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Foundations of Genetics
Chapter 7
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Outline
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Gregor Mendel
 Pea Experiments
 Inheritance Theory
Punnett Squares
Epistasis
Multiple Alleles
Sex Linkage
Human Heredity
 Mutations
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Gregor Mendel
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Monk studied science and mathematics at
University of Vienna.
 Worked with garden peas to study heredity.
- Many varieties available
- Infrequent version of a trait
- Small and easy to grow
- Left alone, flowers do not open
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Mendel’s Design
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Let each variety self-fertilize for several
generations (True breeding).
 P generation
Crossed two pea varieties exhibiting
alternative traits such as white versus purple
flowers.
 Offspring resulted in F1 generation.
Allowed plants produced in crosses to self
fertilize and counted numbers of each type of
offspring.
 F2 Generation
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Mendel’s Results
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F1 Generation
 All F1 flowers were purple.
- Called expressed trait dominant and
unexpressed trait (white flowers)
recessive.
F2 Generation
 Approximately 25% of individuals exhibited
white flowers.
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Mendel’s Results
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F3 Generation
 Ratio of purple:white flowers was 3:1.
- Concluded 3:1 ratio in F2 generation
was actually a 1:2:1 ratio.
- (1) True-breeding dominant: (2) not-true
breeding dominant: (1) true-breeding
recessive.
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Mendel’s Proposed Theory
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Parents transmit information about traits to
offspring (genes).
Each parent contains two copies of the factor
governing each trait.
 Homozygous - same copies
 Heterozygous - different copies
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Mendel’s Proposed Theory
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Alternative forms of a factor lead to
alternative traits (alleles).
 Phenotype - Appearance (Physical
expression of genes).
 Genotype - Alleles governing appearance.
Alleles inherited from each parent do not
affect each other.
Presence of allele does not ensure trait
expression.
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Punnett Squares
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Punnett Squares can be used to visualize
crosses between two individuals.
 Possible gametes of individual are listed
along horizontal side. And possible
gametes of the other individual are listed
along the vertical side.
- Genotypes of potential offspring are
represented by cells within square.
 Frequency expressed as probability.
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Punnett Squares
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Testcross
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Test unknown individual with a homozygous
recessive yields two alternatives:
 Unknown individual homozygous (PP) PP
x pp: all offspring have purple flowers.
 Unknown individual heterozygous (Pp) Pp
x pp: one-half offspring will have white
flowers (pp) and one-half have purple
flowers (Pp).
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Mendel’s Laws
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First Law - Segregation
 Only one allele specifying an alternative
trait can be carried in a particular gamete,
and gametes combine randomly in forming
offspring.
Second Law - Independent Assortment
 Genes located on different chromosomes
are inherited independently of one another.
- Dihybrid Crosses
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Epistasis
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Epistasis - Interaction between products of
two genes where one gene modifies the
other gene’s phenotypic expression.
 Emerson - To produce pigment, a Zea
mays plant must possess at least one
functional copy of each enzyme gene.
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Epistasis
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Multiple Alleles
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Often each allele has its own effect and the
alleles are considered codominant.
 Human ABO Blood type
- Type A individuals only galactosamine
- Type B individuals add only galactose
- Type AB individuals add both sugars
- Type O individuals add neither sugar
 Rh Blood Group
- Rh cell surface marker
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ABO Blood Groups
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Genotypic Modifications
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Pleiotrophic alleles have more than one
effect on the phenotype.
 May have different effects on different
phenotypic consequences.
 Characteristic of many inherited disorders.
Incomplete Dominance
 Some alleles produce a heterozygote
phenotype that is intermediate between
those of the parents.
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Sex Linkage
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A trait determined by a gene on the sex
chromosome is said to be sex-linked.
 In Drosophila, sex is determined by the
number of copies of the x chromosome.
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Human Chromosomes
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An individual’s particular array of
chromosomes is known as a karyotype.
 Failure of chromosomes to separate
correctly during meiosis I or II is called
nondisjunction, which leads to aneuploidy
(abnormal number of chromosomes).
- Down Syndrome caused by trisomy 21.
 Overall odds are 1 in 750.
 Much higher in women over 45.
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Nondisjunction in Sex Chromosomes
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X Chromosome
 XXX or XXY yields Klinefelter Syndrome
 XO yields Turner Syndrome
Y Chromosome
 XYY - Antisocial ?
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Mutations in Human Heredity
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Mutations are accidental changes in genes.
 Rare, random, and usually result in
recessive alleles.
- Pedigrees used to study heredity.
 Hemophilia - Inherited condition where
blood is slow to clot or does not clot at all.
 Only expressed when individual has
no copies of the normal allele.
 Royal Hempohilia - Sex-linked
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Sickle-Cell Anemia
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Sickle-Cell Anemia is a recessive inherited
disorder in which afflicted individuals have
defective hemoglobin, and thus are unable to
properly transport oxygen to tissues.
- Heterozygotes usually appear normal.
- Homozygotes have Sickle-Cell, but are
resistant to malaria.
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Other Disorders
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Tay-Sachs
 Incurable heredity disorder which causes
brain to deteriorates.
- Nonfunctional form of hexosaminidas A
enzyme.
Huntington’s Disease
 Inherited condition caused by dominant
allele causing progressive brain
deterioration.
- Symptoms usually develop late in life.
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Genetic Counseling and Therapy
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Process of identifying parents at risk of
producing children with genetic defects and
assessing genetic state of early embryos.
 Amniocentesis
 Ultrasound
 Chorionic Villi Sampling
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Review
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Gregor Mendel
 Pea Experiments
 Inheritance Theory
Punnett Squares
Epistasis
Multiple Alleles
Sex Linkage
Human Heredity
 Mutations
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies