Ch.5 E.Baker Genetics Fall 2012

Circumstances that appear to contradict Mendel’s law---although the law actually still apply. 1)Lethal alleles-an allele that causes early death Some allele combinations cause such severe problems in an embryo or fetus that development stops Sometimes considered lethal if they cause death before a person is old enough to reproduce Can cause miscarriages Sometimes a double dose of a dominant allele is lethal Ex: Mexican hairless dogs

2)Multiple alleles-a gene that exists in more than 2 allele forms, although a diploid individual only has 1 or 2 of them. Although each person has 2 alleles for any autosomal gene (one on each chromosome) a gene can exist in more than 2 allelic forms Ex: Blood Types

3)Co-dominance-alleles that are co- dominant to each other are both expressed in a heterozygote, such as a person who has AB blood type 4)Incomplete dominance-heterozygous phenotype is intermediate b/t that of either homozygotes. Ex: Snapdragons (red, white, pink)

5)Epistasis-refers to one gene’s interfering with the expression of a different gene. Ex: Labrador Retrievers,Bombay Phenotype 6)Penetrance and Expressivity Penetrance-refers to whether a phenotype is expressed for a particular genotype a.Completely penetrant-everyone who inherits the combination has some symptoms b.Incompletely penetrant-some individuals do not express the phenotype or they have no symptoms Ex: Polydactyly (having extra fingers or toes) Penetrance is expressed numerically as a percentage

Expressivity-refers to the degree to which a phenotype is expressed when it is expressed; intensity or severity of symptoms (ex. Do they have extra fingers and/or toes on 1 foot, both feet, etc?). 7)Pleiotropic-refers to the appearance of several apparently unrelated phenotypic effects caused by a single gene a. Marfan syndrome -autosomal dominant gene produces a defect in an elastic connective tissue. b.Porphyria-inherited as a dominant trait that leads to intermittent attacks of pain and dementia with symptoms first appearing in adulthood. c.Sickle cell anemia -caused by double dose of the recessive allele 8)Phenocopy-an environmentally caused trait that appears to be inherited, the trait either resembles a Mendelian disorder or mimics inheritance by occurring in certain relatives. Ex: children with AIDS

9)Genetic Heterogeneity-occurs when different genes cause the same phenotype, different genes can produce phenotypes that are clinically indistinguishable. Ex: 132 forms of deafness

When Mendel’s Laws do NOT apply. 1)Mitochondrial genes & maternal inheritance- mitochondrial genes are maternally inherited b/c only females transmit them; Ex: mitochondrial illnesses tend to affect cells w/ an abundance of mitochondria (such as muscle cells) 2)Uniparental Disomy-rare inheritance of a double dose of genetic material from 1 parent but none from the other; the term literally means “2 bodies from one parent”; caused by nondisjunction during meiosis II of both parents. Genomic imprinting-a phenotype that is different depending upon the sex of the parent who passes it on. Ex: a) Prader-Willi syndrome- child is obese, small hands & feet, does not develop signs of puberty (double dose of mom’s DNA or missing DNA in 1 parent’s gene to be expressed) b) Angelman’s syndrome- child has extended tongue, poor muscle coordination, large jaw, laughs uncontrollably, & arm flapping (double dose of dad’s DNA).

3) Linkage- refers to transmission of genes on the same chromosome; linked genes do not assort independently and do not result in predicted Mendelian ratios for 2 or more genes; the closer 2 genes are on a chromosome, the greater the probability of them being inherited together, the farther apart 2 genes are on a chromosome, the greater the probability that crossing over will occur b/t them.