Chapter 12.3

Examples of Autosomal Inheritance Patterns

AP BiologyFall 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