Download Alleles Alleles

Chapter 19
Genetics and
Inheritance
Suggested Lecture
Presentation
Robert J. Sullivan
Marist College
Copyright © 2009 Pearson Education, Inc.
Alleles
 Different forms of homologous genes
 Humans have 23 pairs of chromosomes
 Homozygous
 Both alleles identical
 Heterozygous
 Alleles are different
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Alleles
 Genotype
 Complete set of genes and alleles
 Phenotype
 Observed physical and functional traits
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Punnett Squares
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Figure 19.1
Patterns of Genetic Inheritance
 Punnett square analysis
 Predicts patterns of inheritance
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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
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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
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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
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Pedigree Chart: Inheritance Pattern for an XLinked Recessive Disease
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Figure 19.16
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Sex-Linked Inheritance: X and Y
Chromosomes
 Origin of sex chromosomes
 Chance mutations
 Y determines sex
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Sex-Linked Inheritance: X and Y
Chromosomes
 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
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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
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Chromosomes May Be Altered in Number
and Structure
 Deletions and translocations
 Alter chromosome structure
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Inherited Genetic Disorders
 Mechanism
 Most mutations usually involve recessive
alleles
y
 Phenylketonuria
 Tay-Sachs disease
 Dominant lethal allele
 Huntington disease
 Always expressed, though at midlife
 Always lethal
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Genes and Behavior
 Mechanism
 Product of gene-specific proteins
 Proteins have specific functions leading to
phenotypes
y
 Protein functions
 Hormones, enzymes, structural,
neurotransmitters
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