Download Chromosomal Basis of Inheritance

Chromosomal Inheritance
Chapter 12
Mader 2007-08
Chromosomal Inheritance
 Humans
type)
 Humans
are diploid (2 chromosomes of each
have 23 different kinds of chromosomes
 Arranged
 Total
in 23 pairs of homologous chromosomes
of 46 chromosomes (23 pairs) per cell
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


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
Mader 2007-08
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
Mader
2007-08
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
Mader 2007-08
Chromosomal Basis of Sex


XX female
Xy male
 Short segments at either end of they Y
chromosome are homologous to regions
of the X.
 These regions allow the X & Y to pair up
during meiosis (in testes)
Mader 2007-08
Chromosomal Basis of Sex

In Humans
 Anatomical signs of sex begin when an
embryo is 2 months old
 Hormone conditions determine the sex
 Presence of Y chromosomes and the
SRY gene triggers the making of
testes (testosterone)
 Absence of the SRY gene triggers the
making of ovaries
Mader 2007-08
Sex Linked Genes


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Genes carried on the sex chromosomes
Females must be homozygous to be
affected by recessive alleles
Heterozygotes and Homozygotes are do
not exist for males…due to the y
chromosome
Mader 2007-08
X Inactivation of Females

During embryonic development one X
chromosome becomes inactive (Barr
Body).

All mitotic divisions create cells with the
same inactive X.
 Males
and females have equal dose
of gene representation
Mader 2007-08

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Genes in the Barr body are not expressed
at this time
Barr Bodies reactivate in the ovaries
Ova are made so that all gametes receive
the same number of active chromosomes
Barr Body development is random and
independent
Mader 2007-08


Sex linked traits for a heterozygote female
will express half of one allele and half of
the other allele…depending on which is
inactive.
Ex: multicolored tortoise shell in cats
Mader 2007-08
Chromosomal Basis of
Inheritance

Thomas Hunt Morgan


Studied fruit flies (Drosophila melanogaster)
Studied specific genes related to specific
chromosomes

He determined that the gene for eye color in fruit
flies were carried on the X chromosome
Mader 2007-08
Eye Color in Fruit Flies
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:
 Fruit
XRXR
XRXr
XrXr
XRY
XrY
Genotype
Homozygous Dominant
Heterozygous
Homozygous Recessive
Monozygous Dominant
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Monozygous Recessive
Phenotype
Female
Red-eyed
Female
Red-eyed
Female
White-eyed
Male
Red-eyed
Male
White-eyed
Mader 2007-08
X-Linked 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
Mader 2007-08
Gene Linkage
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Linked Genes

Morgan studied 2 traits on his fruit flies
 Body color and wing shape
 Wild type (gray body b+) mutant (black
body b)
 Wild type (normal wings vg+) mutant
(short wings vg)
 b+b vg+vg v. bb vg vg test cross
Mader 2007-08
Linkage Groups
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Expected results:
 All Parental offspring
Actual results:
 The proportion of parental offspring vs.
nonparental offspring was very high
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Genetic Recombination


Explains the nonparental type offspring.
Unlinked genes
 50% frequency of recombination of
unlinked genes
 Due to law of segregation and
independent assortment
 Fig 15.2 and Fig 13.10 in text.
Mader 2007-08
Linked Genes

Linked genes
 Crossing over of linked genes when
tetrads form in meiosis I.
Mader 2007-08
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
Mader
2007-08

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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
Mader 2007-08
Crossing Over
Mader 2007-08
Complete vs. Incomplete Linkage
Mader 2007-08
Linkage Mapping

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
Genetic map based upon recombination
frequencies of genes
1 map unit = 1% recombination frequency
The farther apart they are the higher the
chance that crossing over occurs
Mader 2007-08

Recombination frequency can only have a
value of up to 50%.
 Anything at this point is
indistinguishable from genes on
separate chromosomes.
 If the distance between genes is great
enough on one chromosome, then
linkage is not observed.
Mader 2007-08
Chromosome Number: Polyploidy
 Polyploidy
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 selffertile but isolated from both “parent” species
Some estimate 47% of flowering plants are
polyploids
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Often lethal in higher
animals
Occurs
Chromosome Number:
Aneuploidy
 Monosomy
(2n - 1)
Diploid
individual has only one of a particular
chromosome
Caused
by failure of synapsed chromosomes to
separate at Anaphase I (nondisjunction)
Mader 2007-08

Trisomy (2n + 1) occurs when an
individual has three of a particular type of
chromosome

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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
Mader 2007-08
Nondisjunction
Mader 2007-08
Trisomy 21
Mader 2007-08
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
Mader 2007-08
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,
Mader 2007-08
Turner and Klinefelter Syndromes
Mader 2007-08
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
Mader 2007-08
Abnormal Chromosome Structure
 Deletion
Missing
Lost
segment of chromosome
during breakage
 Translocation
A
segment from one chromosome moves to a
non-homologous chromosome
Follows
breakage of two nonhomologous
chromosomes and improper re-assembly
Mader 2007-08
Deletion, Translocation,
Duplication, and Inversion
Mader 2007-08
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 re-insertion
 Genes occur in reverse order in inverted segment
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Inversion Leading to
Duplication and Deletion
Mader 2007-08
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
Some
syndrome
cancers
Mader 2007-08
Williams Syndrome
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Alagille Syndrome
Mader 2007-08