Transcript Chromosome Number

Chromosomal
Inheritance
Ch. 12 Outline – Chromosomal Inheritance
X-Linked Alleles
Human X-Linked Disorders
Gene Linkage
Crossing-Over
Chromosome Map
Changes in Chromosome Number
Changes in Chromosome Structure
Human Syndromes
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Chromosomal
Inheritance
2
Chromosomal Inheritance
Humans are diploid (2 chromosomes of each type)
Humans have 23 different kinds of chromosomes
Arranged in 23 pairs of homologous chromosomes
Total of 46 chromosomes (23 pairs) per cell
One of the chromosome pairs determines the sex of
an individual (The sex chromosomes)
The other 22 pairs of chromosomes are autosomes
Autosomal chromosomes are numbered from
smallest (#1) to largest (#22)
The sex chromosomes are numbered as the 23rd pair
Chromosomal
Inheritance
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Sex Determination in Humans
Sex is determined in humans by allocation of
chromosomes at fertilization
Both sperm and egg carry one of each of the 22
autosomes
The egg always carries the X chromosome as
number 23
The sperm may carry either and X or Y
If the sperm donates an X in fertilization, the zygote
will be female
If the sperm donates a Y in fertilization, the zygote
will be male
Therefore, the sex of all humans is determined by the
sperm donated by their father
Chromosomal
Inheritance
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X-Linked Alleles
Genes carried on autosomes are said to be
autosomally linked
Genes carried on the female sex chromosome
(X) are said to be X-linked (or sex-linked)
X-linked genes have a different pattern of
inheritance than autosomal genes have
The Y chromosome is blank for these genes
Recessive alleles on X chromosome:
- Follow familiar dominant/recessive rules in
females (XX)
- Are always expressed in males (XY), whether
dominant or recessive
- Males said to be monozygous for X-linked
genes
Chromosomal
Inheritance
Eye Color in Fruit Flies
Fruit flies (Drosophila melanogaster) are common
subjects for genetics research
They normally (wild-type) have red eyes
A mutant recessive allele of a gene on the X
chromosome can cause white eyes
Possible combinations of genotype and phenotype:
XRXR
XRXr
XrXr
XRY
XrY
Genotype
Homozygous Dominant
Heterozygous
Homozygous Recessive
Monozygous Dominant
Monozygous Recessive
Phenotype
Female
Red-eyed
Female
Red-eyed
Female
White-eyed
Male
Red-eyed
Male
White-eyed
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Drosophila Chromosomes
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X-Linked Inheritance
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Human X-Linked Disorders:
Red-Green Color Blindness
Chromosomal
Inheritance
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Color vision In humans:
Depends three different classes of cone cells
in the retina
Only one type of pigment is present in each
class of cone cell
- The gene for blue-sensitive is autosomal
- The red-sensitive and green-sensitive genes are
on the X chromosome
- Mutations in X-linked genes cause RG color
blindness:
males with mutation (XbY) are colorblind
 Only homozygous mutant females (XbXb) are
colorblind
 Heterozygous females (XBXb) are asymptomatic
carriers
 All
Red-Green Colorblindness Chart
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X-Linked Recessive Pedigree
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Human X-Linked Disorders:
Muscular Dystrophy
Chromosomal
Inheritance
Muscle cells operate by release and rapid
sequestering of calcium
Protein dystrophin required to keep calcium
sequestered
Dystrophin production depends on X-linked gene
A defective allele (when unopposed) causes
absence of dystrophin
Allows calcium to leak into muscle cells
Causes muscular dystrophy
All sufferers male
Defective gene always unopposed in males
Males die before fathering potentially homozygous
recessive daughters
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Human X-Linked Disorders:
Hemophilia
Chromosomal
Inheritance
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“Bleeder’s Disease”
Blood of affected person either refuses to clot
or clots too slowly
Hemophilia A – due to lack of clotting factor
VIII
Hemophilia B – due to lack of clotting factor IX
Most victims male, receiving the defective
allele from carrier mother
Bleed to death from simple bruises, etc.
Factor VIII now available via biotechnology
Hemophilia Pedigree
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Human X-Linked Disorders:
Fragile X Syndrome
Chromosomal
Inheritance
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Due to base-triplet repeats in a gene on the X
chromosome
CGG repeated many times
6-50 repeats – asymptomatic
230-2,000 repeats – growth distortions and
mental retardation
Inheritance pattern is complex and
unpredictable
Gene Linkage
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Chromosomal
Inheritance
Gene Linkage
When several genes of interest exist on the
same chromosome
Such genes form a linkage group
Tend to be inherited as a block
If all genes on same chromosome:
- Gametes of parent likely to have exact allele
combination as gamete of either grandparent
- Independent assortment does not apply
If all genes on separate chromosomes:
- Allele combinations of grandparent gametes
will be shuffled in parental gametes
- Independent assortment working
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Linkage Groups
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Chromosomal
Inheritance
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Constructing a Chromosome Map
Crossing-over can disrupt a blocked allele pattern on a
chromosome
Affected by distance between genetic loci
Consider three genes on one chromosome:
 If one at one end, a second at the other and the third in the middle
- Crossing over very likely to occur between loci
- Allelic patterns of grandparents will likely to be disrupted in
parental gametes with all allelic combinations possible
 If the three genetic loci occur in close sequence on the
chromosome
- Crossing over very UNlikely to occur between loci
- Allelic patterns of grandparents will likely to be preserved in
parental gametes
Rate at which allelic patterns are disrupted by crossing over:
 Indicates distance between loci
 Can be used to develop linkage map or genetic map of
chromosome
Crossing Over
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Complete vs. Incomplete Linkage
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Chromosome Number:
Polyploidy
Chromosomal
Inheritance
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Polyploidy
Occurs when eukaryotes have more than 2n
chromosomes
Named according to number of complete sets of
chromosomes
Major method of speciation in plants
- Diploid egg of one species joins with diploid pollen of
another species
- Result is new tetraploid species that is self-fertile but
isolated from both “parent” species
- Some estimate 47% of flowering plants are polyploids
Often lethal in higher animals
Chromosome Number:
Aneuploidy
Chromosomal
Inheritance
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Monosomy (2n - 1)
Diploid individual has only one of a particular
chromosome
Caused by failure of synapsed chromosomes to
separate at Anaphase I (nondisjunction)
Trisomy (2n + 1) occurs when an individual has
three of a particular type of chromosome
Diploid individual has three of a particular chromosome
Also caused by nondisjunction
This usually produces one monosomic daughter cell
and one trisomic daughter cell in meiosis I
Down syndrome is trisomy 21
Nondisjunction
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Trisomy 21
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Chromosomal
Inheritance
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Chromosome Number:
Abnormal Sex Chromosome Number
Result of inheriting too many or too few X or Y
chromosomes
Caused by nondisjunction during oogenesis
or spermatogenesis
Turner Syndrome (XO)
Female with single X chromosome
Short, with broad chest and widely spaced
nipples
Can be of normal intelligence and function
with hormone therapy
Chromosomal
Inheritance
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Chromosome Number:
Abnormal Sex Chromosome Number
Klinefelter Syndrome (XXY)
Male with underdeveloped testes and
prostate; some breast overdevelopment
Long arms and legs; large hands
Near normal intelligence unless XXXY, XXXXY,
etc.
No matter how many X chromosomes,
presence of Y renders individual male
Turner and Klinefelter Syndromes
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Chromosomal
Inheritance
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Chromosome Number:
Abnormal Sex Chromosome Number
Ploy-X females
XXX simply taller & thinner than usual
Some learning difficulties
Many menstruate regularly and are fertile
More than 3 Xs renders severe mental
retardation
Jacob’s syndrome (XYY)
Tall, persistent acne, speech & reading
problems
Chromosomal
Inheritance
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Abnormal Chromosome Structure
Deletion
Missing segment of chromosome
Lost during breakage
Translocation
A segment from one chromosome moves to a
non-homologous chromosome
Follows breakage of two nonhomologous
chromosomes and improper re-assembly
Deletion, Translocation,
Duplication, and Inversion
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Chromosomal
Inheritance
Abnormal Chromosome Structure
Duplication
A segment of a chromosome is repeated in
the same chromosome
Inversion
Occurs as a result of two breaks in a
chromosome
- The internal segment is reversed before reinsertion
- Genes occur in reverse order in inverted
segment
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Inversion Leading to
Duplication and Deletion
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Chromosomal
Inheritance
Abnormal Chromosome Structure
Deletion Syndromes
Williams syndrome - Loss of segment of
chromosome 7
Cri du chat syndrome (cat’s cry) - Loss of
segment of chromosome 5
Translocations
Alagille syndrome
Some cancers
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Williams Syndrome
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Alagille Syndrome
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