Transcript Inheritance of genetic traits

Inheritance of genetic traits
Chapter 12
Making a Pedigree
• A pedigree is a graphic representation of genetic
inheritance
• It is a diagram made up of a set of symbols that identify
males and females, individuals affected by the trait being
studied, and family relationships
Male
Parents
Female
Siblings
A half-shaded circle
or square represents
a carrier, a
heterozygous
individual.
Affected
Known
male
heterozygotes
for recessive
Affected allele
female
Mating
Death
I
In a pedigree, a circle
Male represents a female; a
square represents a
male.
Female
1
2
II
2
1
3
4
5
III
1
2
4
3
IV
1
2
3
4
5
A horizontal line
connecting a circle and
a square indicates that
the individuals are
parents, and a vertical
line connects parents
with their offspring.
Each row shows a
generation with most
recent at the bottom
Simple Recessive Heredity
• Most genetic disorders are caused by recessive alleles
1. Cystic Fibrosis
• Cystic fibrosis (CF) is a fairly common genetic
disorder among white Americans
• About 1 in 28 white Americans carries the
recessive allele, & 1 in 2500 children born to
white Americans inherits the disorder.
• Due to a defective protein in the plasma
membrane, cystic fibrosis results in the
formation and accumulation of thick mucus in
the lungs and digestive tract
2. Tay Sachs Disease
• Tay-Sachs (tay saks) disease is a recessive
disorder of the central nervous system among the
Jewish people in the U.S. & eastern European
descent
• A recessive allele results in the absence of an
enzyme that normally breaks down a lipid
produced and stored in tissues of the central
nervous system
• Because this lipid fails to break down properly, it
accumulates in the cells
Typical pedigree for Tay-Sachs
I
1
2
II
1
2
3
4
1
2
III
3
IV
1
Symptoms are not
usually visual until
around 48 months. Child
gradually becomes blind
and helpless, develops
seizures & eventually
paralyzed.
The child dies by the age
of 3 or 4
No treatment or cure.
Can be detected by
amniocenteses
3. PKU
• Phenylketonuria (fen ul kee tun YOO ree uh),
also called (PKU), is a recessive disorder that
results from the absence of an enzyme that
converts one amino acid, phenylalanine, to a
different amino acid, tyrosine.
• Because phenylalanine cannot be broken down,
it and its by-products accumulate in the body and
result in severe damage to the central nervous
system
Phenylketonuria
• A PKU test is normally performed on all infants a few days after birth
• Infants affected by PKU are given a diet that is low in phenylalanine
until their brains are fully developed
Phenylketonurics: Contains Phenylalanine
Simple Dominant
• Many traits are inherited just as the rule of dominance
predicts
• Remember that in Mendelian inheritance, a single
dominant allele inherited from one parent is all that is
needed for a person to show the dominant trait
Some examples of Simple
Dominance
• A cleft chin
• Widow’s peak hairline
• Hitchhiker’s thumb
• Almond shaped eyes
• Thick lips
• Presence of hair on the middle
section of your fingers
Huntington’s Disease
• Huntington’s disease is a lethal genetic disorder caused
by a rare dominant allele
• It results in a breakdown of certain areas of the brain
• Usually, dominant allele with such severe effects would
result in death before the affected individual could have
children and pass the allele on to the next generation
• But because the onset of Huntington’s disease usually
occurs between the ages of 30 and 50, an individual may
already have had children before knowing whether he or
she is affected.
Typical Pedigree of Huntington’s Disease
I
1
2
II
1
2
3
4
5
4
5
III
1
2
3
Huntington’s Disease
leads to progressive
degeneration of brain
cells, which causes
muscle spasms,
personality disorders &
death in 10-15 years
from onset
When inheritance follows different rules
Section 2
Incomplete Dominance
• When traits are inherited in an incomplete dominance
pattern, the presence of a 3rd phenotype appears.
• For example, if a homozygous red-flowered snapdragon
plant (RR) is crossed with a homozygous white-flowered
snapdragon plant (R′ R′), all of the F1 offspring will have
pink flowers
Appearance of a 3rd Phenotype
Red
White
All
pink
Red
(RR)
White
(R’R’)
Pink
(RR’)
Pink
(RR’)
All pink flowers
1 red: 2 pink: 1 white
Why a 3rd Phenotype?
• The new phenotype occurs because the flowers
contain enzymes that control pigment production
• The R allele codes for an enzyme that produces a
red pigment. The R’ allele codes for a defective
enzyme that makes no pigment.
• Because the heterozygote has only one copy of
the R allele, its flowers appear pink because they
produce only half the amount of red pigment that
red homozygote flowers produce.
Codominance: expression of
both alleles
• Codominant alleles cause the phenotypes of both
homozygotes to be produced in heterozygous
individuals.
• In codominance, both alleles are expressed equally
Sickle Cell Anemia
• In an individual who is homozygous for the
sickle-cell allele, the oxygen-carrying protein
hemoglobin differs by one amino acid from
normal hemoglobin
• This defective hemoglobin forms crystal-like
structures that change the shape of the red blood
cells. Normal red blood cells are disc-shaped,
but abnormal red blood cells are shaped like a
sickle, or half-moon
• HbAHbA individuals are normal; HbS HbS have
sickle-cell trait.
• With sickle-cell disease, red blood cells are
irregular in shape (sickle-shaped) rather than
biconcave, due to abnormal hemoglobin that
the cells contain.
• Symptoms results in poor circulation, anemia,
low resistance to infection, hemorrhaging,
damage to organs, jaundice, and pain of
abdomen and joints.
• Bone marrow transplants pose high risks;
other research focuses on fetal hemoglobin,
etc
Sickle-cell anemia
Abnormally shaped blood cells, slow blood flow, block
small vessels, and result in tissue damage and pain.
Normal red
blood cell
Sickle cell
Multiple Alleles:
• Although each trait has only two alleles in the patterns of
heredity you have studied thus far, it is common for more
than two alleles to control a trait in a population
• Traits controlled by more than two alleles have multiple
alleles
Multiple alleles: govern blood type
Mendel’s laws of heredity also can be applied to traits
that have more than two alleles.
The ABO blood group is a classic example of a
single gene that has multiple alleles in humans.
Determining blood type is necessary before a person
can receive a blood transfusion because the red
blood cells of incompatible blood types could clump
together, causing death
Multiple Alleles Govern Blood
Type
Human Blood Types
Genotypes Surface Molecules Phenotypes
A
A
lA lA or lAli
B
B
lB lB or lBi
lA lB
A and B
AB
None
ii
O
The ABO Blood Group
The gene for blood type, gene l, codes for a molecule
that attaches to a membrane protein found on the
surface of red blood cells.
The lA and lB alleles each code for a different molecule.
Your immune system recognizes the red blood cells as
belonging to you. If cells with a different surface
molecule enter your body, your immune system will
attack them.
Phenotype A
Surface molecule A
The lA allele is dominant to
i, so inheriting either the lAi
alleles or the lA lA alleles
from both parents will give
you type A blood.
Surface molecule A is
produced.
Phenotype B
lB
The allele is also
dominant to i.
To have type B blood, you
must inherit the lB allele from
one parent and either another
lB allele or the i allele from
the other.
Surface molecule B is produced.
Surface molecule B
Phenotype AB
Surface molecule B
The lA and lB alleles are
codominant.
This means that if you inherit
the lA allele from one parent
and the lB allele from the other,
your red blood cells will
produce both surface molecules
and you will have type AB
blood.
Surface molecule A
Phenotype O
The i allele is recessive and
produces no surface
molecules.
Therefore, if you are
homozygous ii, your
blood cells have no
surface molecules and
you have blood type O.
Homework:
• Click below to play the blood typing game
http://nobelprize.org/educational_games/medicine/l
andsteiner/index.html
Sex Determination
• In humans the diploid number of chromosomes
is 46, or 23 pairs
• There are 22 pairs of homologous chromosomes
called autosomes. Homologous autosomes look
alike.
• The 23rd pair of chromosomes differs in males
and females and are known as the sex
chromosomes and are indicated by the letters
with X and Y
Sex Determination
If you are female, your
23rd pair of
chromosomes are
homologous, XX.
X
X
Female
If you are male, your 23rd
pair of chromosomes XY,
look different.
X
Y
Male
XY
Male
Sex
Determination
X
XX
Female
X
Y
XX
Female
XY
Male
XX
Female
XY
Male
X
Sex Linked Inheritance
• Traits controlled by genes located on sex chromosomes
are called sex-linked traits
• The alleles for sex-linked traits are written as
superscripts of the X or Y chromosomes
• Because the X and Y chromosomes are not homologous,
the Y chromosome has no corresponding allele to one on
the X chromosome and no superscript is used
Dystrophy sex linked cross
White-eyed
male (XrY)
F2
Females:
Redeyed
female
(XRXR)
all red eyed
Males:
1/2 red eyed
1/2 white
eyed
F1 All red eyed
Sex-Linked Inheritance
• The pattern of sex-linked inheritance is explained by the
fact that males, who are XY, pass an X chromosome to
each daughter and a Y chromosome to each son
• Two traits that are governed by X-linked recessive
inheritance in humans are
• 1. red-green color blindness
• 2. Hemophilia
• 3. Muscular Distrophy
Red-Green Color Blindness
People who have red-green
color blindness can’t
differentiate these two colors.
Color blindness is caused by
the inheritance of a recessive
allele at either of two gene
sites on the X chromosome.
Hemophilia
Hemophilia A is an X-linked disorder that causes a
problem with blood clotting.
About one male in every 10 000 has hemophilia, but
only about one in 100 million females inherits the
same disorder
Males inherit the allele on the X chromosome from
their carrier mothers. One recessive allele for
hemophilia will cause the disorder in males
Females would need two recessive alleles to inherit
hemophilia
Skin Color: polygenic inheritance
In the early 1900s, the idea that polygenic inheritance
occurs in humans was first tested using data collected on
skin color.
Scientists found that when light-skinned people mate with
dark-skinned people, their offspring have intermediate
skin colors
Skin color: A polygenic
trait
Number of individuals
Number of Genes Involved in Skin Color
Expected
distribution4 genes
Observed
distribution
of skin
color
Light
Expected
distribution1 gene
Range of skin color
Expected
distribution3 genes
Right
This graph shows
the expected
distribution of
human skin color if
controlled by one,
three, or four genes.
Polygenic Inheritance
• Polygenic inheritance occurs when 1 trait is
governed by 2 or more sets of alleles
• A hybrid cross for skin color provides a range of
intermediates
• Includes cleft lip, clubfoot, hypertension,
diabetes, schizophrenia, allergies & cancers
• Behavioral traits include suicide, phobias,
alcoholism, and maybe homosexuality (not
predetermined
Karyotype
• This chart of chromosome pairs is called a karyotype,
and it is valuable in identifying unusual chromosome
numbers in cells
Try this interactive Karyotype
http://learn.genetics.utah.edu/units/disorder
s/karyotype/flash/karyotype_try_it.swf
Downs Syndrome 21:
Trisomy 21
• Down syndrome is the only autosomal trisomy in
which affected individuals survive to adulthood
• It occurs in about one in 700 live births
• Down syndrome is a group of
symptoms that results from
trisomy of chromosome 21
• Some degree of mental
retardation
Turners Syndrome
• Turner (XO) syndrome females have only one sex
chromosome, an X.
• Turner females are short, have a broad chest and webbed neck.
• Ovaries of Turner females never become functional; therefore,
do not undergo puberty
Klinefelter Syndrome
• Klinefelter syndrome males have one Y chromosome
and two or more X chromosomes.
• Affected individuals are sterile males; testes and prostate
are underdeveloped.
• Individuals have large hands and feet and long arms and
legs.
Triplo-X Females
• Triplo-X females have three or more X chromosomes.
• There is no increased femininity; most lack any physical
abnormalities.
• There is an increased risk of having triplo-X daughters or
XXY sons.
• May experience menstrual irregularities, including early
onset of menopause.
Jacob Syndrome
• XYY males with Jacob syndrome have two Y
chromosomes instead of one.
• Results from nondisjunction during meiosis II.
• Usually taller than average; suffer from
persistent acne; tend to have lower intelligence.
• Earlier claims that XYY individuals were likely
to be aggressive are not correct