Chapter 19 Genetics and Inheritance Alleles Different forms

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 Copyright © 2009 Pearson Education, Inc.

Punnett Squares: Copyright © 2009 Pearson Education, Inc. 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 Copyright © 2009 Pearson Education, Inc.

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 Copyright © 2009 Pearson Education, Inc.

Allelic Interactions § Incomplete dominance § 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 Copyright © 2009 Pearson Education, Inc.

Pedigree Chart: Inheritance Pattern for an X-Linked Recessive Disease Copyright © 2009 Pearson Education, Inc. 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 Copyright © 2009 Pearson Education, Inc.

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 Copyright © 2009 Pearson Education, Inc.

Chromosomes May Be Altered in Number and Structure Altered chromosome structure Copyright © 2009 Pearson Education, Inc.

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 Copyright © 2009 Pearson Education, Inc.

Genes and Behavior § Mechanism § Product of gene-specific proteins § Proteins have specific functions leading to phenotypes § Protein functions § Hormones, enzymes, structural, neurotransmitters Copyright © 2009 Pearson Education, Inc.