Transcript Pedigree Charts - hills

Pedigree Charts
A family history of genetics
What is a pedigree chart?
• Used to show records of families or individuals.
• Track the occurrence of diseases such as:
– Huntington’s – simple dominant – lethal allele – causes breakdown of
the brain
– Cystic fibrosis – 1/2500 – mucus accumulates (white North Amer.)
– Tay-Sachs disease – lipids accumulate in CNS (Jewish)
– Phenylketonuria – missing enzyme causes problems in CNS
(Nordic/Swedish)
• Shows how possible conditions may be inherited.
• Uses symbols to represent the individuals and their relationships to each
other
Symbols used in pedigree charts
•
Normal male
•
Affected male
•
Normal female
•
Affected female
•
Marriage
A marriage with five children, two
daughters and three sons. The second
son is affected by the condition.
Eldest child  Youngest child
Organising the pedigree chart
• Generations are identified by Roman numerals
I
II
III
IV
Organising the pedigree chart
•
•
Individuals in each generation are identified by Arabic numerals numbered from
the left
Therefore the affected individuals are II3, IV2 and IV3
I
II
III
IV
Other Symbols Used
Dizygotic twins
Two zygotes - fraternal
Adopted
Monozygotic
Twins
One zygote - identical
Consanguineous marriage
people descended from the
same ancestor
Sex unspecified
Divorced
Number of children of
each sex indicated
Multiple marriage
Affected
Extra marital union
Factors to Consider in Pedigrees
• Is the trait located on a sex chromosome or an autosome?
– Autosomal – not on a sex chromosome
– Sex Linkage – located on one of the sex chromosomes
• Y-linked - only males carry the trait.
• X-linked (recessive) - sons inherit the disease from normal parents
– Sons can never inherit from their father!
• How is the trait expressed?
– Dominant - the trait is expressed in every generation.
– Recessive - expression of the trait may skip generations.
Reading Pedigrees
•
Is the trait dominant or recessive?
•
Trait skips generations
• It is recessive!
Reading Pedigrees
• Is the condition Sex linked or not?
• Appears in both males and females, therefore autosomal
Albinism: An Example
• Expressed in both sexes at approximately equal frequency.
– Thus, autosomal.
• Not expressed in every generation.
– Thus, recessive.
Albinism: Genotype the Affected
Individuals
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•
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Assign codes for the alleles.
– Code “A” for the dominant normal allele.
– Code “a” for the recessive allele for albinism.
Affected individuals must be homozygous for “a.”
First generation parents must be “Aa” because they have normal phenotypes, but
affected offspring.
Albinism: Genotype the Normal
Individuals
• Normal individuals must have at least one “A.”
Albinism: Parent-Offspring
Relationships
• II6 must transmit “a” to each offspring.
• The “A” in the offspring must come from the father.
– Normal father could be either heterozygous or homozygous for an
“A.”
Albinism: Parental Genotypes are
Known
• Both original parents are heterozygous.
• Normal offspring could have received an “A” from either parent, or from
both.
Albinism: One Parental Genotype
is Known
• Only the genotype of the offspring expressing albinism are known.
• Normal offspring must have received an “a” from their affected father.
Hairy Ears: An Example
• Only males are affected.
• All sons of an affected father have hairy ears.
– Sons have the same trait as their fathers
• Thus, hairy ears is Y-linked.
Hairy Ears: Female Sex
Determination
• All females are XX.
Hairy Ears: Male Sex
Determination
• All males are XY.
Hairy Ears: Gene on the Y
Chromosome
• A super-script “H” indicates the allele on the Y chromosome for hairy ears.
Hairy Ears: Wild-Type Allele for
Normal Ears
• Code “+” indicates the allele on the Y chromosome for normal ears.
Hemophilia: An Example
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•
In this pedigree, only males are affected
– Sons do not share the phenotypes of their fathers.
– Thus, hemophilia is linked to a sex chromosome–the X.
Expression of hemophilia skips generations.
– Thus, it is recessive.
Extensive bruising of
the left forearm and
hand in a patient with
hemophilia.
Hemophilia
• All females are XX.
Hemophilia:
Expression of Male Sex Chromosomes
• All males are XY.
Hemophilia: Genotype the
Affected Individuals
•
Assign codes for the alleles.
– Code “H” for the recessive hemophilia allele.
– Code “+” for the wild-type normal allele.
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•
Affected individuals must have an “H” on an X chromosome.
All daughters of affected male must carry recessive allele
H
H
Hemophilia: Father-Daughter
Relationship
• All daughters of an affected father receive an X chromosome with the “H”
allele.
Hemophilia: Genotyping the
Normal Individuals
• Normal individuals must have at least one X chromosome with the wildtype allele, “+.”
Hemophilia: Homozygous or
Heterozygous?
• Only males affected
– Not Y-linked
• Skips a generation: recessive
– X-linked
Pedigree of Queen Victoria