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Chapter 20
Genetics and
Human
Inheritance
Lecture Presentation
Betty McGuire
Cornell University
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Genetics and Human Inheritance
 Principles of inheritance
 Breaks in chromosomes
 Detecting genetic disorders
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Principles of Inheritance
 Genetic information
 Carried on chromosomes that are carried in
the egg and sperm in equal numbers
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Principles of Inheritance
 Homologous pairs of chromosomes
 23 chromosomes received from one
parent pair with 23 chromosomes from
the other parent
 Each member of a homologous pair carries
genes for the same traits
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Principles of Inheritance
 Genes
 Segments of DNA
 Code for a specific protein that will play a
structural or functional role in the cell
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Principles of Inheritance
 Trait
 Characteristic
 Produced by the actions of one or more
gene-directed proteins
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Principles of Inheritance
 Alleles
 Different forms of a gene
 Produce different versions of the trait
they determine
 Example: gene for freckles
 One allele causes freckles to form
 Other allele does not
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Principles of Inheritance
 Homozygous
 Individuals with two copies of the same allele
 Heterozygous
 Individuals with different alleles of a
given gene
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Principles of Inheritance
 Dominant
 When the effects of an allele can be
detected regardless of the alternative allele
 Recessive
 When the effects of an allele are masked in
the heterozygous condition
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Principles of Inheritance
 Genotype
 Alleles that are present
 Genetic composition of an individual
 Phenotype
 Observable physical traits of an individual
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Copyright © 2012 Pearson Education, Inc.
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Principles of Inheritance
 Law of Segregation
 During gamete formation, the two alleles for
each gene separate as the homologous
chromosomes move toward opposite ends of
the cell during meiosis
 Each chromosome is inherited independent
of the other chromosomes, following the Law
of Independent Assortment
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Principles of Inheritance
 Gregor Mendel
 Studied how single genes are inherited from
parent to offspring
 First used one-trait crosses
 Then used two-trait (dihybrid) crosses
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Principles of Inheritance
 Punnett square
 Matrix used to predict genetic makeup
of offspring of individuals of particular
genotypes
 Rows represent possible gametes of one
parent
 Columns represent possible gametes of the
other parent
 Boxes represent possible combinations
of gametes
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Principles of Inheritance
 Monohybrid cross
 Cross in which both parents are
heterozygous for one trait of interest
 Genotypic ratio of offspring
 1 FF : 2 Ff : 1 ff
 Phenotypic ratio of offspring
 3 with freckles (FF and Ff) : 1 without (ff)
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Principles of Inheritance
 Dihybrid cross
 Cross in which both parents are
heterozygous for two traits of interest
 Phenotypic ratio of offspring
9 : 3 : 3 : 1
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Principles of Inheritance
Web Activity: One- and Two-Trait Crosses
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Principles of Inheritance
 Pedigree
 Chart showing the genetic connections
among individuals in a family
 Especially useful in following recessive
alleles that are not visible in the
heterozygote
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Principles of Inheritance
 Genetic disorders
 Often caused by recessive alleles
 Carrier
 Someone who displays the dominant
phenotype but is heterozygous for a trait
 Carries the recessive allele and can
pass it to descendants
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Principles of Inheritance
 Dominant allele
 Often produces a functional protein that the
recessive allele does not
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Principles of Inheritance
 Example: albinism
 Ability to produce brown pigment melanin
is lacking
 Ability to produce melanin depends on the
enzyme tyrosinase
 Dominant allele that results in
pigmentation produces functional
tyrosinase
 Recessive allele that results in albinism
produces nonfunctional tyrosinase
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Principles of Inheritance
 Complete dominance
 Heterozygote exhibits the trait associated
with the dominant allele but not that of the
recessive allele
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Principles of Inheritance
 Codominance
 Effects of both alleles are apparent in a
heterozygote
 Example: blood type AB
 The protein products of both the A and B
alleles are expressed on the surface of
the red blood cell
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Principles of Inheritance
 Incomplete dominance
 Expression of the trait in a heterozygous
individual is in between the way the trait is
expressed in a homozygous dominant or
homozygous recessive person
 Example: sickle-cell allele
 Heterozygote has sickle-cell trait (HbAHbS)
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Principles of Inheritance
Web Activity: Codominance
and Incomplete Dominance
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Principles of Inheritance
 Pleiotropy
 One gene having many effects
 Sickling of red blood cells caused by
abnormal hemoglobin affects many areas of
the body
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Principles of Inheritance
 Multiple alleles
 When three or more forms of a given gene
exist across many people in the population
 Example: ABO blood types
 Gene has three alleles: IA, IB, i
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Principles of Inheritance
 Polygenic inheritance
 Variation in a trait, such as height,
independent of environmental influences
 Involves two or more genes, often on
different chromosomes
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Principles of Inheritance
 Genes on the same chromosome
 Usually inherited together
 Described as being linked
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Principles of Inheritance
 Sex-linked genes
 Y chromosome is much smaller than
X chromosome
 Y carries fewer genes
 Most genes on the X chromosome have no
corresponding alleles on the Y chromosome
 Known as X-linked genes
 Different pattern of inheritance
 Recessive phenotype of X-linked genes
more common in males
 Son can inherit X-linked recessive only
from mother
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Principles of Inheritance
 Examples of disorders caused by X-linked
recessive alleles
 Red-green color blindness
 Two forms of hemophilia
 Duchenne muscular dystrophy
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Principles of Inheritance
Web Activity: Sex-Linked Traits
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Principles of Inheritance
 Sex-influenced genes
 Autosomal genes whose expression is
influenced by sex hormones
 Example: male pattern baldness
 More common in men than in women
because its expression depends on both
the presence of the allele for baldness
and the presence of testosterone
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Breaks in Chromosomes
 Chromosome breakage
 Usually caused by
 Chemicals
 Radiation
 Viruses
 Results in changes in the structure and
function of the chromosome
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Breaks in Chromosomes
 Deletion
 Loss of a piece of chromosome
 Most common deletion occurs when the tip
of a chromosome breaks off
 Example: cri-du-chat syndrome
 Loss of tip of chromosome 5
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Breaks in Chromosomes
 Duplication
 Addition of piece of chromosome
 Effects depend on size and position of
the addition
 Example: Fragile X syndrome
 Duplication of a region on the
X chromosome
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Detecting Genetic Disorders
 Prenatal genetic testing is recommended
 If a defective gene runs in the family
 When the mother is older than 35
 Due to increased risks of nondisjunction
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Detecting Genetic Disorders
 Amniocentesis
 10–20 ml of amniotic fluid is withdrawn,
which contain epithelial cells of the fetus
 Cells are cultured and then examined
 Abnormalities in the number
of chromosomes
 Presence of certain alleles that are likely
to cause specific diseases
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Detecting Genetic Disorders
 Chorionic villi sampling (CVS)
 Involves taking a small piece of chorionic villi
 Fingerlike projections of the chorion
 Cells of chorion have same genetic
makeup as fetus
 Cells are cultured and then chromosomes
examined
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Detecting Genetic Disorders
 Newborn genetic testing
 Blood test screens for phenylketonuria (PKU)
 Adult genetic testing
 Many predictive genetic tests are now
available or being developed
 Some identify people who are at risk or
predisposed for a specific disease before
symptoms appear
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