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8/3/2019 Chromosomes and Inheritance .... by hashim
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Chromosomes and Inheritance
Since genes are carried
on chromosomes,knowledge of
chromosome number
and structure has far-
reaching implications
for basic genetics,human health, and
evolution.
A normal human malekaryotype.
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X-linked InheritanceWhen Men and Woman Play by Different Rules
Behind the 8-ball? Colorblindness is an X-linked recessive trait.
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X-linked Inheritance
There are many X-linked recessive traits.
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Pedigree Analysis is a Key Tool in Human Genetics
Analyzing a pedigree is like puzzle-buildingyou try things (assigning
potential genotypes) until the pieces fit (youre as certain as you can be about
genotypes and modes of transmission (autosomal vs. X-linked; dominant vs.
recessive).
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A Pedigree of a Recessive Human Trait
Note that the trait can appear in offspring of parents without the trait.
Heterozygotes who do not show the trait are termed carriers.
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A Pedigree of a Dominant Human Trait
Note that the trait appears in every generation and the offspring
of an affected heterozygote are expected to show the trait.
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Changes in Chromosome Number and Structure
Changes in chromosome number and structure are important for health and
evolution.
Down syndrome is caused by a change in chromosome number.
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Aneuploidy occurs when one of the chromosomes is present inan
abnormal number of copies.
Aneuploidy
Trisomy and monosomy are two forms of aneuploidy.
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Down Syndrome is Caused by
Trisomy for Chromosome 21
Aneuploidy is remarkably common, causingtermination of at least 25% of human
conceptions.
Aneuploidy is also a driving force in cancer
progression (virtually all cancer cells are
aneuploid).
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Chromosome Non-Disjunction in Meiosis Causes Aneuploidy
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The Frequency of Chromosome Non-Disjunction And Down
Syndrome Rises Sharply with Maternal Age
The phenomenon is clearthe explanation isnt.
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Sex Chromosome Aneuploid Conditions are Common
Klinefelter syndrome
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Polyploidy
Polyploidy occurs when all the chromosomes are present inthreeor more copies.
Polyploidy is common in plants and rare in animals.
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Polyploids Are Created When Chromosome Number Doubles
A common way for this to occur is for the mitotic spindle to fail,
leaving all chromosomes in one cell.
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Polyploidy is a Major Force in Plant Evolution
Roughly 35% of flowering plants (the most familiar plant
species) arose through polyploidization.
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Most Crop Species are Polyploid
Polyploids, like the one on the left, are larger than their diploid
progenitors (strawberry on right).
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There are 4 types of
chromosome structural change
all of them associated with
human disorders
Chromosome Structural Changes
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A Boy with Cri-du-Chat Syndromea Debilitating Disorder
Caused by Chromosome Deletion
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Cri-du-Chat is Caused by the Loss of the Short Arm of One Copy
of Chromosome 5
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Translocations Lead to a Number of Human Cancers
In Burkitts lymphoma, a chromosome translocation
causes a cell cycle-promoting gene to always beactive.
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Searching for Chromosomal Defects - Amniocentesis and Chorionic
Villus Sampling
Many new
techniques forlearning about
individual genes
rather than whole
chromosomes are
available or under
development.
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Searching for Chromosome and Gene DefectsPre-Implantation
Genetic Diagnosis (PGD)
Removing a cell for
diagnosis from a humanembryo.
The diagnosis: trisomy
21 (Down syndrome).
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