chromosomes and human genetics chapter 12. genes units of information about heritable traits in...
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Chromosomes andHuman Genetics
Chapter 12
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Genes
Units of information about heritable traits
In eukaryotes, distributed among
chromosomes
Each has a particular locus Location on a chromosome
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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
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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
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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
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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
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Sex Chromosomes
Autosomes
Karyotype
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Sex Chromosomes
Discovered in late 1800s
Mammals, fruit fliesXX is female, XY is male
In other groups XX is female, XY male
Human X and Y chromosomes function as
homologues during meiosis
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Sex Determination
X
X Y
X
XX
XY
XX
XY
X X
Y
X
x
x
eggs sperm
female(XX)
male(XY)
Figure 12.5Page 198
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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
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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
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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
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Pedigree Symbols
male
female
marriage/mating
Individual showing trait being studied
sex not specified
generationI, II, III, IV...
offspring in order of birth, from left to right
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I
II
III
IV
V
6 7
12
5,5 6,6
5,5 6,6
5,5 6,6
5,5 6,6
5,5 6,6
5,5 6,6
6,6 5,5
6,6 5,5
5,6 6,7
6,6 6,6*Gene not expressed in this carrier.
*
malefemale
Example: Pedigree of Polydactyly
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Hemophilia
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Polydactyly or Extra Digits
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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
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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
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Full Linkage
x
AB ab
50% AB 50% ab
All AaBb
meiosis, gamete formation
Parents:
F1 offspring:
Equal ratios of two types of gametes:
AB
ab
AB
ab
ab
AB
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Incomplete Linkage
Parents:
F1 offspring:
Unequal ratios of four types of gametes:
All AaCc
x
meiosis, gamete formation
AC acA
C A
C
AC
a
c
ac
Ac
aC
a
c
parental genotypes
recombinant genotypes
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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
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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
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Human Genetic Conditions
A genetic abnormality is an uncommon or rare trait E.g. Polydactyly
A genetic disorder is an inherited condition that may cause medical problems E.g. Cystic Fibrosis
A syndrome is a set of symptoms that characterize a disorder E.g. Down Syndome
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Human Genetic Abnormalities/Disorders Different patterns of inheritance are
observed depending on the condition:Autosomal Recessive InheritanceAutosomal Dominant InheritanceX-linked inheritance
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Autosomal Recessive Inheritance
If parents are both
heterozygous, child will
have a 25% chance of
being affected
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Albinism
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Autosomal Dominant Inheritance
Trait typically appears in every generation
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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
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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
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Males show disorder more than females
Son cannot inherit disorder from his father
X-Linked Recessive Inheritance
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Color Blindness
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Hemophilia
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Chromosomal Mutations
Duplication Deletion Inversion Translocation
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Duplication
Gene sequence that is repeated several to
hundreds of times
Duplications occur in normal
chromosomes
May have adaptive advantageUseful mutations may occur in copy
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Duplication
normal chromosome
one segment repeated
three repeats
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Inversion
A linear stretch of DNA is reversed
within the chromosome
segments G, H, I become inverted
In-text figurePage 206
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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
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Translocation
one chromosome
a nonhomologouschromosome
nonreciprocal translocation
In-text figurePage 206
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Deletion
Loss of some segment of a chromosome Most are lethal or cause serious disorder
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Polyploidy
Individuals have three or more of each type of chromosome (3n, 4n)
Common in flowering plants Lethal for humans
99% die before birthNewborns die soon after birth
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PolyploidyIndividuals have three or more of each type of chromosome (3n, 4n)
Common in flowering plants
Found in some fish & amphibians
Lethal for humans
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Aneuploidy
Individuals have one extra or less chromosome
(2n + 1 or 2n - 1) Major cause of human reproductive
failure Most human miscarriages are
aneuploids
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Nondisjunction
n + 1
n + 1
n - 1
n - 1chromosome alignments at metaphase I
nondisjunction at anaphase I
alignments at metaphase II anaphase II
Figure 12.17Page 208
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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
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Aneuploidy
Having one extra or one less chromosome
Downs Syndrome trisomy 21
Turners Syndrome XO
Klinefelters XXY
Downs Syndrome
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Trisomy and monosomy in sex chromosomes
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Turner Syndrome
Inheritance of only one X (XO) 98% spontaneously aborted Survivors are short, infertile females
No functional ovariesSecondary sexual traits reducedMay be treated with hormones, surgery
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Klinefelter Syndrome
XXY condition Results mainly from nondisjunction in
mother (67%) Phenotype is tall males
Sterile or nearly soFeminized traits (sparse facial hair, somewhat
enlarged breasts)Treated with testosterone injections
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Turners Syndrome
Klinefelters Syndrome
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Phenotypic Treatments
Symptoms of many genetic disorders can
be minimized or suppressed byDietary controls
Adjustments to environmental conditions
Surgery or hormonal treatments
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Genetic Screening
Large-scale screening programs detect affected persons
Newborns in United States routinely tested for PKUEarly detection allows dietary intervention
and prevents brain impairment
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Prenatal Diagnosis
Amniocentesis
Chorionic villus sampling
Fetoscopy
All methods have some risks
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Prenatal Diagnosis
Amniocentesis Chorionic villus sampling