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Ch. 11
Important Vocab
Genetics: the study of heredity
 Fertilization: the joining of sperm and egg
 Genes: chemical factors that determine
traits (a specific characteristic)
 Alleles: different forms of a gene

 Ex: tall vs short, yellow vs green
More Important Vocab

True-breeding: individuals that will
produce offspring identical to
themselves if allowed to self-pollinate
 TT, tt, RR, rr

Hybrid: individuals that will not produce
offspring identical to themselves if
allowed to self-pollinate
 Tt, Rr, Ss
Principle of Dominance

States that some alleles are dominant
and others are recessive
 An organism that has a dominant allele for a
particular trait will always show that trait
 An organism with a recessive allele for a trait
will exhibit that form only when the dominant
allele is absent
Who is the Father of Genetics?
Gregor Mendel
 Studied pea plants
 Started with true-breeding plants

 True-breeding tall plant (TT) x true-breeding
short plant (tt)
 All offspring were tall
 Why?
○ Principle of dominance
Probability and Genetics
Probability: the likelihood that a particular
event will occur
 Why study probability with genetics?

 All the traits you receive from your parents
depends on chance!

What do we use to predict the outcome of
a genetic crosses?
 Punnett Squares
Probabilities Predict Averages
Probabilities predict the average
outcome of a large number of individuals
 You are more likely to get the predicted
outcome with 500 individuals rather than
5 individuals.

Chromosomes
Contains genes
 Found in the nucleus in eukaryotes

 Found in the cytoplasm in prokaryotes

Mendel states:
 Each organism must inherit a single copy of
every gene from each parent.
 When an organism produces its own
gametes, those two sets (copies) of genes
must be separated from each other so that
each gamete contains just one set of genes.
Chromosomes
Homologous chromosomes: chromosomes
that each have a corresponding
chromosome from the parent of the opposite
sex
 A cell that contains both sets of homologous
chromosomes is said to be diploid (meaning
2 sets)
 A cell that contains only 1 set of
chromosomes is called haploid – i.e.
gametes

Chromosomes

Humans:
 How many chromosomes does each of your
typical body cells have?
○ 46 (diploid)
 How many chromosomes does each of your
gametes have?
○ 23 (haploid – one set of chromosomes)
Meiosis
Meiosis is a process of reduction
division in which the number of
chromosomes per cell is cut in half
through the separation of homologous
chromosomes in a diploid cell.
 Divided into 2 major steps:

 Meiosis I = http://education-
portal.com/academy/lesson/meiosis-ireductional-cell-division.html#lesson
 Meiosis II = http://educationportal.com/academy/lesson/meiosis-iiequational-cell-division.html#lesson
Meiosis I

Prophase I
 Each chromosome pairs with its
corresponding homologous chromosome to
form a tetrad (made of 4 chromatids)
 Homologous chromosomes then exchange
portions of there chromatids during crossingover
○ Results in the exchange of alleles between
homologous chromosomes and produces new
combinations of alleles
Meiosis I

Metaphase I
 Spindle fibers attach to the chromosomes
 Chromosomes line up across middle of cell
at metaphase plate

Anaphase I
 Fibers pull homologous chromosomes
towards opposite ends of cell

Telophase I and Cytokinesis
 Nuclear membranes reform
 Cell separates into 2 haploid daughter cells
Meiosis II

Prophase II
 The 2 cells created by Meiosis I now enter
into a second meiotic division
 At this point, each cell’s chromosomes
contains 2 sister chromatids (same genes,
but different alleles  one may have “A”
while the other may have “a”)

Metaphase II
 Chromosomes line up at metaphase plate
 Spindle attaches to centromeres
Meiosis II

Anaphase II
 Sister chromatids separate and move
towards opposite ends of cell

Telophase II and Cytokinesis
 Nuclear membrane reforms
 Cytoplasm divides
 Result: 4 genetically different haploid
gametes
Mitosis vs Meiosis
 Mitosis
produces 2 identical
diploid daughter cells
 Meiosis produces 4
genetically different haploid
gametes

Mitosis vs Meiosis = http://www.youtube.com/watch?v=rB_8dTuh73c
(One factor crosses)
Vocab for Punnett Squares

Homozygous: two identical alleles for a
particular trait
 Aka – true-breeding

Heterozygous: two different alleles for a
particular trait
 Aka – hybrid

Phenotype: physical characteristics
 Yellow, green, tall, short

Genotype: genetic makeup
 YY, Yy, yy, TT, Tt, tt
Let’s Practice the Vocab!

T= tall, t = short
 Homozygous tall = TT
 Heterozygous = Tt
 Homozygous short = tt

Y = yellow, y = green
 Homozygous yellow = YY
 Heterozygous = Yy
 Homozygous green = yy
Ratios for Normal Monohybrid Crosses

Phenotypic ratio
 Compares the # of individuals that show the
dominant trait to the # of individuals that show
the recessive trait
 Tall:short, Yellow:green

Genotypic ratio
 Compares the # of homozygous dominant
individuals to the # of heterozygous individuals
to the # of homozygous recessive individuals
 TT:Tt:tt , YY:Yy:yy
Practice with Punnetts!

Heterozygous tall x heterozygous tall
 What are the genotypes?
○ Tt x Tt
 Punnett Square
 Phenotypic ratio?
○ Tall:short
T
○ 3:1
 Genotypic ratio?
t
○ TT:Tt:tt
○ 1:2:1
T
t
TT
Tt
Tt
tt
More Practice with Punnetts!

Homozygous tall x homozygous short
 What are the genotypes?
○ TT x tt
 Punnett Square
 Phenotypic ratio?
○ Tall:short
t
○ 4:0
 Genotypic ratio?
t
○ TT:Tt:tt
○ 0:4:0
T
T
Tt
Tt
Tt
Tt
More Practice with Punnetts!

Heterozygous tall x short
 What are the genotypes?
○ Tt x tt
 Punnett Square
 Phenotypic ratio?
○ Tall:short
t
○ 2:2
 Genotypic ratio?
t
○ TT:Tt:tt
○ 0:2:2
T
t
Tt
tt
Tt
tt
Your turn to try!

Homozygous tall x heterozygous tall
 What are the genotypes?
○ TT x Tt
 Punnett Square
 Phenotypic ratio?
○ Tall:short
T
○ 4:0
 Genotypic ratio?
t
○ TT:Tt:tt
○ 2:2:0
T
T
TT
TT
Tt
Tt
(Two Factor crosses)
Back to Mendel!

Mendel asked: Does the segregation of
one pair of alleles affect the segregation
of another pair of alleles?
 No, they will separate independently.
 Example:
○ What the alleles for height do will not affect
what the color alleles do.
Principle of Independent
Assortment
States that genes for different traits can
segregate independently during the
formation of gametes.
 Gametes: sex cells
 What are the two types of gametes?

 Sperm
 Egg
Dihybrid Crosses

Let’s look at two factors: seed shape
and color
 Round (R) vs wrinkled (r) seed shape
 Yellow (Y) vs green (y) seed color
Homozygous round and yellow = RRYY
 Wrinkled and green = rryy
 Heterozygous for both traits = RrYy

Dihybrid Crosses
Let’s cross 2 truebreeding plants:
Round yellow x wrinkled green
 Step 1: Genotypes

 RRYY
 rryy

Step 2: Gamete
combinations 
RRYY
rryy
RY
RY
RY
RY
ry
ry
ry
ry
Dihybrid Crosses
Another way to figure out the gamete
combinations
 Parent: RRYY
Parent: rryy

R
R
r
r
Y RY
RY
y
ry
ry
Y RY
RY
y
ry
ry
Dihybrid Crosses

Step 3: Punnett Square (RRYY x rryy)
RY
RY
RY
RY
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
Dihybrid Crosses

Step 4: Phenotypic ratio
 Round yellow: round green: wrinkled yellow:
wrinkled green
 16:0:0:0
Now you try!
Cross two individuals that are
heterozygous round and yellow
 What are the genotypes?

 RrYy
R
 RrYy

What are the gametes?
r
Y RY
rY
y
ry
 RrYy = RY, Ry, rY, ry
 RrYy = RY, Ry, rY, ry
Ry
RrYy x RrYy

RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Phenotypic ratio:
 Round/yellow : round/green : wrinkled/yellow : wrinkled green
9:
3:
3:
1
Incomplete Dominance
Codominance
Incomplete Dominance
Case in which one allele is not completely
dominant over another
 Heterozygous phenotype is somewhere
between the two homozygous
phenotypes
 Example: Snapdragons

 Red (R) and white (r)
 RR = red and rr = white
 What about Rr?
○ These are pink!
Let’s Practice!
Red snapdragon x white snapdragon
 Genotypes?
R
R

 RR x rr
Punnett Square
 Ratios:

 Phenotypic
○ Red: pink: white
○ 0:4:0
 Genotypic
○ RR: Rr: rr
○ 0:4:0
r
Rr
Rr
r
Rr
Rr
Now It’s Your Turn:
Pink x Pink
 Genotypes?

R
 Rr x Rr
Punnett Square
 Ratios:

 Phenotypic
○ Red: pink: white
○ 1:2:1
 Genotypic
○ RR: Rr: rr
○ 1:2:1
r
R
RR Rr
r
Rr
rr
Codominance
Situation in which both alleles contribute
to the phenotype
 Heterozygote is usually spotted or
speckled compared to the solid colored
homozygotes
 Example: Chickens





B = black, W = White
BB = black chicken
WW = white chicken
BW = black and white speckled chicken
Let’s Practice!
Black chicken x White Chicken
 Genotypes?

B
 BB x WW
Punnett Square
 Ratios:

 Phenotypic
○ Black: speckled: white
○ 0:4:0
 Genotypic
○ BB: BW: WW
○ 0:4:0
B
W
BW BW
W
BW BW
Your Turn!
Speckled chicken x speckled chicken
 Genotypes?

B
 BW x BW
Punnett Square
 Ratios:

 Phenotypic
○ Black: speckled: white
○ 1:2:1
 Genotypic
○ BB: BW: WW
○ 1:2:1
W
B
BB BW
W
BW WW
Bloodtyping is Codominant!

Types A and B are both dominant to O
Bloodtype
A
B
AB
O
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
Antigens
A
B
A and B
Will take
blood from:
A or O
B or O
AB, A, B, O
Can’t take
blood from:
B or AB
A or AB
---
None
O
A, B, AB
Bloodtyping with Punnetts
Homozygous A x Homozygous B
 Genotypes?

IA
 IAIA X IBIB
Punnett Square
 Probabilities

 A = 0%
 B = 0%
 AB = 100%
 O = 0%
IA
IB
IAIB IAIB
IB
IAIB IAIB
Bloodtyping with Punnetts
AB x O
 Genotypes?

IA
IB
i
IAi
IBi
i
IAi
IBi
 IAIB X ii
Punnett Square
 Probabilities

 A = 50%
 B = 50%
 AB = 0%
 O = 0%
Your Turn!
Heterozygous A x Heterozygous B
 Genotypes?

IA
 IAi X IBi
Punnett Square
 Probabilities

 A = 25%
 B = 25%
 AB = 25%
 O = 25%
i
IB
IAIB IBi
i
IAi
ii
Sex-linked Traits
Human Heredity

Karyotype: a picture of chromosomes
arranged in homologous pairs
Karyotypes

Human karyotypes show 46 chromosomes
 23 came from the sperm (father)
 23 came from the egg (mother)

Chromosome pairs #1-22 are called
autosomes.
 These determine all sorts of traits – eye color,
hair color, height, shapes of features
How do Genetic Disorders Occur?

Non-disjunction
 Occurs during Meiosis I
 Homologous chromosomes fail to separate
during Anaphase I
 Result = gametes have wrong number of
chromosomes
○ One ends up with an extra, while the other will
have one less
Sex Chromosomes

Chromosome pair #23 is called the sex
chromosomes
 Females = XX
 Males = XY
All human egg cells carry 1 X.
 Only ½ of human sperm cells carry an
X, whereas the other ½ carry a Y

 Results in a 50/50 chance of having either a
girl or boy.
Sex-linked Genes
Sex-linked genes are genes located on the X
chromosome.
 Most sex-linked traits are recessive.
 Males are more likely to show sex-linked traits.

 This is because males have only 1 X – if this X is
“bad”, the sex-linked trait will be expressed.
 Females have 2 Xs – both Xs have to be “bad” in
order for the trait to be expressed.

Examples: Colorblindness, Hemophilia,
muscular dystrophy
Colorblindness - Genotypes

Males can be:
 Normal Male = XY
 Colorblind male = XcY

Females can be:
 Normal female (not a carrier) = XX
 Normal female (carrier) = XcX
○ This means she doesn’t show the trait, but
can pass the trait on to her offspring
 Colorblind female = XcXc
Colorblindness with Punnetts

Normal male x colorblind female
 What are the genotypes?
○ XY x XcXc
 Punnett Square
 What is the chance they will
have a colorblind son?
X
Y
Xc XcX XcY
○ 50%
 What is the chance they will
have a colorblind daughter?
○ 0%
Xc XcX
XcY
Colorblindness with Punnetts

Colorblind male x normal female
 What are the genotypes?
○ XcY x XX
 Punnett Square
 What is the chance they will
have a colorblind child?
Xc
Y
X XcX XY
○ 0%
 If they have a daughter, what
chance she will be a carrier?
○ 100%
X XcX
XY