Chapter 19 Definitions Gene DNA sequences that contain instructions for building proteins Chromosomes Structures within the nucleus composed of DNA and protein Humans have 23 pairs on chromosomes 22 pairs of homologus chromosomes these are also known as autosomes 1 pair of sex chromosomes determine gender Inheritance Characteristics or traits that are transmitted from parents to offspring via genes Alleles Alternative forms of a gene Alleles arise form mutation Homozygous Two identical alleles at a particular locus Heterozygous Twp different alleles at a particular locus Mutations A change in the DNA base pair sequence of a cell Genotype An individuals complete set of alleles Phenotype Observable physical and functional traits Phenotype is determined by inherited alleles and environmental EX Hair color eye color skin color blood type disease factors susceptibility Punnett Square Predicts patterns of inheritance Law of Segregation When gametes are formed in the parents the alleles separate from each other so that each gamete gets only one allele of each gene Incomplete Dominance Heterozygous phenotype is intermediate between that of either homosygote Dominant Masks or suppresses the expression of its complementary allele Always expressed even if heterozygous Recessive Will not be expressed if paired with a dominant allele heterozygous Law of Independent Assortment Genes for different traits are separated from each other independently during meiosis Codominance Products of both alleles are expressed Examples Genes for ABO blood types A gene and B gene are codominant An individual heterozygous for the A and B genes will be blood type AB expressing bot A and B antigens on red blood cells Sickle Cell Anemia HbS encodes sickle cell hemoglobin homozygous HbA encodes normal hemoglobin homozygous Affected individual makes both types of hemoglobin heterozygous Polygenic Inheritance Inheritance of phenotypic traits that depend on many genes Usually distributed within a population as a continuous range of values EX Eye color Skin color Height Body size and shape Sex Linked Inheritance Sex Chromosomes 23rd pair of chromosomes Not homologus X and Y chromosomes carry different genes Males have one X and one Y chromosome both 50 Determines gender of the offspring Females have two X chromosomes Deletion Piece of a chromosome breaks off Translocation Piece of chromosome breaks off and attaches to a different Nondisjunction during meiosis Failure of homologus chromosomes or sister chromosome chromatids to seperate