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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 • • • 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 • • 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. • • 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