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Genetics
• The study of the mechanism of heredity
• Basic principles proposed by Mendel in the
mid-1800s
Copyright © 2010 Pearson Education, Inc.
Genetics
• Diploid number of chromosomes
• In all cells except gametes
• Diploid number = 46 (23 pairs of homologous
chromosomes) (For Humans!)
• 1 pair of sex chromosomes
• XX = female, XY = male
• 22 pairs of autosomes
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Chromosomes & the Genome
• Karyotype: diploid chromosomal complement
displayed in homologous pairs
(b)
(a) The slide is
viewed with a
microscope, and
the chromosomes
are photographed.
Copyright © 2010 Pearson Education, Inc.
(c)
The photograph is entered into
a computer, and the chromosomes are electronically
rearranged into homologous
pairs according to size and
structure.
The resulting display is the
karyotype, which is examined
for chromosome number and
structure.
Genotype and Phenotype
• Genome
• all the chromosomes & genes of an organism
• Genotype
• the particular set of gene versions an organism
has
• Phenotype
• the physical/biochemical manifestation of the
genes
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Genetic Terms
loci
centromeres
homologous
chromosomes
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E locus
H locus
sister
chromatids
alleles
sister
chromatids
Alleles
• A version of a particular gene
• Set of genes at the same locus on
homologous chromosomes
• Homozygous genotype
• Both alleles at a locus are identical
• Heterozygous
• The alleles at a locus are different
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Genetic Terms
• Inheritance Patterns
• Dominant
• A trait appears in the phenotype even if only
one allele for that trait is present
• Recessive
• A trait that can be masked
• Only appears in phenotype if it is homozygous
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Genetic Terms
• Inheritance patterns
• Simple dominance
• One allele is dominant one is recessive
• Thumbs, blood type
• Co-dominance
• Both alleles affect a trait – each give different
effect
• AB Blood type
• Intermediate dominance
• Two alleles give full effect, one allele gives half
effect haploinsufficiency
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Sexual Sources of Genetic Variation
• Independent assortment of chromosomes
• Crossover between homologues
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Segregation and Independent Assortment
• During gametogenesis, maternal and paternal
chromosomes are randomly distributed to
daughter cells
• The two alleles of each pair are segregated
during meiosis I
• Alleles on different pairs of homologous
chromosomes are distributed independently
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Segregation and Independent Assortment
• The number of gamete types = 2n, where n is
the number of homologous pairs
• In a man’s testes, 2n = 223 = 8.5 x 106
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Meiotic Recombination
• Genes on the same chromosome are linked
• Chromosomes can cross over and exchange
segments
• Recombinant chromosomes have mixed
contributions from each parent
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Hair color genes
Eye color genes
Homologous chromosomes synapse in
prophase of meiosis I
H Allele for brown hair
h Allele for blond hair
E Allele for brown eyes
e Allele for blue eyes
Paternal chromosome
Maternal chromosome
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Homologous pair
Figure 29.3 (1 of 4)
Chiasma
Non-sister chromatids undergo recombination
(crossing over)
H Allele for brown hair
E Allele for brown eyes
h Allele for blond hair
e Allele for blue eyes
Paternal chromosome
Maternal chromosome
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Homologous pair
Figure 29.3 (2 of 4)
The chromatids break and rejoin
H Allele for brown hair
E Allele for brown eyes
h Allele for blond hair
e Allele for blue eyes
Paternal chromosome
Maternal chromosome
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Homologous pair
Figure 29.3 (3 of 4)
Gamete 1
Gamete 2
Gamete 3
Gamete 4
At end of meiosis, each haploid gamete has 1 of the 4
chromosomes shown. 2 chromosomes are recombinant –
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
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Homologous pair
Figure 29.3 (4 of 4)
Types of Inheritance
• Most traits are determined by multiple alleles
or by the interaction of several gene pairs
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Simple Dominance Inheritance
• Reflects the interaction of dominant and
recessive alleles
• Punnett square
• Tool to represent genetic crosses
• Helps predict possible gene combinations
resulting from the mating
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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
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Dominant-Recessive Inheritance
• Offspring: 25% TT, 50% Tt, 25% tt
• The larger the number of offspring, the greater
the likelihood that the ratios will conform to the
predicted values
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female
male
Heterozygous
female forms
two types
of gametes
Heterozygous
male forms
two types
of gametes
1/2
1/2
1/2
1/2
1/4
1/4
1/4
Possible
combinations
in offspring
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1/4
Figure 29.4
Simple Dominance Inheritance
• Dominant traits (for example, widow’s peaks,
freckles, dimples)
• Polydactyly, Marfan Syndrome
• Huntington’s disease is caused by a delayedaction gene
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Simple Dominance Inheritance
• Simple dominance inheritance patterns
frequently cause disease only when the
genomtype is homozygous for the recessive
alleles
• Albinism, cystic fibrosis, and Tay-Sachs
disease, sickle-cell disease
• Heterozygotes have one mutant allele and
one normal allele but do not have disease
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What is Dominant? What is Recessive?
• Dominant alleles usually encode functional
proteins
• Recessive alleles often encode defective
proteins or no protein at all.
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Multiple-Allele Inheritance
• Genes that exhibit more than two allele forms
• ABO blood grouping is an example
• Three alleles (IA, IB, i) determine the ABO
blood type in humans
• IA and IB are codominant (both are expressed
if present), and i is recessive
Copyright © 2010 Pearson Education, Inc.
Copyright © 2010 Pearson Education, Inc.
Table 29.2
Sex-Linked Inheritance
• Inherited traits determined by genes on the
sex chromosomes
• X chromosomes > 2500 genes
• Y chromosomes ~80 genes
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Sex-Linked Inheritance
• e.g. X-linked genes
• clotting factor (hemophilia)
• red opsin (red-green color blindness)
• Females have two X – so normal allele masks
mutant
• Males – 1 X so mutant is expressed if present
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colorblindness
protanopia
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deuteranopia
tritanopia
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Polygenic Inheritance
• Depends on several different genes at acting
in concert
• Gives more fine-scale phenotypic variation
between two extremes
• Examples: skin color, eye color, height
• Quantitative traits
• QTLs
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Eye color variation
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Polygenic Inheritance of Skin Color
• Alleles for dark skin (ABC) are incompletely
dominant over those for light skin (abc)
• The first-generation offspring each have three
“units” of darkness (intermediate
pigmentation)
• The second-generation offspring have a wide
variation in possible pigmentations
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Environmental Factors in Gene Expression
• Phenocopies: environmentally produced
phenotypes that mimic conditions caused by
genetic mutations
• Environmental factors can influence genetic
expression after birth
• Poor nutrition can affect brain growth, body
development, and height
• Childhood hormonal deficits can lead to
abnormal skeletal growth and proportions
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Non-Mendelian Inheritance
• Influences due to
• Genes of small RNAs
• Epigenetic marks (chemical groups attached to
DNA)
• Extranuclear (mitochondrial) inheritance
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Small RNAs
• MicroRNAs (miRNAs) and short interfering RNAs
(siRNAs)
• Bind to mRNAs
• Cause mRNA to be destroyed or not translated
into proteins
• In future, RNA-interfering drugs may treat
diseases such as age-related macular
degeneration and Parkinson’s disease
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Extranuclear (Mitochondrial) Inheritance
• 37 genes are in human mitochondrial DNA
(mtDNA)
• Transmitted by the mother in the cytoplasm of
the egg
• Errors in mtDNA are linked to rare disorders:
muscle disorders and neurological problems,
possibly Alzheimer’s and Parkinson’s
diseases
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Genetic Screening, Counseling, and
Therapy
• Newborn infants are routinely screened for a
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: for testing
a woman pregnant for the first time after age
35 to see if the baby has trisomy-21 (Down
syndrome)
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Carrier Recognition
• Two major avenues for identifying carriers of
genes: pedigrees and DNA testing
• Pedigrees trace a particular genetic trait
through several generations; helps to predict
the future
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Pedigree
Symbols
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Dominant or Recessive
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Autosomal Recessive
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X-linked recessive
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Key
Male
Affected male
Mating
Female
Affected female
Offspring
Ww
ww
ww
1st generation
Ww grandparents
2nd generation
(parents, aunts,
uncles)
3rd generation
(two sisters)
Widow’s peak
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No widow’s peak
Figure 29.7
Carrier Recognition
• DNA probes can detect the presence of
unexpressed recessive mutations
• Tay-Sachs and cystic fibrosis mutations can
be identified by such tests
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Fetal Testing
• Used when there is a known risk of a genetic
disorder
• Amniocentesis: amniotic fluid is withdrawn
after the 14th week and fluid and cells are
examined for genetic abnormalities
• Chorionic villus sampling (CVS): chorionic villi
are sampled and karyotyped for genetic
abnormalities
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(a) Amniocentesis
Amniotic fluid withdrawn
Fetus
Placenta
1
A sample of
amniotic fluid can be
taken starting at the
14th to 16th week of
pregnancy.
Centrifugation
Uterus Cervix
Fluid
Biochemical
Fetal
tests can be
cells
performed
immediately on
the amniotic fluid
or later on the
cultured cells.
2
3
Fetal cells must
be cultured for
several weeks to
obtain sufficient
numbers for
karyotyping.
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Biochemical
tests
Several
weeks
Karyotyping of
chromosomes
of cultured cells
Figure 29.8
Human Gene Therapy
• Genetic engineering has the potential to
replace a defective gene
• Defective cells can be infected with a
genetically engineered virus containing a
functional gene
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