Chapter 15 - The Chromosomal Basis of Inheritance

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3.a.4 – The inheritance pattern of many traits
cannot be explained by simple Mendelian
genetics (15.1, 15.2, 15.3, 15.5).
3.c.1 – Changes in genotype can result in
changes in phenotype (15.4).
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Developed the “Chromosome
Theory of Inheritance”
1) Mendelian factors or alleles are
located on chromosomes
2) Chromosomes segregate and
show independent assortment
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Embryologist at Columbia University
Chose to use fruit flies as a test organism in
genetics
Allowed the first tracing of traits to specific
chromosomes
 Drosophila melanogaster
 Feeds on fungus growing on fruit
 Early test organism for genetic studies
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Small
Cheap to house and feed
Short generation time
 New generation every 2 weeks
100s of offspring produced
 Few chromosomes
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 4 pairs (8 total)
 3 pairs autosomes, 1 pair sex
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Mendel: use of uppercase or lowercase
letters
 T = tall
 t = short
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Morgan: symbol from the mutant
phenotype
 + = wild phenotype (natural pheno)
 No symbol = mutant phenotype (any pheno
different from wild)
Recessive mutation:
 w = white eyes
 w+ = red eyes
 Dominant mutation:
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 Cy = Curly wings
 Cy+ = Normal wings
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Letters come from 1st
mutant trait observed
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A male fly with a mutation for white eyes
Then, he crossed the white eye male with
normal red eye female
All had red eyes
 Same as Mendel’s F1
This suggests that white eyes
is a genetic recessive
Morgan expected the F2 to have a 3:1 ratio of
red:white
 He got this ratio
 However, all of the white eyed flies were
MALE
 Most red eyed flies were FEMALE
 Therefore, the eye color trait appeared to be
linked to sex
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Sex-linked traits
Genetic traits whose expression are
dependent on the sex of the individual
Genes on sex chromosomes exhibited unique
patterns
Eye color gene
located on X
chromo (with no
corresponding gene
on Y)
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There are many genes, but only a few
chromosomes
Therefore, each chromosome must carry a
number of genes together as a “package”
 There was a correlation between a
particular trait and an individual’s sex
Traits that are located on the same
chromosome (that tend to be inherited
together)
Result:
Failure of (deviation from) Mendel's
Law of Independent Assortment.
Ratios mimic monohybrid crosses.
Body Color
and Wing type
Wild: gray and normal (dom +)
 Mutant: Black and vestigal (rec)
This is why we use “b” for body color
alleles and “vg” for wing alleles
 Symbols:
 Body color - b+: gray; b: black
 Wings - vg+: normal; vg: vestigal
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b+b vg+vg X bb vgvg
#1: b+b = gray; vg+vg = normal
#2: bb = black; vgvg = vestigal
(b+ linked to vg+)
(b linked to vg)
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If unlinked: 1:1:1:1 ratio.
If linked: ratio will be altered
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Occurs during Pro I of
meiosis
Breaks up linkages and
creates new ones
Recombinant offspring
formed that doesn't
match the parental
types
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Independent Assortment of traits fails
Linkage may be “strong” or “weak”
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Degree of strength related to how close the
traits are on the chromosome
 Weak - farther apart
 Strong - closer together
 Usually located closer to centromere
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Constructed from crossing-over
frequencies
1 map unit = 1% recombination frequency
Have been constructed for many traits in
fruit flies, humans and other organism.
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Several systems are known:
Mammals – XX and XY
Diploid insects – X and XX
3. Birds – ZZ and ZW
4. Haploid-Diploid
1.
2.
1.
Mammals
 Determined by whether sperm has X or Y
2.
Diploid insects
 Only X chromosomes present
3.
Birds
 Egg determines sex
4.
Haploid-Diploid
 Females develop from fert egg
 Males develop from unfert egg
Sex determination ALWAYS 50-50
X chromosome - medium sized
chromosome with a large number of
traits
 Y chromosome - much smaller
chromosome with only a few traits
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Eggs – only contain X
Sperm – either X or Y
 Males - XY
Females - XX
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Comment - The X and Y chromosomes
are a homologous pair, but only for a
small region at one tip
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Inheritance of traits on the sex
chromosomes
 NOT TO BE CONFUSED WITH sex-linked traits!!!!!
X Linkage - common; Y- rare
Dads: only to daughters (b/c dads ONLY
give X chromo to daughters)
 Moms: to either sex
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Hemizygous - 1 copy of X chromosome
Show ALL X traits (dominant or recessive)
More likely to show X recessive gene
problems than females
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Disorders on X-chromo:
 Color blindness
 Duchenne's Muscular Dystrophy
 Hemophilia (types a and b)
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Patterns
 Trait is usually passed from a carrier mother to 1
of 2 sons
 Affected father has no affected sons, but passes
the trait on to all daughters (who will be carriers
for the trait)
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Watch how questions with sex linkage are
phrased:
 Chance of children?
 Chance of males?
 Chance of females?
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You MUST practice genetics problems
w/ these traits: Hemophilia, Muscular
dystrophy and colorblindness (they all
work the same!)
 Yes!!!
 ONLY if their mother was a carrier and
their father is affected
 How? Mother contributes X (with
affected allele) and dad contributes all
he can to make a daughter – affected X
25
45
6
29
56
8
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Hairy ear pinnae
Comment - new techniques have found a
number of Y-linked factors that can be
shown to run in the males of a family
Ex: Jewish priests
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Traits that are only expressed in one sex
Ex: prostate development, gonad
specialization, fallopian tube development
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Traits whose expression differs because of
the hormones of the sex
Ex: beards, mammary gland development,
baldness
Baldness:
 Testosterone – makes the trait act as a dominant
 No testosterone – makes the trait act as a
recessive
 Males – have gene = bald
 Females – must be homozygous to have thin hair
(rare)
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In every somatic cell (in females), one X
chromosome is inactivated
 Humans: differs/random
 Kangaroos: always paternal X that is
inactivated
 Called Barr bodies
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Inactive X chromosome observed in the
nucleus
▪ Becomes inactive during embryonic
development
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Way of determining genetic sex
(without doing a karyotype)
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Compact body which lies close to nuclear
envelope
Most genes on this X are NOT expressed
Inside developed ovaries, these are
reactivated (so that each ova will get an
active X)
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Which X inactivated is random
Inactivation happens early in embryo
development by adding CH3 groups to
the DNA
 Changes DNA nucleotide
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Result - body cells are a mosaic/combo
of X types
 Some have active X from mom, others active X
from dad
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Calico Cats
Human examples are known (sweat gland
disorder)
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Why don’t you find many calico
males?
 They must be XB XOY and are always
sterile
 Why?
They MUST have an extra X chromo
(to have an inactive X - you must
have TWO!)
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Two types of alterations:
 Changes in number
 Changes in structure
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Aneuploidy - too many or too few
chromosomes, but not a whole “set” change
Polyploidy - changes in whole “sets” of
chromosomes
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Caused by nondisjunction
 the failure of a pair of chromosomes to separate
during meiosis
 Result: too many or too few chromosomes in a
gamete
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Nondisjunction in Meiosis I produces 4
abnormal gametes.
Nondisjunction in Meiosis II produces 2
normal and 2 abnormal gametes.
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Monosomy: 2N – 1 (very rare)
 Mono = one (missing copy)
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Trisomy: 2N + 1 (more common)
 Tri = three (extra copy)
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Normal: 2N
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Monosomy
2N - 1 or 45 chromosomes
 Genotype: X_ or X0
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Phenotype: female, but very poor secondary
sexual development.
Characteristics:
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Short stature.
Extra skin on neck.
Broad chest.
Usually sterile
Normal mental development except for some spatial
problems.
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Why are Turner Individuals usually sterile?
 Odd chromosome number
 Two X chromosomes needed for ovary
development.
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Kleinfelter Syndrome
Meta female
Supermale
Trisomy
2N + 1 (2N + 2, 2N + 3)
Genotype: XXY (XXXY, XXXXY)
Phenotype: male, sexual development
may be poor/slow
 Often taller than average, mental
development fine (in XXY), usually
sterile
 More X = more mental problems
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May have been a Kleinfelter Syndrome
individual.
Much taller than average
Produced no children/sterile individual
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Trisomy
2N + 1 or 2N + 2
Genotype: XXX or XXXX
Phenotype: female, but sexual development
poor.
 Mental impairment common.
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Trisomy
2N + 1 or 2N + 2
Genotype: XYY or XYYY
Phenotype: male, usually normal
development, fertile w/ normal sex organ
development
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Trisomy 21: Down Syndrome
Trisomy 13: Patau Syndrome
Both have various physical and mental
changes
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Increases with maternal age (especially above
35)
 How? An embryo’s ovaries are halted in meiosis I
(during egg development)
 When ovulation occurs, the eggs resume meiosis
and nondisjunction occurs then
 This is why it is often seen more in older women
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Mental retardation
Heart defects
Characteristic facial features
 Why is trisomy more common than
monosomy?
 Fetus can survive an extra copy of a chromosome,
but being hemizygous for somatic cell is usually
fatal
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Why is trisomy 21 more common in
older mothers?
 Maternal age increases risk of nondisjunction
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Triploid= 3N
Tetraploid= 4N
Usually fatal in animals
Cells receive AN ENTIRE EXTRA COPY of all
homologous chromosomes (including sex
chromo)
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In plants, even # polyploids are often
fertile, while odd # polyploids are sterile.
Why?
 Odd number of chromosomes can’t
be split during meiosis to make
spores.
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Deletions: loss of genetic info
Duplications: extra copies of genetic info
Inversions and translocations:
 Position effects: a gene's expression is
influenced by its location to other genes
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Part of p arm of #5 missing
 Deletion chromosomal abnormality
Good survival rate
Severe mental retardation
Small sized heads common
Malformed larynx w/ vocal/speech problems
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Part of X chromo is missing
 Deletion
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Sterile
Mental retardation
Oversized testes (if male); ovaries (if
female)
“Double jointedness”
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Caused by translocation
An abnormal chromosome produced by an
exchange of portions of chromosomes 9 and
22
Causes chronic myeloid leukemia
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Gene expression and inheritance depends on
which parent passed on the gene
Usually caused by different methylations of
the DNA
CAUSE:
 Imprints are "erased" in gamete producing cells
and re-coded by the body according to its sex
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RESULT:
 Phenotypes don't follow Mendelian Inheritance
patterns because the sex of the parent does
matter
Prader-Willi Syndrome and
Angelman Syndrome
Both lack a small gene region from
chromosome 15
Male gene contribution missing: Prader-Willi
Female: Angelman
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Method that cells might use to detect that
TWO different sets of chromosomes are in
the zygote
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Inheritance of genes not located on the
nuclear DNA
Where does it come from?
 DNA in organelles (Mitochondria and chloroplasts)
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Result:
 Mendelian inheritance patterns fail.
 Maternal Inheritance of traits where the trait is
passed directly through the egg to the offspring
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Myoclonic Epilepsy
Ragged Red-fiber Disease
Leber’s Optic Neuropathy
All are associated with ATP generation
problems and affect organs with high ATP
demands
 Muscle, brain
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Gives non-green areas in leaves
 Called variegation
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Several different types known
Very common in ornamental plants
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Recognize the relationships between Mendelian
inheritance patterns and chromosomes.
Identify linked genes and their effect on inheritance
patterns.
Recognize the chromosomal basis of recombination
in unlinked and linked genes.
Recognize how crossover data is used to construct a
genetic map.
Identify the chromosomal basis of sex in humans.
Recognize examples of sex-linked disorders in
humans.
Identify X-inactivation and its effect in females.
Recognize sources and examples of
chromosomal alterations in humans.
 Identify examples of abnormalities in sex
chromosome number in humans.
 Recognize the basis and effects of parental
imprinting of genes in human inheritance
patterns.
 Recognize the basis and effect of extranuclear
inheritance on genetic inheritance patterns.
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