CH. 12
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1900s: Thomas Morgan: began breeding
experiments with Drosophila (Fruit fly)
Have 4 pairs of homologous chromosomes
 One chromosome pair was different in males than in
females
 Females: 2 chromosomes were identical
 Males: one was same as both females, and one was
shorter and hook shaped
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 Chromosome that was same in both male and females: X
chromosome
 Shorter-hook shaped chromosome: Y chromosome
 X and Y chromosomes = sex chromosomes
 All other chromosomes = autosomes
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Sex chromosomes form pairs & segregate into
separate cells during meiosis I gametes will
have either X or Y
In mammals & most insects, males = XY,
females = XX
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Gametes produced by females only have X
Gametes produced by males can have either X or Y
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Morgan hypothesized that more genes are
carried by the X chromosome than by the
smaller Y chromosome
Genes found on X chromosome = X-linked
genes
Genes found on Y chromosome = Y-linked
genes
Presence of a gene on a sex chromosome = sex
linkage
R-eyed M, R-eyed F
W-eyed M, R-Eyed F
F1 XrY X XRXR
Xr
XR
XR
XRXr
XRXr
F2
Y
XRY
XRY
XR
Xr
XRY X XRXr
XR
Y
XRXR
XRY
XRXr
XrY
-No white-eyed females gene for eye color is carried on the X chromosome
-Eye color in Drosophila is an X-linked trait
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There are thousands more genes than
chromosomes each chromosome carries many
genes
Genes located on one chromosome = linkage
groups
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Linked-genes tend to be inherited together
Morgan demonstrated that existence of linkage groups
with Drosophila body color and wing length do not sort
independent of each other BUT crossing-over can allow
for different variations that would not typically happen
crossing-over does not create new genes or delete old
ones, instead it changes the locations of genes among the
chromosomes that carry them, producing new gene
combinations
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The likelihood that crossing-over will result in
separation of two genes depends on genes’
distance from each other on chromosome
Farther apart two genes are more likely
separated by crossovers
Chromosome map: diagram that shows linear
sequence of genes on chromosome
Can make this based on following
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Percentage of crossing-over btwn the genes for two traits
is proportional to distance btwn them on chromosome
 Example: two genes that are separated by crossing-over 1%
of time are one map unit apart.
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Change in DNA of organism is Mutation
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Can involve whole chromosome or single DNA
nucleotide & can take place in any cell
Germ-cell mutations: occur in germ cells (gametes);
doesn’t affect organism but can be passed to
offspring
Somatic mutations: take place in organism’s body
cells & can affect organism not passed to offspring
 Ex. Certain skin cancer, some types of leukemia
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Lethal mutations: cause death, usually before birth
Some mutations offer advantage to organisms
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Deletion: loss of piece of
chromosome due to breakage (info
carried by missing piece is lost)
Inversion: chromosomal segment
breaks off and then reattaches in
reverse orientation to same
chromosome.
Translocation: chromosome piece
breaks off and reattaches to another,
non-homologous chromosome
Nondisjunction: failure of
chromosome to separate from its
homologue during meiosis
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One gamete receives extra copy of
chromosome and other gametes lacks
chromosome entirely
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May involve large segments of DNA or single
nucleotide within codon
Point mutation: substitution, addition, removal of
single nucleotide
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Substitution: one nucleotide in codon is replaced with different
nucleotide, resulting in new codon
 If new codon codes for same AA, no problem
 If new codon codes for different AA, resulting protein will be
affected
Insertion: one or more nucleotide added to gene
Deletion: one or more nucleotide deleted from gene
 Frame-shift mutation: insertion or deletion of single
nucleotide causes remaining codon to be incorrectly grouped
Substitution Mutation
Deletion Mutation
Insertion Mutation
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How does the inheritance of sex chromosomes result in
approximately equal number of males and females among
the offspring of fruit flies?
Offer an explanation for why organ did not find white-eyed
female Drosophila in the F2 generation when he crossed
white-eyed males with red-eyed females.
How does crossing-over show that genes are found on
chromosomes?
How can crossing-over btwn two alleles be used to map
their locations on chromosomes?
What are point mutations?
Biologists have observed that chromosome mutations often
occur during nuclear division. Why do you think this is so?
Explain your answer.
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The male sex chromosomes are X and Y. One-half of males gametes will
contain a Y chromosomes and one-half will contain an X chromosome;
the number of male offspring and the number of female offspring are
approximately equal as a result.
The mutant allele for white eyes is X-linked, so a female Drosophila must
have two copies of the mutant allele in order to be white-eyed. A male
who has only one mutant allele for white eyes will be white-eyed because
he has only one X chromosome.
Traits known to be controlled by genes located on the same chromosome
are normally inherited together, but they can be inherited individually if
crossing-over has separated them
The farther apart two alleles are on a chromosome, the more often they
will be separated by crossing-over and thus the more often they will
appear in new combinations in the offspring
A point mutation is the substitution, addition, or removal of a single
nucleotide within a segment of DNA
Just before nuclear division, chromatin condenses to form chromosomes,
which move throughout the cell during the different stages of nuclear
division and thus are at higher risk of breakage
HUMAN GENETICS
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Patterns of inheritance are significantly more
complicated to study among humans than
among Drosophila
Humans have 20X as many genes & our 23
pairs of chromosomes are made up of ~100,000
genes
Geneticists often study disease-causing genes
because they are passed from one generation to
the next
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Biologists discover how traits are inherited by
studying phenotypes among members of same
species from generation to next
Often study members of same family
Pedigree: family record that shows how trait is
inherited over several generations
CARRIERS:
individuals who have
one copy of a
recessive autosomal
allele
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Genetic disorders: diseases or debilitating conditions that have a
genetic basis
Single Allele Dominant
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Single Allele Recessive
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Colorblindness
Hemophila
Muscular Dystrophy
Polygenic
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Albinism
Cystic fibrosis
PKU
Hereditary Deafness
X-Linked
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HD
Polydactyly (extra fingers or toes)
Dwarfism
Foot size
Height
Skin, hair & eye color
Multiple Alleles
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ABO blood groups
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Single-celled Traits: controlled by single allele of
gene
More than 200 human traits governed by single dominant
allele
 Ex. Huntington’s Disease
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Genetic Marker: short section of DNA that is
known to have a close association with particular
gene located nearby
Geneticists can inform person of presence of
marker before he or she becomes parent
250 single-allele traits are controlled by
homozygous recessive alleles
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Cystic fibrosis & sickle cell anemia
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Controlled by 3 or more alleles of same gene that
code for single trait
ABO blood groups
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IA, IB, i
Each individual’s blood group consists of 2 of the 3
IA & IB are dominant to i
IA & IB are codominant (both expressed together)
Genotype
Blood Type
I AI A
A
I Ai
A
IBIB
B
IBi
B
I AI B
AB
ii
O
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Trait controlled by two or more genes:
Polygenic trait
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Ex. Skin color is caused by 3-6 genes
 Each gene results in certain amount of a
brownish black pigment called melanin
more melanin = darker skin
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Ex. Eye color
 Light blue eyes = little melanin
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Show many degrees of variation
Expression of many traits, those
governed by many genes, is influenced
by environment
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Ex. Height is influenced by unknown
number of genes
 But also influenced by nutrition & disease
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Genes found only on X chromosome
Ex. Colorblindness
Many variations exist, but most common in inability
to distinguish red from green
 ~8% males are colorblind
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Ex. Hemophilia
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Impairs ability of blood to clot following a cut,
bruise or injury
Duchenne muscular dystrophy
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Weakens and progressively destroys muscle tissue
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Presence of male or female sex hormones
influences expression of certain human traits
called sex-influenced traits
Males and females have different phenotypes,
even when they share same genotype
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Ex. Pattern baldness different levels of hormones
in men and women will cause different phenotypes
even when they have the same genotype
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Nondisjunction occurs during meiosis & can cause
gametes to lack a chromosome or have an extra
chromosome
If nondisjunction occurs during egg formation, one
egg will have 22 chromosomes instead of normal
23 another egg will have 24
If one of these eggs combines with sperm, resulting
zygote will have 45 or 47 chromosomes
 45 chromosomes = monosomy
 47 chromosomes = trisomy
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 Ex. Trisomy 21 = Down Syndrome
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Abnormalities are often lethal
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Males with extra X will have Klinefelter’s
syndrome (XXY)  these persons have
feminine characteristics, sometimes mentally
retarded and infertile
Individuals with single X chromosome will
have Turner’s syndrome (XO)
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Persons have female appearance, but do not mature
sexually and remain infertile
Zygotes that receive only a single Y chromosome
will not survive because X chromosome contains
info essential for development
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Genetic screening: examination of person’s
genetic makeup
May involve constructing karyotype
Other techniques test individual’s blood for
presence or absence of certain proteins
Genetic counseling: form of medical guidance
that informs parents about problems that could
affect offspring
Physicians can diagnose more than 200 genetic
disorders in fetus using variety of tools
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Two forms prenatal (before birth) testing that can reveal
fetal abnormalities
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Amniocentesis: doctor removes small amount of amniotic fluid
from amnion, sac that surrounds fetus, between 14-16 weeks of
pregnancy
 Fetal cells and proteins from fluid can be analyzed and karyotype
prepared
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Chorionic villi sampling: doctor obtains sample of chorionic villi,
tissue that grows btwn mother’s uterus and placenta, btwn 8-10
weeks
 Tissue can be used to produce karyotype
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Some genetic screening is performed immediately after
birth
1 out of 10,000 babies is afflicted with phenylktonuria (PKU)
body cannot metabolize AA phenylalanine accumulation can
cause severe brain damage
 PKU can be detected by blood test administered during first few
days of life
 Dangers can be eliminated by placing these infants on special diet
lacking phenylalanine
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AMNIOCENTESIS
CHORIONIC VILLI SAMPLING
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A husband and wife are heterozygous for cystic
fibrosis. Their son has cystic fibrosis. Their second
child, a daughter, does not. Prepare a pedigree for this
family
A husband and wife have the ABO blood group
genotypes IAIB and ii. What ABO blood types can their
children have?
Compare the inheritance of Huntington’s disease with
the inheritance of sickle cell anemia
Is pattern baldness a sex-linked trait or a sexinfluenced trait? Explain your answer
How can nondisjunction change chromosome number?
Colorblindness is rare among females. Why? Explain
your answer.
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Parents have normal dominant allele and the recessive CF allele
and thus are carriers for CF. Their son is homozygous recessive;
their daughter has at least one copy of the normal dominant allele.
Their children could have the ABO blood types A or B
Huntington’s disease (HD) is inherited as a dominant autosomal
allele. Anyone who receives the allele for HD will have the
disease. Sickle cell anemia is inherited as a recessive autosomal
allele. A person heterozygous for sickle cell anemia is a carrier of
the disease.
Pattern baldness is a sex-influenced trait because it is controlled
by an autosomic allele, but it is expressed differently in males and
females
Nondisjunction can produce gametes with either one too few or
one too many chromosomes. If such a gamete is fertilized, a
monosomic or trisomic individual can result.
Because colorblindness is an X-linked recessive trait, a female
would have to receive two copies of the allele for colorblindness,
one from each parent, in order to exhibit colorblindness