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Chapter 10:
Patterns of Inheritance
Thinking Time!!!!!!!!!
• Why is it, in some cases, that two brown-eyed
parents have all brown-eyed children, and in
other cases two brown-eyed parents have some
brown-eyed children and some blue-eyed
children?
• What do you think would be the result of a cross
between a red flower and a white flower?
Cracking the code of life
1. The Blending Hypothesis:
–
Predicts that offspring appearances will be blends of the phenotypes
of their parents.

+
red
white
all pink
2. The Particulate Hypothesis (Mendel):
–
–
One copy of each factor is inherited from each parent.
The factors remain intact and do not physically blend together.

+
red
white
+
red
white
Genetic Vocabulary:
• Trait: Variation of a particular character.
– Ex: eye color is a character,

while
traits
• Hybrids: The offspring of 2 different true-breeding species.
+

mule
• Monohybrid cross: a pairing in which the parents differ in
only one (mono) character.
– Ex: Flower color
P generation: Parent plants
F1 generation: The hybrid
offspring
F2 generation: When F1 plants
self-fertilize or fertilize each
other, their offspring makes up
the F2 generation.
Terms need to know:
• Allele: The different forms of genes.
– Organisms have two alleles for each trait, one from each parent.
• Dominant: appear to hide other traits.
– capital letters ABCDEFGH
• Recessive: can be hidden in one generation and then appear in
the next.
– lower-caser letters abcdefgh
• Phenotype - the way an organisms’ genes express themselves
(physical type, description)
– either short, tall, purple, white, & etc
• Genotype - the gene type of an organism (genes, letters)
– TT, Tt, tt
Homozygous vs. Heterozygous
In pea plants,
is dominant over
• Homozygous - 2 alleles are the same.
– homozygous dominant
• 2 dominant genes : PP
– homozygous recessive
• 2 recessive genes : pp
• Heterozygous – 2 alleles are different.
– 1 dominant gene and 1 recessive gene: Pp.
Gregor Mendel
• Often called the "Father of Genetics”
• Performed many experiments with plants,
mostly garden peas.
• His 1st step: identify true-breeding plant.
• 2nd step: He crossed true-breeding plants
that had two distinct traits.

?
Mendel’s First Experiment:
¾ of plants are purple
¼ of plants are white
Mendel’s Results:
• He examined the inheritance patterns of 7 different pea-plant
characters.
• For each character, one of the two parent traits disappeared in
the F1, but reappeared in ¼ of the F2 generation.
The Testcross:
• It is not possible to predict the
genotype of an organism with a
dominant phenotype.
• It could be homozygous dominant
PP or heterozygous Pp.
• A test cross can determine the
identity of the unknown allele.
Writing Genotypes from phenotypes:
For Height:
tall (T) is dominant and short (t) is recessive.
• 1. homozygous dominant for height
– Answer: TT
• 2. heterozygous for height
– Answer: Tt
• 3. homozygous tall
– Answer: TT
• 4. recessive for height
– Answer: tt
• 5. heterozygous tall
– Answer: Tt
Writing phenotypes from genotypes:
For Height:
tall (T) is dominant and short (t) is recessive.
For Flower color:
red (R) is dominant and white (r) is recessive.
• 6. TT
– Answer: homozygous dominant, tall
• 7. tt
– Answer: homozygous recessive, short
• 8. Tt
– Answer: heterozygous, tall
• 9. Rr
– Answer: heterozygous, red
• 10. rr
– Answer: homozygous recessive, white
Activity Time:
The Coin Activity
• When tossing one penny, what is the probability
of getting one head?
• When tossing one penny, what is the probability
of getting one tail?
When tossing two pennies, what is the probability of
getting one head and one tail?
Punnett Squares
• It shows the possible outcomes for the
Phenotypes of the individual.
Predicting Genotypes & Phenotypes
• In pea plants,
is dominant
over
Tall
• Cross: homozygous tall
Short
X
homozygous short
Step 1:
• Write down the genotypes of each parent
– ABCDEFGH  dominant traits
– abcdefgh  recessive traits
• Homozygous tall:
• Homozygous short:
TT
tt
Step 2:
• Draw a Punnett square
• Write the gametes of one parent across the top and the
other parent along the side
T
t
t
T
Step 3:
• Fill in each box of the Punnett square
– Capital letter  goes 1st
T
T
t
Tt
Tt
t
Tt
Tt
Step 4:
• List the possible genotypes and phenotypes of the offspring.
• Possible genotypes:
Tt
• Possible phenotypes:
Tall
Results
Parent 1:
Parent 2:
T
T
Tt
Tt
t
t
Tt
Tt
Results
• Each of the offspring has a ____
4 /4 or _____%
100 chance of
showing ______
Tt genotype.
• Since T (tall) is dominant over t (short). Therefore, each of
the offspring has a ____/4
or _____%
4
100 chance of being
__________.
tall
Practice Time:
Key: brown eyes (B) is dominant, blue eyes (b) is recessive
is dominant
over
Homozygous dominant X Heterozygous
• Genotypes: BB & Bb
B
b
B
B
BB
BB
Bb
Bb
Questions:
1. Phenotype of offspring? all brown
2. Genotype of offspring? BB & Bb
3. Phenotypic ratio: 4 brown: 0 blue
4. Genotypic ratio: 2 BB: 2Bb
Your Turn
In humans,
is dominant
over
Questions:
1.
2.
3.
4.
Phenotype of offspring? brown & blue
Genotype of offspring? BB, Bb & bb
Phenotypic ratio: 1 BB: 2 Bb: 1 bb
Genotypic ratio: 3 Brown: 1 blue
heterozygous brown X heterozygous brown
Dihybrid Cross
• In one dihybrid cross experiment, Mendel studied the inheritance of
seed color and seed shape.
is dominant over
Yellow (Y)
green (y)
is dominant over
Round (R)
wrinkled (r)
Dihybrid cross:
• Mendel crossed true-breeding plants that had yellow,
round seeds (YYRR) with true-breeding plants that
has green, wrinkled seeds (yyrr).
• The first parent could only produce RY gametes. The
other could only produce ry gametes.
• The union of these gametes results in RrYy.
Dihybrid Cross:
•
Four classes of gametes
(YR, Yr, yR, and yr) are
produced in equal
amounts.
•
Which results in 16 equally probable ways in which
the alleles can combine in the F2 generation.
Phenotypic ratio - 9:3:3:1
•
Intermediate Inheritance:
• In Mendel’s pea crosses, the F1 offspring always looked like the
dominant homozygous parent.
– Because the recessive phenotype required two recessive
alleles.
YY  yellow
Yy  yellow
yy  green
– But, for some characters of organisms, neither allele is
dominant  intermediate inheritance.
Dominant
Intermediate Inheritance:
• Heterozygous F1 generation hybrids do not show the
phenotype of the dominant allele or the recessive allele but
somewhere in between.
– For example, in a particular breed of chickens called
Andalusians, black and white offspring produce blue F1
hybrid offspring.
– Because neither the black nor white is dominant, capital or
lower case letters are not used.
– Instead a C for color is paired with a superscript B for black
or W for white.
B
B
W
W
B
W
C C
C C
C C
black
white
blue
X
B
B
W
C C
W
C C
The predicted phenotypes
in F2 are
1 black: 2 blue: 1 white
B
W
C C
B
B
C C
B
W
C C
B
W
C C
W
W
C C
Multiple Alleles:
• Codominant: both alleles are expressed
• The gene for human blood type expresses four phenotypes:
– A, B, AB or O.
• The alleles IA and IB are codominant and the i allele is recessive to both.
Phenotypes
(Blood type)
Genotypes
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
O
ii
• The letters refer to two carbohydrates,
designated A and B, which are found on the
surface of red blood cells.
FYI:
• Type O-negative blood does not have any antigens. It is
called the "universal donor" type because it is compatible
with any blood type.
• Type AB-positive blood is called the "universal recipient"
type because a person who has it can receive blood of any
type.
Polygenic inheritance
• When a single phenotype results from the expression of two or
more genes.
• For instance, phenotypes like high blood pressure are not the
result of a single "blood pressure" gene with many alleles.
• The phenotype is an interaction between a person's weight,
cholesterol level, kidney function, smoking, and probably lots of
others too.
The Importance of Environment
• An individual’s phenotype depends on environment as well as genes.
• In human, nutrition influences height, exercise affects build, and
exposure to sunlight darkens skin.
• The product of a genotype is generally not a single defined phenotype,
but a range of possibilities influenced by the environment.
Chromosome Theory of Inheritance:
• It states that:
– Genes are located on chromosomes.
– Homologous Chromosomes separate during meiosis so
that alleles are segregated.
– Chromosomes under go segregation and independent
assortment during meiosis.
Mendel’s Law of Genetics:
1. Alleles are different forms of the same gene that segregate
during gamete formation (Law of Segregation)
2. Alleles of different genes segregate independently (Law of
independent assortment)
Sex-linked Genes
• Located on a sex chromosome
• In humans, most sex-linked genes are found on the X
chromosome, which is much larger than the Y chromosome.
• Discovered by Thomas Hunt Morgan.
P:
Morgan mated a
white-eyed male fly with a
red-eyed female fly.
r
X
R
X
Y
R r
X X
female
R
X Y
male
F1:
R
X
R r
X X
female
R
X Y
male
All the F1 offspring had red eyes.
X
F1:
R r
X X
X Y
female
male
R
R
X
R
X
F2:
r
X
Then, he bred the F1
offspring together.
Y
R R
X X
female
R r
X X
female
R
X Y
male
r
XY
male
He got the classical 3:1
phenotypic ratio.
However, none of the
flies with white eyes
were female.
He concluded that the
gene involved is only
on the X chromosome.
Are you colorblind?????
Are you colorblind?????
Are you colorblind?????