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Pedigree Analysis in Human Genetics Chp.4 Human Pedigrees


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Title: Pedigree Analysis in Human Genetics Chp.4 Human Pedigrees

Pedigree Analysis in Human Genetics Chp.4 Human
  • The use of pedigrees is an important method for
    analyzing the inheritance of traits in human

Fig. 4-CO, p. 70
4.1 Pedigree Analysis and Construction is a Basic
Method in Human Genetics
  • Analysis of pedigrees using knowledge of
    Mendelian principles allows us to
  • Determine whether the trait has a dominant or
    recessive pattern of inheritance
  • Determine whether the gene in question is located
    on an X or Y chromosome or on an autosome
  • This kind of information can be used to predict

Patterns of Inheritance
  • Patterns in the pedigree are used to determine
    how a trait is inherited
  • Autosomal dominant
  • Autosomal recessive
  • X-linked dominant
  • X-linked recessive
  • Y-linked
  • Mitochondrial inheritance

Keep In Mind
  • Pedigree construction and analysis are basic
    methods in human genetics

Pedigree Analysis
  • Pedigree analysis proceeds in several steps
  • Rule out patterns of inheritance that are
    inconsistent with the pedigree
  • If only one pattern of inheritance is consistent
    with the pedigree, it is accepted as the pattern
    for that trait
  • If more than one pattern in consistent with the
    pedigree, which one is expected to be more
  • If due to small sample size, it is impossible to
    choose among pattern of inheritance, the
    inclusion of more family members may be necessary

4.2 Autosomal Recessive Traits
  • Characteristics of autosomal recessive traits
  • For rare traits, most affected individuals have
    unaffected parents
  • All children of affected parents are affected
  • The risk of an affected child with heterozygous
    parents is 25
  • The trait is expressed in both males and females

Pedigree A Rare Autosomal Recessive Trait
Fig. 4-2, p. 73
Example of an Autosomal Recessive Trait Cystic
  • Cystic fibrosis A fatal recessive genetic
    disorder associated with abnormal secretions of
    the exocrine glands

Fig. 4-3a, p. 75
The Frequency of the Gene for Cystic Fibrosis in
the Human Population
  • 1 in 25 Americans of European descent
  • 1 in 46 Americans of Hispanic descent
  • 1 in 65 African Americans
  • 1 in 250 Asian Americans

Fig. 4-4, p. 75
Cystic Fibrosis Gene Product (CTRF)
  • The CFTR gene was identified in 1989
  • CFTR protein controls the movement of chloride
    ions across the plasma membrane

Fig. 4-5, p. 76
Exploring GeneticsWas Noah an Albino?
  • Noahs flesh was white as snow
  • From the Book of Enoch the Prophet
  • Phenotype Lack of pigmentation
  • Inheritance of albinism
  • An autosomal recessive trait
  • Normal, heterozygous parents (may be closely
  • Homozygous recessive offspring (albino)

4.3 Autosomal Dominant Traits
  • Characteristics of autosomal dominant traits
  • Heterozygotes have an abnormal phenotype
  • Every affected individual has at least one
    affected parent (except in traits with high
    mutation rates)
  • If an affected individual is heterozygous and has
    an unaffected mate, each child has a 50 chance
    of being affected
  • Two affected individuals can have an unaffected
  • Usually an affected family member in each

Pedigree An Autosomal Dominant Trait
Fig. 4-6, p. 77
Example of an Autosomal Dominant Trait Marfan
  • Marfan syndrome
  • An autosomal dominant genetic disorder that
    affects the skeletal system, cardiovascular
    system, and eyes
  • Individuals are tall, thin, long arms and legs.
    Thin fingers
  • Heart defects

Fig. 4-7, p. 77
Cardiovascular Effects of Marfan Syndrome
  • Marfan syndrome weakens connective tissue around
    the base of the aorta

4.4 Sex Linked Inheritance
  • Genes on sex chromosomes have a distinct pattern
    of inheritance
  • Males (XY) pass their X chromosome to all of
    their daughters but none of their sons
  • Females (XX) pass an X chromosome to all of their
  • Most genes on the X chromosome are not on the Y
  • Males carrying an X-linked recessive allele
    express the recessive phenotype

Distribution of Sex Chromosomes from Generation
to Generation
Fig. 4-10, p. 79
Sex Linked Traits
  • X-linked
  • Pattern of inheritance that results from genes
    located on the X chromosome
  • Y-linked
  • Pattern of inheritance that results from genes
    located only on the Y chromosome

In Males, Genes on the X chromosome Hemizygous
  • Hemizygous
  • A gene present on the X chromosome that is
    expressed in males in both the recessive and
    dominant condition

X-Linked Dominant Traits
  • Quite rare inheritance pattern
  • Affected males produce all affected daughters and
    no affected sons
  • A heterozygous affected female will transmit the
    trait to half of her children
  • Sons and daughters are equally affected
  • On average, twice as many daughters as sons are

Pedigree of an X-linked Dominant Trait
Fig. 4-11, p. 79
X-Linked Recessive Traits
  • X-linked recessive traits affect males more than
    females because males are hemizygous for genes on
    the X chromosome

X-Linked Recessive Inheritance
  • Affected males receive the mutant X-linked allele
    from their mother and transmit it to all of their
    daughters, but not to their sons
  • Daughters of affected males are usually
  • Sons of heterozygous females have a 50 chance of
    being affected
  • Hemizygous males (only one X) and females
    homozygous for the allele are affected

Pedigree X-Linked Recessive Inheritance
Fig. 4-12, p. 80
Example of an X-linked Recessive Trait Color
  • Color blindness
  • Defective color vision caused by reduction or
    absence of visual pigments
  • Three forms red, green, and blue blindness
  • About 8 of the male population in the US

Fig. 4-13, p. 80
Testing For Color Blindness
  • People with normal color vision see the number 29
    in the chart those who are color-blind cannot
    see the number

Fig. 4-14, p. 81
Color Blindness Defect in the Retina
  • Defects in photoreceptor cells of the retina
    (cone cells) cause color blindness

Fig. 4-15, p. 81
Example of an X-linked Recessive Trait Muscular
  • Muscular dystrophy
  • A group of genetic diseases associated with
    progressive degeneration of muscle tissue
  • Duchenne and Becker muscular dystrophy are
    inherited as X-linked recessive traits
  • Duchenne muscular dystrophy (DMD) affects 1 in
    3,500 males in the US

Molecular Characteristics ofDuchenne Muscular
  • Dystrophin proteins are flexible and that
    normally stabilize the muscle cells during
    contraction are defective
  • Plasma membranes are torn apart during muscle
    contraction, causing death of muscle tissue

Fig. 4-16, p. 82
4.5 Paternal Inheritance Y Chromosome
  • Only males have Y chromosomes
  • Genes on the Y chromosome are passed directly
    from father to son
  • All Y-linked genes are expressed
  • Males are hemizygous for genes on the Y
  • To date only 36 Y-linked traits have been

Pedigree Y-Linked Traits
Fig. 4-18, p. 84
4.6 Non-Mendelian Maternal Inheritance
Mitochondrial Genes
  • Mitochondria
  • Cytoplasmic organelles that convert energy from
    food into ATP (ATP powers cellular functions)
  • Carry DNA for 37 mitochondrial genes
  • Genetic disorders in mitochondrial DNA are
    associated with defects in energy conversion

Mitochondrial Inheritance
  • Mitochondria (and genetic disorders caused by
    mutations in mitochondrial genes) are maternally
  • Mitochondria are transmitted from mothers to all
    their offspring through the cytoplasm of the egg

Pedigree Mitochondrial Inheritance
Fig. 4-19, p. 84
Exploring GeneticsHemophilia and History
  • Queen Victoria passed the X-linked recessive gene
    for hemophilia to several of her children

p. 85
4.7 An Online Catalog of Human Genetic Traits
  • OMIM
  • Genetic traits are described, cataloged, and
    numbered in a database called Online Mendelian
    Inheritance in Man
  • OMIM is updated daily and contains information
    about all known human genetic traits
  • Each trait is assigned an OMIM number
  • There are more that 10,000 entries

4.8 Many Factors can Affect the Pattern of
  • Variations in gene expression affect pedigree
    analysis and assignment of genotypes to members
    of the pedigree
  • Several factors can affect gene expression
  • Interactions with other genes in the genotype
  • Interactions between genes and the environment

4.8 Many Factors can Affect the Pattern of
  • Phenotypes are often age related
  • Example Huntington disease
  • Penetrance and expressivity cause variations in
  • Penetrance the probability the the phenotype
    will appear
  • Expressivity The range of phenotypes from a
    given genotype

An Example of Incomplete Penetrance and Variable
  • Camptodactyly
  • A dominant trait (immobile, bent little fingers)
    with variable expression

Fig. 4-22, p. 88
Keep In Mind
  • Patterns of gene expression can be influenced by
    many different environmental factors

Common recessive alleles can produce pedigrees
that resemble dominant inheritance
  • Common alleles can enter a pedigree from outside
    the family and thus appear dominant

Fig. 4-23, p. 88
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