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chromosomes sans pictures. ppt by HC120211094444

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									Chromosomes and human inheritance


Genes are distributed in chromosomes. Each
gene has a specific position (locus) on a
chromosome.

Each pair of homologous chromosomes has
the same arrangement of genes. Genes on the
same chromosome are linked.

The genes may be identical or different.
Variant forms of genes are called alleles.
Alleles form through mutation (changes
in the DNA sequence)

There may be many different alleles within
the human population, but each individual
will have only two.

Combinations of alleles can be beneficial,
have no particular effect, or contribute
to disease
Humans have tens of thousands of genes
arranged on 46 chromosomes:

22 pairs of autosomes (homologous pairs)
1 pair of sex chromosomes
X and Y are NOT homologous

Genes located on “sex” (X or Y) chromosomes
are called sex- (or X or Y)-linked genes
The sex of an individual is determined by the
sperm:

Female gametes- X or X
Male gametes- X or Y

             X       X

        X    XX     XX

        Y    XY     XY
The Y chromosome is much smaller than the
X chromosome. For many years it was thought
that there were no genes on the Y chromosome.


It is now known that there are several genes
on the Y chromosome, that contribute to male
sexual development.


There are thousands of genes on the X
chromosome. Do females get a “double dose”
of these genes?
No- because of X inactivation

In each cell in a female, one of the X chromosomes
is condensed into a Barr body.




                                nucleus


                                Barr body
What happens if a zygote is formed with the
wrong number of chromosomes?
   Aneuploidy- too many or too few chromosomes
  Polyploidy- three or more sets of chromosomes
  Triploidy- three sets of chromosomes
      fairly common- 15-18% of miscarriages
      A few are born alive- most die within
           a month
      Usually occurs when one egg is fertilized
           by two sperm (75% of triploids)

  In contrast, polyploid plants are common
How does aneuploidy happen?
    addition or deletion of an individual
    chromosome

This usually happens through nondisjunction
(a chromosome pair fails to separate during
meiosis)
n+1


n-1




 n


 n
When an aneuploid gamete fuses with a normal
gamete, the offspring will be aneuploid.

When this happens with an autosome the condition
is usually lethal. About 50% of all abnormalities
in fetal death involve trisomies.


A few survive.
Trisomy 13: Patau syndrome
     most survive fewer than 6 months
     defects in face, eyes, feet, nervous
     system and heart

More common if maternal age is higher
     (over 32). Also contributes to

Trisomy 18: Edwards syndrome
     babies are very small, grow slowly,
     usually female, and die within 2-4 months
     due to heart or respiratory failure

Diagnosis by karyotyping
Trisomy 21 (Down syndrome)
     occurs in about 1:900 live births
     characteristic physical features
     growth and mental development are usually
          retarded

     approximately 40% of all people with this
     syndrome have heart defects

     later in life, have very high incidence of
     leukemia and/or Alzheimer’s disease

     few live past age 50
Maternal age is a risk factor for trisomy

1:2000 births at age 20

1:100 at age 35

1:30 at age 45

Amniocentesis or CVS (chorionic villus sampling)
    recommended for this reason
Aneuploidy of sex chromosomes
   more common (1:400 in males, 1:650
    in females

I.   Turner syndrome (XO)
     About 1:10,000 live births
     females lack an X chromosome
     short, broad-chested, do not undergo
       secondary sexual development
     infertile
     not associated with mental retardation
II. Klinefelter syndrome (XXY)
       about 1:1000 live births
       not apparent until puberty

     immature sexual development- occasion-
           ally breast development
     infertile
     may have mild retardation

     advanced maternal age increases risk

     affected male may have several
     X chromosomes- the more chromosomes,
     the more severe the effect
III. XYY syndrome
       about 1:1000 live births
       affected men tend to be unusually tall
       MAY have personality disorders or
            low intelligence (most don’t)

     Once thought to make men predisposed to
     criminal behavior (frequency of XYY men
     is unusually high in prisons and mental
     institution). No causal relationship has
     been proven, though
Conclusions

At least one X chromosome is required for
      survival (OY is lethal)

Extra X chromosomes seem to interfere with
     normal development (even though
     all but one is inactivated in cells)

The more X chromosomes are present, the
     more severe the effect
Chromosomes can also be structurally altered
    common (1:400 frequency)

Deletions- part of chromosome missing

Translocation- part of one chromosome
     joined to another
          associated with a hereditary form
          of Down syndrome, some cancers

Duplications- part of chromosome is duplicated
     effects vary
        Recombination: portions of chromosomes
        are rearranged


Genes are moved “out of position”
     inversions (ABCDACBD)

     duplications (ABCDABBCD)

     translocations (ABCDEFABCJKL

                    (GHIJKLGHIDEF)

Effects can be drastic (and permanent)
      What kind of cell is affected and what
      Are the consequences?

Germ-line cell (gamete)
     these mutations are inherited
     these give rise to alleles
Somatic cell
     these affect the particular tissue
     but cannot be inherited
     example: tumors

Implications for gene therapy
Mutations- changes in gene sequence
     damage
     errors in copying gene sequence

Cells have mechanisms to repair this damage
      but they are not always completely
      successful

These changes lead to alleles

Sometimes they are harmless, sometimes they
    lead to disease (p. 178)
How do mutations happen?
Spontaneous errors of replication

Chemicals (“mutagens”)
Radiation
Viruses


If damage to DNA is not repaired, the mutation
becomes incorporated into the genome.
Genetic disorders

Most are recessive (can be passed from two
     healthy, “carrier” parents to a child)

Some alleles are more common than others;
    some are more common in certain ethnic
    groups than others

Sickle cell allele- African descent
Cystic fibrosis- European descent

Many of these alleles can now be identified through
    testing
Genetic counseling

What contributes to a high-risk pregnancy?

Disease-causing alleles
     dominant or recessive alleles-
     relative risk of each?

Maternal age

(Paternal age not clear)

								
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