Transcript sex linkage and disorders

Tracing the Inheritance of the
Human Y Chromosome
Women have sex chromosomes of XX, and
men have sex chromosomes of XY.
 Which of a man's grandparents could not be
the source of any of the genes on his Ychromosome?
–
–
–
–
Father's Mother.
Mother's Father.
Father's Father.
Mother's Mother, Mother's Father, and Father's
Mother.
– Mother's Mother.
Tracing the Inheritance of the
Human X Chromosome
Women have sex chromosomes of XX, and
men have sex chromosomes of XY.
 Which of a women's grandparents could not
be the source of any of the genes on either of
her X-chromosomes?
–
–
–
–
–
Mother's Father.
Father's Mother.
Mother's Mother.
Father's Father.
Mother's Mother and Mother's Father
Is there a special pattern of
inheritance for genes located on
the X or Y chromosome?
Yes, they are called sex-linked genes.
 More than 100 sex-linked genetic
disorders have been mapped on the X
chromosome.
 Y chromosomes are smaller than X
chromosomes and appear to only
contain a few genes.

Red-Green Colorblindness

The most common type of red-green color
perception defect is due to a mutation on the Xchromosome (i.e. a red-green color blind allele).
X-linked red-color blindness is a recessive trait.
Females heterozygous for this trait have normal
vision. The color perception defect manifests itself
in females only when it is inherited from both
parents. By contrast, males inherit their single Xchromosome from their mothers and become red
green color blind if this X-chromosome has the
color perception defect.
Hemophilia

Hemophilia in humans is due to an Xchromosome mutation. What will be the
results of mating between a normal
(non-carrier) female and a hemophilac
male?
– half of daughters are normal and half of
sons are hemophilic.
– all sons are normal and all daughters are
carriers.
Chromosomal Disorders

In some disorders, entire chromosomes, or
large segments of them, are missing,
duplicated, or otherwise altered.
 The most common error is nondisjunction,
when homologous chromosomes fail to
separate during meiosis.
 If nondisjunction occurs, the number of
chromosomes in the gametes can be
abnormal, and a disorder may result.
Nondisjunction
Section 14-2
Homologous
chromosomes
fail to separate
Meiosis I:
Nondisjunction
Go to
Section:
Meiosis II
Nondisjunction
Section 14-2
Homologous
chromosomes
fail to separate
Meiosis I:
Nondisjunction
Go to
Section:
Meiosis II
Nondisjunction
Section 14-2
Homologous
chromosomes
fail to separate
Meiosis I:
Nondisjunction
Go to
Section:
Meiosis II
Down’s Syndrome

Occurs in 1/800-1,000 births
 Caused by nondisjunction
 Trisomy 21 = three copies of chromosome 21
Age (years)
Frequency of Fetuses with Down
Syndrome to Normal Fetuses
at 16 weeks of pregnancy
Frequency of Live Births of
Babies with Down Syndrome
to Normal Births
15 - 19
----
1 / 1250
20 - 24
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1 / 1400
25 - 29
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1 / 1100
30 - 31
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1 / 900
32
----
1 / 750
33
1 / 420
1 / 625
34
1 / 325
1 / 500
35
1 / 250
1 / 350
36
1 / 200
1 / 275
37
1 / 150
1 / 225
38
1 / 120
1 / 175
39
1 / 100
1 / 140
40
1 / 75
1 / 100
41
1 / 60
1 / 85
42
1 / 45
1 / 65
43
1 / 35
1 / 50
44
1 / 30
1 / 40
45 and older
1 / 20
1 / 25
Down’s Syndrome

The image shows a karyotype of a person
with Down’s Syndrome, Trisomy 21
Sex Chromosome Disorders

Turner’s Syndrome (XO)
– underdeveloped ovaries, short stature, webbed neck,
and broad chest. Individuals are sterile, and lack
expected secondary sexual characteristics. Mental
retardation typically not evident.
Sex Chromosome Disorders

Klinefelter’s Syndrome (XXY)
– Nondisjunction in males
– Some development of breast tissue, little body hair is
present; typically tall, with or without evidence of mental
retardation. Males with XXXY, XXXXY, and XXXXXY
karyotypes have a more severe presentation, and mental
retardation is expected.
Some Autosomal Disorders in Humans
Type of Disorder
Disorder
Major Symptoms
Disorders
caused by
recessive alleles
Albinism
Lack of pigment in hair, skin, and
eyes
Cystic Fibrosis
Excess mucus in lungs, digestive
tract, liver; increased susceptibility
to infections; death in childhood
unless treated
Phenylketonuria
Accumulation in brain cells; lack
of normal pigment; mental
retardation
Tay-Sachs Disease
Lipid accumulation in brain cells;
mental deficiency; blindness;
death in early childhood
Disorders
Caused by
dominant alleles
Achondroplasia
Dwarfism (one form)
Huntington’s
Disease
Mental deterioration and
uncontrolled movements; appears
in middle age
Disorders
caused by
Sickle Cell Anemia
Sickled red blood cells; damage
to many tissues
codominant alleles
Albinism
Phenylketonuria
Achondroplasia
Sickle Cell Anemia
Causes of Genetic Disorders
Dominant Alleles
 Recessive Alleles
 Sex-linked genes
 Autosomal genes
 Nondisjunction
 Gene mutations
 Chromosomal mutations

Gene Mutations
Changes in the DNA sequence that
affect genetic information.
 Result from changes in one gene.
 Types:

– Point mutations
– Frameshift mutations
Chromosomal Mutations

Mutations involving changes in whole
chromosomes.
 Changes in the number of copies or structure
of a chromosome.
 Types:
–
–
–
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Deletions
Duplications
Inversions
Translocation
Deletion
Duplication
Inversion
Translocation