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Genetics Chapter 8 Three Principles of Mendel: Principle of dominance & recessiveness.- In heterozygous individuals, ONLY the dominant allele achieves expression. The recessive allele is present but remains unexpressed. In order to express a recessive allele, one has to be homozygous for the trait (they must have 2 recessive alleles) pg. 119 #5 Principle of segregation.-The members of each pair of alleles separate when gametes are formed. A gamete will receive one allele or the other. pg. 122 Principle of independent assortment.-2 or more pairs of alleles segregate independently of one another during gamete formation. Pg.122 Origins of Genetics Heredity The passing of characters from parents to offspring Character Inherited characteristic (ex. flower color) Trait Single form of a characteristic (ex. purple flower) Genes Control the expression of traits (i.e. flower color) Have two parts - alleles One allele is from Mom other from Dad. Alleles Dominant - the expressed allele of the gene Capital letter Ex. P Recessive - the allele that was not expressed Lower case letter Ex. P Homozygous (Homo = same) Two alleles are the same Ex. PP or pp Heterozygous (Hetero = different) The alleles are different Ex. Pp Modern Terms Genotype The set of alleles for a Gene Ex. PP, Pp or pp Phenotype Physical appearance Ex. Purple or White Flowers The dominant allele determines phenotype Ex. Pp Purple Flower or PP Purple Flower If both alleles are recessive then the individual will express the recessive phenotype. Ex. pp White Flower Probability The likelihood that a specific event will occur Expressed in multiples ways: Fractions Percentages Decimals Ratios Probability = ½ 50% .5 3:1 or 3 to 1 # of one kind of possible outcome Total # of all possible outcomes Incomplete Dominance Inheritance that is intermediate between two parents Ex. Snapdragon with red flowers are crossed with a snapdragon with white flowers a snap dragon with pink flowers is produced. Neither the allele for red flowers nor the allele for white flowers is dominant. Codominance Case in which both forms of the character are displayed. Ex. The ABO blood groups Neither IA nor IB are dominant is dominant over the other. Both are dominant over i. When both IA and IB are present they are codominant and the individual is type AB Blood Typing Blood Types A IA IB i AB B O IA IB i IAIA IAIB IAi IAIB IBIB IBi IAi IBi ii Sex-Linked Traits Body cell has 46 chromosomes 44 autosomes, 2 sex chromosomes. Gamete (egg/sperm) has 23 chromosomes – 22 autosomes, 1 sex chromosome Sex-Linked Gene A gene located only on the X or Y Most are carried on the X chromosome Males have only one X chromosome Male who carries a recessive allele on the X chromosome will exhibit the sex-linked condition Hemophilia (Heme: blood, philia: liking of) Condition that impairs the blood’s ability to clot A sex-linked trait Carriers and victims contain the recessive allele on their X-chromosomes Only females can be carriers XHXh Because males only have one X chromosome Males CAN NOT be carriers only victims. Genetic Disorders Sickle Cell Anemia Recessive disorder among African Americans Caused by a mutated allele that produces a defective hemoglobin protein. Hemoglobin in RBC’s bind and transport oxygen through the body Genetic Disorders Cystic Fibrosis Fatal hereditary recessive disorder among Caucasians. Clogging of the airways and lungs with thick mucus and the blockage of ducts of the liver and pancreas. There is no known cure. Gene Mutations: changes to the DNA that result in a change in the protein. Mutagen: substance or condition that causes or increases the rate of mutation. Ex: viruses, x-rays, cigarettes, etc… Mutation: permanent change in the genetic material of a cell. codon of a gene. – set of 3 DNA bases Ex: GAG to GUG D. Frameshift Mutation: insertion or deletion of a nitrogen base that produces a type of gene mutation. Genetic Disorders Albinism Recessive Body is unable to produce an enzyme necessary to make melanin. Melanin is a pigment that gives color to hair, skin and eyes Chromosome Rearrangements: Inversion: when part of a chromosome is broken & becomes reinserted backwards. Translocation: A broken piece of chromosome attaches to a chromosome of a different pair. Deletion: when part of a chromosome breaks off and is lost. D. Nondisjunction: homologous chromosomes fail to separate. Monosomy: only has one chromosome of a pair. Lethal G. Trisomy: 3 copies of a chromosome. Ex. Down’s Syndrome Pedigrees A family history that shows how a trait is inherited over several generations Helps to track down the carriers (heterozygotes) of recessive disorders. Reading Pedigrees A shaded box – affected male ………… A shaded circle- affected female………. A clear box - normal male………….. A clear circle - normal female………...