Chapter 19: Genetics and Inheritance Alleles Different forms of homologous genes Humans have 23 pairs of chromosomes Homozygous Both alleles identical Heterozygous Alleles are different Genotype Complete set of genes and alleles Phenotype Observed physical and functional traits Pearson - The Biology Place

Punnett Squares: predicts patterns of inheritance Figure 19.1

Patterns of Genetic Inheritance Mendel developed basic rules of inheritance Law of segregation Reproductive cells carry only one copy of each gene Law of independent assortment Genes for different traits are separated from each other independently during meiosis Applies in most cases

Allelic Interactions Genotype and environment affect phenotype Dominant alleles Gene always expressed, even if heterozygous Recessive alleles Two copies of gene needed to be expressed; must be homozygous

Allelic Interactions Incomplete dominance Codominance Heterozygote is an intermediate phenotype Codominance Both phenotypes equally expressed Polygenic inheritance Multiple genes involved Linked alleles May be inherited together if close on a chromosome

Pedigree Chart: Inheritance Pattern for an X-Linked Recessive Disease Figure 19.16

Sex-Linked Inheritance: X and Y Chromosomes Origin of sex chromosomes Chance mutations Y determines sex Sex-linked genes located on sex chromosomes Sex-linked or X-linked inheritance Characteristics Mostly males with disease Passed to sons by mother Father cannot pass the gene Sex-influenced traits Affected by presence of testosterone, estrogen

Chromosomes May Be Altered in Number and Structure Nondisjunction during meiosis Examples Down syndrome: trisomy 21 Alterations of the number of sex chromosomes: XYY: double-Y syndrome XXY: Klinefelter syndrome XXX: trisomy-X syndrome XO: Turner syndrome; no Y

Chromosomes May Be Altered in Number and Structure Altered chromosome structure

Inherited Genetic Disorders Mechanism Most mutations usually involve recessive alleles Phenylketonuria Tay-Sachs disease Dominant lethal allele Huntington disease Always expressed, though at midlife Always lethal

Genes and Behavior Mechanism Protein functions Product of gene-specific proteins Proteins have specific functions leading to phenotypes Protein functions Hormones, enzymes, structural, neurotransmitters