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The Basis of Heredity
“Chapter 18”
1
Curriculum Outcomes
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Key Terms
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Questions
Questions 1-3 page 596
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Genes and Heredity
Can you identify members of a family by physical traits?
Heredity – is the transmission of biological traits from
parent to offspring.
Genetics – Study of inheritance of biological traits.
Biological traits are determined by genes, which are
specific segments of DNA.
Humans are able to use this information to their
advantage.
Cows and Dogs produced
Crop plants
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Activity
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Mendelelian
Genetics
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Gregor Mendel
(1822-1884)
Responsible
for the Laws
governing
Inheritance of
Traits
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Gregor Johann Mendel
ƒAustrian monk
ƒStudied the
inheritance of
traits in pea plants
ƒDeveloped the laws
of inheritance
ƒMendel's work was
not recognized until
the turn of the
20th century
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Gregor Johann Mendel
ƒBetween 1856 and
1863, Mendel
cultivated and
tested some 28,000
pea plants
ƒHe found that the
plants' offspring
retained traits of
the parents
ƒCalled the “Father
of Genetics"
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Site of
Gregor
Mendel’s
experimental
garden in the
Czech
Republic
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Particulate Inheritance
ƒMendel stated that
physical traits are
inherited as “particles”
ƒMendel did not know
that the “particles”
were actually
Chromosomes & DNA
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Gregor Mendel – Pioneer of
Genetics
Mendel tracked and recorded the transmission
of seven visible traits through several
generations of the garden pea. To Keep track he
called the first parents P and than Filial General
F1 and so on.
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Why did he work with a garden pea?
Garden peas have a number of Characteristics
How it reproduces- reproduces through self
pollination.
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Seven Characteristics Studied By Mendel
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The Principle of Dominance
When Mendel used pollen from a pea plant with round
seeds to fertilize a pea plant with wrinkled seeds, he
found that all the offspring (progeny) in the F1
generation had round seeds.
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Progeny – new individual that result from reproduction;
offspring.
Did this mean that pollen determines shape?
So, he did the opposite and again all the progeny had
round seeds.
Round-seed shape was always the dominant trait.
Mendel called the other wrinkled shaped seeds the
recessive trait.
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Questions
Questions 1-5 page 600
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Genetic Terminology
ƒ Trait - any characteristic that
can be passed from parent to
offspring
ƒ Heredity - passing of traits
from parent to offspring
ƒ Genetics - study of heredity
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Types of Genetic Crosses
ƒ Monohybrid cross - cross
involving a single trait
e.g. flower color
ƒ Dihybrid cross - cross involving
two traits
e.g. flower color & plant height
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Punnett Square
Used to help
solve genetics
problems
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Designer “Genes”
ƒ Alleles - two forms of a gene
(dominant & recessive)
ƒ Dominant - stronger of two genes
expressed in the hybrid;
represented by a capital letter (R)
ƒ Recessive - gene that shows up less
often in a cross; represented by a
lowercase letter (r)
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More Terminology
ƒ Genotype - gene combination for
a trait (e.g. RR, Rr, rr)
ƒ Phenotype - the physical feature
resulting from a genotype (e.g.
red, white)
ƒ Segregation – the separation of
alleles during meiosis
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Genotype & Phenotype in Flowers
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes
RR
Rr
rr
Phenotypes
RED
RED
YELLOW
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Genotypes
ƒ Homozygous genotype - gene
combination involving 2 dominant
or 2 recessive genes (e.g. RR or
rr); also called pure
ƒ Heterozygous genotype - gene
combination of one dominant &
one recessive allele
(e.g. Rr);
also called hybrid
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Result of Peas being Crossed
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Probability and Inheritance of
Single Traits
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Phenotypic ratio – the ratio of offspring with a
dominant trait to the alternative recessive trait
Punnett Square – a chart used to determine the
predicted outcome of a genetic cross.
Genotypic ratio – the ratio of offspring with
each possible allele combination from a particular
cross.
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P = # of ways that a given outcome can occur
Total # of possible outcomes
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Review Questions
Page 475 Questions 1-4 (old text)
Questions 1-3 page 604
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Genes and Environment
Determine Characteristics
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Mendel’s Pea Plant
Experiments
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Why peas, Pisum sativum?
ƒCan be grown in a
small area
ƒProduce lots of
offspring
ƒProduce pure plants
when allowed to
self-pollinate
several generations
ƒCan be artificially
cross-pollinated
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Reproduction in Flowering Plants
Pollen contains sperm
Produced by the
stamen
Ovary contains eggs
Found inside the
flower
Pollen carries sperm to the
eggs for fertilization
Self-fertilization can
occur in the same flower
Cross-fertilization can
occur between flowers
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Mendel’s Experimental
Methods
Mendel hand-pollinated
flowers using a
paintbrush
He could snip the
stamens to prevent
self-pollination
He traced traits
through the several
generations
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How Mendel Began
Mendel
produced
pure
strains by
allowing the
plants to
selfpollinate
for several
generations
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Eight Pea Plant Traits
Seed shape --- Round (R) or Wrinkled (r)
Seed Color ---- Yellow (Y) or Green (y)
Pod Shape --- Smooth (S) or wrinkled (s)
Pod Color --- Green (G) or Yellow (g)
Seed Coat Color ---Gray (G) or White (g)
Flower position---Axial (A) or Terminal (a)
Plant Height --- Tall (T) or Short (t)
Flower color --- Purple (P) or white (p)
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Mendel’s Experimental Results
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Did the observed ratio match
the theoretical ratio?
The theoretical or expected ratio of
plants producing round or wrinkled seeds
is 3 round :1 wrinkled
Mendel’s observed ratio was 2.96:1
The discrepancy is due to statistical
error
The larger the sample the more nearly
the results approximate to the
theoretical ratio
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Generation “Gap”
Parental P1 Generation = the parental
generation in a breeding experiment.
F1 generation = the first-generation
offspring in a breeding experiment. (1st
filial generation)
From breeding individuals from the P1
generation
F2 generation = the second-generation
offspring in a breeding experiment.
(2nd filial generation)
From breeding individuals from the F1
generation
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Following the Generations
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
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Monohybrid
Crosses
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P1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Wrinkled seeds
RR
x
rr
Genotype: Rr
r
r
R
Rr
Rr
Genotypic
Ratio: All alike
R
Rr
Rr
Phenotypic
Ratio: All alike
Phenotype:
Phenotype Round
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P1 Monohybrid Cross Review
ƒ Homozygous dominant x Homozygous
recessive
ƒ Offspring all Heterozygous
(hybrids)
ƒ Offspring called F1 generation
ƒ Genotypic & Phenotypic ratio is ALL
ALIKE
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F1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Round seeds
Rr
x
Rr
R
r
R
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype:
Phenotype Round &
wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
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F1 Monohybrid Cross Review
ƒ Heterozygous x heterozygous
ƒ Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
ƒ Offspring called F2 generation
ƒ Genotypic ratio is 1:2:1
ƒ Phenotypic Ratio is 3:1
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What Do the Peas Look Like?
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…And Now the Test Cross
Mendel then crossed a pure & a
hybrid from his F2 generation
This is known as an F2 or test
cross
There are two possible
testcrosses:
Homozygous dominant x Hybrid
Homozygous recessive x Hybrid
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F2 Monohybrid Cross
st
(1 )
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Round seeds
RR
x
Rr
Genotype: RR, Rr
R
r
R
RR
Rr
Genotypic
Ratio: 1:1
R
RR
Rr
Phenotypic
Ratio: All alike
Phenotype:
Phenotype Round
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F2 Monohybrid Cross (2nd)
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Wrinkled seeds x Round seeds
rr
x
Rr
R
r
r
Rr
Rr
r
rr
rr
Genotype: Rr, rr
Phenotype:
Phenotype Round &
Wrinkled
G. Ratio: 1:1
P.Ratio: 1:1
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F2 Monohybrid Cross Review
ƒ Homozygous x heterozygous(hybrid)
ƒ Offspring:
50% Homozygous RR or rr
50% Heterozygous Rr
ƒ Phenotypic Ratio is 1:1
ƒ Called Test Cross because the
offspring have SAME genotype as
parents
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Practice Your Crosses
Work the P1, F1, and both
F2 Crosses for each of the
other Seven Pea Plant
Traits
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Mendel’s Laws
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Results of Monohybrid Crosses
Inheritable factors or genes are
responsible for all heritable
characteristics
Phenotype is based on Genotype
Each trait is based on two genes,
one from the mother and the
other from the father
True-breeding individuals are
homozygous ( both alleles) are the
same
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Law of Dominance
In a cross of parents that are
pure for contrasting traits, only
one form of the trait will appear in
the next generation.
All the offspring will be
heterozygous and express only the
dominant trait.
RR x rr yields all Rr (round seeds)
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Law of Dominance
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Law of Segregation
During the formation of gametes
(eggs or sperm), the two alleles
responsible for a trait separate
from each other.
Alleles for a trait are then
"recombined" at fertilization,
producing the genotype for the
traits of the offspring.
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Applying the Law of Segregation
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Law of Independent
Assortment
Alleles for different traits are
distributed to sex cells (&
offspring) independently of one
another.
This law can be illustrated using
dihybrid crosses.
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Dihybrid Cross
A breeding experiment that tracks
the inheritance of two traits.
Mendel’s “Law of Independent
Assortment”
a. Each pair of
independently
b. Formula: 2n
alleles segregates
during gamete formation
(n = # of heterozygotes)
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Question:
How many gametes will be produced
for the following allele arrangements?
Remember: 2n (n = # of heterozygotes)
1. RrYy
2. AaBbCCDd
3. MmNnOoPPQQRrssTtQq
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Answer:
1. RrYy: 2n = 22 = 4 gametes
RY
Ry
rY ry
2. AaBbCCDd: 2n
ABCD ABCd
aBCD aBCd
= 23 =
AbCD
abCD
8 gametes
AbCd
abCD
3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64
gametes
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Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
RrYy
RY Ry rY ry
x
RrYy
RY Ry rY ry
All possible gamete combinations
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Dihybrid Cross
RY
Ry
rY
ry
RY
Ry
rY
ry
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Dihybrid Cross
RY
RY RRYY
Ry RRYy
rY RrYY
ry
RrYy
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy
RrYy
Rryy
RrYy
rrYY
rrYy
Rryy
rrYy
rryy
Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
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Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
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Test Crosses
Wool producers often prefer white wool, since
black wool is brittle and difficult to dye.
Black sheep can be avoided by breeding only
homozygous white rams. However the allele for
white wool (W) is dominant over the allele for
black wool (w), so white rams can be
heterozygous. How can a woo producer be sure
that a white ram is homozygous?
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Test Cross – the cross of an individual of
unknown genotype to an individual that is fully
recessive.
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Test Cross
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Test Cross
A mating between an individual of unknown
genotype and a homozygous recessive
individual.
Example: bbC__ x bbcc
BB
Bb
bb
=
=
=
brown eyes
brown eyes
blue eyes
CC = curly hair
Cc = curly hair
cc = straight hair
bC
b___
bc
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Test Cross
Possible results:
bc
bC
b___
C
bbCc
bbCc
or
bc
bC
b___
c
bbCc
bbcc
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Summary of Mendel’s laws
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT
CROSS
OFFSPRING
TT x tt
tall x short
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green
x
round & green
9/16 round seeds & green
pods
3/16 round seeds & yellow
pods
3/16 wrinkled seeds & green
pods
1/16 wrinkled seeds & yellow
pods
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Incomplete Dominance
and
Codominance
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Incomplete Dominance
Incomplete dominance – the expression of both
forms of an allele in heterozygous individual in
cells of an organism, producing an intermediate
phenotype.
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Incomplete Dominance
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Incomplete Dominance
F1 hybrids have an appearance somewhat
in between the phenotypes of the two
parental varieties.
Example: snapdragons (flower)
red (RR) x white (rr)
r
r
RR = red flower
rr = white flower
R
R
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Incomplete Dominance
r
r
R Rr
Rr
R Rr
Rr
produces the
F1 generation
All Rr = pink
(heterozygous pink)
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Incomplete Dominance
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Codominance
Codominance – the expression of both forms of
an allele in heterozygous individual in different
cells of the same organism
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Codominance
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Codominance
Two alleles are expressed (multiple
alleles) in heterozygous individuals.
Example: blood type
1.
2.
3.
4.
type
type
type
type
A
B
AB
O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
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Codominance Problem
Example: homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
IA
i
IB
IAIB
IB i
IB
IAIB
IB i
1/2 = IAIB
1/2 = IBi
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Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IB i
i
IAi
IB i
1/2 = IAi
1/2 = IBi
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Codominance
Question:
If a boy has a blood type O and
his sister has blood type AB,
what are the genotypes and
phenotypes of their parents?
boy - type O (ii)
AB (IAIB)
X
girl - type
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Codominance
Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
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Sex-linked Traits
Traits (genes) located on the sex
chromosomes
Sex chromosomes are X and Y
XX genotype for females
XY genotype for males
Many sex-linked traits carried on
X chromosome
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Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
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Sex-linked Trait Problem
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
XRY
x
XrXr
Remember: the Y chromosome in males
does not carry traits.
Xr
Xr
RR = red eyed
Rr = red eyed
R
X
rr = white eyed
XY = male
Y
XX = female
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Sex-linked Trait Solution:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyed
female
50% white eyed
male
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Female Carriers
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Genetic Practice
Problems
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Breed the P1 generation
tall (TT) x dwarf (tt) pea plants
t
t
T
T
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Solution:
tall (TT) vs. dwarf (tt) pea plants
t
t
T
Tt
Tt
produces the
F1 generation
T
Tt
Tt
All Tt = tall
(heterozygous tall)
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Breed the F1 generation
tall (Tt) vs. tall (Tt) pea plants
T
t
T
t
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Solution:
tall (Tt) x tall (Tt) pea plants
T
t
T
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:1 phenotype
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Pedigree Charts
Pedigree chart – a chart used to record the
transmission of a particular trait or traits over
several generations. (like a family tree)
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Pedigree Chart
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Questions
Questions 1-3 page 607
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Other Patterns of Inheritance
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Pleiotrophic Genes – affect many different
characteristics. Ex: Sickle cell anemia. A blood
disorder. Normal hemoglobin has the allele HbA.
Sickle cell occurs in individuals who have two
copies of the mutated allele HbS. This mutation
causes abnormally shaped hemoglobin that
interlock with one another. People with sickle
cell, are fatigued, weak and have an enlarged
spleen. Often show signs of heart lung, and
kidney failure.
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Multiple Alleles
Multiple Alleles – when trait are determined by
more that two alleles. Most commonly seen trait
is call the wild type.
Mutant – any allele of a gene other than the wild
type allele.
Ex: fruit fly can have any one of four eye
colours. Red is the wild type, eyes may also be
apricot, honey and white. They have two alleles
for eye colour.
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Review Questions
Page 478 Questions 1-8 (old text)
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Environment and Phenotype
Himalayan rabbits have black fur when raised in
low temperatures and white in high temperatures
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Questions
Questions 1-3 page 612
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Dihybrid Crosses and Polygenic
Traits
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Dihybrid cross – a genetic cross involving two
genes, each of which has more than one allele.
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Dihybrid
Crosses
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Punnett Square
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Probability and Dihybrid Crosses
P = # of ways that a given outcome can occur
total # of possible outcomes
Questions page 615
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Selective breeding
Selective breeding – the crossing of desired
traits from plants or animals to produce
offspring with both characteristics
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Selective Breeding
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Inbreeding – the process whereby breeding
stock is drawn from a limited number of
individuals possessing desirable phenotypes.
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Polygenetic traits – inherited characteristics
that are determined by more than one gene
Epistatic gene – a gene that masks the
expression of another gene or genes.
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Review
Questions
Page 483 Questions 9-11 (old text)
Questions 1-4 page 619
Review Questions page 623- 625
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Review
Questions
Page 489 Questions 12-14
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Social Issue
Social Issue Page 491
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Lab
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