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Biology 311 Human Genetics Lecture 4 "Pedigree Analysis" Fall 2004 Reading: Chap. 4 pp. 102-106 Lecture Outline: 1. Mendelian genetics 2. Pedigree symbols (review) 3. Pedigree patterns 4. Mitochondrial inheritance Lecture: 1. Mendelian genetics simplest genetic characters=Mendelian traits one genetic locus mainly determines characteristic Mendel deduced: Genes are inherited as discrete units Two versions (alleles) of a gene determine each trait Terms to know: Dominance Recessiveness Homozygous Heterozygous Hemizygous Genotype Phenotype Be able to carry out monohybrid and dihybrid crosses. 2. Pedigree Symbols See Fig. 4.1 Male Female Sex unknown Affected 1 Unaffected Carrier Marriage or mating Consanguineous marriage Twins Dead Generations I, II, etc. Individuals 1, 2, 3, etc. Proband or propsitus 3. Basic Pedigree Patterns a. b. c. d. e. Box 4.1 and Fig. 4.2 autosomal dominant inheritance autosomal recessive inheritance X-linked recessive inheritance X-linked dominant inheritance Y-linked inheritance a. Autosomal dominant A=dominant a=recessive Usually involves matings of an affected individual with an unaffected individual. Aa x aa 1/2 Aa dominant trait 1/2 aa recessive trait 2 Examples: Familial hypercholesterolemia Hereditary colon cancer Huntington disease Neurofibromatosis Achondroplasia (dwarfism) b. autosomal recessive A=dominant trait (normal) a=recessive trait (unaffected) matings are usually between two unaffected individuals transmission of the trait occurs when two heterozygotes mate Aa x Aa 1/4 AA 2/4 Aa 1/4 aa 3/4 A- normal 1/4 aa recessive trait Examples: Tay-Sachs Cystic fibrosis Sickle cell anemia Phenylketonuria c. X-linked recessive trait Xa=recessive trait XA=normal dominant trait X-linked recessive trait affects mainly males Usually is passed from a carrier mother to her son XaXA x XAY 1/4 XAXa 1/4 XAXA 1/4 XAY 1/4XaY female carrier (normal) female normal male normal male affected 3 Examples: Hemophilia A Duchenne muscular dystrophy Red-green color blindness d. X-linked dominant inheritance XA=dominant trait (affected) Xa=recessive trait (normal) Affects males and females, but occurs more often in females Usually an affected individual marries an unaffected individual XAXa x XaY 1/4 XAXa 1/4 XaXa 1/4 XAY 1/4 XaY affected female normal female affected male normal male Example: Vitamin D-resistant rickets e. Y-linked inheritance affects only males affected males have an affected father very few Y-linked traits; maleness only known Y-linked character XX x XY* XX XY* normal female affected male No Y-linked diseases known 4. Mitochondrial Inheritance Mendel's laws apply to nuclear genes, not mitochondrial genes. A few genes are carried in the small circular DNA of mitochondria. Defects in mtDNA are responsible for some human diseases. 4 These diseases are maternally inherited because at fertilization only female parent (egg) contributes mitochondria to zygote, portion of sperm containing the mitochondrion does not enter egg. Example: MtDNA associated deafness shown in Fig. 4.4 5. Complementation Box 4.2 In complementation, mutant characters are combined and the result is restoration of the wild type phenotype. Use to determine whether two recessive characters are due to alleles at the same gene or to two different genes. Example: autosomal deafness due to two distinct genes. Fig. 4.3 Pedigree shows that offspring of two parents affected by two different types of deafness are unaffected. Locus heterogeneity: Where the same clinical phenotype can result from mutations from any of several different genes. 5