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Heredity
AP Biology: Chapters 14- 15
3/10/16
Genetic Problems
• Most of the material in this unit
prepares you for solving
genetic problems.
• A genetics problem is an
analysis of the characteristics
(traits) of parents and
offspring.
• Given the traits of one of these
generations, you are required
to determine the traits of the
other generation.
Probability Rules
• If a coin is tossed, there is a ½ (50%)
chance, or probability that it will be
heads.
• If a coin is tossed again, there is
again, a ½ (50%) chance that it will
be heads.
• The first toss does not affect the
second toss, that it, the two tosses
are independent.
Probability Rules
• To determine the probability of two or more
independent event occurring together,
you merely multiply the probabilities of
each event happening separately.
• This is the rule of multiplication of
probability.
• For 2 consecutive tosses of a coin, the
probability of getting 2 heads is ½ x ½ = ¼
• For 3 tosses, the probability of 3 heads
would be ½ x ½ x ½ = 1/8
Genetic Terms:
• A gene represents the genetic
material on a chromosome that
contains instructions for creating a
particular trait. (coding for a protein)
• An allele is one of several varieties of
a gene. In pea plants, for example,
there are two alleles of the gene for
flower color: purple allele and white
allele.
Genetic Terms:
• A locus refers to the location on a
chromosome where a gene is
located. Every gene has a unique
locus on a particular chromosome.
• Every cell contains two copies of each
chromosome, one inherited from
each parent. This pair is called a
homologous pair.
3/10
Homologues
• This pair of chromosomes
(homologues) each contains a gene
for the same trait exactly at the same
loci.
• At any one particular locus, the two
genes on a pair of homologues may
represent the different alleles for that
gene because they originated from
different parents.
Genetic Terms:
• If the two alleles inherited for a
gene are different, one allele
may be dominant, while the
other is recessive.
• The trait coded by the
dominant allele is the actual
trait expressed. In pea plants,
the purple flower is dominant,
the white is recessive.
Genetic Terms:
• Homozygous dominant refers to the
inheritance of two dominant alleles (PP).
In this case, the dominant trait is
expressed.
• In the homozygous recessive condition,
two recessive alleles are inherited (pp),
and the recessive trait is expressed.
• Heterozygous refers to the condition
where the two inherited alleles are
different (Pp). In this condition, only the
dominant allele is expressed.
Genetic Terms:
• The phenotype is the actual expression of a
gene. Purple flowers, blue eyes, and brown
hair.
• The genotype represents the actual alleles
(genetic make up). For example, PP
describes the genotype for the homozygous
dominant condition. If P represents the
allele for purple flowers, and p represents
the allele for white flowers, the genotype Pp
would express the phenotype of purple
flowers.
Law of Segregation
• Remember, during Meiosis I, the two
members of a homologous pair of
chromosomes migrate to opposite
poles.
• As a result, each gamete will contain
one allele for each gene. This is
random!
• Thus, the Law of Segregation refers to
this random segregation of alleles and
their chromosomes to separate
gametes.
Law of Independent Assortment
• In addition, the migration of
homologues within one pair of
homologous chromosomes to opposite
poles does not influence the migration
of any OTHER homologous pair.
• Each homologue will assort to one
pole or the other of a cell separately
and independently of each other. This
is called independent assortment.
Segregation & Assortment
• Because both laws of segregation and
independent assortment are random
processes, the rules of probability can
be used to describe how the different
chromosomes (and their alleles) in
parents assemble in gametes and
offspring.
Mendel
• Mendel, a 19th century monk, is
credited with the discovery the laws
of segregation and independent
assortment
• In his experiments, he crossed two
varieties of pea plants to form
offspring, or hybrids.
• A monohybrid cross involves only one
trait (color or size, etc)
Monohybrid Crosses:
• In monohybrid crosses, a single trait is
examined.
• Using pea plants as an example, let P
represent the allele for purple flowers and p
represents the allele for white flowers.
• Suppose a plant heterozygous for purple
flowers (Pp) is crossed with a white flowered
plant (pp). The cross can be represented by
the genotypes Pp x pp
Punnett Square:
One parent along the
top, one parent
along the side.
p
p
P
Pp
Pp
P
Pp
Pp
Punnett Square
• In a Punnett square for a
monohybrid cross, the gametes for
one parent are represented in two
spaces a the top of the diagram.
• The gametes for the second parent
are represented along the left side
of the diagram.
• Combine the top and side
genotypes and place them in the
corresponding box.
Punnett Square
• Can you find
the frequencies
of the F2
generation for
the cross of Pp
x Pp?
• ¼ are:_______
• ½ are: ______
• ¼ are: ______
P
P
p
p
Understanding check point
• Yellow • Cross a homozygous yellow
pages? pea with a green pea
• Cross a heterozygous Tall pea
with a short pea
• Cross a Heterozygous Purple
flower with a Homozygous
Purple flower
• Cross a Homozygous Round
pea with a wrinkled pea
Test Cross
• Suppose you wanted to know the
genotype of a white- flowered pea
plant.
– That’s easy, since white is recessive, and
the only way to get white is with pp.
• Suppose you wanted to know the
genotype of a purple- flowered pea
plant. Is it PP or Pp?
– You must perform a Test Cross to
determine the unknown genotype.
Test Cross - p. 267
• To determine the unknown genotype,
you cross the unknown with a
genotype that is known. (pp).
– Always cross your unknown with double
recessive!
• This is represented as: P__ x pp.
– Since we do not know the other allele, we
use an underscore.
– To determine the unknown allele we use a
Punnett Square and do a PP x pp cross
and a Pp x pp cross.
Dihybrid Crosses
• In a dihybrid cross, genes for two
different traits are observed.
• For pea plants a dihybrid cross might
observe flower color (white/purple)
and plant height (short/tall)
• Another example of a dihybrid cross
might involve two characteristics of
seeds: color and texture.
Dihybrid Crosses
• If Y and y are used to represent the two
alleles yellow (Y) and green (y) and the
letters (R) for round and (r) for wrinkled
what would the cross look like?
• A cross between one pea plant
homozygous for both traits and second
plat homozygous recessive for both
traits would be:
– YYRR x yyrr
Dihybrid Cross w/ Punnett
Square
• Use the distributive property to distribute
the alleles and write the pairs along the
top and side of the Punnett Square:
– YYRR x yyrr
– YR, YR, YR, YR
– yr, yr, yr, yr
• Example ?
• Blue packet questions?
• Note the F2 generation phenotypic
ratio
•9:3:3:1
• Usual ratio with dihybrid hetero
cross
Problems - Sample
• Cross a green, heterozygous round pea
plant with a homozygous yellow and
heterozygous round pea plant.
– Determine parents genotype
– Determine the gamete combination
possibility
– Use punnett square or math to determine
the Genotypic RATIO and the Phenotypic
RATIO (GR / PR) of the offspring
What did you get?
• Parents Genotypes
– yyRr
– YYRr
• Determine gamete combinations
– yR and yr
– YR and Yr
• (½ yR + ½ yr) X (½ YR + Yr) OR
Punnett Square
Punnett Square method
YR
yR
YyRR
yr
YyRr
Yr
YyRr
Yyrr
Final Answer?
Genotypic Ratio
• ¼ YyRR
• ¼ YyRr
• ¼ YyRr
• ¼ Yyrr
Phenotypic Ratio
• ¾ Yellow and Round
• ¼ Yellow and wrinkled
Mode of Inheritance • Complete Dominance
• Monohybrid & Dihybrid Crosses
• Incomplete Dominance
• Codominance
• Multiple Alleles
• Epistasis
• Polygenic
– ABO Blood typing Activity!!!
Incomplete Dominance
• Sometimes, the alleles for a gene do
not exhibit the dominant and recessive
behaviors above. Instead, the
combined expression of two different
alleles in the heterozygous condition
produces a blending of the individual
expressions.
Incomplete Dominance
• In snapdragons, for example, the
heterozygous condition of one allele for
red flowers (R) and one allele for white
flowers (r) is pink (Rr).
• White board problem:
– Cross a white snapdragon with a pink
snapdragon. Determine the GR & PR for
offspring
Codominance
• Another kind of inheritance pattern is
codominance.
• In this pattern, both inherited alleles are
completely expressed.
• For example, the M and N blood types
produce two molecules that appear on the
surface of human red blood cells.
• The M (or LM) allele produces a certain blood
molecule.
• The N (or LN) allele produces a different kind
of molecule
Codominance
• Individuals who are MM (LMLM)
produce one kind of molecule.
• Individuals who are NN (LNLN) produce
a second kind of molecule.
• Individuals who are MN (LMLN) produce
BOTH kinds of molecule
(Codominance)
• White Board Problem
– Cross a MM with a MN
Multiple Alleles
• In the blood group that produces A,B, and O
blood types, there are three possible alleles,
represented by IA, IB, or i.
• Superscripts are used because the two
alleles, A and B are codominant.
• A lower case i is used for the 3rd allele
because it is recessive when expressed with IA
or IB .
• There are 6 possible genotypes representing
all 3 alleles.
Multiple Alleles
•
•
•
•
•
•
•
•
•
IAIA and IAi represent A blood type.
IBIB and IBi represent B blood type.
IAIB represents AB blood type.
ii represents O blood type.
The four phenotypes (A, B, AB, and O)
correspond to the presence or absence an A
or B carbohydrate glycoprotein on the surface
of an RBC.
IAIA/ IAi = A carbohydrate
IBIB/ IBi = B carbohydrate
IAIB = both carbohydrates
ii = no carbohydrates
ABO Blood Typing Activity
• White Board Problem:
– Cross an AB with a O blood type
• Using synthetic blood… you will type
4 patients’ blood samples…
• Finish some notes first before activity
• Complete today!
Epistasis
• Epistasis occurs when one gene
affects the phenotypic expression of a
second gene.
• This frequently occurs in the expression
of pigmentation.
• One gene turns on (or off) the
production of pigment, while a
second gene controls either the
amount of pigment produced or the
color of the pigment.
• Dog example page 273
Epistasis – mice example
• Epistasis occurs in the pigmentation of fur in
mice. One gene codes for the presence or
absence of pigmentation. A second gene
codes for the color (black or brown) of the
fur.
• Thus C and c represent the alleles for the
presence or absence of color.
• B and b represent the alleles for black or
brown fur.
• A mouse has to inherit at least one C to have
colored fur, while the B or b determines what
color.
• What fur type is observed with a genotype of
cc?
Pleiotropy
• Pleiotropy occurs when a single gene
has more than one phenotypic
expression.
• For example, the gene in pea plants
that expresses the round or wrinkled
texture of seeds also influences the
phenotypic expression of starch
metabolism and water absorption.
Pleiotropy
• The allele for round seeds codes for a greater
conversion of glucose to starch than does
the allele for wrinkled seeds.
• In wrinkled seeds, then, there is more
unconverted glucose.
• A higher concentration of glucose increases
osmotic gradient, which increases the
absorption of water.
• Immature wrinkled seeds thus contain more
water. When these seeds dehydrate, the
water is lost, which results in a wrinkled
pattern.
Pleiotropy
• Many disease-causing genes exhibit
pleiotropy.
• Sickle-cell anemia, a human blood disease,
is caused by an allele that incorrectly codes
for hemoglobin, valine instead of glutamic
acid… leads to rigid protein structure of hgb
• As a result, the abnormal hemoglobin
molecule causes the RBC to become sickle
shaped, especially in low Oxygen conc.
• In response, the RBC’s do not flow through
capillaries freely and oxygen is not
adequately delivered throughout the body.
Polygenic Inheritance –3/13/15
• Polygenic – more than one gene
effecting a single phenotype such as
skin color
• Additive effect of two or more genes
• AABBCC --- very dark skin color
• Note graphic on p274 (simplified
model of skin color inheritance)
p274
Need extra practice??
• More genetic problem practice?
• Check out Page 282 for extra
questions and review…
• Includes helpful suggestions as well.
Nature vs Nurture
• Effect of environment on
phenotype occurs as well
• Note fig 14.14 (p274) –
hydrangea flower’ color varies
due to pH of the soil
• What about skin color
variation due to environment?
Blood typing Activity NOW!
Pedigree 3/13/15
• Family Tree record that is used
to determine the genotypes
and phenotypes of related
organisms
• Essential to research of a
genetic condition
• P 276
Using Pedigrees
• Check out Pedigree A and
Pedigree B
• Work on worksheet together
Technology provides tools
• Genetic counselors in many hospitals can
provide information to prospective parents
concerned about a family history for a
genetic disorder
• Fetal testing can be done to determine the
presence of a disease on a fetus
• Chorionic Villus Sampling (CVS) provides a
small amount of fetal tissue to karyotype
quickly
• PKU testing determines the presence of
phenylketonuria
???????not done 2015????
• Genetic Disorders ?????…
• You will be doing a report of a
genetic disorder or condition of your
choice.
– Present/poster walk to class next Friday –
3/30/12
The following describes a few interesting
additional points regarding Genetic
Inheritance
Recessively inherited disorders
• Recessive alleles that cause human
disorders are usually defective
versions of normal alleles
• Defective alleles code for either a
malfunctional or no protein
• Heterozygotes can be phenotypically
normal, if one copy of the normal
allele is all that is needed to produce
sufficient quantities of specific protein
• (example – Sickle Cell Carrier)
?Recessively inherited disorders
• Recessively inherited disorders range in
severity from nonlethal traits (albinism) to
lethal disease (cystic fibrosis). Since these
disorders are caused by recessive alleles:
– The phenotypes are expressed ONLY in the
recessive homozygotes (aa) who inherit one
recessive allele from each parent
– Heterzygotes (Aa) can be phenotypically normal
and act as carriers possibly transmitting the
recessive allele to their offspring
• Most people with recessive disorders are
born to unaffected parents, both of whom
are carriers
?Recessively inherited disorders
• Human genetic disorders that are
recessively inherited include:
– Cystic Fibrosis
– Tay-Sachs
– Sickle-Cell Anemia
• The probability of inheriting a
recessively inherited disorder is greater
if the parents are closely related.
?Dominantly inherited disorders
• Some human disorders are dominantly
inherited
– For example, achondroplasia (a type of dwarfism)
affects 1 in 10,000 people who are heterozygous
for this gene
– Homozygous dominant condition results in
spontaneous abortion of the fetus
• Lethal dominant alleles are much rarer than
lethal recessives
• Late-acting lethal dominants can escape
elimination of the disorder does not appear
until an advanced age, after the afflicted
individuals may have transmitted the lethal
gene to their children. (Huntington’s disease)
?Genetic condition research
• POSTER ACTIVITY WILL BE DONE WHEN
MRS. SHAW RETURNS
• Poster presentation – Poster Walk
• Modeling college-level practice of
showing others what you know and
what you have researched
• Note handout for detail info
• You will evaluate 3 posters
Chapter 15
begin
3/20/15 …start
Figure 15.0 Chromosomes
Relating Mendelism to Chromosomes:
• Mendelian inheritance has its
physical basis in the behavior
of chromosomes during sexual
life cycles
• A convergence of Genetics
and Cytology (the study of
cells and cell history) in the
early 1900’s to describe
chromosomal behaviors
Relating Mendelism to Chromosomes:
• Based on these observations,
biologists developed the
CHROMOSOME THEORY OF
INHERITENCE. According to
this theory:
– Mendelian factors or genes are
located on chromosomes
– It is the chromosomes that
segregate and independently
assort
Figure 15.2 The chromosomal basis of Mendel’s laws p287
Linked Genes
• If two genes are on different
chromosomes, such as seed color and
seed texture of pea plants, the genes
segregate independently of one
another.
• Linked genes are genes that reside on
the SAME chromosome and thus
cannot segregate because they are
physically connected.
• Genes that are linked are usually
inherited together.
Linked Genes
• In the fruit fly, Drosophila
melanogaster, flies reared in the
laboratory have occasionally been
found to exhibit abnormal traits. These
traits originate from gene mutations, or
molecular changes in the DNA.
• Two such mutations, affecting body
color and wing structure, are linked.
Evidence for linked genes in Drosophila (p293)
Figure 15.5a Recombination due to crossing over (p295)
Linkage Map/Gene Distance
• The greater the distance between two
genes on a chromosome, the more
places between the genes than the
chromosome can break and thus the
more likely the two genes will cross over
during synapsis.
• As a result, recombination frequencies are
used to give a picture of the arrangement
of genes on a chromosome. (The % of
crossover is the recombination
frequency/gene distance).
Page 296
Morgan’s choice of an experimental
organism
• Wild-type: normal or most
frequently observed
phenotype
• Mutant Phenotypes:
phenotypes which are
alternatives to the wild type
due to mutations in the wildtype gene
Sex Determination
• Note that sex determination
varies with organisms (p290)
• Chromosomal Numbers vary
• SRY Gene in humans
– Required for development of
testes
– Found ONLY on Y chromosome
*p290
*Sex-Linked Inheritance
• There is one pair of homologous
chromosomes in animals that does not have
exactly the same genes.
• These two chromosomes, the X and Y
chromosomes are called the sex
chromosomes , or autosomes.
• Sex-linked genes (or X-linked) are genes that
reside on the X chromosome.
• Y-linked genes are possible, but few genes
reside on the Y chromosome, Y-linkage is rare
*p291
*Discovery of a sex linkage
• After a year of breeding
Drosophila to find variant
phenotypes, Morgan
discovered a single male fly
with white eyes instead of the
wild-type red.
• Morgan mated this mutant
white-eyed with a red-eyed
female.
*Figure 15.3 Morgan’s first mutant p288
*Sex-Linked
• There are additional considerations when
working with sex-linked genes because when
females (XX) inherit a sex-linked gene, they
receive two copies of the gene, one on each
X chromosome.
• In contrast, a male (XY) will inherit only one
copy of the gene because only the X
chromosome delivers the gene.
• Whichever allele is on the X chromosome
(dominant or recessive) is expressed.
INQUIRY Figure 15.4 p289
3/15
• With a partner review the problem
presented. Note the Genotypic and
Phenotypic ratio of F2 generation!
• Solve and record on white board the
answer to the following cross.
– White eyed female fruit fly crossed with a
wild eyed male… what will be their
Genotypic and Phenotypic ratio for their
F1generation?
Sex-Linked
• Hemophilia is caused by a sex-linked
recessive gene (h) in humans.
• Hemophiliacs cannot properly form blood
clots and in the worst cases can die from
minor injuries by bleeding to death.
• Females and males who inherit the normal
allele (H) and XHXH and XHY.
• In order for a female to be affected she must
have 2 copies of the defective allele: XhXh.
• A male, however need only inherit one copy
of the affected allele XhY
• Therefore, sex-linked traits are much more
common in males.
• Females who carry the XHXh are carriers.
Sex Chromosomes
• Problem
A woman whose father was a
hemophiliac marries a man
without hemophilia. Determine
the Genotypic and Phenotypic
ratio for their possible offspring.
• Chi-Square…Were you able to finish all
crosses???
– Practice problem…
• Df = ?
• Did you calculate the chi-square?
• Volunteer to calculate on board!
• Solve following problem (board)
– A white-eyed female Drosophila is mated
with a red-eyed (wild type) male. What are
the genotypic and phenotypic ratio of
their offspring?
X-Inactivation (p292)
• During embryonic development in female
mammals, one of the two X chromosomes in
each does not uncoil into chromatin.
• Instead, X-inactivation occurs, and one
chromosome remains coiled, as a dark
compact body, called a Barr body.
• Barr bodies are mostly inactive X
chromosomes, and the alleles on them do
not get expressed.
• Thus, only the genes on one active X
chromosome are expressed by the cell.
X -Inactivation
• Calico cats have yellow, black, and
white hair, randomly arranged in
patches over their bodies.
• The yellow and black colors are coded
by a gene on the X chromosome.
(white is controlled by a different gene)
• When the gene for yellow is
inactivated, black patches of fur
appear. The same for yellow fur.
X- Inactivation
• Subsequent daughter cells will have
the same X chromosome inactivated
as did the embryonic parent cell from
which they originated.
• The inactivated X chromosome is
reactivated during meiosis.
Nondisjunction p297
• In the normal process of meiosis,
chromosomes pair at the metaphase
plate and subsequently separate
and migrate to opposite poles.
• When nondisjunction occurs, the
chromosome pair migrate to the
same pole (do not separate).
Nondisjunction
Video Clip - Non-disjunction
3/25
15_13Nondisjunction.mpg
Nondisjunction
• As a result, half the gametes will have
an extra chromosome, and the other
half will be missing a chromosome.
Nondisjunction
• Down syndrome occurs when an egg
or sperm with an extra number 21
chromosome fuses with a normal
gamete. (3 - #21 chromosome)
• The result is trisomy of chromosome 21.
• Down syndrome results in mental
retardation, heart defects, respiratory
problems, and abnormalities of external
features.
Nondisjunction
• Turner syndrome results when there is
nondisjunction of the sex chromosomes.
• Sperm or an egg will have no chromosome
(O)
• Zygote will result with an XO
• Turner syndrome zygotes (X0) is female with
no second sex chromosome.
• Turner syndrome individuals are physically
abnormal and sterile.
Errors and Exceptions in Chromosomal
Inheritance
• Aneuploidy: A condition of having an
abnormal number of certain
chromosome
– An aneuploid cell that has a chromosome
triplet is said to be trisomic
– An aneuploid with a missing chromosome
is said to be monosomic
– Abnormal gene dosage in aneuploids
causes characteristic symptoms in
survivors. Trisomy of chromosome 21 is…?
Errors and Exceptions in Chromosomal
Inheritance
• Polyploidy: A chromosome number
that is more than two complete
chromosome sets
– Triploidy: a polyploid chromosome
number of 3N
– Tetraploidy: a polyploid chromosome
number of 4N
Alterations of chromosome structure
• Chromosome breakage can alter
chromosome structure in four ways:
– Deletion (fragment lacking)
– Duplication (fragment joined to another
homologue)
– Translocation (fragments joins to
nonhomologue)
– Inversion (fragment attaches in reverse
order)
Figure 15.x1 Translocation
Karyotype Changes
• Human disorders due to
chromosomal alterations
– Down Syndrome: affects 1 out of 700
children in the U.S.
– Trisomy of chromosome 21
– Phenotypic facial features, short
stature, heart defects, mental
retardation, respiratory infections,
proneness to leukemia and
alzheimers
Figure 15.14 Down syndrome p273
Karyotype Changes
• Klinefelter’s Syndrome: Usually an XXY
male (can be multiple X’s) affected
individual may have female phenotypes
• Triple X Syndrome: Usually XXX, fertile and
can show normal phenotype
• Turner’s Syndrome: Genotype of XO
(human monosomy) short stature, some
sexual characteristic fail to develop,
internal sex organs do not develop
• Cri du chat Syndrome: caused by a
deletion on chromosome 5, symptoms
include mental retardation, small head,
with unusual facial features and a cry
that sounds like a mewing cat
Figure 15.x2 Klinefelter syndrome
Figure 15.x3 XYY karyotype
Translocation
(ex: chronic Myelongenous leukemia)
Translocation
• Chronic Myelongenous Leukemia: portion
of chromosome 22 switches places with
chromosome 9, symptoms include those
seen in other types of leukemia
• Fragile X Syndrome: most common genetic
cause of mental retardation. It is an
abnormal X chromosome, the tip of the X
chromosome hangs on the rest of the
chromosome by a thin DNA strand
• Prader-Wili Syndrome: caused by a deletion
at chromosome 15, characterized by
mental retardation, obesity, short stature,
small hands and feet, uncontrollable
laughter and jerky movement, often
associated with uncontrollable desire to eat,
never feeling satiated
Some inheritance patterns are exceptions to
standard Mendelian inheritance
• There are two normal exceptions to
Mendelian genetics:
• One exception involves genes located
in the nucleus, and the other exception
involves genes located outside the
nucleus
• In both cases, the sex of the parent
contributing an allele is a factor in the
pattern of inheritance
Genomic Imprinting (p301)
• For a few mammalian traits, the
phenotype depends on which parent
passed along the alleles for those traits
• Such variation in phenotype is called
genomic imprinting
• Genomic imprinting involves the
silencing of certain genes that are
“stamped” with an imprint during
gamete production
• It appears that imprinting is the result of
the methylation (addition of —CH3) of
cysteine nucleotides
• Genomic imprinting is thought to affect
only a small fraction of mammalian
genes
• Most imprinted genes are critical for
embryonic development
© 2011 Pearson Education, Inc.
Additional situations
• Methylation --- (CH3)
• Adding methyl groups to certain
nucleotides at a specific loci on
chromosomes
• This causes the gene to be inactivated
(silenced)
• At this point the animal probably uses
the allele that is not imprinted
Page 302 (mutations in plastids – inherited from maternal parent)
Figure 15.UN04
Extranuclear genes
• Small circles of DNA found in
mitochondria & plastids (chloroplasts)
• They reproduce themselves
• Passed to daughter organelles
• Mitochondrial DNA
– Mother’s family genes ONLY
(Mitochondrial DNA originates in the OVUM)
Used in many geneology work -”Journey of
Man”
• Some defects in mitochondrial genes
prevent cells from making enough ATP
and result in diseases that affect the
muscular and nervous systems
– For example, mitochondrial myopathy
and Leber’s hereditary optic neuropathy
© 2011 Pearson Education, Inc.
Technology provides tools
• Genetic counselors in many hospitals can
provide information to prospective parents
concerned about a family history for a
genetic disorder
• Fetal testing can be done to determine the
presence of a disease on a fetus
• Chorionic Villus Sampling (CVS) provides a
small amount of fetal tissue to karyotype
quickly
• PKU testing determines the presence of
phenylketonuria
Research Project
• Genetic Disorders
• You will be doing a report of a
genetic disorder or condition of your
choice.
– Poster Walk will occur Friday, 3/17/15
The following describes a few interesting
additional points regarding Genetic
Inheritance
Human Genetic Defects
• Genetic defects can be caused by the
inheritance of an allele, or it can be
caused by chromosomal abnormalities.
–
–
–
–
A missing chromosome (nondisjunction)
An extra chromosome (nondisjunction)
Chromosome portions deleted (deletion)
Chromosome portions duplicated
(duplication)
– Chromosome portion moved to another
chromosome (translocation)
– Chromosome portions rearranged in reverse
orientation (inversion)
Recessively inherited disorders
• Recessive alleles that cause human
disorders are usually defective
versions of normal alleles
• Defective alleles code for either a
malfunctional or no protein
• Heterozygotes can be phenotypically
normal, if one copy of the normal
allele is all that is needed to produce
sufficient quantities of specific protein
• (example – Sickle Cell Carrier)
Recessively inherited disorders
• Recessively inherited disorders range in
severity from nonlethal traits (albinism) to
lethal disease (cystic fibrosis). Since these
disorders are caused by recessive alleles:
– The phenotypes are expressed ONLY in the
recessive homozygotes (aa) who inherit one
recessive allele from each parent
– Heterzygotes (Aa) can be phenotypically normal
and act as carriers possibly transmitting the
recessive allele to their offspring
• Most people with recessive disorders are
born to unaffected parents, both of whom
are carriers
Recessively inherited disorders
• Human genetic disorders that are
recessively inherited include:
– Cystic Fibrosis
– Tay-Sachs
– Sickle-Cell Anemia
• The probability of inheriting a
recessively inherited disorder is greater
if the parents are closely related.
Dominantly inherited disorders
• Some human disorders are dominantly
inherited
– For example, achondroplasia (a type of dwarfism)
affects 1 in 10,000 people who are heterozygous
for this gene
– Homozygous dominant condition results in
spontaneous abortion of the fetus
• Lethal dominant alleles are much rarer than
lethal recessives
• Late-acting lethal dominants can escape
elimination of the disorder does not appear
until an advanced age, after the afflicted
individuals may have transmitted the lethal
gene to their children. (Huntington’s disease)
Human Genetic Defects:
• The AP Exam expects you to know
the more common genetic defects
in humans and their underlying
causes.
• Research Project… begin today!
• Poster presentation due Friday!
• Take Home test for Genetics Unit &
Begin Evolution Unit on Monday!
Genetic condition research
• Poster presentation – Poster Walk
• Modeling college-level practice of
showing others what you know and
what you have researched
• Note handout for detail info
• You will evaluate at least 2 other
posters