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prepared by
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Atlantic Cape Community
College
CHAPTER
29
Heredity
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
Genetics
• Study of mechanism of heredity
• Basic principles proposed by Mendel in
mid-1800s
© 2013 Pearson Education, Inc.
The Vocabulary of Genetics
• Diploid number (46) of chromosomes in all
cells except gametes
– 23 pairs of homologous chromosomes
– 1 pair-sex chromosomes
• Determine genetic sex (XX = female, XY = male)
– 22 pairs–autosomes
• Guide expression of most other traits
© 2013 Pearson Education, Inc.
The Vocabulary of Genetics
• Karyotype diploid chromosomal
complement displayed in homologous
pairs
• Genome -genetic (DNA) makeup; two sets
of genetic instructions (maternal and
paternal)
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Figure 29.1 Preparing a karyotype.
1 The slide is
viewed with a
microscope, and
the chromosomes
are photographed.
© 2013 Pearson Education, Inc.
2 The photograph is entered into a
computer, and the chromosomes are
electronically rearranged into
homologous pairs according to size
and structure.
3 The resulting display is the
karyotype, which is examined for
chromosome number and structure.
Gene Pairs
• Alleles
– Matched genes at same locus (location) on
homologous chromosomes
– Homozygous–alleles same for single trait
– Heterozygous–alleles different for single trait
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Gene Pairs
– Dominant-one allele masks (suppresses) its
recessive partner
• Dominant denoted by capital letter; recessive by
same letter in lower case
• Dominant expressed if one or both alleles
dominant
• Recessive expressed only if both alleles recessive
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Genotype and Phenotype
• Genotype–genetic makeup
• Phenotype–expression of genotype
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Sexual Sources of Genetic Variation
• Each person unique due to three events
– Independent assortment of chromosomes
– Crossover of homologues
– Random fertilization of eggs by sperm
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Chromosome Segregation and Independent
Assortment
• During gametogenesis, maternal and
paternal chromosomes randomly
distributed to daughter cells
• Alleles for each trait segregated during
meiosis I
–  Gamete receives only one allele of the four
alleles (2 maternal / 2 paternal) for each trait
• Alleles on different pairs of homologous
chromosomes distributed independently
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Segregation and Independent Assortment
• Independent assortment  incredible
variety
– Number of gamete types = 2n, where n =
number of homologous pairs
– In testes, 2n = 2223 = 8.5 million
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Figure 29.2 Gamete variability resulting from independent assortment.
Maternal chromosomes
Paternal chromosomes
© 2013 Pearson Education, Inc.
Crossover and Genetic Recombination
• Genes on same chromosome linked
– Normally pass to daughter cells as unit
• Homologous chromosomes can break,
even between linked genes  precisely
exchange gene segments (crossover or
chiasma)
–  Recombinant chromosomes
• Chromosomes have mixed contributions from each
parent
•  Tremendous variability
© 2013 Pearson Education, Inc.
Figure 29.3 Crossover and genetic recombination. (1 of 4)
Hair color
genes
Eye color
genes
Homologous chromosomes synapse during prophase of
meiosis I. Each chromosome consists of two sister chromatids.
H Allele for brown hair
E Allele for brown eyes
h Allele for blond hair
e Allele for blue eyes
Paternal chromosome
Maternal chromosome
© 2013 Pearson Education, Inc.
Homologous pair
Figure 29.3 Crossover and genetic recombination. (2 of 4)
Chiasma
One chromatid segment exchanges positions with a
homologous chromatid segment—in other words, crossing
over occurs, forming a chiasma.
H Allele for brown hair
E Allele for brown eyes
h Allele for blond hair
e Allele for blue eyes
Paternal chromosome
Maternal chromosome
© 2013 Pearson Education, Inc.
Homologous pair
Figure 29.3 Crossover and genetic recombination. (3 of 4)
The chromatids forming the chiasma break, and the broken-off
ends join their corresponding homologues.
H Allele for brown hair
E Allele for brown eyes
h Allele for blond hair
e Allele for blue eyes
Paternal chromosome
Maternal chromosome
© 2013 Pearson Education, Inc.
Homologous pair
Figure 29.3 Crossover and genetic recombination. (4 of 4)
Gamete 1
Gamete 2
Gamete 3
Gamete 4
At the conclusion of meiosis, each haploid gamete has one
of the four chromosomes shown. Two of the chromosomes
are recombinant (they carry new combinations of genes).
H Allele for brown hair
E Allele for brown eyes
h Allele for blond hair
e Allele for blue eyes
Paternal chromosome
Maternal chromosome
© 2013 Pearson Education, Inc.
Homologous pair
Random Fertilization
• Single egg fertilized by single sperm in
random manner
• Independent assortment and random
fertilization ~ 72 trillion (8.5 million  8.5
million) zygote possibilities
• Random fertilization increases number of
possibilities exponentially
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Types of Inheritance
• Few phenotypes traced to single gene
• Most traits determined by multiple alleles
or by interaction of several gene pairs
© 2013 Pearson Education, Inc.
Dominant-Recessive Inheritance
• Reflects interaction of dominant and
recessive alleles
• Punnett square
– Predicts possible gene combinations resulting
from mating of parents of known genotypes
– Only probability of particular genotype
(phenotype)
© 2013 Pearson Education, Inc.
Dominant-Recessive Inheritance
• Example
– Probability of genotypes from mating two
heterozygous parents
– Dominant allele—capital letter; recessive
allele—lowercase letter
– T = tongue roller and t = cannot roll tongue
– TT and tt are homozygous; Tt is heterozygous
© 2013 Pearson Education, Inc.
Dominant-Recessive Inheritance
• Offspring: 25% TT; 50% Tt; 25% tt
• Larger number of offspring  greater
likelihood ratios conform to predicted
values
• Probability of two children with same
genotype
– Multiply probabilities of separate events
– E.g., probability of two children who cannot
roll tongue = 1/4 * 1/4 = 1/16
© 2013 Pearson Education, Inc.
Figure 29.4 Genotype and phenotype probabilities resulting from a mating of two heterozygous parents.
Tt female
Tt male
Heterozygous
female forms
two types
of gametes
Heterozygous
male forms
two types
of gametes
1/2
T
T
1/2
TT
1/2
t
t
1/4
tT
Tt
1/4
1/4
tt
1/4
Possible
combinations
in offspring
© 2013 Pearson Education, Inc.
1/2
Dominant Traits
• Dominant traits (for example, widow's
peaks, freckles, dimples)
• Dominant disorders uncommon; many
lethal  death before reproductive age
• Huntington's disease caused by delayedaction gene
– Activated ~ age 40
– Dominant gene possibly passed to offspring–
50% chance
© 2013 Pearson Education, Inc.
Recessive Inheritance
• Some recessive genes more desirable
condition
– E.g., normal vision—recessive;
astigmatism—dominant
• Most genetic disorders inherited as simple
recessive traits
– Albinism, cystic fibrosis, and Tay-Sachs
disease
• Heterozygotes–carriers; do not express
trait but can pass to offspring
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Table 29.1 Traits Determined by Simple Dominant-Recessive Inheritance.
© 2013 Pearson Education, Inc.
Incomplete Dominance
• Heterozygous individuals have
intermediate phenotype
• Rare in humans; example—sickling gene
– SS = normal Hb made
– Ss = sickle-cell trait; both aberrant and
normal Hb made; can suffer sickle-cell crisis
under prolonged reduction in blood O2
– ss = sickle-cell anemia; only aberrant Hb
made; more susceptible to sickle-cell crisis
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Figure 17.8b Sickle-cell anemia.
Val His Leu Thr Pro Val Glu …
1
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2
3
4
5
6
7
146
Sickled erythrocyte results from a
single amino acid change in the
beta chain of hemoglobin.
Multiple-Allele Inheritance
• Genes that exhibit more than two allele
forms
– E.g., ABO blood grouping
• Three alleles (IA, IB, i) determine ABO
blood type in humans
– Each person has only two
– IA and IB-codominant (both expressed if
present)
– i - recessive
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Table 29.2 ABO Blood Groups.
© 2013 Pearson Education, Inc.
Sex-Linked Inheritance
• Inherited traits determined by genes on
sex chromosomes
• X chromosomes bear over 2500 genes
(many for brain function); Y chromosomes
carry about 78 genes
– Few regions can participate in crossover
© 2013 Pearson Education, Inc.
Sex-Linked Inheritance
• X-linked genes - found only on X
chromosome
– X-linked recessive alleles always expressed
in males
• Never masked or damped in males (no Y
counterpart)
• Most X-linked conditions expressed in males
– Passed from mothers to sons (e.g.,
hemophilia or red-green color blindness);
never father to sons
– Passed from mothers to daughters, but
females require two alleles to express
© 2013 Pearson Education, Inc.
Polygene Inheritance
• Traits reflecting actions of several gene
pairs at different locations
• Results in continuous phenotypic variation
between two extremes
• Examples: skin color, eye color, height,
intelligence, metabolic rate
© 2013 Pearson Education, Inc.
Polygene Inheritance of Skin Color
• Alleles for dark skin (ABC) incompletely
dominant over those for light skin (abc)
• First-generation offspring of AABBCC X
aabbcc cross  all heterozygous
(intermediate pigmentation)
• Second-generation offspring have wide
variation in possible pigmentations  bellshaped curve
© 2013 Pearson Education, Inc.
Figure 29.6 Simplifiedmodel for polygene inheritance of skin color based on three gene pairs.
Parents
3
aabbcc
AABBCC
(very light)
(very dark)
3
Proportion of second-generation population
First-generation
offspring
© 2013 Pearson Education, Inc.
AaBbCc
AaBbCc
15/64
15/64
20/64
15/64
6/64
1/64
1/64
6/64
20/64
6/64
1/64
Environmental Factors in Gene Expression
• Maternal factors (e.g., drugs, pathogens)
alter normal gene expression during
embryonic development
• E.g., thalidomide
– Embryos developed phenotypes not directed
by their genes
• Phenocopies-environmentally produced
phenotypes; mimic conditions caused by
genetic mutations
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Environmental Factors in Gene Expression
• Environmental factors influence gene
expression after birth
– Poor nutrition affects brain growth, body
development, and height
– Childhood hormonal deficits can lead to
abnormal skeletal growth and proportions
© 2013 Pearson Education, Inc.
Nontraditional Inheritance
• Genetic outcomes influenced by
– Small RNAs
– Epigenetic marks (chemical groups attached
to DNA or histone proteins)
– Extranuclear inheritance (mitochondrial DNA)
© 2013 Pearson Education, Inc.
Nontraditional Inheritance: Regulation of
Gene Expression
• Three levels of controls in human genome
– First layer - protein-coding genes
• Less than 2% of DNA
– Second layer–small RNAs
• In non-protein-coding DNA
– Third layer–epigenetic marks
• Stored in proteins and chemical groups that bind to
DNA; and in way chromatin packaged
© 2013 Pearson Education, Inc.
Small RNAs
• MicroRNAs (miRNAs) and short interfering
RNAs (siRNAs)
– Act directly on DNA, other RNAs, or proteins
– Inactivate transposons, genes that tend to
replicate themselves and disable or
hyperactivate other genes
– Control timing of apoptosis during
development
– Prevent translation of another gene
– Mutations linked to prostate and lung cancers,
and schizophrenia
© 2013 Pearson Education, Inc.
Small RNAs
• Nucleotide sequences for small RNAs now
determined
• Extensive gene therapy research to
develop RNA-interfering drugs for
diseases such as
• Age-related macular degeneration, Parkinson's
disease, cancer, and other disorders
© 2013 Pearson Education, Inc.
Epigenetic Marks
• Information stored in proteins and
chemical groups (e.g., methyl and acetyl
groups) bound to DNA; and in way
chromatin packaged in cell
• Determine whether DNA available for
transcription (acetylation) or silenced
(methylation)
• May predispose cell to cancerous changes
or devastating illness
© 2013 Pearson Education, Inc.
Epigenetic Marks
• Genomic imprinting
– Tags certain genes with methyl group during
gametogenesis as maternal or paternal;
essential for normal development
• Allows embryo to express only mother's or
father's gene
• Each new generation erases imprinting
when new gametes produced
© 2013 Pearson Education, Inc.
Epigenetic Marks
• Mutations of imprinted genes may 
pathology
• Same allele can have different effects
depending on which parent it comes from
– For example, deletions in chromosome 15
result in disorders with different symptoms
• Prader-Willi syndrome if inherited from father
• Angelman syndrome if inherited from mother
© 2013 Pearson Education, Inc.
Extranuclear (Mitochondrial) Inheritance
• Some genes (37) in mitochondrial DNA
(mtDNA)
• Transmitted by mother in cytoplasm of egg
• Errors in mtDNA linked to rare disorders
– Muscle disorders and neurological problems,
possibly Alzheimer's and Parkinson's
diseases
© 2013 Pearson Education, Inc.
Genetic Screening, Counseling, and
Therapy
• Newborn infants routinely screened for
number of genetic disorders
– Congenital hip dysplasia, imperforate anus,
PKU, and other metabolic disorders
• Other examples
– Screening adult children of parents with
Huntington's disease; testing fetus of 35 yo
woman for trisomy-21 (Down syndrome)
© 2013 Pearson Education, Inc.
Carrier Recognition
• Two ways to identify gene carriers
– Pedigrees and blood tests
• Pedigrees
– Trace genetic trait through several
generations; helps predict future
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Carrier Recognition
• Blood tests and DNA probes can detect
presence of unexpressed recessive genes
– E.g., Tay-Sachs and cystic fibrosis genes
© 2013 Pearson Education, Inc.
Fetal Testing
• Used when known risk of genetic disorder;
both invasive; both carry risk
• Amniocentesis
– Amniotic fluid withdrawn after 14th week; fluid
and cells examined for genetic abnormalities
– Testing takes several weeks
• Chorionic villus sampling (CVS)
– Chorionic villi sampled at 8-10 weeks;
karyotyped for genetic abnormalities; testing ~
immediate
© 2013 Pearson Education, Inc.
Figure 29.7 Fetal testing—amniocentesis and chorionic villus sampling.
Chorionic villus sampling (CVS)
Amniocentesis
Amniotic
fluid
withdrawn
Fetus
1 A sample of
amniotic fluid can
be taken starting at
the 14th to 16th
week of pregnancy.
1 A sample of
chorionic villus tissue
can be taken as early
as the 8th to 10th week
of pregnancy.
Placenta
Fetus
Centrifugation
Uterus
Suction tube
inserted through
cervix
Placenta
Cervix
Chorionic villi
Fluid
2 Biochemical tests
(tests for genetic
disorders) can be
performed immediately on
the amniotic fluid or later
on the cultured cells.
Fetal
cells
Fetal
cells
Biochemical
tests
3 Fetal cells must be
cultured for several weeks
to obtain sufficient
numbers for karyotyping
(testing for chromosomal
disorders).
Several
hours
Several
weeks
Karyotyping of
chromosomes
of cultured cells
© 2013 Pearson Education, Inc.
2 Karyotyping and biochemical
tests can be performed on the
fetal cells immediately, providing
results within a day or so.
Figure 29.7a Fetal testing—amniocentesis and chorionic villus sampling.
Slide 1
Amniocentesis
Amnioti fluid
withdrawn
1 A sample of amniotic
fluid can be taken starting
at the 14th to 16th week of
pregnancy.
Fetus
Placenta
Centrifugation
Uterus
2 Biochemical tests
(tests for genetic
disorders) can be
performed immediately
on the amniotic fluid or
later on the cultured
cells.
3 Fetal cells must be
cultured for several
weeks to obtain
sufficient numbers for
karyotyping (testing for
chromosomal
disorders).
© 2013 Pearson Education, Inc.
Cervix
Fluid
Fetal
cells
Biochemical
tests
Several
weeks
Karyotyping of
chromosomes
of cultured cells
Figure 29.7a Fetal testing—amniocentesis and chorionic villus sampling.
Amniocentesis
Amnioti fluid
withdrawn
1 A sample of amniotic
fluid can be taken starting
at the 14th to 16th week of
pregnancy.
Fetus
Placenta
Centrifugation
Uterus
Cervix
Fluid
Fetal
cells
© 2013 Pearson Education, Inc.
Slide 2
Figure 29.7a Fetal testing—amniocentesis and chorionic villus sampling.
Slide 3
Amniocentesis
Amnioti fluid
withdrawn
1 A sample of amniotic
fluid can be taken starting
at the 14th to 16th week of
pregnancy.
Fetus
Placenta
Centrifugation
Uterus
2 Biochemical tests
(tests for genetic
disorders) can be
performed immediately
on the amniotic fluid or
later on the cultured
cells.
© 2013 Pearson Education, Inc.
Cervix
Fluid
Fetal
cells
Biochemical
tests
Figure 29.7a Fetal testing—amniocentesis and chorionic villus sampling.
Slide 4
Amniocentesis
Amnioti fluid
withdrawn
1 A sample of amniotic
fluid can be taken starting
at the 14th to 16th week of
pregnancy.
Fetus
Placenta
Centrifugation
Uterus
2 Biochemical tests
(tests for genetic
disorders) can be
performed immediately
on the amniotic fluid or
later on the cultured
cells.
3 Fetal cells must be
cultured for several
weeks to obtain
sufficient numbers for
karyotyping (testing for
chromosomal
disorders).
© 2013 Pearson Education, Inc.
Cervix
Fluid
Fetal
cells
Biochemical
tests
Several
weeks
Karyotyping of
chromosomes
of cultured cells
Figure 29.7b Fetal testing—amniocentesis and chorionic villus sampling.
Chorionic villus sampling (CVS)
1 A sample ofchorionic
villus tissue can be
taken as early as the 8th
to 10th week of
pregnancy.
Fetus
Placenta
Biochemical
tests
Chorionic
villi
Fetal
cells
Several
hours
Karyotyping of
chromosomes
of cultured cells
© 2013 Pearson Education, Inc.
Suction tube
inserted through
cervix
2 Karyotyping and
biochemical tests can
be performed on the
fetal cells immediately,
providing results within
a day or so.
Slide 1
Figure 29.7b Fetal testing—amniocentesis and chorionic villus sampling.
Chorionic villus sampling (CVS)
1 A sample ofchorionic
villus tissue can be
taken as early as the 8th
to 10th week of
pregnancy.
Fetus
Placenta
Chorionic
villi
Fetal
cells
© 2013 Pearson Education, Inc.
Suction tube
inserted through
cervix
Slide 2
Figure 29.7b Fetal testing—amniocentesis and chorionic villus sampling.
Chorionic villus sampling (CVS)
1 A sample ofchorionic
villus tissue can be
taken as early as the 8th
to 10th week of
pregnancy.
Fetus
Placenta
Biochemical
tests
Chorionic
villi
Fetal
cells
Several
hours
Karyotyping of
chromosomes
of cultured cells
© 2013 Pearson Education, Inc.
Suction tube
inserted through
cervix
2 Karyotyping and
biochemical tests can
be performed on the
fetal cells immediately,
providing results within
a day or so.
Slide 3
Human Gene Therapy
• Gene therapy may alleviate or cure
disorders
• Genetic engineering has potential to
replace defective gene, including
– Defective cells infected with genetically
engineered virus containing functional gene
– Inject "corrected" gene directly into cells
• Prohibitively expensive; ethical, religious,
societal questions
© 2013 Pearson Education, Inc.