1. 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

2. Punnett Squares: predicts patterns of inheritance
Figure 19.1

3. 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

4. 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

5. 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

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

7. 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

8. 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

9. Chromosomes May Be Altered in Number and Structure
Altered chromosome structure

10. 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

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