Classical Genetics

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Classical Genetics
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Gregor Mendel Father of Genetics
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Law of Segregation alleles separate during the
formation of sex cells. Sex cells contain only
one member of the pair of alleles
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If genes are located on separate chromosomes,
they are inherited independently of each other
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Law of Dominance when an organism carries
alleles for two contrasting traits, only the
dominant trait is shown
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• How could one determine whether an individual is
  homozygous dominant AA or heterozygous dominant
  Aa?
• Answer
• Perform a Test Cross
• This is done by crossing it with an individual
  that is homozygous recessive for the same trait,
  then looking at the offspring

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MendAlien Genetics

• http//www.phschool.com/science/biology_place/bioc
  oach/inheritance/genxfile.html
• Try some crosses and see each concept we covered
  explained.

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Incomplete Dominance
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Incomplete Dominance

• When two alleles are equally dominant, they
  interact to produce a new phenotype that is an
  intermediate between the two alleles.
• Example
• RED SNAPDRAGON X WHITE SNAPDRAGON ? PINK
  SNAPDRAGON

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Incomplete Dominance

• A capital letter represents one incomplete
  dominance allele
• A different capital letter represents the other
  incomplete dominant allele

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Incomplete Dominance
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Incomplete Dominance
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• Note Nelson Textbook uses
• CR CR red
• CW CW white
• CR CWpink
• See Figure 1 on page 144

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Incomplete Dominance

• In humans, curly hair (HH) is incompletely
  dominant to straight hair (HH).  The
  heterozygous individual has wavy hair (HH).

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Codominance

• In codominance, both alleles are expressed
  independently and are uniquely recognizable.
• Example
• Red flower X white flower? red and white spotted
  flowers

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Codominance
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Codominance

• A capital letter represents one of the codominant
  alleles
• A different letter represents the other
  codominant allele
• Note Nelson Textbook uses
• CR CR red
• CW CW white
• CR red and white spots
• See Figure 2 on page 145

   
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Codominance
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This roan horse is heterozygous for coat color.
Its fur appears grey because it has a mixture of
black and white hairs, not because the individual
hair colors are grey.
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Roan Cow
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In horses, gray horses (GG) are codominant to
white horses (WW).  The heterozygous horses(GW)
is an appaloosa horse (a white horse with gray
spots on the rump and loins).
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Codominance Tabby Pattern
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Dihybrid Crosses

• Mendel also studied the inheritance of two
  separate traits in crossbreeding following the
  same procedures he had used for studying single
  traits.
• Dihybrid Cross a type of cross that involves two
  genes, each consisting of non-identical alleles.

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Dihybrid Cross

• Mendel proposed that alleles of different genes
  are assorted independently of one another during
  the formation of gametes.
• This proved to be true.
• This is called the Law of Independent Assortment

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Dihybrid Cross Punnett Square

• in a dihybrid cross question, all 4 possible
  gametes for one parent are placed along the top
  of the Punnett Square and all 4 gametes for the
  other parent are placed along the side.
• Example Heterozygous round, yellow pea plants
  RrYy
• 4 gametes for RrYy are
• RY, Ry, rY, ry

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RrYy x RrYy
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P Generation RRYY x rryy ? F 1
Generation RrYy ? F 2 Generation 9/16
yellow, round 3/16
yellow wrinkled 3/16
green, round 1/16 green
wrinkled
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9331 Phenotype Ratio

• A phenotype ratio of 9331 occurs in the
  offspring of a mating of two organisms
  heterozygous for two traits.

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Eye Colour Dihybrid Cross BbGg x BbGg
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F.Y.I. Eye Colour Details

• At one locus (sitegene) there are two different
  alleles segregating the B allele confers brown
  eye color and the recessive b allele gives rise
  to blue eye color. At the other locus (gene)
  there are also two alleles G for green or hazel
  eyes and g for lighter colored eyes.The B
  allele will always make brown eyes regardless of
  what allele is present at the other locus. In
  other words, B is dominant over G. In order to
  have true blue eyes your genotype must be bbgg.
  If you are homozygous for the B alleles, your
  eyes will be darker than if you are heterozygous
  and if you are homozygous for the G allele, in
  the absence of B, then your eyes will be darker
  (more hazel) that if you have one one G allele.

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Sex-Linked Traits
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Sex-Linked Traits

• Sex-linked genes are located on one of the sex
  chromosomes (X or Y) but not the other.
• Since, typically the X chromosome is longer, it
  bears a lot of genes not found on the Y
  chromosome, thus most sex-linked genes are
  X-linked genes.

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Morgans Experiments

• In 1910, Thomas Hunt Morgan discovered that eye
  colour in fruit flies was sex-linked.
• Morgan's results is due to the fact that the gene
  for eye colour in Drosophila is located on the X
  chromosome.
• White eye colour is predominantly found in male
  flies.

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Sex-Linked Traits

• Morgans work on sex-linkage in fruit flies has
  been called one of the most important events in
  genetics.
• His work with Drosophila proved true the theory
  that Mendel's "traits" are found on chromosomes.

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Why are recessive sex-linked traits found more
often in males?

• Since females have two copies of the X
  chromosome, a female must inherit two defective
  genes.
• Since a male has only one X chromosome, if it is
  defective, he will have the disorder. XcY
• A female who appears normal but has one defective
  chromosome is called a carrier. XCXc

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Carriers

• X-linked traits are maternally inherited from
  carrier mothers. Each son born to a carrier
  mother has a 50 probability of inheriting the
  X-chromosome carrying the mutant allele.
• There are a few Y-linked traits these are
  inherited from the father.

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Sex-Linked Disorders

• hemophilia
• red-green color blindness
• congenital night blindness
• some high blood pressure conditions
• duchene muscular dystrophy

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Symbols for Colour-Blindness
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Punnett Square for Hemophilia Carrier Female x
Normal Male
XH Xh  
XH XH XH XH Xh  
Y XH Y Xh Y
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Normal colour vision should read the number
74.Red-Green colour deficiencies should read the
number 21.Total colour blindness should not be
able to read any numeral.
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Those with normal colour vision should read the
number 8. Those with red-green colour vision
deficiencies should read the number 3. Total
colour blindness should not be able to read any
numeral.
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Normal vision should read the number 29.
Red-green deficiencies should read the number
70.Total colour blindness should not read any
numeral
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Normal colour vision should read the number
5.Red-Green colour deficiencies should read the
number 2.Total colour blindness should not be
able to read any numeral.
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MendAlien Genetics

• http//www.whsd.k12.pa.us/courses/H0058/Interactiv
  e_Study_Partner/activity/ch15/a02/s01/fr150201.htm
  http//www.whsd.k12.pa.us/courses/H0058/Interactiv
  e_Study_Partner/activity/ch15/a02/s01/fr150201.htm