BEYOND MENDELIAN GENETICS Conditions that Mendel didn

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BEYOND MENDELIAN GENETICSConditions that Mendel
didnt exploreIncomplete DominanceCo-dominance
Multiple AllelesPolygenic TraitsLethal gene
KCI 2014
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Limitations of Mendelian genetics

• Some genes are NOT independently assorted simply
  because they are too close to each other on a
  chromosome
• some alleles are incompletely dominant
• some genes are sex-linked
• A single trait can be controlled by many genes
  which can have multiple alleles

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Incomplete dominance
Flower color of snapdragons

• A condition in which neither allele for a gene
  completely conceals the presence of the other it
  results in intermediate expression of trait

3 phenotypes are observed in F2 in a 12(blended
traits)1 ratio
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Incomplete dominance

• However, the alleles of flower color do segregate
  according to Mendels law of independent
  assortment
• In human, traits showing incomplete dominance

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How to represent incomplete dominance?

• Use superscripts rather than upper-case and
  lower-case letters since neither allele is
  dominant over the other
• Eg. Snapdragon flower colour both alleles affect
  the colour of flower C
• 2 alleles are therefore CR (red) and CW (white)

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Incomplete dom.
https//www.youtube.com/watch?vR4-c3hUhhyc
Sickle cell disease in humans. Trait concerned
Shape of blood cells Hb letter chosen to
represent hemoglobin

• HbAHbA normal biconcave disk
• HbSHbS sickle cell S shape
• HbAHbS sickle cell trait (some normal and some
  sickle blood cells under prolonged low O2
  condition) ? resistant to malaria giving the
  individuals a heterozygous advantage

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If both parents have sickle cell trait
(heterozygous), what is the chance of their
offspring having sickle cell anemia?

• P HbAHbS x HbAHbS
• F1

Probability of offspring in F1 Probability of offspring in F1 Probability of offspring in F1
Genotype 25 HbA HbA 50 HbA HbS 25 HbS HbS
Phenotype 25 normal blood shape 50 with sickle cell trait 25 crescent blood shape (sickle cell anemia)
HbA HbS
HbA HbA HbA HbA HbS
HbS HbA HbS HbS HbS
Answer 25 chance their offspring will have
sickle cell anemia (HbSHbS)
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Codominance

• Is the condition in which both alleles for a
  trait are equally expressed in a heterozygous
  offspring both alleles are dominant

E.g a. Shorthorn roan cow Cattle can be red
(HRHR all red hairs), white (HWHW all white
hairs), or roan (HRHW red white hairs
together) b. Human AB blood type (two types of
protein ("A" "B") appear together on the
surface of blood cells.)
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Codominance
Correct the letter HR, not Hr as expressed in
your textbook
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Whats the difference between incomplete
dominance and codominance?

• E.g.
• a. Incomplete dominance
• - Both alleles are partially expressed
• - Offspring shows a third phenotype that is
  something between the parents phenotypes (i.e. a
  blend)
• b. Co-dominance
• - Both alleles are fully expressed
• - Offspring shows a third phenotype that is
  completely different from both the parents
  phenotype

Correct the letter r to R in this co-dominance
cross ?
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Multiple alleles

• So far weve studied traits controlled by one
  gene with two alleles
• An individual has only 2 alleles for a gene but
  many different alleles for a gene can exist
  within a population
• A gene with more than 2 alleles is said to have
  multiple alleles

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Reminder
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Human blood types example of both codominance
and dominance

• One single gene determine a persons blood type
  A, B or O
• This gene is designated as I and have 3 alleles
  that code for antigen proteins attached to the
  RBCs membrane

Alleles of the I gene What do they enable RBCs membrane to have?
IA Polysaccharide A
IB Polysaccharide B
i No polysaccharide
Genotype Blood type
IAIA IAi Type A
IBIB IBi Type B
IAIB Type AB
ii Type O
Possible alleles from a female
IA IB i
IA IAIA IAIB IAi
IB IAIB IBIB IBi
i IAi IBi ii
Possible alleles from a male
RECAP http//wps.aw.com/bc_goodenough_boh_3/104/2
6722/6840887.cw/content/index.html
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Multiple Alleles (Cont.) Rabbit Coat
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• Predict the genotypic ratio and phenotypic ratio
  of a cross between a heterozygous Chinchilla
  (cchc) with a heterozygous dark grey (Cch).

P cchc x Cch F1
Genotypic Ratios 1 Ccch 1Cc 1 cchch 1
chc Phenotypic Ratios 2 dark grey 1 light
grey 1 Himalayan
cch c
C Ccch Cc
ch cchch chc
One trait (i.e. rabbit coat color) is influenced
by multiple alleles
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Complete attached practice problems on
incomplete-, co-dominance and multiple
allelesTaking up Practice problem
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Incomplete dominance practice problem
SpongeBob loves growing flowers for his pal
Sandy! Her favorite flowers, Poofkins, are found
in red, blue, and purple. 1. Write the correct
genotype for each color if R represents a red
gene and B represents a blue gene. Red - Blue -
Purple - 2. What would happen if SpongeBob
crossed a Poofkin with red flowers with a Poofkin
with blue flowers. (a) Give the genotypes and
phenotypes for the offspring. (b) How many of the
plants would have red flowers? _____ (c) How
many of the plants would have purple flowers?
_____ (d) How many of the plants would have
blue flowers? _____
3. What would happen if SpongeBob crossed two
Poofkins with purple flowers? (a) Give the
genotypes and phenotypes for the offspring. (b)
How many of the plants would have red flowers?
_____ (c) How many of the plants would have
purple flowers? _____ (d) How many of the
plants would have blue flowers? _____
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Codominance practice problem

• 1. In some chickens, the gene for feather color
  is controlled by codominance. The allele for
  black is B and the allele for white is W. The
  heterozygous phenotype is known as erminette.
• a. What is the genotype for black chickens? ____
• b. What is the genotype for white chickens? ____
• c. What is the genotype for erminette chickens?
  ____

2. If two erminette chickens were crossed, what
is the probability that a. They would have a
black chick? ____ b. They would have a white
chick? ____
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3. In a certain fish, blue scales and red scales
are codominant. When a fish has the genotype B R,
it has a patchwork of blue and red scales. What
happens if you breed this fish with a fish that
only has Blue Scales.
 Step 1 Determine the genotypes of the
parents CB CR CB CB CR CR
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Multiple alleles practice problem

• 1. In the 1950s, a young woman sued film
  star/director Charlie Chaplin for parental
  support of her illegitimate child. Charlie
  Chaplins blood type was already on record as
  type AB. The mother of the child had type A and
  her son had type O blood.
• Complete a Punnett square for the possible cross
  of Charlie and the mother.
• b. The judge ruled in favor of the mother and
  ordered Charlie Chaplin to pay child support
  costs of the child. Was the judge correct in his
  decision based on blood typing evidence? Explain
  why or why not.
• Use Punnett squares to support your answer.

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• Alien Genotyping
• 2. In these aliens, horns are controlled by three
  allelles
• One center horn (A) is codominant with two horns
  (B). If an alien inherits both alleles (AB), then
  the alien has 3 horns. A recessive allele (O)
  results in an alien have no horns. Match the
  genotype to each of the pictures below.

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Incomplete dom.
Show a. Pink x Red b. Pink x Pink c. White x
White