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Chromosomes and Human Genetics

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Title: Chromosomes and Human Genetics


1
Chromosomes andHuman Genetics
  • Chapter 15

2
Chromosomes Cancer
  • Some genes on chromosomes control cell growth and
    division
  • If something affects chromosome structure at or
    near these loci, cell division may spiral out of
    control
  • This can lead to cancer

3
Philadelphia Chromosome
  • First abnormal chromosome to be associated with a
    cancer
  • Associated with a chronic leukemia
  • Overproduction of white blood cells

4
A Reciprocal Translocation
1
2
  • Chromosome 9 and chromosome 22 exchanged pieces

6
13
15
19
20
5
An Altered Gene
  • When the reciprocal translocation occurred, a
    gene at the end of chromosome 9 fused with a gene
    from chromosome 22
  • This hybrid gene encodes an abnormal protein that
    stimulates uncontrolled division of white blood
    cells

6
Understanding Chromosomes
  • 1882 - Walter Fleming
  • 1887 - August Weismann
  • 1900 - Rediscovery of Mendels work

7
Genes
  • Units of information about heritable traits
  • In eukaryotes, distributed among chromosomes
  • Each has a particular locus
  • Location on a chromosome

8
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

9
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

10
Sex Chromosomes
  • Discovered in late 1800s
  • Mammals, fruit flies
  • XX is female, XY is male
  • In other groups XX is male, XY female
  • Human X and Y chromosomes function as homologues
    during meiosis

11
Karyotype Preparation - Stopping the Cycle
  • Cultured cells are arrested at metaphase by
    adding colchicine
  • This is when cells are most condensed and easiest
    to identify

12
Karyotype Preparation
  • Arrested cells are broken open
  • Metaphase chromosomes are fixed and stained
  • Chromosomes are photographed through microscope
  • Photograph of chromosomes is cut up and arranged
    to form karyotype diagram

13
Human Karyotype
1 2 3 4 5
6 7 8 9 10
11 12
13 14 15 16 17 18
19 20 21 22 XX (or
XY)
14
Sex Determination
eggs
sperm
Female germ cell
Male germ cell
sex chromosome combinations possible in new
individual
15
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

16
Effect of YChromosome
appearance of structures that will give rise
to external genitalia
appearance of uncommitted duct system of
embryo at 7 weeks
7 weeks
Y present
Y absent
Y present
Y absent
testes
ovaries
10 weeks
ovary
testis
birth approaching
17
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

18
Discovering Linkage
One cross
homozygous dominant female
recessive male
x
Gametes
heterozygous female
heterozygous male
All F1 offspring have red eyes
19
Discovering Linkage
Reciprocal cross
homozygous recessive female
dominant male
x
Gametes
X
X
X
Y
heterozygous females
recessive males
F1 offspring
Half are red-eyed females, half are white-eyed
males
20
Discovering Linkage
  • Morgans crosses showed relationship between sex
    and eye color
  • Females can have white eyes
  • Morgan concluded gene must be on the X chromosome

21
Linkage Groups
  • Genes on one type of chromosome
  • Fruit flies
  • 4 homologous chromosomes
  • 4 linkage groups
  • Indian corn
  • 10 homologous chromosomes
  • 10 linkage groups

22
Full Linkage
AB
ab
x
Parents
F1 offspring
All AaBb
meiosis, gamete formation
50AB
50ab
With no crossovers, half of the gametes have one
parental genotype and half have the other
23
Incomplete Linkage
AC
ac
x
Parents
F1 offspring
All AaCc
meiosis, gamete formation
Unequal ratios of four types of gametes
a
a
A
A
C
c
C
c
Most gametes have parental genotypes
A smaller number have recombinant genotypes
24
Crossover Frequency
Proportional to the distance that separates genes
A
B
C
D
Crossing over will disrupt linkage between A and
B more often than C and D
25
Linkage Mapping in Humans
  • Linkage maps based on pedigree analysis through
    generations
  • Color blindness and hemophilia are very closely
    linked on X chromosome
  • Recombination frequency is 0.167

26
Pedigree
  • Chart that shows genetic connections among
    individuals
  • Standardized symbols
  • Knowledge of probability and Mendelian patterns
    used to suggest basis of a trait
  • Conclusions most accurate when drawn from large
    number of pedigrees

27
Pedigree for Polydactly
male
female
5,5 6,6

5,5 6,6
6,6 5,5
6,6 5,5
6
7
5,5 6,6
5,5 6,6
5,5 6,6
5,5 6,6
5,6 6,7
12
6,6 6,6
28
Genetic Abnormality
  • A rare, uncommon version of a trait
  • Polydactyly
  • Unusual number of toes or fingers
  • Does not cause any health problems
  • View of trait as disfiguring is subjective

29
Genetic Disorder
  • Inherited conditions that cause mild to severe
    medical problems
  • Why dont they disappear?
  • Mutation introduces new rare alleles
  • In heterozygotes, harmful allele is masked, so it
    can still be passed on to offspring

30
Autosomal Recessive Inheritance Patterns
  • If parents are both heterozygous, child will have
    a 25 chance of being affected

31
Galactosemia
  • Caused by autosomal recessive allele
  • Gene specifies a mutant enzyme in the pathway
    that breaks down lactose

enzyme 1
enzyme 2
enzyme 3
GALACTOSE-1- PHOSOPHATE
GALACTOSE-1- PHOSOPHATE
LACTOSE
GALACTOSE
glucose
intermediate in glycolysis
32
Autosomal Dominant Inheritance
  • Trait typically appears in every generation

33
Huntington Disorder
  • Autosomal dominant allele
  • Causes involuntary movements, nervous system
    deterioration, death
  • Symptoms dont usually show up until person is
    past age 30
  • People often pass allele on before they know they
    have it

34
Acondroplasia
  • 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

35
X-Linked Recessive Inheritance
  • Males show disorder more than females
  • Son cannot inherit disorder from his father

36
Examples of X-Linked Traits
  • Color blindness
  • Inability to distinguish among some of all colors
  • Hemophilia
  • Blood-clotting disorder
  • 1/7,000 males has allele for hemophilia A
  • Was common in European royal families

37
Fragile X Syndrome
  • An X-linked recessive disorder
  • Causes mental retardation
  • Mutant allele for gene that specifies a protein
    required for brain development
  • Allele has repeated segments of DNA

38
Hutchinson-Guilford Progeria
  • Mutation causes accelerated aging
  • No evidence of it running in families
  • Appears to be dominant
  • Seems to arise as spontaneous mutation
  • Usually causes death in early teens

39
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

40
Duplication
normal chromosome
one segment repeated
three repeats
41
Inversion
  • A linear stretch of DNA is reversed
  • within the chromosome

42
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

43
Translocation
chromosome
nonhomologous chromosome
reciprocal translocation
44
Deletion
  • Loss of some segment of a chromosome
  • Most are lethal or cause serious disorder

45
Aneuploidy
  • Individuals have one extra or less chromosome
  • (2n 1 or 2n - 1)
  • Major cause of human reproductive failure
  • Most human miscarriages are aneuploids

46
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

47
Nondisjunction
n 1
n 1
n - 1
n - 1
chromosome alignments at metaphase I
nondisjunction at anaphase I
alignments at metaphase II
anaphase II
48
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

49
Turner Syndrome
  • Inheritance of only one X (XO)
  • 98 spontaneously aborted
  • Survivors are short, infertile females
  • No functional ovaries
  • Secondary sexual traits reduced
  • May be treated with hormones, surgery

50
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

51
XYY Condition
  • Taller than average males
  • Most otherwise phenotypically normal
  • Some mentally impaired
  • Once thought to be predisposed to criminal
    behavior, but studies now discredit

52
Phenotypic Treatments
  • Symptoms of many genetic disorders can be
    minimized or suppressed by
  • Dietary controls
  • Adjustments to environmental conditions
  • Surgery or hormonal treatments

53
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

54
Prenatal Diagnosis
  • Amniocentesis
  • Chorionic villus sampling
  • Fetoscopy
  • All methods have some risks

55
Preimplantation Diagnosis
  • Used with in-vitro fertilization
  • Mitotic divisions produce ball of 8 cells
  • All cells have same genes
  • One of the cells is removed and its genes
    analyzed
  • If cell has no defects, the embryo is implanted
    in uterus
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