Transcript Revised Parikh Ch 11

Biology 1 Notes
Chapter 11 (Introduction to Genetics)
Prentice Hall; pages 262-274, 279-285
•
11–1 The Work of Gregor Mendel
A. Gregor Mendel’s Peas
B. Genes and Dominance
C. Segregation
1. The F1 Cross
2. Explaining the F1 Cross
•
11–2 Probability and Punnett Squares
A. Genetics and Probability
B. Punnett Squares
C. Probability and Segregation
D. Probabilities Predict Averages
•
11–3 Exploring Mendelian Genetics
A. Independent Assortment
1. The Two-Factor Cross: F1
2. The Two-Factor Cross: F2
B. A Summary of Mendel’s Principles
C. Beyond Dominant and Recessive Alleles
1. Incomplete Dominance
2. Codominance
3. Multiple Alleles
4. Polygenic Traits
D. Applying Mendel’s Principles
E. Genetics and the Environment
•
11–5 Linkage and Gene Maps
A. Gene Linkage
B. Gene Maps
Early Concept of Genetics
• Every living thing has a set of characteristics inherited from its parents.
• People didn’t always understand the process of inheritance, but they had an
idea that certain characteristics are passed from generation to generation
• Examples:
– Native Americans developed more than 300 varieties of corn
– People of Peru developed potatoes by selecting and breeding wild starchy
plants
– The domestic dog came from selectively breeding wolves
• It was not until the mid-nineteenth century that Gregor
Mendel, an Austrian monk, carried out important studies
of heredity
• Genetics- the scientific study of heredity
• Heredity- the passing on of characteristics from parents to
offspring.
• Characteristics that are inherited are called traits.
• Mendel was the first person to succeed in predicting how
traits are transferred from one generation to the next.
• Mendel chose to use the garden pea in his experiments for
several reasons.
Mendel chose his subject carefully
• Garden pea plants reproduce sexually, which means that they produce male
and female sex cells, called gametes (egg and sperm).
• In a process called fertilization, the male gamete unites with the female gamete.
• The resulting fertilized cell, called a zygote, then develops into a seed.
• The transfer of pollen grains from a male to a female reproductive organ in a
plant is called pollination.
Pollen
grains
Remove
male parts
Transfer pollen
Female
part
Male
parts
Cross-pollination
• When he wanted to breed, or cross, one plant with another, Mendel opened the
petals of a flower and removed the male organs.
• He then dusted the female organ with pollen from the plant he wished to cross
it with. This process is called cross-pollination.
• By using this technique, Mendel could be sure of the parents in his cross.
Mendel was a careful researcher
• He studied only one trait at a time to
control variables, and he analyzed his
data mathematically.
• The tall pea plants he worked with were
from populations of plants that had been
tall for many generations and had
always produced tall offspring. Such
plants are said to be true breeding for
tallness.
• Likewise, the short plants he worked
with were true breeding for shortness.
• True-breeding means that if they were
allowed to self-pollinate, they would
produce offspring identical to
themselves.
Mendel’s Research
• Mendel studied seven different traits
and each trait had two contrasting
characteristics:
– Flower color
– Flower position
– Seed color
– Seed shape
– Pod shape
– Pod color
– Stem length
• A trait is a specific characteristic, such
as seed color or plant height, that
varies from one individual to another
• Mendel crossed plants with each of the
seven contrasting characters and
studied their offspring.
• He called each original pair of plants
the P (parental) generation.
• He called the offspring the F1, or “first
filial,” generation.
Language of Genetics
• Mendel called the observed trait dominant and the trait that disappeared
recessive.
• Always use the same letter for different forms of the same trait (alleles).
– capital letter shows dominance, lowercase letter shows recessive
– Example: Stem Height
Tall allele (T)
Short allele (t)
• The way an organism looks and behaves is called its phenotype.
– Example: tall or short
• The allele combination (genetic make-up) an organism contains is known as its
genotype.
– Example: TT, Tt, or tt
• An organism’s genotype can’t always be known by its phenotype.
• An organism is homozygous for a trait if its two alleles for the trait are the
same.
– Example: TT or tt
• An organism is heterozygous for a trait if its two alleles for the trait differ from
each other.
– Example: Tt
Punnett Squares
• In 1905, Reginald Punnett, an English biologist, devised a quick way of
finding the expected genotypes possibilities (Punnett square)
• Punnett Squares are used to make phenotype and genotype predictions
in order to predict/compare the genetic variations that will result from
a cross
• A Punnett square for this cross is two boxes tall
and two boxes wide because each parent can
produce two kinds of gametes for this trait.
• The two kinds of gametes from one parent are
listed on top of the square, and the two kinds of
gametes from the other parent are listed on the
left side.
• It doesn’t matter which set of gametes is on top
and which is on the side.
• Each box is filled in with the gametes above and
to the left side of that box. You can see that each
box then contains two alleles—one possible
genotype.
• After the genotypes have been determined, you
can determine the phenotypes.
Heterozygous
tall parent
T
T
T
t
T
t
Heterozygous
tall parent
t
t
Mendel’s Experiment #1
• Parents (P):
• (phenotype) Purebred tall x Purebred short
• (genotype)
TT x tt
• F1 all hybrids:
• Phenotype: tall
• Gentoype: Tt
T
T
t
Tt
Tt
t
Tt
Tt
Mendel’s Experiment
• Mendel completed similar experiments for all of the seven traits
• The offspring of crosses between parents with different traits
are called hybrids.
Seed
Shape
Seed
Color
Round
Yellow
Seed Coat
Color
Gray
Pod
Shape
Pod
Color
Smooth
Green
Flower
Position
Plant
Height
Axial
Tall
Short
Wrinkled
Green
White
Constricted
Yellow
Terminal
Round
Yellow
Gray
Smooth
Green
Axial
• All of the offspring had the character of only one of the
parents
Tall
Conclusions from Experiment #1
• Biological inheritance is determined by factors that
are passed from one generation to the next.
–
–
–
–
factors that determine traits are genes
each trait is controlled by one gene
(genes are found on chromosomes)
each trait has two different forms called alleles
P
• One form is always dominant over the other
– if the dominant allele is present, it will show up
(expressed by a capital letter)
– the recessive allele will be exhibited only when the
dominant allele is absent (expressed by a lower case
letter)
– This is known as The Principle of Dominance
• In Mendel’s experiments, the allele for tall (T) plants
was dominant and the allele for short (t) plants was
recessive.
• Pea plants will be tall unless the allele for tallness
is absent
– TT- Tall plant
– Tt- Tall plant
– tt- short plant
Short plant
Tall plant
t
T T
t
t
T
F1
All tall plants
T t
Going further …
• Mendel wondered if the recessive allele had disappeared completely.
• So, he crossed the F1 plants to produce the F2 (second filial) generation by
letting the F1 plants self-pollinate.
• In every case, he found the recessive trait of the pair seemed to disappear in
the F1 generation, only to reappear unchanged in one-fourth of the F2 plants.
Seed
shape
Seed
color
Flower
color
Flower
position
Pod
color
Pod
shape
Plant
height
Dominant
trait
round
yellow
purple
axial
(side)
green
inflated
• The recessive
allele
reappeared.
tall
Recessive
trait
wrinkled
green
white
terminal
(tips)
yellow
constricted
• All of the F2
generation
produced plants
with the similar
results
short
• All of the traits
(including
height) showed
a 3:1 ratio of 3
dominant to 1
recessive
Mendel’s Experiment #2
• F1:
• (phenotype) (Hybrid) tall x (Hybrid) Tall
• (genotype)
Tt x Tt
•
•
•
•
•
•
F2: TT Tt Tt tt
T
t
T
TT
Tt
t
Tt
tt
(tall) (tall) (tall) (short)
Genotypic ratio:
1 TT: 2Tt: 1tt
Phenotypic ratio:
3 tall: 1 short
Homologous Chromosome 4
Conclusions from Experiment #2
a
• Mendel concluded from his second
experiment that alleles for shortness and
Terminal
tallness had segregated (separated) during
the formation of gametes (reproductive
cells)
• Each allele is located on different copies of a
chromosome, one inherited from each
Inflated
parent.
Tall
A
Axial
D
d Constricted
T
t
Short
• During meiosis the two alleles
separate so that each gamete carries
only a one copy of a gene
• During fertilization, these gametes
can randomly pair to produce (up
to) four various combinations of
alleles.
The Principle of Segregation
Tt  Tt cross
Law of segregation
• The Principle of
Segregation states that
every individual has
two alleles of each
gene and when
gametes are produced,
each gamete receives
one of these alleles.
F1
Tall plant
Tall plant
T
T
t
t
F2
Tall
Tall
T T
T
Tall
t
3
T
Short
t
t
t
1
Probability
• Probability is the likelihood an event will occur
• The principles of probability can be used to predict the outcomes of
genetic crosses
• A Punnett square can be used to determine the probability of the
number of desired outcomes by the total number of possible
outcomes
r
R
RR
• Example: What is the probability of
getting a plant that produces round
R
seeds when two plants that are
heterozygous (Rr) are crossed?
• Punnett square shows three plants
with round seeds out of four total
Rr
plants, so the probability is 3/4.
• It is important to remember that the r
results predicted by probability are
more likely to be seen when there is a
large number of offspring.
Rr
rr
Monohybrid Cross (one trait)
G= green pea pods
g= yellow pea pods
1) Cross of homozygous dominant x homozygous recessive
GG
_______
• P
g
g
X
G
G
Gg
Gg
Gg
Gg
gg
_______
(genotype)
F1 genotype:
all Gg
F1 phenotype:
all Green
2) Cross two of F1 generation
Gg
Gg
________ X _______ (genotype)
• F1
G
g
G
GG
Gg
g
Gg
gg
• F2 genotype & ratio:
• 1 GG: 2 Gg: 1 gg
• F2 phenotype & ratio:
• 3 green: 1 yellow
Test Cross - used to distinguish between
homozygous and heterozygous organisms
• unknown organism is crossed with a
homozygous recessive individual
• if unknown is heterozygous, then 1/2 of
offspring will be dominant and 1/2 will
be recessive
• if unknown is homozygous, then all
offspring will be dominant
Test Cross Example
• Example: (unknown vs. homozygous recessive)
• If unknown is heterozygous, then ½ the offspring will be
dominant and ½ will be recessive
• If unknown is homozygous, then all of the offspring will be
dominant
• A breeder has a yellow cat and wants to know if it is a purebred.
To determine what the cat’s genotype is the breeder does a test
cross with a brown cat (homozygous recessive) . All of the
offspring turn out yellow. What is the genotype of the original
yellow cat?
Yellow cat:
either YY or Yy
Brown cat: yy
Y
Y
y
Yy
Yy
y
Yy
Yy
Y
y
y
Yy
yy
y
Yy
yy
Monohybrid vs. Dihybrid Crosses
Plant height
Tall
• Mendel’s first experiments are
called monohybrid crosses
because mono means “one” and
the two parent plants differed
from each other by a single
trait—height.
Seed
shape
Seed
color
Short
• Mendel performed another set of crosses in
which he used peas that differed from each
other in two traits rather than only one.
• Such a cross involving two different traits is
called a dihybrid cross.
round
wrinkled
yellow
green
Dihybrid crosses
• Mendel took true-breeding pea plants that had round yellow seeds (RRYY) and
crossed them with true-breeding pea plants that had wrinkled green seeds
(rryy).
• He already knew the round-seeded (R) trait was
dominant to the wrinkled-seeded (r) trait.
• He also knew that yellow (Y) was dominant to green (y).
• All of the F1 offspring were round and yellow with the genotype RrYy.
• Mendel then let the F1 plants
pollinate themselves.
• He found some plants that producedP
round yellow seeds and others that
produced wrinkled green seeds.
• He also found some plants with
round green seeds and others with F
wrinkled yellow seeds.
• He found they appeared in a definite
ratio of phenotypes—9 round yellow:
3 round green: 3 wrinkled yellow: 1 F
wrinkled green.
Dihybrid Cross
round yellow x wrinkled green
1
Wrinkled green
Round yellow
All round
yellow
1
2
9
Round yellow
3
Round green
3
Wrinkled yellow
1
Wrinkled green
Dihybrid Cross (two traits)
• P RRYY x rryy
• F1 RrYy x RrYy
RY
RrYy
RY
All possible
gamete combinations Ry
_______
RY
_______
Ry
rY
_______
rY
________
ry
ry
Ry
rY
ry
RY
Ry
rY
ry
RY
RRYY RRYy RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
F2 genotype
and ratio:
RY
RY
Ry
rY
RRYY RRYy RrYY
ry
RrYy
1 RRYY
2 RRYy
1 RRyy
Ry
RRYy
RRyy
RrYy
Rryy
2 RrYY
4 RrYy
rY
RrYY
RrYy
rrYY
rrYy
2 Rryy
1 rrYY
ry
RrYy
Rryy
rrYy
rryy
2 rrYy
1 rryy
RY
Ry
rY
ry
F2
phenotype
and ratio:
RY
RRYY RRYy RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
3: round,
green
rY
RrYY
RrYy
rrYY
rrYy
3: wrinkled,
yellow
ry
RrYy
Rryy
rrYy
rryy
1 wrinkled,
green
9: round,
yellow
The Principle of Independent Assortment
Gametes from RrYy parent
RY
Gametes from RrYy parent
• Mendel’s found that
genes for different
traits—for example,
seed shape and seed
color—are inherited
independently of
each other.
• This conclusion is
known as the
Principle of
Independent
Assortment
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRYy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
RY
Ry
rY
ry
A Summary of Mendel’s Principles
• The inheritance of biological characteristics is
determined by individual units known as genes.
Genes are passed from parents to offspring.
(Mendel called genes, “factors.”)
• Dominance- if two alleles in a gene pair are different, the
dominant allele will control the trait and the recessive
allele will be hidden
• Segregation - each adult has two copies of each gene-one
from each parent. These genes are segregated from each
other when gametes are formed.
• Independent Assortment - genes for different traits are
inherited independently of each other.
Exceptions to Mendel’s Principles
• Some alleles are neither dominant nor
recessive, and many traits are controlled by
multiple alleles or multiple genes.
• Patterns of Inheritance
–
–
–
–
Incomplete Dominance
Codominance
Multiple Alleles
Polygenic Traits
Incomplete Dominance
• Some alleles are neither dominant nor recessive
• Heterozygous phenotype is somewhere in between the two
homozygous phenotypes
• Phenotype appears to be blended but alleles remain separate &
distinct
• There are no dominant or recessive alleles therefore uppercase
and lowercase letters are not used
• Example: genotype of four o’clock plants
red flowers = FrFr
white flowers = FwFw
pink flowers = FrFw
• Alleles for red and white flowers show incomplete dominance
• Heterozygous plants have pink flowers—a mix of red and
white coloring
Incomplete Dominance
Flower color of four o’clock plants
Cross a red plant
with a white plant.
Fr Fr
X
Fw Fw
Fr
Fr
Fw FrFw
FrFw
FrFw
FrFw
Genotype: all FrFw
Phenotype:
all pink
Fw
Codominance
• both alleles in the heterozygote express
themselves fully and contribute to the
phenotype
• Example: Feather color in Chickens
– Allele for black feather is codominant with white
feathers
– A heterozygous phenotype is “erminette”speckled black and white feathers
– It is not a blend, the colors appear separate
• Example: Blood Type in Humans
– A blood type
= IAIA or IAi
– B blood type
= IBIB or IBi
– AB blood type
= IAIB
– O blood type
= ii
B.
The letters stand for A
and B antigens.
Humans have antigens
to A, B, both A and B,
or neither A nor
Multiple Alleles
• Although individuals can’t have more than 2 alleles, more than 2 alleles
can exist in a population
• Example: Coat color in rabbits
– Coat color is determined by a single gene that has four alleles
– Four alleles display a pattern of dominance that produces four
possible coat colors
• Example: Blood types in humans
– alleles are IA, IB, i
Codominance and Multiple Allele Example
(Blood Type in Humans)
• Cross an individual with
AB blood and a person
with O blood
IAIB
IA
Genotype:
2 IAi: 2 IBi
Phenotype:
2 A: 2 B
X
ii
IB
i
IAi
IBi
i
IAi
IBi
Polygenic Traits
• results from an interaction of several genes
• traits are controlled by two or more genes
• Example: Eye Color of Fruit Flies
– three genes make the reddish-brown pigment of fruit fly
eyes
– wide range of phenotypes can occur
• Example: Eye Color of Humans
– controlled by genes for pigment, tone, amount
of pigment, and distribution of pigment
Applying Mendel’s Principles
• Mendel formed hypotheses about inheritance without
knowing what genes are or where they are located in cells
• In the 1900’s Thomas Hunt Morgan studied the fruit fly
– (Drosophila melanogaster )
– easy to feed and maintain
– new generation produced every 2 weeks
– produce large numbers of offspring
– have only four pair of chromosomes
• Morgan found that Mendel’s principles
applied to fruit flies as well as plants.
Applying Mendel’s Principles to Humans
•
•
•
•
•
Many doctors used Mendel’s principles to study human disorders
Albinism is lack of the pigment melanin that gives human skin its color
Individuals with the dominant allele (A) produce skin coloration
Individuals homozygous for the recessive form of the allele (a) have albinism
If two people with normal skin color have a child with albinism, what are the
odds that a second child will also have albinism?
25%
Aa
Aa
Genotypes of the parents _______
x________
• If the first child has albinism (aa), then both parents must have at least one allele
that is a. Since both parents have normal skin color, they must also have A.
A
a
A
AA
Aa
a
Aa
aa
There is a 25%
chance that the
second child
will also have
albinism
Genetics and the Environment
• Characteristics of any organism is
not determined by genes alone
• Characteristics are determined by
the interaction between genes and
the environment
• Genes may affect a sunflower’s
height and color of its flowers, but
they are influenced by climate, soil,
and availability of water (if the
• Hydrangea with the same
genotype for flower color
sunflower doesn’t have enough
express different
water it can’t grow)
phenotypes depending on
the acidity of the soil.
• 1903 - Walter Sutton - stated the chromosome
theory of heredity - the material of inheritance
is carried by the chromosomes
- Sutton recognized “factors” were genes on
the chromosomes he saw
- both occur in pairs
- both separate during meiosis
- both sort independently
- Sutton realized that there must be many
different genes on a chromosome and that
they are inherited together
Gene Linkage
• What we know: genes from different chromosomes sort
independently in meiosis
• But. . What about genes on the same chromosome?
• At first, it seems they would be inherited together, but it is a
little more complicated
• Morgan in 1910 noticed that some genes were linked (which
violates the principle of independent assortment)
• Morgan and his associates studied 50 genes of the fruit fly and
were able to place all of the genes in 4 linkage groups which
assorted independently, but all the genes in one group were
inherited together
– Linked genes are located on the same chromosome and are inherited
together
• 4 linkage groups- fruit fly has 4 chromosomes
• It is the chromosomes however, that assort independently, not
individual genes
Gene Linkage
• Morgan discovered that chromosomes sort
independently and not genes
• Why didn’t Mendel discover this
– 6 of the 7 traits he studied were on different
chromosomes
– The two genes on the same chromosomes were so far
away from each other they assorted independently
• Showing that genes on the same chromosomes are
not forever linked and crossing over occurs
• Crossing over can separate and exchange linked genes
and produce new combinations of alleles.
• This is important in genetic diversity of a species or
population
An example of gene linkage
• When crossing long,
purple sweet peas
with round, red peas,
the expected F2 ratio
(9:3:3:1) did not show
up
• new ratio appeared to
indicate that the traits
did not sort
independently
Genetic recombination - the shuffling of genes into new
combinations by crossing over during prophase I of
meiosis
• If genes were being
sorted together, then
crossing over was the
only explanation for the
appearance of red, long
and purple, round peas
• recombinant - an
organism or a
chromosome with a
recombined set of genes
Gene mapping - locating genes on
chromosomes
• Geneticists work with two traits at a time
and look for the number of recombinants in
the offspring
• if two genes are far apart on a chromosome,
there are more places between them where
crossing over can occur, therefore more
recombinants are formed
• if two genes are close together, few
recombinants occur
Gene
Mapping
• In 1911, Alfred Strutevant hypothesized that the rate
at which crossing-over separates linked genes could
be used to map were genes are located on a
chromosome
• The farther apart genes are, the more likely they
were to be separated by a cross-over
• Sturtevant created a gene map showing the relative
location of each known gene on a chromosome
How to construct a gene map
• The recombinant rate (frequency of crossing-over
between genes) is used to construct genetic maps
• Genetic recombination-the shuffling of genes into new
combination by crossing-over during prophase I of
meiosis
A
A
A 10 D
orA 10 D
or
D 10 A
D 10 A
D
D
50
50
B
B
B 5 C
or B 5 C
or
C 5 B
C 5 B
35
35
C
orC
or
C
C
35
35
D
D
Chromosome Mapping
•
Suppose there are four genes— A, B, C, and D—on a
chromosome.
A
10
D
35
C
5
B
50
• Geneticists determine that the frequencies of recombination
among them are as follows: between A and B—50%; between
and D— 10%; between B and C—5%; between C and D—35%.
• The recombination frequencies can be converted to map units:
A-B = 50; A-D =10; B-C = 5; C-D = 35.
Chromosome Mapping
• These map units are not actual distances on the
chromosome, but they give relative distances between
genes. Geneticists line up the genes as shown.
• The genes can be arranged in the sequence that reflects the
recombination data.
50
B
A
50
B
A
A 10 D
B 5 C
10
A
D
orB 5 C
or
or
or
D 1010 A
C 55 B
A
D
B
C
D
35
C
D
35 orC
or
35
C
D
35
C
D
• This sequence is a
chromosome map.
A
10
D
35
50
C
5
B
Comparative Scale of a Gene Map
Mapping of
Earth’s Features
Mapping of Cells,
Chromosomes, and Genes
Cell
Earth
Country
Chromosome
State
Chromosome
fragment
City
People
Gene
Nucleotide
base pairs