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Sex Linked Inheritance
Introduction
• A human female, has 23 pair of
chromosomes
• A human male, has 22 similar pairs and
one pair consisting of two chromosomes
that are dissimilar in size and structure.
• The 23 rd pair in both the sexes is called
sex chromosomes
• the female, XX. the male, XY
X-linked diseases
• X-linked diseases are those for which the gene is
present on the X chromosome.
• X-linked diseases show inheritance patterns that
differ from autosomal diseases.
• This occurs because males only have one copy of
the X chromosome (plus their Y chromosome) and
females have two X chromosomes.
• Because of this, males and females show different
patterns of inheritance and severity of manifestation.
While there are both dominant and recessive Xlinked diseases, there are some characteristics that
are common to X-linked disorders in general
• X-linked genes are never passed from
father to son.
• The Y chromosome is the only sex
chromosome that passes from father to
son.
• Males are never carriers – if they have a
mutated gene on the X chromosome, it
will be expressed.
• Males are termed hemizygous for
genes on the X chromosome.
X-linked dominant
• hereditary pattern in which a dominant gene on the X
chromosome causes a characteristic to be manifested in the
offspring.
• X-linked dominant diseases are those that are expressed in
females when only a single copy of the mutated gene is present.
•
Very few X-linked dominant diseases have been identified (e.g.
hypophosphatemic rickets, Alport syndrome, diabetes
insipidus)
hypophosphatemic rickets or vitamin D resistant rickets >>>low
serum phosphorus, skeletal abnormalities
• Alport syndrome, which involves progressive hearing loss and
progressive kidney problems.
Characteristics of X-linked
dominant diseases include:
• Never passed from father to son.
• Affected males produce only affected females. An affected
male only has one X chromosome to pass on to his daughters
• Affected females produce 50% normal and 50% affected
offspring.. >>>> heterozygous
• Males are usually more severely affected than females. Some
X-linked dominant traits may even be lethal to males.
• Females are more likely to be affected. Since females have 2 X
chromosomes, they have 2 “chances” to inherit the mutated
allele.
Pattern for inheritance
• Mating A
Mating B
Pattern for inheritance
• Mating A
Mating B
Amelogenesis Imperfecta-1
: X-linked dominant
Gene name: Amelogenin, Chromosome location: Xp22.3-p22.1
Altered Cellular function: Abnormal tooth enamel
Symptoms: Very hard enamel, thin enamel, small teeth, and/or rough teeth
Incidence: Rare
X-linked recessive
• hereditary pattern in which a recessive gene
on the X chromosome results in the
manifestation of characteristics in male
offspring and a carrier state in female
offspring
• X-linked recessive diseases are those in
which a female must have two copies of the
mutant allele in order for the mutant
phenotype to develop.
• Many X-linked recessive disorders are wellknown, including color blindness,
hemophilia, and Duchenne muscular
dystrophy.
•
•
Pattern of x linked recessive inheritance
Hemophilia
• The blood fails to clot normally
• Lacking a blood clotting factor
VIII(antihemophilic globulin, AHG),IX
• bleeding from even minor cuts
• in 1,500 newborn males. Most (75%) have
hemophilia A, a lack of clotting factor VIII.
• Hemophilia B- "Christmas Disease" is a defect in
clotting factor IX.
• Transfusions of fresh whole blood or plasma or
factor concentrates control bleeding
Inheritance of hemophilia
f
Typical features of X-linked
recessive inheritance
• Never passed from father to son.
• Males are much more likely to be affected because they only
need one copy of the mutant allele to express the phenotype.
•
• Affected males get the disease from their mothers and all of
their daughters are obligate carriers.
• Sons of heterozygous females have a 50% chance of receiving
the mutant allele.
The Marker X syndrome
• Fragile X syndrome (Marker X syndrome )
is a genetic condition involving changes in
the long arm of the X chromosome. It is
characterized by mental retardation.
Alternative Names
Martin-Bell syndrome; Marker X syndrome
Causes, & Risk Factors
• Fragile X syndrome is the most common form of
inherited mental retardation in males and a
significant cause in females.
• The inheritance is different from common dominant
or recessive inheritance patterns.
• A fragile area on the X chromosome tends to repeat
bits of the genetic code.
• The more repeats, the more likely there is to be a
problem.
• Boys and girls can both be affected, but because
boys have only one X chromosome, a single fragile X
is more likely to affect them more severely.
Fragile X Syndrome Symptoms & Signs
• Family history of fragile X syndrome, especially
a male relative
• Mental retardation
• Large testicles (macro-orchidism)
• Large size
• Tendency to avoid eye contact
• Hyperactive behavior
• Large forehead and/or ears with a prominent jaw
Fragile X Syndrome Symptoms
• Sex-limited inheritance:
• A trait that appears in only one sex is called sex-limited
.
• This is different from X-linked inheritance, which refers to
traits carried on the X chromosome.
• Sex hormones and other physiologic differences
between males and females may alter the expressivity of
a gene.
• For example, premature baldness is an autosomal
dominant trait, but presumably as a result of female sex
hormones, the condition is rarely expressed in the
female, and then usually only after menopause.
• Thus, sex-limited inheritance, perhaps more correctly
called sex-influenced inheritance, is a special case of
limited expressivity and penetrance
Sex limited traits
• sex linked traits are generally expressed much
more often in males than in females.
• some traits which affect one sex more than
another are not necessarily sex linked.
• Examples are cases of sex limited expression
which might include genes affecting beard
growth or breast size, and (in cattle), horn
growth and milk yield.
Sex-influenced traits
• characteristic may appear in both sexes but
expression of the phenotype differs.
• Example: Early balding (pattern baldness) in
humans. Heterozygous men (b+/b) lose their
hair; heteroyzgous women do not have
significant hair loss.
• Homozygous men or women (b/b) become bald.
The trait is therefore dominant in men, recessive
in women. (We used b to designate the mutant
baldness allele even though the allele is
dominant in males.)