Transcript Document
Chromosomes and
Human Genetics
Chapter 12
Genes
Units of information about heritable traits
In eukaryotes, distributed among
chromosomes
Each has a particular locus
Location
on a chromosome
Homologous Chromosomes
Homologous autosomes are identical in
length, size, shape, and gene sequence
Sex chromosomes are nonidentical but
still homologous
Homologous chromosomes interact,
then segregate from one another during
meiosis
Alleles
Different molecular forms of a gene
Arise through mutation
Diploid cell has a pair of alleles at each
locus
Alleles on homologous chromosomes may
be same or different
Studying Human Genetics
Studying Human Genetics is much more
complicated than using other model systems
(e.g. Pea Plants)
Humans reproduce slowly, have few offspring, and it
is unethical to breed humans for experiments
There are many techniques that are used to
study human genetics indirectly
Karyotypes
Pedigree analysis
Linkage maps
Karyotype
Picture of an individual’s
chromosomes
Making a Karyotype:
Metaphase
chromosomes are fixed
and stained
Chromosomes
are photographed
through microscope
Photograph
of chromosomes is cut up
and arranged to form karyotype
diagram
Karyotype
Autosomes
Sex Chromosomes
Sex Chromosomes
Discovered in late 1800s
Mammals, fruit flies
XX
is female, XY is male
In other groups XX is female, XY male
Human X and Y chromosomes function as
homologues during meiosis
Sex
Determination
Figure 12.5
Page 198
female
(XX)
male
(XY)
eggs
sperm
X
x
Y
X
x
X
X
X
X
XX
XX
Y
XY
XY
The Y Chromosome
Fewer than two dozen genes identified
One is the master gene for male sex
determination
SRY
gene (sex-determining region of Y)
SRY present, testes form
SRY absent, ovaries form
The X Chromosome
Carries more than 2,300 genes
Most genes deal with nonsexual traits
Genes on X chromosome can be
expressed in both males and females
Pedigree
A chart showing the genetic connections
between individuals
A genetic family tree
Often groups of people whose heritage is well
documented or who are somewhat isolated from
others are used to develop pedigrees.
E.g.
Icelanders, Mormons, Ashkenazi Jews, Amish,
Sardinians
male
Pedigree
Symbols
female
marriage/mating
offspring in order of birth,
from left to right
Individual showing trait
being studied
sex not
specified
I, II, III, IV...
generation
Example: Pedigree of Polydactyly
female
I
male
II
5,5
6,6
*
III
IV
5,5
6,6
6
6,6
5,5
6,6
5,5
7
5,5
6,6
5,5
6,6
5,5
6,6
5,5
6,6
5,6
6,7
12
V
*Gene not expressed in this carrier.
6,6
6,6
Polydactyly or Extra Digits
Linked Genes
Genes found on one type of chromosome
Linked genes can assort separately from
on another only through crossing over
The closer to genes are to each other on a
chromosome, the more tightly linked they
are (i.e. the more likely they are to assort
together during meiosis
A B
C
D
a b
c
d
A B
C
D
a b
c
d
A B
c
d
a b
C
D
Markers A & B are linked
Markers A & C are not linked
Full Linkage
B
a
A
Parents:
A
AB
b
x
a
b
B
F1 offspring:
All AaBb
meiosis, gamete formation
Equal ratios of two
types of gametes:
A
B
50% AB
a
b
50% ab
ab
Incomplete Linkage
AC
A
C
Parents:
ac
a
x
c
A
a
c
C
F1 offspring:
All AaCc
meiosis, gamete formation
Unequal ratios of four
types of gametes:
a
A
C
c
parental
genotypes
a
A
c
C
recombinant
genotypes
Crossover Frequency
Proportional to the distance that separates genes
Crossing over will disrupt linkage between A and B
more often than C and D
A
B
C
D
Linkage mapping
Done by using known “landmarks” or markers on
chromosomes
These markers are either genes or DNA fragments
whose location has already been worked out
By watching how these markers are inherited by those
with & without the disorder, geneticists can predict the
location of the gene responsible for the disorder
If a gene and a marker are found together 100% of the
time, they are completely linked
If a gene and a marker are found together 50% of the
time they are completely unlinked
Human Genetic Conditions
A genetic abnormality is an uncommon or rare
trait
E.g.
A genetic disorder is an inherited condition that
may cause medical problems
E.g.
Polydactyly
Cystic Fibrosis
A syndrome is a set of symptoms that
characterize a disorder
E.g.
Down Syndome
Human Genetic
Abnormalities/Disorders
Different patterns of inheritance are
observed depending on the condition:
Autosomal Recessive Inheritance
Autosomal Dominant Inheritance
X-linked inheritance
Autosomal Recessive
Inheritance
If parents are both
heterozygous, child will
have a 25% chance of
being affected
Albinism
Autosomal Dominant
Inheritance
Trait typically appears in
every generation
Huntington Disorder
Autosomal dominant allele
Causes involuntary movements, nervous system
deterioration, death
Symptoms don’t usually show up until person is
past age 30
People often pass allele on before they know
they have it
Achondroplasia
Autosomal dominant allele
In homozygous form usually
leads to stillbirth
Heterozygotes display a
type of dwarfism
Have short arms and legs
relative to other body parts
X-Linked Recessive Inheritance
Males show disorder more
than females
Son cannot inherit
disorder from his father
Color Blindness
Hemophilia
Chromosomal Mutations
Duplication
Deletion
Inversion
Translocation
Duplication
Gene sequence that is repeated several to
hundreds of times
Duplications occur in normal
chromosomes
May have adaptive advantage
Useful
mutations may occur in copy
Duplication
normal chromosome
one segment
repeated
three repeats
Inversion
A linear stretch of DNA is reversed
within the chromosome
segments
G, H, I
become
inverted
In-text figure
Page 206
Translocation
A piece of one chromosome becomes
attached to another nonhomologous
chromosome
Most are reciprocal
Philadelphia chromosome arose from a
reciprocal translocation between
chromosomes 9 and 22
Translocation
one chromosome
a nonhomologous
chromosome
nonreciprocal translocation
In-text
figure
Page 206
Deletion
Loss of some segment of a chromosome
Most are lethal or cause serious disorder
Polyploidy
Individuals have three or more of each
type of chromosome (3n, 4n)
Common in flowering plants
Lethal for humans
99%
die before birth
Newborns
die soon after birth
Polyploidy
Individuals have three or more of each type of
chromosome (3n, 4n)
Common in flowering plants
Found in some fish & amphibians
Lethal for humans
Aneuploidy
Individuals have one extra or less
chromosome
(2n + 1 or 2n - 1)
Major cause of human reproductive
failure
Most human miscarriages are
aneuploids
Nondisjunction
n+1
n+1
n-1
chromosome
alignments at
metaphase I
n-1
nondisjunction alignments at
at anaphase I metaphase II
anaphase II
Figure 12.17
Page 208
Down Syndrome
Trisomy of chromosome 21
Mental impairment and a variety of
additional defects
Can be detected before birth
Risk of Down syndrome increases
dramatically in mothers over age 35
Aneuploidy
Having one extra or one less chromosome
Downs Syndrome
trisomy 21
Turners Syndrome
XO
Klinefelters
XXY
Downs Syndrome
Trisomy and monosomy in sex
chromosomes
Turner Syndrome
Inheritance of only one X (XO)
98% spontaneously aborted
Survivors are short, infertile females
No
functional ovaries
Secondary
May
sexual traits reduced
be treated with hormones, surgery
Klinefelter Syndrome
XXY condition
Results mainly from nondisjunction in
mother (67%)
Phenotype is tall males
Sterile
or nearly so
Feminized traits (sparse facial hair, somewhat
enlarged breasts)
Treated with testosterone injections
Turners Syndrome
Klinefelters Syndrome
Phenotypic Treatments
Symptoms of many genetic disorders can
be minimized or suppressed by
Dietary
controls
Adjustments
Surgery
to environmental conditions
or hormonal treatments
Genetic Screening
Large-scale screening programs detect
affected persons
Newborns in United States routinely
tested for PKU
Early
detection allows dietary intervention
and prevents brain impairment
Prenatal Diagnosis
Amniocentesis
Chorionic villus sampling
Fetoscopy
All methods have some risks
Prenatal Diagnosis
Amniocentesis
Chorionic villus sampling