Transcript Genetics Notesx

Genetics
Explain the basic rules and
processes associated with the
transmission of genetic
characteristics
Describe the evidence for
dominance, segregation and
the independent assortment of
genes on different
chromosomes, as investigated
by Mendel
Introduction
• Heredity
– is the passing of traits from parents to offspring
• Genetics
– is the study of the patterns of inheritance as
hereditary characteristics or traits
• Gregor Mendel
– the "father of modern genetics“
– an Austrian monk
– published his completely new and thoroughly
documented ideas of inheritance in 1866
Introduction
• His model was so simple that scientists who
read it at that time considered it "trivial“
– it received little attention and no recognition until it
was rediscovered in 1900 after his death
(simultaneously by 3 different people)
• In the meantime,
– chromosomes had been named
– their movements during mitosis and meiosis
observed and described
Introduction
• 1902
– scientists realized that chromosomes moved
precisely as reported by Mendel
• Once the connection between chromosomes
and heredity was established, the science of
genetics was reborn
• Mendel was the first person to realize that
genetic traits are inherited as separate
particles
Introduction
• He did not actually see these particles
– but he predicted their existence based on patterns of
inheritance
• He proposed that organisms have a pair of
"factors" for each trait
– one from each parent
• We NOW know that the particles of inheritance
are segments of DNA
– which we call genes
Mendel's Experiments
• Mendel worked with garden peas
– available in many different varieties
• E.g. pure breeding tall, pure breeding dwarf
• Pea flowers contain both male and female
parts and normally self-pollinate
– but can easily be artificially cross-pollinated with
other pea plants
• Mendel crossed plants of two varieties with
contrasting traits (such as tall and dwarf) to
see what would happen.
Mendel's Experiments
• Mendel worked with seven traits, each which
occurred in two distinct forms
• Mendel began by studying crosses involving
only one trait at a time
• Example: Flower color
– P1: (parental generation)
• pure-breeding red flowered plants X purebreeding white flowered plants
– F1: (first filial generation)
• red-flowered hybrids (genetically mixed
offspring)
– F2: (second filial generation)
• 3/4 red-flowered and 1/4 white-flowered
Mendel's Laws
1. Inherited characteristics are controlled by
pairs of factors
– genes
– one from each parent.
2. One gene may “mask” the effect of another.
– The gene which is expressed is dominant, while
the one which is masked is recessive.
3. Pairs of genes segregate during gamete
formation
– so each sex cell contains only one member of a
pair of genes.
Terms
• Alleles
– Two or more alternate forms of a gene, which
produce contrasting effects for a certain trait
– e.g. red (R) and white (r) for flower colour of peas
• Homozygous
– having two of the same allele
• eg. red/red (RR)
• purebred
• Heterozygous
– having two different alleles eg. red/white (Rr)
Terms
• Genotype
– the genetic makeup of an individual
• Phenotype
– the expression of the genes, or appearance of an
individual
• Purebred
– an organism having all homozygous gene pairs
• Hybrid
– an organism having at least one heterozygous
gene pair
Terms
• Monohybrid
– an organism having only one
heterozygous gene pair
• Dihybrid
– an organism having two heterozygous
gene pairs
Monohybrid Cross
Monohybrid Punnet Square
Male
Female
R
R
r
Rr
Rr
r
Rr
Rr
Monohybrid Cross
Monohybrid Cross
Monohybrid Cross
• The phenotypic ratio for a
monohybrid cross is
always 3:1
–dominant trait : recessive trait
Predicting the Outcome of a
Genetic Cross
• When we know the genotypes of
parents used in a genetic cross, we can
predict the genotypes of the offspring
and their expected ratios
– Must use a Punnett square is used
• named after some guy named Punnett
• A geneticist
– Basically it’s a just a fancy chart
Monohybrid Punnet Square
Male
Female
R
R
r
Rr
Rr
r
Rr
Rr
Practice
If an organism has the
dominant phenotype, how can
you determine whether it is
homozygous or
heterozygous?
Test Cross
• You conduct a test cross.
– Mate it with an organism of a known
genotype and see what you get.
• The only known genotype that is observable is
HOMOZYGOUS RECESSIVE (rr)
• For example:
– determine whether a red-flowered pea
plant is homozygous or heterozygous
Test Cross
P: Red flowered X White-flowered
R?
rr
F1: Suppose the cross produces all red flowers
Rr Rr Rr Rr
• If no offspring showing the recessive
phenotype are produced, the unknown
parent must be …
– homozygous
Test Cross
P: Red flowered X White-flowered
R?
rr
F1: Suppose the cross produces 50% red flowers and
50% white flowers
• The only way a white flower could appear is if it
received a recessive allele from the unknown
parent.
– The unknown parent must be …
– HETEROZYGOUS
Dihybrid Cross
• In addition to his monohybrid crosses,
Mendel performed dihybrid crosses of
plants with two different pairs of
contrasting alleles
• In one experiment, Mendel crossed
plants homozygous for seeds that were
both round and yellow with plants
homozygous for wrinkled, green seeds
Dihybrid Cross
• All the F1 offspring had round, yellow seeds
• Self-fertilization of the F1 plants produced
and F2 generation of seeds with the following
phenotypes:
315 round yellow
108 round green
101 wrinkled yellow
32 wrinkled green
Dihybrid Cross
• To find the ratio among the F2
phenotypes,
– we take the number of offspring in the
smallest category - 32 - and divide it into
the number of offspring in the other
categories:
– then the quotient is rounded to the nearest
whole number
Dihybrid Cross
• Thus Mendel determined the phenotypic
ratio in the F2 generation to be 9:3:3:1
– This is the ratio typical of a dihybrid cross
in which both pairs of alleles show a
dominant-recessive relationship
Dihybrid Cross
• Mendel explained these data by assuming that the
genes governing seed color and seed shape move
independently during gamete formation
• In the process of independent assortment, each
pair of alleles behaves as it would in a monohybrid
cross - independently of the other pair
• A dihybrid can produce four possible gene
combinations (with equal probability)
• If the alleles are:
R = round
Y = yellow
r = wrinkled
y = green
• The possible gamete combinations are
RY
Ry
rY
ry
Dihybrid Cross
Parent:
P Gametes:
F1:
F1 Gametes:
F2:
RRYY x rryy
RY
ry
RrYy
RY, Ry, rY, ry
9/16 round, yellow
3/16 round, green
3/16 wrinkled, yellow
1/16 wrinkled, green
(R_Y_)
(R_yy)
(rrY_)
(rryy)
Practice
Multiple Alleles
&
Incomplete Dominance
• Compare ratios and probabilities of
genotypes and phenotypes for dominant
and recessive, multiple, incomplete
dominant, and codominant alleles
• Explain the relationship between
variability and the number of genes
controlling a trait
Multiple Alleles
•
•
•
The genes which we have studied so far
have only two different alleles:
Many genes actually exist in more than two
allelic forms,
Although only two genes control coat colour
in rabbits it is controlled by a series of four
alleles for the same gene:
1. C
Full colour
2. Cch
Chinchilla
3. ch
Himalayan
4. c
Albino
Multiple Alleles
•
•
The dominance hierarchy of these
alleles is C > Cch > ch > c
Determine the genotypes for the
following phenotypes:
Phenotype
Possible Genotype
1.
2.
3.
4.
Full color
Chinchilla
Himalayan
Albino
C + any of the 4
Cch+ anything but C
c h c h, c h c
cc
Incomplete Dominance
• In some cases, a heterozygous
organism shows a blending of
genes because neither gene is
dominant:
– this is termed incomplete dominance
– for example, in snapdragons, neither the
red nor the white allele is dominant…
Incomplete Dominance
Parent: Red Flowers x White Flowers
RR x rr
F1:
Pink Flowers
Rr, Rr, Rr, Rr
F2:
¼ Red flowers
RR
Rr
½ Pink flowers
¼ White flowers rr
Codominance
•
•
•
If two different alleles each contribute to a
phenotype, they are termed codominant
One of the best-known example of
codominant genes occurs in humans, and
determine ABO blood types
There are three possible alleles:
1.
2.
3.
IA
IB
i
Codominance
Blood Type
Phenotype
Possible Genotypes
A
IA IA,
IA i
B
IB IB,
IB i
AB
IA IB
O
i i
Practice
Chromosome Mapping
• Explain the influence of gene linkage
and crossing over on variability
Chromosome Mapping
Chromosomal Theory of Inheritance
1. Genes are located on chromosomes
2. Chromosomes undergo segregation
during meiosis
3. Chromosomes assort independently
during meiosis
4. Each chromosome contains many
different genes
Chromosome Mapping
• Each chromosome contains
hundreds or thousands of genes
• Genes located on the same
chromosome are inherited together,
– they are part of a single chromosome
that is passed along as a unit
– such genes are said to be linked
Chromosome Mapping
• during meiosis, chromosomes may exchange
segments of DNA by crossing-over
A A
B B
C C
a a
b b
c c
D D
E E
F F
d d
e e
f f
Chromosome Mapping
• during meiosis, chromosomes may exchange
segments of DNA by crossing-over
A A
B B
C C
a a
b b
c c
A a
B B
C C
A a
b b
c c
D D
E E
F F
d d
e e
f f
D D
E e
F f
d d
E e
F f
Chromosome Mapping
• The closer two genes are on a chromosome,
the fewer the possible points of crossover are
between them, and the less frequently such a
cross-over will occur
• In other words, if two genes are close
together on a chromosome it is likely they will
stay together and not be exchanged between
chromatids during meiosis
• To determine the location of genes along a
chromosome is called MAPPING a
chromosome
Chromosome Mapping
• A chromosome map indicates
1. The order in which specific
genes occur on a chromosome
2. The distances between the
genes
Chromosome Mapping
• Example:
– In Drosophila, the following data was obtained
from genetic crosses:
• 13% recombination between bar eye and garnet eye
– High percentage recombination indicates that these
two genes are far apart from each other
– High likelihood that crossing over will occur between
these two genes.
• 7% recombination between garnet eye and scalloped
wings
– These two genes are closer together than bar eye
and garnet eye
• 6% recombination between scalloped wings and bar eye
Chromosome Mapping
• This data can be used to map the
chromosome:
Bar eye
Scalloped wings
6 units
Garnet eye
7 units
13 units
Chromosome Mapping
Sex Determination
• There are two chromosomes involved in the
determination of sex of most animals,
– the sex chromosomes (X & Y)
• Any other chromosome not involved in sex
determination is called an autosome
– for example, humans have 22 pairs of autosomes
and 1 pair of sex chromosomes
• in most mammals, females are homozygous
and have 2 X chromosomes, while males are
heterozygous and carry an X and a Y
chromosome
Sex Determination
• Female = XX
• Male = XY
• Females can produce eggs carrying
only an X chromosome,
• Males produce sperm carrying either an
X or a Y chromosome (50% of each)
• Thus, it is the male who determines the
sex of his offspring !
Sex Linkage
• Compare the pattern of inheritance
produced by genes on the sex
chromosomes to that produced by
genes on autosomes, as investigated by
Morgan and others.
A True Story
• One day a geneticist named Thomas Hunt
Morgan discovered a mutant white-eyed male
fly among his hundreds of red-eyed flies. He
bred the white-eyed male with several redeyed females and observed the offspring: as
expected, all the F1 flies had red eyes,
showing red to be the dominant allele. He
allowed the F1 generation to interbreed freely
and observed the F2 generation. Again, as
expected, the ratio of red-eyed flies to whiteeyed flies was 3:1, except that …
every white-eyed fly was
male!
Explanation …
Morgan concluded that the gene
controlling eye-colour was carried
on the… X-chromosome
Sex Linkage
• Like other chromosomes, the sex
chromosomes carry many genes
• Some of the regions of the Xchromosome have a homologous
region on the Y- chromosome
– There are also large non-homologous
portions:
• That is, the X chromosome carries
some genes that have no
counterparts on the Y chromosome
Sex Linkage
• Genes for sex-linked traits are carried
on the X chromosome but not on the Y
chromosome
– Therefore in a male, the gene on the X
chromosome is expressed whether it is
dominant or recessive
– In a female, she must have two recessive
alleles to have the recessive phenotype
Fly Solution
•
•
•
•
•
•
•
R = Red
r = white
XR XR = Red eyed Female
XR Xr = Red eyed Female
Xr Xr = White eyed Female
XR Y = Red eyed Male
Xr Y = White eyed Male
Fly Solution
XR XR
x
Xr Y
XR
XR
Xr
XR Xr
XR Xr
Y
XR Y
XR Y
XR Xr x
XR Y
XR
Xr
XR
XR XR
XR Xr
Y
XR Y
Xr Y
Example of hemophilia trait – (h) recessive disease
Sex Linkage
• Some sex-linked traits in humans are
– Colour vision
– Hemophilia
– Duchenne's Muscular Dystrophy
Sex-Influenced Genes
Sex Influenced Genes
• The main role of sex hormones is to
influence the reproductive system
and its related organs,
– These hormones, however, also affect
many other parts of the body
• Genes that are expressed to a
greater or lesser degree as a result
of the level of sex hormones are
called sex-influenced genes.
Sex Influenced Genes
• These genes are usually located on the
autosomes
• Males and females with the same
genotype may differ greatly in phenotype
because the levels of sex hormones
• For example:
– A bull may have a gene for high milk
production, but he will not produce milk
because he has low levels of female
hormones.
Sex Influenced Genes
• In humans, the gene for male pattern
baldness is autosomal and sex-influenced
• A man will become bald even if he has
only one allele for baldness, because
– the male sex hormones somehow stimulate
the expression of the allele
• In a woman, however, the allele acts as a
recessive allele so that she must have
two balding genes before she loses her
hair
Lethal Alleles
Lethal Alleles
• If An organism has a mutation that
destroys the genetic code for a protein
essential to life, the organism will often die
prematurely.
• This gene that fails to code for a functional
protein is called a lethal allele
• It is possible for lethal alleles to be
dominant, but most are rapidly eliminated
from a population because they cause
death before the individual carrying the
allele reproduces.
Lethal Alleles
• An exception of a lethal dominant allele that
remains in a population is the one
responsible for Huntington's Disease in
humans, because this allele is not
expressed until later in life (35 - 45 years of
age)
• An example of a recessive lethal allele in
humans is the one for Brachydactyly:
– Heterozygotes have a short middle finger bone
that makes the fingers appear to have only two
bones instead of three
– Homozygous babies lack fingers and have
abnormal development of the skeleton that
result in death in infancy
Lethal Alleles
• Some human examples of lethal
alleles are:
– Sickle cell anemia
– Tay-sachs disease
– Cystic fibrosis
– Huntington's disease
Pedigrees
Pedigrees
Pedigrees
• Because geneticists are unable to manipulate
the mating patterns of people, they must
analyze the results of matings that have
already occurred.
• As much information as possible is collected
about a family's history for a particular trait,
and this information is assembled into a
family tree describing the interrelationships of
parents and children across the generations
– This is called a pedigree
Pedigrees
• From a pedigree you should be able to
determine if a particular trait is dominant or
recessive, sex-linked or autosomal
• A pedigree not only helps us understand the
past but also helps us predict the future
• Geneticists, physicians, and genetic
counselors use pedigrees
– for analysis of genetic disorders
– to advise prospective parents of genetic risks
involved
• Complete the pedigree studies on page 632
and page 611 as a class.