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  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

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

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

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

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)

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

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