Download Mendelian genetics

Bell Ringer
1. Fill in the table correctly.
Mitosis Meiosis
Creates somatic cells
Concerns sexual reproduction
Forms haploid daughter cells
Used for growth and repair
Forms unique cells
Mendelian Genetics
Can we predict what offspring will look like?
Mendelian Genetics
• We can determine an organism’s physical traits
(characteristics) by examining its chromosomes.
• Chromosomes are made up of a series of genes,
sequences that code for proteins.
• In the mid-1800s, Gregor Mendel experimented with pea
plants. He discovered that he could predict what the
offspring of pea plants would look like, as the parents
were passing down their physical traits to their offspring.
• This lead to two vital laws: the laws of segregation and
independent assortment.
Segregation
• The law of segregation states that every physical trait
has a unique gene that codes for it. All of the different
possibilities for each trait are called alleles.
• Example: alleles for eye color: black, brown, blue, etc.
• Since you have two of every chromosome, you have
two alleles for ALL of your traits. Your alleles are split
up when forming new gametes!
Independent Assortment
• The DNA in your cells is
randomly separated in
anaphase I.
• For each chromosome
pair, there is a 50%
chance of getting either
chromosome.
• Independent
assortment: the idea
that each gamete
randomly receives
chromosomes in meiosis.
Independent Assortment
Alleles
• One pair of alleles will
determine one specific
physical trait. In traditional
Mendelian genetics, all
traits have two possible
alleles:
• A dominant allele, which
will ALWAYS dictate the
physical trait if present
• A recessive allele, which
ONLY dictates the
physical trait if there are
NO dominant alleles
present
Alleles
• The exact pair of alleles
is referred to as a
genotype. Your physical
trait is called a
phenotype.
• Genotype – “genes” –
genetic sequences
• Phenotype –
“physical” – physical
appearance
• Genotype ALWAYS
controls phenotype!
Alleles
Rules:
• Dominant alleles are typically given a capital letter.
Recessive alleles receive a lowercase letter.
• Example: if yellow peas are dominant, and green peas
recessive, then Y = yellow, and y = green.
• If the genotype contains ANY dominant alleles, the
phenotype will be dominant.
• So, BOTH YY and Yy will produce YELLOW peas.
• You will ONLY see the recessive trait if you have TWO
recessive alleles.
• In other words, the ONLY way to make green peas is with
the genotype yy!
Alleles
Rules:
• If you have two of the same alleles, you have a
homozygous pair. “Homo-” means the same.
• If you have two different alleles, you have a
heterozygous pair. “Hetero-” means different.
• Using our previous example of yellow and green peas:
• The genotype YY is homozygous dominant, and makes
yellow peas.
• The genotype Yy is heterozygous, and makes yellow peas.
• The genotype yy is homozygous recessive, and makes
green peas.
Punnett Square
• We can use a device
called a Punnett
square to predict
what offspring will
look like.
• A Punnett square is
a grid that shows all
possible offspring.
Punnett Square
• Monohybrid squares
are very easy to use:
1. Take the genotype
of one parent, and
split the alleles.
Place them on the
top of the square.
• Example: YY crossed
with yy.
Punnett Square
• Monohybrid squares
are very easy to use:
2. Take the genotype
of the other
parent, and split
the alleles. Place
them to the left of
the square.
• Example: YY crossed
with yy.
Y
Y
Punnett Square
• Monohybrid squares
are very easy to use:
3. Fill in your square
by crossing the
alleles together.
• Example: YY crossed
with yy.
Y
y
y
Y
Punnett Square
• What genotypes will
our offspring be?
• What phenotypes
will our offspring be?
Y
Y
y
Yy Yy
y
Yy Yy
Common Monohybrid Crosses
• We will examine
some of the most
common monohybrid
crosses you will see.
• Pay close attention to
the genotypic and
phenotypic
outcomes!
• For our examples, P =
purple flowers, p =
white flowers.
Common Monohybrid Crosses
• Crossing a pure
dominant with a
pure recessive.
• Example: PP x pp
• Genotypic ratio:
100% Pp (hybrid)
• Phenotypic ratio:
100% purple flowers
P
P
p
Pp Pp
p
Pp Pp
Common Monohybrid Crosses
• Crossing two
hybrids.
• Example: Pp x Pp
• Genotypic ratio: 1 PP:
2 Pp: 1 pp
• 25%/50%/25%
• Phenotypic ratio:
75% purple flowers:
25% white flowers
• 3:1 purple to white
P
p
P
PP Pp
p
Pp pp
Common Monohybrid Crosses
• Crossing a hybrid with
a pure recessive.
• Example: Pp x pp
• Genotypic ratio: 50%
Pp: 50% pp
• 1 Pp: 1 pp
• Phenotypic ratio: 50%
purple flowers: 50%
white flowers
• 1:1 purple to white
P
p
p
Pp pp
p
Pp pp
Dihybrid Crosses
• Say you want to cross
a tall purple flower
with a short white
flower. You will be
crossing two
different traits.
• Can we find out what
the offspring will look
like?
Dihybrid Crosses
• According to Mendel’s
law of independent
assortment, the genes
for two different traits
will always be on
different chromosomes.
• We CAN determine how
two different traits can
be passed down, though.
This requires the use of a
dihybrid cross.
Dihybrid Cross
• In a dihybrid cross, you
will cross two traits at
the same time.
• Since each gamete has to
have one copy of each
chromosome, you need
to assign each side of the
Punnett square TWO
letters; one for each
trait!
• http://www.youtube.
com/watch?v=PdQPX
VZqyT8
Dihybrid Crosses
• All organisms in a
dihybrid cross will
have four alleles.
• You’ll need to
separate them into
gametes using the
FOIL method!
• Example: Parent has
the genotype AaBb.
• Front: AB
• Outside: Ab
• Inside: aB
• Last: ab
Dihybrid Crosses
• Then, place each
gamete pair along
the top of the
Punnett square.
AB Ab aB ab
Dihybrid Crosses
• Do the same for the
other parent.
• Example: Parent has
the genotype AaBb.
• Front: AB
• Outside: Ab
• Inside: aB
• Last: ab
AB Ab aB ab
Dihybrid Crosses
• Now, fill in each cell
of the dihybrid
Punnett square.
• Always keep the
same letters
together.
• Also, keep the letters
in the same order!
AB Ab aB ab
AB
Ab
aB
ab
Dihybrid Crosses
AB
Ab
aB
ab
AB
AABB AABb AaBB AaBb
Ab
AABb AAbb AaBb Aabb
aB
AaBB AaBb aaBB aaBb
ab
AaBb Aabb aaBb aabb
Dihybrid Crosses
• From there, you can determine the genotypic
and phenotypic ratios.
• Remember, genotypic ratios are the number of
different sequences, and phenotypic ratios are
the number of different appearances.
• When counting genotypes, ALWAYS work from
most to least dominant.
Dihybrid Crosses
Sequence
Number
AABB
1
AABb
2
AAbb
1
AaBB
2
AaBb
4
Aabb
2
aaBB
1
aaBb
2
aabb
1
AB
Ab
aB
ab
AB
AABB AABb AaBB AaBb
Ab
AABb AAbb AaBb Aabb
aB
AaBB AaBb aaBB aaBb
ab
AaBb Aabb aaBb aabb
Dihybrid Crosses
• For this particular
cross, the genotypic
ratio is
1:2:1:2:4:2:1:2:1
• Don’t worry – you
won’t see any
crosses this complex.
You will have much
simpler ones to do!