BIOLOGY 171

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BIOLOGY 171 CHAPTER 14
MENDEL AND THE GENE IDEA
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MENDEL AND GENETICS

• Gregor Mendels work with garden peas became
  the foundation for the science of genetics.
• Education and profession
• Presented work in 1866
• Mendel had worked out the basic concepts of
• Independent assortment
• The law of segregation
• The concept of gamete development
• The concept of dominance and recessiveness

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GREGOR MENDEL
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GENETIC CROSSES IN PLANTS
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DEFINITIONS

• Gene - The unit of DNA along a chromosome that
  codes for a single characteristic (mendelian
  definition) or the unit of DNA that codes for
  a specific polypeptide
• (molecular definition).
• Allele - Different forms of the same gene.
• Locus - The location of the gene on the
  chromosome.

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DEFINITIONS

• Genotype - The actual genetic make up of an
  individual.
• Phenotype - The actual expression of a
  characteristic.
• Recessive - An expression that is expressed
  only when its allele is the only form of the
  gene present in the genotype.

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DEFINITIONS

• Dominant - An expression that is always
  expressed if its gene is present in the genotype.
  
• Homozygous - The condition in which both forms
  of the gene are the same, ie. AA or aa.
• Heterozygous - The condition in which the two
  expressions of a gene pair are different.

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DEFINITIONS

• Sex chromosomes - Chromosomes involved in sex
  determination. In man these are the X and Y
  chromosomes.
• Autosomes - Chromosomes not involved in sex
• determination although they may carry genes
• for secondary sexual characteristic.
• Karyotype - A display of metaphase chromosomes
• in which homologs are paired together and
• photographed.

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DEFINITIONS

• Characteristic - A heritable feature such as
  flower color.
• Trait - Variations of a character such as white
  or purple flowers.
• True Breeding - In plants means that when plants
  self pollinate, the offspring are like the
  parents.
• Hybridization - The mating of individuals with
  different expressions of a character.

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MENDELS OBSERVATIONS

• An organism has two genes for each character.
• Law of Segregation states that the members of a
  gene pair separate during gamete formation
  meaning that each gamete receives only one
  gene from each pair during meiosis.
• The Concept of Independent Assortment states
  that the members of one pair of homologous
  chromosomes segregate independent of all other
  homologous pair during gamete formation.

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INDEPENDENT ASSORTMENT
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MENDELS CROSSES

• Monohybrid Cross - Cross that involves only one
  pair of contrasting alleles.
• Dihybrid Cross - Cross that examines the
  inheritance
• of two different pair of contrasting alleles.
• Test Cross - A cross between an individual
  expressing
• a dominant phenotype with an individual
• expressing the recessive expression of the same
  
• character. The cross is done to determine if
  the
• dominant individual is homozygous or
  heterozygous.

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MONOHYBRID CROSS
In garden peas purple flowers (P) are dominant to
white flowers(p). Cross a pure breeding plant
with purple flowers with a white flowering
plant. Give the phenotypic and genotypic ratios
of the F1 and F2 generations.
P1 PP X pp Gametes

Phenotypic Ratio All Purple Genotypic
Ratio All Pp (Heterozygous)
F1 Pp
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MONOHYBRID CROSS F2
P
p
PP
Pp
P
F2
Phenotypic Ratio 3 Purple 1 White Genotypic
Ratio 1 PP 2 Pp 1 pp
p
Pp
pp
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DIHYBRID CROSS F2
In garden peas, yellow seed (Y) are dominant to
green seed (y) and round seed (R) are dominant
to wrinkled seed (r). Cross a plant homozygous
for both yellow and round seed with a plant that
produces green wrinkled seed. Give the
phenotypic and genotypic ratios of the F1 and F2.
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(No Transcript)
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F2 Cross YyRr X YyRr
Gametes
X
yR
YR
Yr
yr
YR
YYRR
YYRr
YyRR
YyRr
Yr
YYRr
YYrr
YyRr
Yyrr
F2
yR
YyRR
YyRr
yyRR
yyRr
yr
yyRr
YyRr
Yyrr
yyrr
Phenotype - 9 Yellow Round 3 Yellow Wrinkled
3 Green Round 1 Green Wrinkled Genotype
- 111122224
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TEST CROSS

• Test Cross - A cross between an individual
  expressing
• a dominant phenotype with an individual
• expressing the recessive expression of the same
  
• character. The cross is done to determine if
  the
• dominant individual is homozygous or
  heterozygous.

In garden peas purple flowers (P) are dominant to
white flowers (p). Is a purple flowering plant
homozygous or heterozygous? PP or Pp
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TEST CROSS
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STATISTICAL APPLICATIONS
Rule of Multiplication The
probability that independent events will occur
simultaneously is the product of their
individual probabilities. EX. The probability
of giving birth to a daughter is 1/2 at any one
birth. The probability of three consecutive
daughters being born to a couple is 1/2 X 1/2 X
1/2 1/8.
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STATISTICAL APPLICATIONS
Rule of Addition The
probability of an event that can occur in two or
more ways is the sum of the separate
probabilities of the different ways. EX.
Parents are both heterozygous for a given
characteristic "Aa". What is the probability of
their having a child that is also "Aa"? The
probability of the "A" allele being in the egg
and "a" in the sperm is 1/2 X 1/2 1/4. The
probability of the "a" allele being in the egg
and the "A" in the sperm is 1/2 X 1/2 1/4.
Thus the probability of an "Aa" child is 1/4
1/4 1/2.
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INCOMPLETE DOMINANCE
Neither allele is dominant to the other, thus
each is expressed in the heterozygous genotype
producing an intermediate phenotype. EX. Red
(RR) and White (RR) snapdragons produce pink
(RR).
Pink crossed with pink. RR X RR
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CODOMINANCE
Inheritance characterized by full expression of
both alleles in the heterozygote. Ex. Short
Horn Cattle. Red (RR), Roan (RR), White (RR)
Roan crossed with roan RR X RR
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Multiple Alleles
In this type of inheritance there are more than
two alleles for the same characteristic in the
general population, although any one individual
only has two genes for the characteristic. EX.
ABO blood group. This characteristic shows
multiple allele inheritance, dominant/recessive
and codominance.
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Multiple Alleles
Alleles IA - Type A IB - Type B i -
Type O
Genotype vs. Phenotype IAIA or IAi Type
A IBIB or IBi Type B IAIB
Type AB ii Type O
IAIB X IAIB
1 Type A 2 Type AB 1 Type B
IA
IB
IB
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VARIATIONS IN INHERITANCE

• Pleiotrophy - The ability of a single gene pair
  to have
• multiple effects. EX. Gene for Sickle-cell
  Anemia.
• Epistasis - A condition in which one gene pair
  has
• the ability to mask or prevent the expression
  of
• another gene pair. EX. Coat color and pigment
• deposition in mice.
• Polygenetic Inheritance - A characteristic that
  is
• determined by the quantitative effect of
  multiple pair of genes.

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POLYGENETIC INHERITANCE
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OTHER CONSIDERATIONS

• Environmental Impact on Phenotype
• Norm of Reaction - Phenotypic range, may be
• caused by environment.
• Pedigrees - Family history for a particular
  trait.
• Shows the relationship of parents to children
• over many generations.

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PEDIGREES
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HUMAN DISORDERS

• Cystic Fibrosis - Lethal genetic disorder,
  recessive characteristic.
• Tay-Sachs Disease - Lethal disorder, recessive
  for a gene that regulates the breakdown of a
  class of lipids in the brain.
• Sickle Cell Disease - Recessive gene for
  hemoglobin. Heterozygous individuals are
  usually normal, but homozygous sickle cell
  individuals show severe symptoms. The gene
  shows pleiotropy.

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DOMINANT DISORDER

• Achondroplasia - Heterozygous individuals show
  a type of dwarfism.
• Huntingtons Disorder - A lethal degenerative
  disease of the nervous system. The gene usually
  becomes active around age 35 to 45.

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TECHNOLOGY AND GENETIC TESTING

• Carrier Recognition
• Fetal Testing
• Amniocentesis
• Chorionic Villus Sampling

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AMNIOCENTESIS AND CHORIONIC VILLUS SAMPLING