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BIOL 1406: Ch. 14 (Mendel's path to the Theory of Inheritence…
BIOL 1406: Ch. 14
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Inheritance Patterns
Pedigree: A diagram of a family tree with conventional symbols, showing the occurrence of heritable characters in parents and offspring over multiple generations.
Carriers: Individuals who is heterozygous at a given genetic locus for a recessively inherited disorder. The heterozygote is generally phenotypically normal for the disorder but can pass on the recessive allele to offspring.
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The expression of a genotype can be affected by environmental influences, resulting in a range of phenotypes. Polygenic characters that are also influenced by the environment are called multifactorial characters.
An organism's overall phenotype , including its physical appearance, internal anatomy, physiology, and behavior, reflects its overall genotype and unique environmental history. Even in more complex inheritance patterns. Mendel's fundamental laws still apply.
Quantitative characters: A heritable feature that varies continuously over a range rather than in an either-or fashion.
Multifactorial: Referring to a phenotypic character that is influenced by multiple genes and environmental factors.
Many human diseases are multifactorial, having both genetic and environmental components and do not follow simple Mendelian inheritance patterns.
Probability Laws
Mendel's laws of segregation and independent assortment reflect the same rules of probability that apply to tossing coins, rolling dice, and drawing cards from a deck.
The multiplication rule states that to determine this probability, we multiply the probability of one event by the probability of the other event .
The multiplication rule tells us that the probability of an F2 plant having wrinkled seeds (rr) is 1/4 and the probability of an F2 plant carrying both dominate alleles for seed shape (RR) is 1/4.
According to the addition rule, the probability that any one of two or more mutually exclusive events will occur is calculated by adding their individual probabilities.
Using the rule of addition, the calculated probability of an F2 heterozygote is 1/4 + 1/4 = 1/2.
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A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously.
In calculating the chances of the various offspring genotypes from such crosses, each character is first considered separately and then the individual probabilities are multiplied.
Inherited Disorders
Huntington's disease: A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20 years after the onset of symptoms.
Multifactorial disorders have a genetic component plus a significant environmental influence. Examples include heart disease, diabetes, cancer, alcoholism, and certain mental illnesses such as schizophrenia and bipolar disorder.
Sickle-cell disease: A recessively inherited human blood disorder in which a single nucleotide change in the alpha-globin gene causes hemoglobin to aggregate, changing red blood cell shape and causing multiple symptoms in afflicted individuals.
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Cystic fibrosis: A human genetic disorder caused by a recessive allele for a chloride channel protein; characterized by an excessive secretion of mucus and consequent vulnerability to infection; fatal if untreated.
Tay-Sachs disease: A human genetic disease caused by a recessive allele for a dysfunctional enzyme, leading to accumulation of certain lipids in the brain. Seizures, blindness, and degeneration of motor and mental performance usually become manifest a few months after birth, followed by death within a few years.