Pedigrees Spinal Muscular Atrophy Severe muscle weakness from

Pedigrees

Spinal Muscular Atrophy Ø Severe muscle weakness from birth Ø Loss of motor neurons in the spinal cord

Evaluating Mode of Inheritance Autosomal Dominant?

Evaluating Mode of Inheritance Autosomal Recessive?

Fred’s Pedigree

Get to Know Your Classmate Ø Pair up (preferably) with someone you do not know Ø Pick a trait or disease found in your family or you partner’s family Ø Draw a pedigree depicting the inheritance of the trait/disease Ø Try to determine the mode of inheritance

Joe Joey Jimmy Jerry Elizabeth Mildred Jeffrey Elizabeth Mom Dad Blondeness in Professor Morrissette’s Family Vicki Bob Doug Stuart Marion Johnny Zoey DAM Norm Billy Roger Savannah Jasmine

Variations to Mendel’s Laws Extensions and Exceptions

Alterations to Mendel’s Ratios Ø In these cases, genotypic ratio is as Mendel predicted but phenotypic ratio is altered l l l l Lethal allele combinations Multiple alleles Different dominance relationships Epistasis Penetrance and expressivity Pleiotropy Genetic heterogeneity Phenocopies

Lethal Allele Combinations

Multiple Alleles Ø Give a range of phenotypes Ø Each of us has 2 alleles for any given gene l 1 from mom and 1 from dad Ø There can be many different alleles for a gene l Different alleles are formed by mutations

Example of Multiple Alleles -PKU Ø Phenylketonuria (PKU) l Enzyme that breaks down phenylalanine is deficient • Phenylalanine accumulates l l There are hundreds of possible alleles Allelic combinations give rise to different phenotypes • • Severe mental retardation Moderate PKU Mild PKU Asymptomatic PKU

Different Dominance Relationship – Incomplete dominance The heterozygous phenotype is intermediate between the homozygous dominant and the homozygous recessive Ø Blended phenotype Ø

Different Dominance Relationship – codominance Ø Phenotypes of both alleles are expressed

Codominance –Blood Type

Codominance –Blood Type

Epistasis Ø One gene affects the expression of another l Example: If a dog has the hairless gene, the genes that affect hair color will not be expressed

Penetrance and Expressivity Ø Ø Describe degrees of inheritance Due to multifactorial inheritance l Ø Penetrance =all-or-none expression of a gene l Ø Expression of a gene is influenced by other genes and by environment 100% of the people who inherit mutant amyloid precursor protein (m. APP) develop Alzheimer disease so m. APP is 100% penetrant Expressivity =severity of gene expression l Polydactyly has variable expressivity • Some have an extra toe and others have an extra toe and an extra finger.

Pleiotropy Ø Gene affects several functions l Ø Phenotype is varied Example: Porphyria variegata

Genetic Heterogeneity Different genes produce the same phenotype Ø Example: Ø l Hearing loss may be due to one of 132 different genes that follow autosomal recessive inheritance B, b =gene for hearing loss type 1 R, r =gene for hearing loss type 2 BBrr bb. RR deaf Bb. Rr NOT deaf

Phenocopy Ø An environmentally caused trait that appears to be inherited l l Example: Phocomelia is a rare genetic disorder whose effects are mimicked by the teratogen, thalidomide Example: AIDs transmission from mother to offspring