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GENETICS AND INHERITANCE
CHAPTER 19
Alleles
• Different forms of homologous genes: humans
have 23 pairs of chromosomes
• Homozygous: both alleles identical
• Heterozygous: alleles different
• Genotype: complete set of genes and alleles
• Phenotype: observed physical and functional
traits
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings.
Punnett Squares
Figure 19.2
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings.
Patterns of Genetic Inheritance
• Punnett square analysis: predicts patterns of
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 © 2003 Pearson Education, Inc. publishing as Benjamin Cummings.
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
• Incomplete dominance: heterozygote is an
intermediate phenotype
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings.
Allelic Interactions
(cont.)
• Codominance: both phenotypes equally
expressed
• Polygenic inheritance: multiple genes involved
• Linked genes: may be inherited together if close
on a chromosome
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings.
Pedigree Chart: Inheritance Pattern
for an X-linked Recessive Disease
Figure 19.12
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings.
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 © 2003 Pearson Education, Inc. publishing as Benjamin Cummings.
Genes and Behavior
• Mechanism
• Product from gene-specific proteins
• Proteins have specific functions leading to
phenotypes
• Protein Functions: hormones, enzymes,
structural, neurotransmitters
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings.