Bell Ringer1. 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 GeneticsCan 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=PdQPXVZqyT8

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 CrossesSequence 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!