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Chapter 12.3. Examples of Autosomal Inheritance Patterns. AP Biology Fall 2010. Autosomal Dominant Inheritance. The dominant allele is nearly always expressed, even in heterozygotes

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Chapter 12 3 l.jpg

Chapter 12.3

Examples of Autosomal Inheritance Patterns

AP Biology

Fall 2010


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Autosomal Dominant Inheritance

  • The dominant allele is nearly always expressed, even in heterozygotes

  • If one parent is heterozygous and the other homozygous recessive, there is a 50 percent chance that any child will be heterozygous

    • Draw a punnette square to illustrate this

    • Cross a normal mother with an affected father

      • aa x Aa

      • aa = normal

      • Aa = affected


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Autosomal Dominant Inheritance

  • If the gene (and its resulting disorder) reduces the chance of surviving or reproducing, its frequency should decrease

    • May not due to mutations, nonreproductive effects, and post reproduction onset


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Autosomal Dominant Inheritance

  • Achondroplasia (dwarfism): is a benign abnormality that does not affect persons to the point that reproduction is impossible

    • The gene is passed on in heterozygotes


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Autosomal Dominant Inheritance

  • Huntington disease is a series degeneration of the nervous system with an onset past the mid-thirties

    • By which time the gene has (usually) been passed from parent to offspring unknowingly


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Autosomal Recessive Inheritance

  • Characteristics of this condition:

    • Either parent can carry the recessive allele on an autosome

    • Heterozygotes are symptom-free carriers

    • Homozygotes are affected

    • Two heterozygous parents have a 50 percent chance of producing heterozygous children and a 25 percent chance of producing a homozygous recessive child

    • When both parents are homozygous, all children will be affected

  • Try crossing a carrier mother (Aa) with a carrier father (Aa)


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Autosomal Dominant Inheritance

  • Galactosemia: the inability to metabolize lactose

    • Is an example of autosomal recessive inheritance in which a single gene mutation prevents manufacture of an enzyme needed in the conversion pathway


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What About Neurobiological Disorders?

  • Patterns of Mendelian genetics are not followed by human neurobiological disorders

  • In most cases a lone gene does not give rise to disorders such as: depression, schizophrenia, or bipolar

  • Researches predict that having certain mutant autosomal alleles increases the chance of developing schizophrenia

    • Mutant alleles are also linked to bipolar disorder and depression