recessive!
How do I know if it’s recessive?
• If it has two little letters, it is
recessive. (tt)
Homozygous vs. Heterozygous
• When both alleles
(letters) are the same,
the individual is
homozygous for that
trait. This is also called
true-breeding.
• When the alleles are
different, the
individual is
heterozygous for the
trait. This is also called
hybrid.
1.
2.
3.
What if the letters are both big,
like SS or TT?
• It is Homozygous.
What if the letters are both little,
like ss or tt?
• It is Homozygous.
What if one letter is big and one
letter is little, like Ss or Tt?
• It is Heterozygous because
they are different sizes.
Genotype vs. Phenotype
• The genetic makeup of
an organism is called
its genotype. Write
this as letters, like TT.
• The organism's
physical traits are its
phenotype. Write this
as a description, like
tall, red..
1.
2.
3.
Is Aa a genotype or phenotype?
• Genotype! Because it’s just
the letters.
Are purple, tall, short, white, and
round genotypes or phenotypes?
• Phenotypes! Because it’s a
physical description.
What is an organism’s genotype if
it receives two recessive alleles
for height (T=tall, t=short)?
• tt. Because little letters are
recessive. The phenotype
would be short.
How can you predict which traits will
be expressed using monohybrid
crosses?
• When doing a genetic cross,
the parent organisms are the P,
or parental generation.
• The offspring are called the F1,
or first filial generation.
• A monohybrid cross is a cross
in which there are two
contrasting alleles for a single
trait.
• A Punnett square is a diagram
that helps predict combinations
in genetic crosses. Punnett
squares are easy to construct.
• Do this animation:
http://www.siskiyous.edu/class
/bio1/genetics/monohybrid_v2
.html
• You can watch this for more
practice and explanation:
http://biologyanimations.blogspot.com/2011
/07/punnett-squareanimation.html
Practice with monohybrid crosses on
the whiteboard!
What if you are using more than
one trait, like someone that has
blue eyes and is short?
• You would use a dihybrid cross
instead of a monohybrid cross.
• A dihybrid cross is a cross that
shows the possible offspring for two
traits.
How do you predict the outcomes of
dihybrid crosses?
Let’s do an example!
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
In this example, we will cross:
A heterozygous black/heterozygous rough
individual with another heterozygous
black/heterozygous rough individual. Their
genotypes will be: __________.
BbRr x BbRr
How do you predict the outcomes of
dihybrid crosses?
BR, Br
bR, br
BR, Br
BbRr x BbRr bR, br
First, you must find ALL possible gametes that can be made from each parent.
Remember, each gamete must have one B and one R.
How do you predict the outcomes of
dihybrid crosses?
BbRr x BbRr
Possible gametes:
Next, arrange all possible
BR
gametes for one parent along the
Br
top of your Punnett Square, and
all possible gametes for the other
bR
parent down the side of your
Punnett Square…
br
How do you predict the outcomes of
dihybrid crosses?
BbRr x BbRr
BR
Br
bR
br
Fur Color:
B: Black
b: White
BR
Coat Texture:
R: Rough
r: Smooth
Br
Then, find
the possible
genotypes
of the
offspring
bR
br
How do you predict the outcomes of
dihybrid crosses?
BR
Br
bR
br
BbRr x BbRr
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
BR
BBRR
BBRr
BbRR
BbRr
Br
BBRr
BBrr
BbRr
Bbrr
bR
BbRR
BbRr
bbRR
bbRr
br
BbRr
Bbrr
bbRr
bbrr
How do you predict the outcomes of
dihybrid crosses?
How many of the
offspring would have a
black, rough coat?
How many of the
offspring would have a
black, smooth coat?
How many of the
offspring would have a
white, rough coat?
How many of the
offspring would have a
white, smooth coat?
BR
Br
bR
br
BR BBRR
BBRr
BbRR
BbRr
Br
BBRr
BBrr
BbRr
Bbrr
bR
BbRR
BbRr
bbRR
bbRr
br
BbRr
Bbrr
bbRr
bbrr
Fur Color:
Coat Texture:
B: Black
R: Rough
b: White
r: Smooth
How do you predict the outcomes of
dihybrid crosses?
How many of the
offspring would have
black, rough coat?
How many of the
offspring would have a
black, smooth coat?
How many of the
offspring would have a
white, rough coat?
How many of the
offspring would have a
white, smooth coat?
BR
Br
bR
br
BR BBRR
BBRr
BbRR
BbRr
Br
BBRr
BBrr
BbRr
Bbrr
bR
BbRR
BbRr
bbRR
bbRr
br
BbRr
Bbrr
bbRr
bbrr
Phenotypic Ratio
9:3:3:1
More dihybrid practice..
In pea plants, yellow seeds (Y) are dominant over green
seeds (y), and rounded peas (R) are dominant over
wrinkled peas (r).
1. Cross a plant that is heterozygous for both traits
with a plant that is homozygous recessive for both
traits. Draw a Punnett square to show all possible
offspring, and determine the genotypic and
phenotypic ratios.
YyRr X yyrr
YR
Yr
yR
yr
yr
YyRr
Yyrr
yyRr
yyrr
yr
YyRr
Yyrr
yyRr
yyrr
yr
YyRr
Yyrr
yyRr
yyrr
yr
YyRr
Yyrr
yyRr
yyrr
Non-mendelian genetics: What
are some other patterns of
inheritance?
Some forms of inheritance are distinctly non-Mendelian
inheritance because they do not follow the principles
described by Gregor Mendel.
Incomplete Dominance
and
Codominance
Incomplete Dominance
• Neither allele is dominant, so the traits blend together.
• Phenotypes combine to create a separate trait.
• Example, red and white snapdragons = offspring pink
snapdragons
Incomplete Dominance
• You can use two
different letters
now!
• RR (red) X WW
(white)
R
R
W
RW RW
pink pink
W
RW RW
pink pink
Incomplete Dominance Practice
• Cross of two purple flowers (what 2 colors make purple?)
RB X ____
RB
____
• What are the genotype ratios?
1RR : 2RB : 1BB
• What are the phenotype ratios?
R
1red : 2 purple : 1 blue
• Can you have a
B
heterozygous red or
no
hybrid blue flower? ___
R
B
RR
red
RB
purple
RB
purple
BB
blue
Codominance
• Expression of both alleles equally
• Phenotype of both traits are seen in
heterozygous individuals
• Example black rooster and white
chicken produce offspring with both
black and white feathers.
Punnett Square of Codominant cross
• Expressed as
two separate
traits in a
Punnett Square
Cross
• BB (black) X WW
(white)
W
W
B
BW
(Black and
white)
BW
(Black and
white)
B
BW
(Black
and
white)
BW
(Black
and
white)