Download File ap notes chapter 15

Chapter 15: The chromosomal
basis of inheritance
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Chromosome Theory of inheritance
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Genes have specific loci on chromosomes
Chromosomes undergo segregation &
independent assortment
Thomas Hunt Morgan
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Worked with Drosophila melanogaster (fruit
fly)
4 pairs of chromosomes; 3 autosomes; 1 pair of
sex chromosomes
 Notation w+ (wild type) w- (mutant type)
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Drosophila melanogaster
Variation in fruit fly
red eye (wild type) vs. white eye
(mutant)
Sex-linked traits
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Genes located on the sex chromosomes
Males more likely to inherit sex linked
traits; only one mutant allele required for
trait (XY vs. XX female)
Provides evidence for specific genes being
located on specific chromosomes
Linked genes & genetic
recombination
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Genes on the same chromosome at close
loci tend to be inherited together
Recombinants= offspring which exhibit
non-parental phenotypes
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When 50% frequency of recombinants exist
genes are located on different chromosomes
When recombinant frequency is less than
50% genes are located on the same
chromosome; recombinants result from
crossing over; amount of recombinants is
related to the distance between the two
gene’s loci
Linkage mapping
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Genetic map
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Ordered list of gene loci along a chromosome
Higher frequency of recombinants=bigger distance
between 2 loci
Provide positional information of loci only in relation
to other loci
Linkage map
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Cytogenetic maps
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Genetic map based on recombination frequencies
Locate genes on a chromosome based on
chromosome features such as banding
Ultimate maps
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Show physical distance between loci in DNA
nucleotides
Concept Check
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Genes A, B, & C are located on the same
chromosome. Testcrosses show that the
recombination frequency between A & B is
28% and between a & C is 12%. What
possible linear orders exist between these
genes?
Sex-linked
Genes
XA = normal gene
Xa = mutated gene
Human sex-linked disorders
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Duchenne Muscular Dystrophy
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Hemophilia
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Defective dystrophin protein
Leads to weakened muscles & loss of
coordination
Absence of one or more proteins required for
clotting in blood
Prolonged bleeding
Color-blindness
X inactivation in Female mammals
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One chromosome from a pair of
chromosomes becomes inactivated during
embryonic development
Inactive chromosome condenses into Barr
body
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Lies on inner edge of nuclear envelope
Genes on Barr body are typically not
expressed
Barr body formation is independent event in
each cell
Methyl groups attached to DNA to cause
inactivation
Concept Check
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A white eyed female Drosophila is mated
with a red eyed male. What phenotypes &
genotypes would be likely in the offspring?
Neither Tom nor Rhoda has Duchenne
Muscular Dystrophy but their first born
son does. What is the probability that their
second child will have the disease?
Disorders caused by chromosome
alteration
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Non-disjunction
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Pairs of homologues do not separate during
meiosis
Aneuploidy-abnormal number of a particular
chromosome
Trisomy
 monosomy
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Polyploidy- two complete chromosome sets
Common in plants
 Can occur in animals though rare
 Likely to look normal in appearance
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Disorders caused by chromosome
alteration
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Alterations in chromosome structure
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Deletion
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Duplication
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Deleted fragment attaches as an extra segment to
a homologous chromosome
Inversion
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Chromosome fragment is lost; certain genes are
missing
Deleted fragment re-attaches to orginal
chromosome but in reverse orientation
Translocation
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Deleted fragment attaches to a non-homologous
chromosome
Human disorders due to
chromosomal alteration
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Down Syndrome (Trisomy 21)
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Klinefelter syndrome
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XO; sterile female
Cri-du-chat syndrome
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XXY; sterile male with some female characteristics
(i.e. breasts)
Turner syndrome
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Non-disjunction in meiosis leads to mental
retardation, characteristic features
Deletion on chromosome 5 leads to mental
retardation, characteristic features & cry sounding like
a cat
Chronic myelogenous leukemia
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Translocation between chromosomes 22 & 9
Down Sydrome
Klinefelter Syndrome
Cri-du-chat
Concept Check
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How could a male can have the
tortoiseshell phenotype?
Genomic imprinting
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Effect of a gene may be dependant on
whether gene was inherited from mother
or father
Methyl group is added to one of the alleles
in the gamete to cause inactivation (or in
a few cases activation)
Zygote expresses only one allele of an
imprinted gene; this same allele is the
active gene in all resulting cells
Inheritance of organelle genes
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Some genes located outside the nucleus
(extranuclear genes)
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Mitochondria
chloroplasts
Organelles located in zygote originate in
egg (egg has more cytosol); all
extranuclear genes originate from the
mother