Chapter 12- Human Genetics
Homologous Chromosomes
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Autosomes- All other chromosomes
except the X and Y chromosome
Generally speaking, homologous
chromosomes are alike in length, shape
and gene sequence
Sex chromosomes= X and Y
chromosomes These are physically
different but able to synapse during
meiosis
Karyotype
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Karyotype= number of metaphase
chromosomes and their defining
characteristics
Cells are cultured in vitro (in glass)
Colchicine is added- blocks formation of
microtubule spindles- arrests cells in
metaphase
Karyotype
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Karyotypes are
pictures of paired
human
chromosomes
Used in identifying
chromosome
abnormalities
Sex Determination in Humans
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XX= female
XY= male
X chromosome carries over 300 genes
Y chromosome carries “maledetermining gene” . Expression of
this gene leads to formationof testes.
Absence of this gene- ovaries
automatically form
SRY Gene
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SRY= Sex-determining region of the
Y chromosome
First 4 weeks of its existence, human
embryo is neither male or female
SRY gene regulates many proteins
that regulate reactions that are
necessary for sex determination
Early Questions About Gene
Locations
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Sutton- Chromosome Theory of
Inheritance= Chromosomes are the
units of heredity
Morgan- genes have specific locations
on specific chromosomes
Morgan performed experiments using
Drosophila melanogaster
Found evidence of the gene for eye
color on the X chromosome
Characteristics of Drosophila
melanogaster
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1. Easy to maintain and breed
2. About 0.1 inch long- so tiny that
hundreds can be kept in a jar
3. Have a reproductive cycle of 10-15
days, therefore they can produce many
generations of offspring in a matter of
weeks
Morgan’s Experiment
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In fruit flies- alleles for red eyes (R) are
dominant over alleles for white eyes (r).
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Morgan hypothesized that the allele for eye
color is carried on the X chromosome
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P1 generation – Cross a white-eyed male and
a homozygous red-eyed female
Sex-linked Traits
Xr
Y
XR
XR Xr
XRY
XR
XR Xr
XRY
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Example: red eye color
is a dominant trait
carried on the X
chromosomes in fruit
flies; white is recessive
Cross red eyed female
with a white eyed male
Get: 2 heterozygous red
eyed females and 2 red
eyed males
Sex-linked Traits
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Now cross the F1: red
eyed male with
heterozygous red eyed
female
You get:
XR
Y
XR
XR XR
XR Y
r
– 1 homozygous red eyed X
female
– 1 heterozygous red eyed
female
– 1 red eyed male
– 1 white eyed male
XR Xr
Xr Y
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Parental Generation
(P1)
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F1 generation
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F2 generation
Conclusion
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The gene for eye color in Drosophila must
be carried on the X chromosome
Females have two X chromosomes
Males have only one X chromosome so
whatever allele in on the X chromosome for
eye color is expressed
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Morgan concluded that genes for certain
traits are carried on the X chromosomes
and that chromosomes and their genes
segregate during meiosis
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The discovery of sex-linked traits
explained why some characteristics
caused by recessive genes are appear
far more often in males since males have
only one X chromosome and females
have two X chromosomes
Sex- Linked Trait
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1. Sex- linked trait = a trait that is determined
by alleles carried only on an X chromosome
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2.In humans sex linked traits are found on the
X chromosome which is much larger than the
Y chromosome
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3.In most organisms:
Males are XY
Females are XX
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Inheritance of human traits
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Sex-linked Traits - determined by X and
Y chromosomes (sex chromosomes)
Recessive traits rarely occur in females
because they have two X
chromosomes, the dominant gene on
one X can mask the recessive gene on
the second X
Recessive traits in males are expressed
because they only have one X
Inheritance of human traits examples of sex-linked traits
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Color vision
– the gene for color
vision is located on
the X
– the recessive gene
results in
colorblindness
(inability to see
certain colors)
– Genotypes: XC XC,
XC Xc, Xc Xc, XCY,
and XcY
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Hemophilia
– a protein needed for
normal blood clotting
is located on the X
– the recessive gene
results in hemophilia
– individuals with this
disease can bleed to
death from a tiny cut
- their blood does not
clot normally
Inheritance of human traits
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Sex-Influenced Traits - traits that are
generally associated with one sex but is
produced by genes carried on autosomes
Example: Baldness
– the allele coding for baldness HB is dominant in
males and recessive in females
– The allele that coding for normal hair HN is
dominant in females and recessive in males
– HN HN - most likely keep hair male and female
– HN HB - male will lose hair, female will keep hair
– HB HB - most likely lose hair male and female
Linkage Groups
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Linkage groups- the genes located on
each type of chromosome
Drosophila has 4 linkage groups
Humans have 23 linkage groups
Crossing over happens among linkage
groups
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Probability of a crossover is proportional
to the distance between genes
The further apart two genes are, the
more likely they will be to cross over
Use patterns of crossing over to map
genes on the chromosome
Changes in Chromosome
Structure
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Deletion
Inversion
Translocation
Duplication
Deletions
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Loss of a chromosome region
May be loss of one or more genesalmost always causes problems
Caused by irradiaiton, viral attck,
chemical action, & other environmetnal
factors
Ex: Cat-cry disorder- deletion form
chromosome 5
Deletions- cont
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Symptoms of cat-cry disorder
– abnormally shaped larynx
– Infant produces mewing sounds
– Mental retardation
Inversion
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Segment of DNA detached from
chromosome and reattaches in the
reverse order
Reversal alters the position and order of
the chromosome’s genes
Affects the way the base sequence is
read and translated
Translocation
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Part of one chromosome exchanges
places with the corresponding part of
another nonhomologous chromosome
Ex; #8 and #14 translocate- form of
cancer results
Duplication
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Gene sequences are repeated several
to many times
Contained even on normal
chromosomes- some DNA duplications
are built into the species
EX: hemoglobin in humans and
primates- have multiple copies of similar
gene sequences
Duplication cont.
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18 of 23 pairs of human chromosomes
are nearly identical to corresponding
chromosome in chimpanzees and
gorillas
Ex: fragile X syndrome-abnormally
constricted region on X chromosome
Changes in Chromosome
Number
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Aneuploidy= one more or one less
chromosome
Causes many miscarriages
Polyploidy= inheritance of three or more
of each type of chromosome
Ex: 1/2 flowering plants, some insects,
fishes & other animals
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Polyploidy is lethal for humans
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Chromosome number can change
during
– mitosis
– meiosis
– fertilization
Changes in Chromosome # cont.
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Nondisjunction= one or more pairs of
chromosomes fails to separate properly
during mitosis or meiosis
Results in:
Trisomy= 3 of one type of chromosome
Monosomy= 1 of one type of
chromosome
Changes in Autosome Number
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Down’s syndrome= trisomy 21
Syndrome means a set of symptoms
that characterize a disorder
Symptoms- mental impairment,
abnormal skeletal development, some
have heart defects
Changes in the Number of Sex
Chromosomes
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Turner’s syndrome= XO
75% from nondisjunction in dad
98% of XO zygotes spontaneously abort
Symptoms- short, infertile, no functional
ovaries
Some benefit form hormone therapy
and corrective surgery
Changes in Sex Chromosome #
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Klinefelter Syndrome= XXY
67% from nondisjunction in mom
Symptoms develop after puberty- taller,
sterile or low fertility, smaller testes,
sparse facial hair, breast enlargement
Testosterone injections can minimize
feminized traits
Changes in Sex Chromosome
Number
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XYY
Symptoms- taller, may be mildly
retarded, basically phenotypically
normal
Once thought to be genetically
predisposed to be criminals