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Patterns of Gene Inheritance Chapter 23 Genetics is the study of genes What is a gene? A. A factor that controls a heritable characteristic B. Something on a chromosome C. Information stored in a segment of DNA D. Something that encodes a protein Gregor Mendel LOTS O’ DEFINITIONS • Gene---A piece of DNA that encodes a particular trait. EX a gene for eye-color • Allele—an alternate form of a gene. EX—allele for blue eyes and allele for brown eyes • Phenotype—the physical expression of a gene or allele. Ex. Blue eyes • Genotype—the genetic composition of an individual. MORE DEFINITIONS • Locus—the location of a gene on a chromosome. Plural=loci • Dominant allele—expressed whether alone or in pairs. Symbolized by a capital letter. EX Brown eye allele=B • Recessive allele—expressed only in the absence of a dominant allele. Symbolized by lowercase. EX blue eye allele=b • What is the maximum # of alleles a diploid individual can have at any given locus? STILL MORE DEFINITIONS!!! • Genotype—the genetic composition of an individual – Homozygous----containing a pair of the same alleles. Can be • Homozygous recessive—two recessive alleles EX. bb, or • Homozygous dominant– two dominant alleles EX BB – Heterozygous—containing two different alleles. EX Bb Law of Segregation • Each individual has two factors (called genes today) for each trait. • Factors segregate during gametogenesis. Ploidy level of gametes? • Fertilization gives each new individual two factors again. • Ploidy level after fertilization? Gene locus Defs Example--Widow’s Peak Gametogenesis • Homologous pairs separate during meiosis. When?  ∴ a gamete has only one allele from each pair of alleles.  ∴ If the primary spermatocyte is genotype Ww, the sperm cell would contain either a W or a w, but not both. • Ww represents the genotype of an individual. • Gametes are represented by W or w. [...]... n OR n n N N n n N N Dihybrid cross What are the possible genotypes of this individuals gametes? Probabilities Two-trait Testcross-How? Genetic Disorders • Pedigree charts- show patterns of inheritance Autosomal Dominant Disorders • Autosomes -Non-sex chromosomes • Dominant allele ∀ ∴when under what genotype(s)will an individual show the disorder? Examples: Autosomal Dominant Disorder • Neurofibromatosis... Huntington Disease – Progressive degeneration of the nervous system – Normal until middle age – No treatment – Gene has been isolated (chromosome 4), genetic testing can diagnose Huntington disease Autosomal recessive pedigree chart • Autosomes -Non-sex chromosomes • Recessive allele ∀ ∴when under what genotype(s)will an individual show the disorder? Autosomal Recessive Disorders • Tay-Sachs Disease –... independently of each other • When will different alleles not sort independently? Independent Assortment r he ther ot a M F E E e E e e n N N N n N e E e n e OR n n E Alignment of Homologs at Metaphase I Replication E e N E N N n Telophase II n Segregation and Independent Assortment MEIOSIS & GENETIC VARIATION Independent Assortment • Diploid organisms can produce 2n diff gametes • Each homologous pair... Most common lethal genetic disorder among Caucasians – Defective chloride ion transport protein – Osmotic imbalance results in thick mucous in lungs and pancreatic ducts Cystic fibrosis therapy Phenylketonuria (PKU) • allele on chromosome 12 • lack an enzyme needed for metabolism of phenylalanine (an amino acid) • Urine test diagnostic • Brain damage unless controlled by diet Polygenic Inheritance... bell-shaped curve EX: Skin color EX 2 Polygenic Inheritance : Polygenic Disorders • Examples: cancer, schizophrenia, hypertension, diabetes, etc – several genes involved – also environmental influences Multiple Allelic Traits • Often more than two alleles exist for a particular gene locus • Maximum # of alleles at any given locus? • Each individual inherits only two alleles for these genes!!! • EX: Human... can orient in either of two ways • Ea homologous pair can orient in two different ways Humans: 223 = 8,388,608 (est 8.4 million) HYPOTHETICAL EXAMPLE OF INDEPENDENT ASSORTMENT Eye color Hair color Gene for brown eyes Gene for blue eyes r Gene for black hair Gene for red hair INDEPENDENT During meiosis I, tetrads can line up 2 different ways ASSORTMENT n OR Meiosis I & II Brown eyes Black hair Blue . maximum # of alleles a diploid individual can have at any given locus? STILL MORE DEFINITIONS!!! • Genotype—the genetic composition of an individual – Homozygous----containing. VARIATION Independent Assortment • Diploid organisms can produce 2 n diff. gametes • Ea. homologous pair can orient in two different ways • Humans: 2 23 =