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BIOLOGY OF HUMANS
Concepts, Applications, and Issues
Fifth Edition
Judith Goodenough Betty McGuire
20
Genetics and
Human Inheritance
Lecture Presentation
Anne Gasc
Hawaii Pacific University and
University of Hawaii–Honolulu Community College
© 2014 Pearson Education, Inc.
Genetics and Human Inheritance
OUTLINE:
 Principles of Inheritance
 Breaks in Chromosomes
 Detecting Genetic Disorders
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Principles of Inheritance
 Genetic information is carried on chromosomes that are
in the egg and sperm in equal numbers
 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
 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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Figure 20.1 Important terms in genetics.
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Principles of Inheritance
 Homozygous (homo, same; zygo, joined together)
 Individuals with two copies of the same allele
 Heterozygous (hetero, different)
 Individuals with different alleles of a given gene
 Dominant (use upper case—example “A”)
 When the effects of an allele can be detected
regardless of the alternative allele
 Recessive (use lower case—example “a”)
 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
 For example, the freckled phenotype has two
genotypes: FF and Ff
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Figure 20.2 Genotypes for selected human phenotypes.
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Table 20.1 Review of Common Terms in Genetics
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Gamete Formation
 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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Figure 20.3 Gamete formation.
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Mendelian Genetics
 Gregor Mendel was a monk in the nineteenth
century who grew up in a region of what was then
Austria and is now part of the Czech Republic
 Studied how single genes are inherited from parent to
offspring
 Used pea plants
 First used one-trait crosses
 Then used two-trait (dihybrid) crosses
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Mendelian Genetics
 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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Figure 20.4 Punnett square.
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One- and Two-Trait Crosses
 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)
 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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One- and Two-Trait Crosses
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Web Activity: One- and Two-Trait Crosses
Figure 20.5 (a) Gamete formation (b) Punnett square.
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Figure 20.6 A person who is heterozygous’ gametes.
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Figure 20.7 A dihybrid cross.
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Pedigrees
 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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Figure 20.8 Pedigrees.
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Pedigrees
 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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Dominant and Recessive Alleles
 Dominant allele
 Often produces a functional protein that the recessive
allele does not
 Example: albinism
 Lacking the ability to produce brown pigment melanin
 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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Codominant Alleles
 Complete dominance
 Heterozygote exhibits the trait associated with the
dominant allele but not that of the recessive allele
 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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Incomplete Dominance
 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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Incomplete Dominance
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Web Activity: Codominance and Incomplete Dominance
Figure 20.11 The inheritance of sickle-cell trait.
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Pleiotropy
 One gene having many effects
 Besides providing an example of incomplete
dominance, sickle-cell anemia is an example of
pleiotropy
 Sickling of red blood cells caused by abnormal
hemoglobin affects many areas of the body
 The sickled cells can break down, clog blood vessels,
and accumulate in the spleen
 These effects can affect the heart, brain, lungs,
kidneys, and muscles and joints
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Figure 20.12 Sickle-cell anemia.
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Multiple Alleles
 When three or more forms of a given gene exist
across many people in the population
 Example: ABO blood types
 Blood type is determined by the presence of certain
polysaccharides (sugars) on the surface of red blood
cells
 Type A blood has the A polysaccharide
 Type B has the B polysaccharide
 Type AB has both A and B polysaccharides
 Type O has neither
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Multiple Alleles
 When three or more forms of a given gene exist
across many people in the population (cont’d)
 Gene has three alleles: IA, IB, I
 Alleles IA and IB specify the A and B polysaccharides,
respectively
 When both of these alleles are present, both
polysaccharides are produced
 IA and IB are, therefore, codominant
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Table 20.2 The Relationship between Genotype and ABO Blood
Types
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Polygenic Inheritance
 Variation in a trait, such as height, independent of
environmental influences
 Involves two or more genes, often on different
chromosomes
 Many traits, including height, skin color, and eye
color, vary almost continuously from one extreme to
another
 Environment can play a role in creating such a
smooth continuum
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Figure 20.13 Human height varies along a continuum.
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Genes on the Same Chromosome
 Usually inherited together
 Described as being linked
 Linked genes usually do not assort independently
 Usually is emphasized here because there is a
mechanism that can unlink genes on the same
chromosome: crossing over
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Sex-Linked Genes
 Y is much smaller than X and 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 because son can inherit X-linked
recessive only from mother
 Examples of disorders
 Red-green color blindness
 Two forms of hemophilia
 Duchenne muscular dystrophy
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Sex-Linked Genes
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Web Activity: Sex-Linked Traits
Figure 20.14 Genes that are X linked.
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Sex-Influenced Genes
 Autosomal genes whose expression is influenced by sex
hormones
 Their expression differs in males and females
 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
 Men can be heterozygous for the trait and still show
pattern baldness
 Women who are homozygous for the trait will develop
pattern baldness later in life
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Breaks in Chromosomes
 Usually caused by chemicals, radiations, viruses
 Results in changes in the structure and function of
the chromosome
 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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Figure 20.15 Cri-du-chat syndrome.
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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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Figure 20.16 Fragile X syndrome.
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Detecting Genetic Disorders
 Prenatal genetic testing is recommended when
 A defective gene runs in the family
 The mother is older than 35, due to increased risks of
nondisjunction
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Detecting Genetic Disorders
 Amniocentesis
 10 to 20 ml of amniotic fluid is withdrawn, which
contains 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
 A disadvantage of CVS is that it has a slightly
greater risk of triggering miscarriage than does
amniocentesis
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Figure 20.17 Amniocentesis and chorionic villi sampling.
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Genetic Testing
 Newborn genetic testing
 Blood test screens for phenylketonuria (PKU)
 Allows doctors and parents to prevent brain damage
by keeping the infant on a strict diet that excludes
most phenylalanine
 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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You Should Now Be Able To:
 Know the principles of inheritance and the definitions
for gene, allele, dominant, recessive, phenotype,
genotype, homozygous, heterozygous
 Understand the Mendelian genetic: Mendel, Punnett
square, monohybrid and dihybrid crosses
 Understand codominance, incomplete dominance,
polygenic inheritance, pleiotropy, X-linked gene, and
sex-influenced trait
 Know the breaks in chromosomes
 Understand the detection of genetic disorders
© 2014 Pearson Education, Inc.