Transcript Sex linked inheritance, sex linkage in Drosophila and man

Pedigree Analysis
Lecture 8
Dr. Attya Bhatti
Pedigree

A pictorial
representation
of
a family
history, essentially a family tree that
outlines the inheritance of one or more
characteristics.
Introduction:
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A “family tree,” drawn with standard genetic symbols, showing
inheritance patterns for specific phenotypic characters is called
pedigree.
Analysis of inheritance pattern of phenotypic characters in a
pedigree is called pedigree analysis.
Propositus/Proband: A member of a family who first comes to the
attention of a geneticist.
The investigator then traces the history of the phenotype in the
propositus back through the history of the family and draws a family
tree, or pedigree.
Goals of Pedigree Analysis

1.
Determine the mode of inheritance:
recessive,
partial
dominance,
sex-linked,
dominant,
autosomal,
mitochondrial, maternal effect.

2. Determine the probability of an affected offspring for
a given cross.
Pedigree Analysis

Males in a pedigree are represented by squares, females by circles. A
horizontal line drawn between two symbols representing a man and a
woman indicates a mating; children are connected to their parents by
vertical lines extending below the parents.
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Persons who exhibit the trait of interest are represented by filled
circles and squares.
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Unaffected persons are represented by open circles and squares.
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Each generation in a pedigree is identified by a Roman numeral; within
each generation, family members are assigned Arabic numerals, and
children in each family are listed in birth order from left to right.
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Deceased family members are indicated by a slash through the circle
or square.
Basic Symbols
Generations labelled roman numerals I, II, ...
Individuals labelled arabic numerals 1, 2, ...
Inheritance Pattern
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Autosomal Dominant Inheritance
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Autosomal Recessive Inheritance
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X-Linked Inheritance
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X-Linked Recessive Inheritance

X-Linked Dominant Inheritance
Y-linked Inheritance
Autosomal recessive disorders
1. Appears in both sexes with equal
frequency.
2. Trait tends to skip generations.
3. Affected offspring are usually
born to unaffected parents.
4. When both parents are
heterozygous, approximately 1/4
of the offspring will be affected.
5. Appears more frequently among
the children of consanguine
Marriages.
For example, the human disease
phenylketonuria is inherited in a simple
Mendelian manner as a recessive
phenotype.
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The affected phenotype of an
autosomal recessive disorder
is determined by a recessive
allele, and the corresponding
unaffected
phenotype
is
determined by a dominant
allele.
Autosomal recessive disorders
Pedigree of a rare recessive phenotype determined by a recessive allele.
Autosomal Dominant disorders
1. Appears in both sexes with equal frequency.
2. Both sexes transmit the trait to their offspring.
3. Does not skip generations.
4. Affected offspring must have an affected parent, unless they
possess a new mutation.
5. When one parent is affected (heterozygous) and the other
parent is unaffected, approximately 1/2 of the offspring will be
affected.
6. Unaffected parents do not transmit the trait.
Autosomal dominant disorders

1.
Key points are,
The main clues for identifying a dominant disorder with
Mendelian inheritance are that the phenotype tends to appear
in every generation of the pedigree
and
2.
That affected fathers and mothers transmit the phenotype to
both sons and daughters.
Autosomal dominant disorders
Fig: Pedigree of a dominant phenotype determined by a
dominant allele.
X-Linked Recessive Inheritance
1. More males than females are affected.
2. Affected sons are usually born to unaffected mothers; thus,
the trait skips generations.
3. A carrier (heterozygous) mother produces approximately
1/2 affected sons.
4. Is never passed from father to son.
5.
All daughters of affected fathers are carriers.
E.g; Hemophilia
X-Linked Recessive Inheritance
X-linked recessive disorders
Fig: Pedigree showing that X-linked recessive alleles
expressed in males are then carried unexpressed by
their daughters in the next generation, to be expressed
again in their sons. Note that III-3 and III-4 cannot be
distinguished phenotypically.
X-Linked Dominant Inheritance
1. Both males and females are affected; often more females
than males are affected.
2. Does not skip generations. Affected sons must have an affected
mother; affected daughters must have either an affected mother or an
affected father.
3. Affected fathers will pass the trait on to all their daughters.
4. Affected mothers (if heterozygous) will pass the trait on to 1/2 of
their sons and 1/2 of their daughters.
X-Linked Dominant Inheritance
X-linked dominant disorders.
Fig: Pedigree showing that all the daughters of a
male expressing an X-linked dominant phenotype
will show the phenotype.
X-linked dominant disorders.
Pedigree showing that females affected by an X-linked
dominant condition are usually heterozygous and pass the
condition to half their sons and daughters
Y-linked inheritance
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Only males are affected.
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Is passed from father to all sons.
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Does not skip generations.