Download Section 3 Studying Heredity

Chapter 8 Mendel and Heredity
Section 1 The Origins of Genetics
Section 2 Mendel’s Theory
Section 3 Studying Heredity
Section 4 Complex Patterns of Heredity
Section 1 The Origins of Genetics
• Heredity is the passing of traits from parents to
offspring. This led to the scientific study we
know today known as genetics.
• Heredity began with Austrian
monk named Gregor Johann
Mendel whom carried out
experiments with Pisum sativum
(Garden Pea Plant).
• Mendel was the first to develop
rules that accurately predicted
patterns of heredity for plants and animals.
Section 1 The Origins of Genetics
Useful Features in Pea Plants
1. Several traits of the garden pea exist in two
clearly different forms: flower color is either
purple or white with no intermediate forms.
Section 1 The Origins of Genetics
2.
Male and female reproductive parts of garden
peas are enclosed within the same flower. This
allowed Mendel to control
(self-fertilization) or (cross-fertilization).
3.
Garden pea is small, grows easy, matures
quickly, and produces many offspring with
results that can be quickly obtained.
Section 1 The Origins of Genetics
Section 1 The Origins of Genetics
Traits Expressed as Simple Ratios
• A monohybrid cross is a cross that involves
one pair of contrasting traits.
– Example: crossing a plant with purple
flowers and a plant with white flowers.
Mendel carried out his experiments in 3 parts:
1. Mendel allowed each variety of garden pea
plant to self-pollinate for several
generations. This ensured that each variety
was true-breeding for a particular trait or
pure for a particular trait. These
true-breeding plants served as the parental
generation in Mendel’s experiment. The
P generation are the first two individuals
that are crossed in a breeding experiment.
Section 1 The Origins of Genetics
2.
Mendel then cross-pollinated two P
generation plants that had contrasting forms
such as purple and white flowers. Mendel
called the offspring of the P generation the
first filal generation, or F1 generation.
3.
Mendel allowed the F1 generation to
self-pollinate. He called the offspring of the
F1 generation plants the second filal
generation, or F2 generation.
Section 1 The Origins of Genetics
Mendel’s Results
• A ratio is a comparison of two numbers and can
be written as a fraction.
•
Look at pg 164 Figure 3 (the F2 generation)
•
Purple flowers to white flowers is a ratio of
(705:224)
•
To reduce (705:224) into a ratio one must divide.
705
224
224
224
3.15:1 Ratio
3.15 Purple Flower : 1 White Flower
Section 1 The Origins of Genetics
Mendel’s Results
• 6,022 yellow and 2,001 green or (6,022 : 2,001)
6,022 yellow = 3.01 y 2,001 green = 1 g
2,001 green
2,001 green
So thus you have a three to one ratio. (3.01:1)
Practice: complete the chart on page 165 titled,
“Calculating Mendel’s Ratios”
Section 2 Mendel’s Theory
Mendel’s Theory of Heredity Hypotheses
1. For each inherited trait, an individual has two
copies of the gene—one from each parent.
2.
There are alternative versions of genes.
Example: The gene for flower color in peas
can exist in a purple version or a white
version. Today different versions of a gene are
called its alleles which can be passed on
which the individual reproduces.
Section 2 Mendel’s Theory
Mendel’s Theory of Heredity Hypotheses
3. When two different alleles occur together, one
of them may be completely expressed, while
the other may have no observable effect on the
organism’s appearance.
Example: The expressed form of the trait is
called the dominant. The trait that was not
expressed when the dominant form was
present this is described as recessive.
Section 2 Mendel’s Theory
Mendel’s Theory of Heredity Hypotheses
4. When gametes are formed, the alleles for each
gene is an individual separate independently of
one another. Thus, gametes carry only one
allele for each inherited trait. When gametes
unite during fertilization, each gamete
contributes one allele.
Example: Each parent can contribute only
one of the alleles because of the way
gametes are produced during the process of
meiosis.
Section 2 Mendel’s Theory
Mendel’s Findings in Modern Terms
Purple Flowers are
Homozygous Dominant
“PP”
Purple Flowers are
Heterozygous
“Pp”
White Flowers are
Homozygous Recessive
“pp”
Alleles are one trait or gene.
P is one allele.
p is another allele
Homozygous traits are traits that
have the same alleles of a
particular gene.
Example purple & white: PP, pp
Heterozygous traits are traits that
have different alleles of a particular
gene. Example purple: Pp
The set of alleles that an individual
has for a character is called its
genotype.
Example: Pp,PP,pp are all genotypes
Phenotypes are physical
characteristics that make up the
genotypes. Purple, white, short, tall
are all considered phenotypes.
Section 2 Mendel’s Theory
The Law of Segregation
• The first law of heredity describes the behavior
of chromosomes during meiosis.
• Homologous chromosomes and then
chromatids are separated.
• The law of segregation states that two alleles for
a character segregate (separate) when gametes
are formed.
Section 2 Mendel’s Theory
The Law of Independent Assortment
• Does one character or trait influence other
different characters or traits. NO
• A dihybrid cross is a cross that considers two
pairs of contrasting characters.
Example: a cross that considers both plant
height and flower color is a dihybrid cross.
• The law of independent assortment states that
the alleles of different genes separate
independently of one another during gamete
formation.
Section 2 Mendel’s Theory
The Law of Independent Assortment
• Two different traits are compared with one
another.
SsBb
Independent Assortment
SB
sB
Sb
sb
Section 3 Studying Heredity
Punnett Squares
• A Punnett square is a diagram that predicts the outcome
of a genetic cross by considering all possible
combinations of gametes in a test cross.
Monohybrid Cross: Homozygous Pea Plants
Cross a homozygous dominant yellow seed pea with a pea
plant that is homozygous recessive plant for green seeds.
YY x yy
Y
Genotypes Phenotypes
Y
Yy = 100% Yellow = 100%
y
Yy
Yy
y
Yy
Yy
Section 3 Studying Heredity
Monohybrid Cross: Heterozygous Pea Plants
Cross a heterozygous dominant yellow seed pea plant with
another heterozygous dominant yellow pea plant.
Yy x Yy
Y
Y
YY
y
Yy
Genotypes Phenotypes
y
YY = 25% Yellow = 75%
Yy = 50% Green = 25%
Yy
yy = 25%
yy
Ratio
1:2:1
Ratio
3:1
Section 3 Studying Heredity
Practice
Give me phenotypes and genotypes for
each monohybrid cross.
1.
2.
3.
Cross a dominant homozygous yellow
pea to a recessive green pea.
Cross a heterozygous dominant green
pea pod to itself.
Black color is dominant to brown in
bears. Cross a heterozygous black
bear with a recessive brown bear.
Section 3 Studying Heredity
Dihybrid Cross: Predict the results of a cross between two
pea plants that are heterozygous for seed shape (R = round,
r = wrinkled) and seed color (Y = yellow, y = green).
RY
Ry
r
Y
ry
Genotype
RRYY = 1/16
rrYy = 2/16
RRYy = 2/16
Rryy = 2/16
rryy = 1/16
RrYy x RrYy
r
RY
Ry
Y
RRYY RRYy RrYY
RRYy RRyy RrYy
RrYY RrYy rrYY
RrYy Rryy rrYy
RrYY = 2/16
rrYY = 1/16
RrYy = 4/16
RRyy = 1/16
ry
RrYy
Rryy
rrYy
rryy
Phenotype
Round/Yellow = 9/16
Round/Green = 3/16
Wrinkled/Yellow = 3/16
Wrinkled/Green = 1/16
Section 3 Studying Heredity
Inheritance of Traits
– Geneticists often prepare a pedigree, or
a family history that shows how a trait is
inherited over several generations.
– Pedigrees are particularly helpful if the
trait causes a genetic disorder and the
family members want to know if they are
carriers of if their children might get the
disorder.
– Carriers can pass the allele for the
disorder to their offspring.
Section 3 Studying Heredity
Pedigree Characteristics
Section 3 Studying Heredity
Pedigree Characteristics
Section 3 Studying Heredity
Pedigree Example
• Apply genotypes to the follow recessive albinoism disease.
aa
aa
Section 3 Studying Heredity
Inheritance of Traits
Autosomal or Sex-Linked
– If a trait is autosomal it will appear in
both sexes equally.
– If a trait is found only in females or
males in a pedigree it is known to be
sex linked or found just on the X or Y
chromosome.
Dominant or Recessive
– If a gene is autosomal dominant, every
individual with the condition will have a
parent with the condition.
Section 3 Studying Heredity
– If the condition is recessive, an
individual with the condition can have
one, two, or neither parent exhibit the
condition.
Heterozygous or Homozygous
– If individuals are homozygous recessive,
their phenotype will show the recessive
allele.
– Two people who are heterozygous
carriers of a recessive mutation will not
show the mutation, but they can
produce children with the trait.
Section 3 Studying Heredity
Sex Linked Monohybrid Cross Genetics
When a problem is sex linked, the trait or disease is
directly linked to the x or y chromosome making the
individual male or female.
Example Hemophilia is recessive trait but it is
sex-linked.
Example Genotypes:
Male with Hemophilia: XhY
Male without Hemophilia: XHY
Female with Hemophilia: XhXh
Female without Hemophilia: XHXH
Female Carrier without Hemophilia: XHXh
Section 3 Studying Heredity
Example Problem:
Hemophilia in humans is due to an X-chromosome
mutation. What will be the results of mating between a
normal (non-carrier) female and a hemophiliac male?
XHXH
XH
XH
X hY
Genotypes
Xh XHXh XXHHXXh h= 50%
XHY = 50%
Y XHY Phenotypes
XHY
Normal Females = 50%
Normal Males = 50%
Section 4 Complex Patterns of Heredity
Complex Control of Traits
– Traits that can range in many different
locations on a chromosome or even many
different chromosomes are called polygenic
traits.
– Example: Eye Color, Height, Weight, Hair, and
even Skin Color.
Intermediate Traits
– Incomplete dominance is where there is no
dominant trait and most of the other traits are
intermediate between the two characteristics.
Section 4 Complex Patterns of Heredity
Incomplete Dominance
Snapdragon Red Flowers
Snapdragon White Flowers
RR
WW
Snapdragon Pink Flowers
Yy x y
RW
Section 4 Complex Patterns of Heredity
Incomplete Dominant Monohybrid Cross:
Snapdragon plants are incomplete dominant for their traits.
Cross a pink snapdragon with a red snapdragon.
RW x RR
R
R
RR
R
RR
W
Genotypes Phenotypes
RR = 50% Red = 50%
RW
RW = 50% Pink = 50%
RW
Section 4 Complex Patterns of Heredity
Multiple Alleles
– Genes with three or more alleles are said to have
multiple alleles.
– In the human population the ABO blood groups (blood
types) are determined by three alleles:
IAIB and i
Or
A or IA Phenotype : (AA,AO) (IAIA,IAi) Genotypes
B or IB Phenotype : (BB,BO) (IBIB,IBi) Genotypes
AB or IAIB Phenotype : (AB) (IAIB) Genotypes
O or i Phenotype : (OO) (ii) Genotypes
O or i will be recessive
Section 4 Complex Patterns of Heredity
Blood Type Monohybrid Cross:
A man with blood type BO is married to a
woman with blood type AO. What are the
genotypes and phenotypes of the mating.
IAi x IBi
IA
IB IAIB
i
IAi
i
Genotypes
IBi
IAIB = 25%
IBi = 25%
A ii
I i = 25%
ii = 25%
Phenotypes
AB = 25%
B = 25%
A = 25%
O = 25%
Section 4 Complex Patterns of Heredity
Blood Type Monohybrid Cross:
A man with blood type A, whose genotype is AO,
is married to a woman with the same blood type
and genotype. What is the probability that they will
have a child with blood type phenotype of A?
IAi x IAi
IA
i
There is a 75% chance that the child
25% chance of
I I I
I i
having an phenotype
of O
A have
A A
A
will
an phenotype
of A and a
i
IAi
ii