Pedigree Charts

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Pedigree Charts

• A family history of genetics

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What is a pedigree chart?

• Used to show records of families or individuals.
• Track the occurrence of diseases such as
• Huntingtons 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

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Symbols used in pedigree charts

• Normal male
• Affected male
• Normal female
• Affected female
• Marriage

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Organising the pedigree chart

• Generations are identified by Roman numerals

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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

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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
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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.

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Reading Pedigrees

• Is the trait dominant or recessive?
• Trait skips generations
• It is recessive!

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Reading Pedigrees

• Is the condition Sex linked or not?
• Appears in both males and females, therefore
  autosomal

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Albinism An Example

• Expressed in both sexes at approximately equal
  frequency.
• Thus, autosomal.
• Not expressed in every generation.
• Thus, recessive.

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Albinism Genotype the Affected Individuals

• 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.

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Albinism Genotype the Normal Individuals

• Normal individuals must have at least one A.

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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.

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Albinism Parental Genotypes are Known

• Both original parents are heterozygous.
• Normal offspring could have received an A from
  either parent, or from both.

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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.

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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.

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Hairy Ears Female Sex Determination

• All females are XX.

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Hairy Ears Male Sex Determination

• All males are XY.

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Hairy Ears Gene on the Y Chromosome

• A super-script H indicates the allele on the Y
  chromosome for hairy ears.

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Hairy Ears Wild-Type Allele for Normal Ears

• Code indicates the allele on the Y chromosome
  for normal ears.

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Hemophilia An Example

• In this pedigree, only males are affected
• Sons do not share the phenotypes of their
  fathers.
• Thus, hemophilia is linked to a sex
  chromosomethe X.
• Expression of hemophilia skips generations.
• Thus, it is recessive.

Extensive bruising of the left forearm and hand
in a patient with hemophilia.
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Hemophilia

• All females are XX.

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Hemophilia Expression of Male Sex Chromosomes

• All males are XY.

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Hemophilia Genotype the Affected Individuals

• Assign codes for the alleles.
• Code H for the recessive hemophilia allele.
• Code for the wild-type normal allele.
• Affected individuals must have an H on an X
  chromosome.
• All daughters of affected male must carry
  recessive allele

H
H
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Hemophilia Father-Daughter Relationship

• All daughters of an affected father receive an X
  chromosome with the H allele.

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Hemophilia Genotyping the Normal Individuals

• Normal individuals must have at least one X
  chromosome with the wild-type allele, .

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Hemophilia Homozygous or Heterozygous?

• Only males affected
• Not Y-linked
• Skips a generation recessive
• X-linked

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