Transcript What makes us human?

Georgia Standards:
SB2c:Using Mendel’s laws, explain the role of meiosis
in reproductive variability.
SB2e: Compare the advantages of sexual reproduction
and asexual reproduction in different situations.
Essential Questions:
•How do you predict the probability of various
genotypes inherited and the expressed phenotypes?
•How does meiosis generate variation in offspring?
Complex Inheritance &
Human Heredity
Chapter 11
11.1: Basic Patterns of Inheritance
• Many human genes have become known
through the study of genetic disorders.
• Genetic Disorders can be caused by
– recessive alleles
– dominant alleles
– Codominant alleles
• What makes an allele dominant, recessive, or
codominant?
– It all depends on the nature of a gene’s protein
product and its role in the cell.
Chapter
11
Complex Inheritance and Human Heredity
11.1 Basic Patterns of Human Inheritance
Recessive Genetic Disorders
 A recessive
trait is
expressed
when the
individual is
homozygous
recessive for
the trait.
Chapter
11
Complex Inheritance and Human Heredity
11.1 Basic Patterns of Human Inheritance
Cystic Fibrosis
 Affects the mucus-producing glands,
digestive enzymes, and sweat glands
 Chloride ions are not absorbed into the
cells of a person with cystic fibrosis but
are excreted in the sweat.
 Without sufficient chloride ions in the
cells, a thick mucus is secreted.
Section 14-1
Figure 14-8 The Cause of Cystic Fibrosis
Chromosome
#7
CFTR
gene
Go to
Section:
The most common
allele that causes
cystic fibrosis is
missing 3 DNA
bases. As a result,
the amino acid
phenylalanine is
missing from the
CFTR protein.
Normal CFTR is a
chloride ion
channel in cell
membranes.
Abnormal CFTR
cannot be
transported to the
cell membrane.
The cells in the
person’s airways are
unable to transport
chloride ions. As a
result, the airways
become clogged with
a thick mucus.
Chapter
11
Complex Inheritance and Human Heredity
11.1 Basic Patterns of Human Inheritance
Albinism
 Caused by altered genes, resulting in the
absence of the skin pigment melanin in hair
and eyes
 White hair
 Very pale skin
 Pink pupils
Chapter
11
Complex Inheritance and Human Heredity
11.1 Basic Patterns of Human Inheritance
Tay-Sachs Disease
 Caused by the absence of the enzymes
responsible for breaking down fatty acids
called gangliosides
 Gangliosides accumulate in the brain,
inflating brain nerve cells and causing
mental deterioration.
Chapter
11
Complex Inheritance and Human Heredity
11.1 Basic Patterns of Human Inheritance
Galactosemia
 Recessive genetic disorder characterized
by the inability of the body to digest
galactose.
Chapter
11
Complex Inheritance and Human Heredity
11.1 Basic Patterns of Human Inheritance
Dominant Genetic Disorders
 Huntington’s disease affects the nervous
system.
 Achondroplasia is a genetic condition that
causes small body size and limbs that are
comparatively short.
Chapter
11
Complex Inheritance and Human Heredity
11.1 Basic Patterns of Human Inheritance
Comparing Dominance & Recessiveness
• Determining whether an allele is dominant
or recessive is critical in medical genetics
because it helps predict which individuals
are at high risk of inheriting a particular
condition (phenotype).
Comparing Dominance & Recessiveness
DOMINANT:
RECESSIVE:
• Can appear in either sex
because an autosome
carries the gene.
•
Can appear in either sex & can skip
generations.
•
Affected individuals have a
homozygous recessive genotype,
whereas in heterozygotes (carriers) the
wild type allele masks expression of
the mutant allele.
•
Parents of an affected individual are
heterozygous or have the trait.
•
Most occur unexpectedly
•
Incest increases the risk of having a
child with an autosomal recessive trait
• If the child has it, then at
least one parent has it.
• Do not skip generations
• If no offspring inherit the
trait in one generation, its
transmission stops
because the offspring can
pass on only the recessive
form of the gene.
Human Traits
• A pedigree chart, which shows the
relationships within a family, can be used
to help determine how genetic disorders
are inherited.
• Many human traits are polygenic
(controlled by many genes)
• Environmental effects on gene expression
are not inherited; genes are.
Pedigree Analysis
Pedigree of Queen Victoria
Have you ever seen a family tree… do you have one??
Graphic representation of family inheritance.
What is a pedigree?
• Shows a pattern of inheritance in a family for a
specific trait (phenotype)
• Genotypes can usually be determined
• Why would we want to use a pedigree in
genetics?
• Track the occurrence of diseases such as:
– Huntington’s – simple dominant – lethal allele – causes breakdown of
the brain
– Cystic fibrosis – 1/2500 – mucus accumulates (white North Amer.)
– Tay-Sachs disease – lipids accumulate in CNS (Jewish)
– Phenylketonuria – missing enzyme causes problems in CNS
(Nordic/Swedish)
The Symbols used:
Sample pedigree:
•generations are numbered with Roman Numerals
•oldest offspring are on the left
How many males are present? How many females?
How many females show the trait being studied?
What is the sex of offspring III-9?
How many offspring did the generation I parents have?
What is the difference between the II-3 & 4 and IV-2 & 3?
Inheritance patterns:
• Autosomal dominant:
The disease is passed from the
father (II-3) to the son (III-5), this
never happens with X-linked traits.
The disease occurs in three
consecutive generations, this
never happens with recessive
traits.
Males and females are affected,
with roughly the same probability.
–Examples: Polydactyly
–Huntington’s disease
Inheritance patterns:
• Autosomal recessive
Males and females are equally likely to be
affected.
The recurrence risk to the unborn sibling
of an affected individual is 1/4.
The trait is characteristically found in
siblings, not parents of affected or the
offspring of affected.
Parents of affected children may be
related. The rarer the trait in the general
population, the more likely a
consanguineous mating is involved.
–Cystic fibrosis
–Tay-Sach’s disease
Inheritance patterns:
• Sex-linked recessive conditions
The disease is never passed from
father to son.
Males are much more likely to be
affected than females.
•All affected males in a family are
related through their mothers.
–Examples:
–Colour-blindness
Trait or disease is typically passed
from an affected grandfather,
through his carrier daughters, to half
of his grandsons.
–Duchenne Muscular Dystrophy
Draw a pedigree to depict the following family
•One couple has a son and a daughter with normal skin
pigmentation.
•Another couple has one son and two daughters with
normal skin pigmentation.
•The daughter from the first couple has three children with
the son of the second couple.
•Their son and one daughter have albinism (OMIM 203100);
their other daughter has normal skin pigmentation.
11.2: Complex Patterns of
Inheritance
• Complex inheritance of traits does not follow
inheritance patterns described by Mendel
•They are not inherited in a dominant or recessive
way.
Exceptions to the Laws:
• Some alleles are neither dominant nor
recessive, and many traits are controlled by
multiple alleles or multiple genes.
• Incomplete dominance occurs when one
allele is not completely dominant over
another
• In incomplete dominance, the heterozygous
phenotype is somewhere in between the two
homozygous phenotypes.
Incomplete Dominance
• Some alleles are neither
dominant nor recessive.
• In four o’clock plants, for
example, the alleles for
red and white flowers
show incomplete
dominance.
• Heterozygous (RW)
plants have pink
flowers—a mix of red and
white coloring
Incomplete Dominance Practice
• In Mendel's experiments, if the gene for tall (T)
plants was incompletely dominant over the gene
for short (t) plants, what would be the result of
crossing two Tt plants?
•
•
•
•
•
A. 1/4 would be tall; 1/2 intermediate height; 1/4 short
B. All the offspring would be tall.
C. 1/2 would be tall; 1/4 intermediate height; 1/4 short.
D. All the offspring would be intermediate.
E. 1/4 would be tall; 1/4 intermediate height; 1/2 short.
Incomplete Dominance Practice
•
Disappearance of parental
phenotypes in the F1 generation
•
A. pink flower color is epistatic to red
flower color.
• A genetic cross of inbred
snapdragons with red flowers
with inbred snapdragons with
white flowers resulted in F1hybrid offspring that all had
pink flowers. When the F1
plants were self-pollinated, the
resulting F2-generation plants
had a phenotypic ratio of 1 red:
2 pink: 1 white. The most likely
explanation is:
•
B. pink flowers are the result of a
blending of the red and white
genotypes.
•
C. flower color is due to 2 or more
complementary genes.
•
D. heterozygous plants have a
different phenotype than either inbred
parent because of incomplete
dominance of the dominant allele.
•
E. flower color inheritance in
snapdragons does not behave as a
Mendelian trait.
Codominance
• In codominace, both • In certain varieties of
alleles contribute to
chickens, the allele
the phenotype of the
for black feathers is
organism.
codominant with the
allele for white
• For example, in cattle
the allele for red hair
feathers.
is codominant with
– Heterozygous
the allele for white
chickens appear
hair.
speckled with black
– Cattle with both alleles
are roan, or pinkish
brown, because their
coats are a mixture of
both red and white hairs.
and white feathers.
Sickle Cell Disease
• Sickle cell disease is a
common genetic disorder
inherited in a
codominant fashion, and
mainly found in African
Americans.
• Sickle cell disease is
characterized by the bent
and twisted shape of the
red blood cells
• These sickle-shaped red blood
cells are more rigid than
normal cells and tend to get
stuck in the capillaries, the
narrowest blood vessels in the
body.
• As a result, blood stops
moving through these vessels,
damaging cells and tissues
beyond the blockage.
• Sickle cell disease produces
physical weakness and
damage to the brain, heart,
and spleen. In some cases, it
may be fatal.
Sickle Cell Disease
• Hemoglobin is the protein
that carries oxygen in the
blood.
• Mutation: the amino acid
valine in place of glutamic
acid.
• As a result, the abnormal
hemoglobin is somewhat
less soluble than normal
hemoglobin. Blood gets
stuck in cappillaries.
Why do so many African Americans carry
the sickle cell allele?
• Most African Americans can trace their ancestry
to west central Africa.
• Malaria, a serious parasitic disease that infects
red blood cells, is common in this region of
Africa.
• People who are heterozygous for the sickle
cell allele are generally healthy and are
resistant to malaria.
Concept Map
Section 14-1
Autosomol
Disorders
caused by
Dominant alleles
Codominant
alleles
include
include
include
Huntington’s
disease
Sickle cell
disease
Galactosemia
Albinism
Cystic
fibrosis
Go to
Section:
Recessive
alleles
Phenylketonuria
Tay-Sachs
disease
Achondroplasia
Hypercholesterolemia
Multiple Alleles
• Many genes have
more than two alleles
and are therefore said
to have multiple
alleles.
• One of the best-known
examples is coat color
in rabbits.
• This does not mean
that an individual can
have more than two
alleles. It only means
that more than two
possible alleles exist
in a population.
• A rabbit’s coat color is
determined by a single
gene that has at least
four different alleles.
– Full color, albino,
himalayan, chinchilla
• ABO blood groups have
three forms of alleles that
can be inherited in a
codominant way.
Chapter
11
Complex Inheritance and Human Heredity
11.2 Complex Patterns of Inheritance
Coat Color of Rabbits
Chinchilla
Albino
Light gray
Dark gray
Himalayan
Multiple Alleles Inherited in a Codominant
Way: The human ABO markers
Phenotype
(Blood Type
Go to
Section:
Genotype
Antigen on
Red Blood Cell
Safe Transfusions
To
From
Multiple Alleles Inherited in a Codominant Way:
The human ABO markers
• Human blood type is
determined by codominant
alleles. There are three
different alleles, known as IA,
IB, and i. The IA and IB alleles
are co-dominant, and the i
allele is recessive.
• The possible human
phenotypes for blood group
are type A, type B, type AB,
and type O. Type A and B
individuals can be either
homozygous (IAIA or IBIB,
respectively), or heterozygous
(IAi or IBi, respectively).
• A woman with type A blood
and a man with type B blood
could potentially have offspring
with which of the following
blood types?
•
•
•
•
•
A. type A
B. type B
C. type AB
D. type O
E. all of the above
Multiple Alleles Inherited in a Codominant Way:
: Predicting human blood types
• What are the possible blood types of the
offspring of a cross between individuals that are
type AB and type O? (Hint: blood type O is
recessive)
•
•
•
•
•
A. AB or O
B. A, B, or O
C. A or B
D. A, B, AB, or O
E. A, B, or AB
Epistasis:
• The result of one allele hiding the effects
of another allele.
• Ex: Labrador’s coat color can vary from
yellow to black
Chapter
11
Complex Inheritance and Human Heredity
11.2 Complex Patterns of Inheritance
Epistasis
 Variety is the result of one allele hiding the
effects of another allele.
eebb
eeB_
No dark pigment present in fur
E_bb
E_B_
Dark pigment present in fur
Polygenic Traits
• Many traits are produced
by the interaction of
several genes.
• Traits controlled by
two or more genes
are said to be
polygenic traits.
• Polygenic traits often
show a wide range of
phenotypes.
– Ex: skin color,
eye color, height,
and fingerprint
pattern
Polygenic Traits:
• Most traits are controlled by
two or more genes and are,
therefore, called polygenic
traits.
• Each gene of a polygenic
trait often has two or more
alleles.
• As a result, one polygenic
trait can have many possible
genotypes and even more
possible phenotypes.
EX: height (A bell-shaped curve is
also called a normal distribution)
Checkpoint Point Questions:
1. Explain what independent assortment means.
2. Describe two inheritance patterns besides
simple dominance.
3. What is the difference between incomplete
dominance and codominance?
4. Identify examples of polygenic traits and
multiples alleles. (one example for each)
Human Chromosomes
• Two of your 46 chromosomes are known as sex
chromosomes, because they determine an
individual’s sex. (Chromosomes # 23)
• Females have two copies of a large X
chromosome. Males have one X and one small
Y chromosome.
• The remaining 44 chromosomes are known as
autosomal chromosomes, or autosomes.
(Chromosomes # 1-22)
Sex Chromosomes:
• All egg cells carry a single X chromosome (23X).
• Half of all sperm cells carry an X chromosome
(23X) and half carry a Y chromosome (23Y).
• This ensures that just about half of the zygotes
will be 46XX and half will be 46XY.
Sex-Linked Genes
• Is there a special pattern
of inheritance for genes
located on the X
chromosome or the Y
chromosome?
• The answer is yes.
Because these
chromosomes determine
sex, genes located on
them are said to be sexlinked genes.
Sex-Linked Genes
• Males have just one X
chromosome. Thus, all Xlinked alleles are expressed
in males, even if they are
recessive.
• This means that the recessive
phenotype of a sex-linked
genetic disorder tends to be
much more common among
males than among females.
• In order for a recessive allele,
such as the one for
colorblindness, to be
expressed in females, there
must be two copies of the
allele, one on each of the two
X chromosomes.
• In addition, because men pass
their X chromosomes along to
their daughters, sex-linked
genes move from fathers to
their daughters and may then
show up in the sons of those
daughters.
Expression of X-Linked Alleles:
Colorblindness
•
X-linked alleles are
always expressed in
males, because males
have only one X
chromosome.
• Males who receive the
recessive Xc allele all
have colorblindness.
• Females, however, will
have colorblindness only
if they receive two Xc
alleles.
Sex-Linked Genes: Hemophilia
•
Hemophilia is another
example of a sex-linked
disorder.
• Two important genes carried
on the X chromosome help
control blood clotting.
• A recessive allele in either of
these two genes may
produce a disorder called
hemophilia
• In hemophilia, a protein
necessary for normal blood
clotting is missing.
• About 1 in 10,000 males is
born with a form of hemophilia.
• People with hemophilia can
bleed to death from minor cuts
and may suffer internal
bleeding from bumps or
bruises.
• Fortunately, hemophilia can be
treated by injections of normal
clotting proteins.
Sex-Linked Genes: Duchenne
Muscular Dystrophy
• Duchenne muscular
dystrophy is a sex-linked
disorder that results in the
progressive weakening and
loss of skeletal muscle.
• People with Duchenne
muscular dystrophy rarely live
past early adulthood. In the
United States, one out of every
3000 males is born with
Duchenne muscular dystrophy.
• Duchenne muscular
dystrophy is caused by a
defective version of the gene
that codes for a muscle
protein.
• Researchers in many
laboratories are trying to find a
way to treat or cure this
disorder, possibly by inserting
a normal allele into the muscle
cells of Duchenne muscular
dystrophy patients.
X-Chromosome Inactivation
• Females have two X
chromosomes, but males
have only one.
• In female cells, one X
chromosome is randomly
switched off.
• If just one X chromosome
is enough for cells in
males, how does the cell
“adjust” to the extra X
chromosome in female
cells?
• That turned-off
chromosome forms a dense
region in the nucleus known
as a Barr body.
• Barr bodies are generally
not found in males because
their single X chromosome
is still active.
X-Chromosome Inactivation: Cats
• In cats, for example, a gene that controls the
color of coat spots is located on the X
chromosome.
• One X chromosome may have an allele for
orange spots and the other may have an allele
for black spots.
• In cells in some parts of the body, one X
chromosome is switched off. In other parts of the
body, the other X chromosome is switched off.
X-Chromosome Inactivation: Cats
• As a result, the cat’s fur will
have a mixture of orange and
black spots, as shown in the
figure below.
• Male cats, which have just one
X chromosome, can have
spots of only one color.
• By the way, this is one way to
tell the sex of a cat. If the cat’s
fur has three colors—white
with orange and black spots,
for example—you can almost
be certain that it is female.
Chapter
11
Complex Inheritance and Human Heredity
11.2 Complex Patterns of Inheritance
Environmental Influences
 Environmental factors
 Diet and exercise
 Sunlight and water
 Temperature
Chapter
11
Complex Inheritance and Human Heredity
11.2 Complex Patterns of Inheritance
Twin Studies
 Helps scientists separate genetic
contributions from environmental
contributions
 Traits that appear frequently in identical
twins are at least partially controlled by
heredity.
 Traits expressed differently in identical
twins are strongly influenced by
environment.
11.3: Chromosomes & Human Heridity
• To analyze chromosomes, cell biologists
photograph cells in mitosis, when the
chromosomes are fully condensed and easy to
see.
• The biologists then cut out the chromosomes
from the photographs and group them together
in homologous pairs.
• A picture of homologous chromosomes
arranged in decreasing size is known as a
karyotype.
Human Chromosomes
• A Human body cell contains 46 chromosomes.
• A haploid sperm, carrying just 23
chromosomes, fertilized a haploid egg, also
with 23 chromosomes.
– 22 autosomes (# 1-22)
– 1 sex chromosome (X or Y)
• The diploid zygote, or fertilized egg, contained
the full complement of 46 chromosomes.
Chapter
11
Complex Inheritance and Human Heredity
11.3 Chromosomes and Human Heredity
Karyotype Studies
 Karyotype—micrograph in which the pairs
of homologous chromosomes are arranged
in decreasing size.
 Images of chromosomes stained during
metaphase
 Chromosomes are arranged in decreasing
size to produce a micrograph.
Karyotype
• These human
chromosomes have
been cut out of a
photograph and
arranged to form a
karyotype.
Chapter
11
Complex Inheritance and Human Heredity
11.3 Chromosomes and Human Heredity
Telomeres
 Telomere caps consist of DNA associated
with proteins.
 Serves a protective function for the
structure of the chromosome
Chromosomal Disorders
• The most common error in meiosis occurs when
homologous chromosomes fail to separate.
• This is known as nondisjunction, which means “not
coming apart.” Nondisjunction can occur either during
meiosis I, as shown in the figure below, or in meiosis II
• Nondisjunction can occur in sex chromosomes and in
autosomes.
Chromosomal Disorders
• Nondisjunction
causes gametes to
have abnormal
numbers of
chromosomes.
• The result of
nondisjunction may
be a chromosome
disorder such as
Down syndrome.
Down Syndrome
•
If two copies of an autosomal
chromosome fail to separate
during meiosis
(nondisjunction) an individual
may be born with three copies
of a chromosome.
• This is known as a trisomy,
meaning “three bodies.” The
most common form of trisomy
involves three copies of
chromosome 21 and is
called Down syndrome.
• In the United States,
approximately 1 baby in 800 is
born with Down syndrome.
• Down syndrome produces mild
to severe mental retardation.
• It is also characterized by an
increased susceptibility to
many diseases and a higher
frequency of some birth
defects.
Sex Chromosomal Disorders:
• Disorders also occur
among the sex
chromosomes.
• Two of these
abnormalities are
Turner’s syndrome and
Klinefelter’s syndrome.
• In females,
nondisjunction can lead
to Turner’s syndrome.
• A female with Turner’s
syndrome inherits only
one X chromosome
(genotype XO).
• Women with Turner’s
syndrome are sterile
because their sex organs
do not develop at
puberty.
Sex Chromosomal Disorders:
• In males, nondisjunction causes Klinefelter’s
syndrome (genotype XXY).
• The extra X chromosome interferes with meiosis and
usually prevents these individuals from reproducing.
• Cases of Klinefelter’s syndrome have been found in
which individuals were XXXY or XXXXY.
• There have been no reported instances of babies being
born without an X chromosome, indicating that the X
chromosome contains genes that are vital for normal
development.
• Video
Chapter
11
Complex Inheritance and Human Heredity
Chapter
36
Human Reproduction and Development
36.2 Human Development Before Birth
Diagnosis in the Fetus
 Ultrasound
 Procedure in which sound waves are
bounced off the fetus
 Determines if the fetus is growing properly
 Determines the position of the fetus in
the uterus
 Determines the gender of the fetus
Chapter
36
Human Reproduction and Development
36.2 Human Development Before Birth
Amniocentesis
 Amniocentesis is
performed in the
second trimester.
 Fluid from the amniotic
sac is removed and
analyzed.
 Diagnosis of
chromosome
abnormalities and other
defects
Chapter
36
Human Reproduction and Development
36.2 Human Development Before Birth
Chorionic Villus Sampling
 Chorionic villus
sampling is performed
during the first
trimester.
 Cells from the chorion are
removed and analyzed by
karyotyping.
 Diagnosis of chromosome
abnormalities and other
genetic defects
Fetal Blood Sampling
• Diagnosis of genetic or chromosome
abnormality
• Checks for fetal blood problems and
oxygen levels
• Medications can be given to the fetus
before birth.
Checkpoint Questions:
1. Why are sex-linked disorders more common in males
than in females?
2. How does nondisjunction cause chromosome number
disorders?
3. List at least two examples of human sex-linked
disorders.
4. Describe two sex chromosome disorders.
5. Distinguish between sex-linked disorders and sex
chromosome disorders.