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22/05/2015
1
Biology 3201 – Genetics Unit #2: Mendelian
Genetics #2Mendelian Genetics (part 2) and Beyond
Incomplete Dominance
O Incomplete dominance: a situation where NEITHER of the two alleles for a trait are dominant O Ex. Snapdragon flowers (red, white, heterozygous
is pink)
O Ex. Four o'clock flowers (red, white, heterozygous is pink)
O We represent the alleles with the same capital letter with a ‘ to show the differenceO Red flowers = RR
O White flowers = R’R’
O Pink flowers = RR’
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Incomplete Dominance
O Ex. What would be the genotypic and
phenotypic ratios of a cross between a red
flower and white flower?
O Parents: Red = RR; White = R’R’
R R
R’ RR’ RR’
R’ RR’ RR’
Genotypic ratio: 100% RR’
Phenotypic ratio: all pink
Incomplete Dominance
O Ex. 2 What would be the expected ratios of a
cross between two pink flowers?
O Parents : RR’ x RR’
R R’
R RR RR’
R’ RR’ R’R’
Genotypic ratio: ¼ RR: ½ RR’: ¼ R’R’
Phenotypic ratio: ¼ red: ½ pink: ¼ white
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Co-dominance
O Co-dominance: a situation where both alleles for a
trait may be dominant. Both alleles are expressed in
a heterozygous individual.
O Ex.
O Feather colour in chickens (black and white
expressed at the same time is barred plummage)
O Roan horses (red and white hair expressed at the
same time
O Blood type (AB – type A and B blood expressed at
the same time
Co-dominance crosses
O In chickens, the gene for black feather
colour is co-dominant to the gene for white
fur colour. What are the expected genotypic
and phenotypic ratios of a cross between a
black rooster and a white hen?
O Parents: BB x WW
O Note: in co-dominant situations, each allele
is given a different capital letter.
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Co-dominance crosses
B B
W BW BW
W BW BW
Genotypic ratio: 100 % BW
Phenotypic Ratio: all barred plummage (black and white)
Multiple Alleles
O Some genetic traits are expressed by
multiple (many) alleles. Such a trait is blood
type in humans. The blood type in humans
is controlled by 3 alleles resulting in 4
distinct blood types:
O Type A
O Type B
O Type AB
O Type O
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Blood types Genotype Phenotype
IAIA or IAi Type A
IBIB or IBi Type B
IAIB Type AB (A and B are co-
dominant). Most common
blood type
ii O (recessive) Least common
blood type
Blood Types
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Mixing Blood
O Only certain types of blood can be mixed. If
you mix the wrong types of blood
agglutination (clumping) will occur. This will
cause the blood cells in the person being
affected to clump together and the person
would die from a blood clot in the brain,
heart, etc.
Red blood cell compatibility
Blood group AB individuals have both A and B antigens on the
surface of their RBCs, and their blood plasma does not contain
any antibodies against either A or B antigen. Therefore, an
individual with type AB blood can receive blood from any group
(with AB being preferable), but cannot donate blood to any
group other than AB. They are known as universal recipients.
Blood group A individuals have the A antigen on the surface of
their RBCs, and blood serum containing IgM antibodies against
the B antigen. Therefore, a group A individual can receive blood
only from individuals of groups A or O (with A being preferable),
and can donate blood to individuals with type A or AB.
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Red blood cell compatibility
O Blood group B individuals have the B antigen on the surface of
their RBCs, and blood serum containing IgM antibodies against
the A antigen. Therefore, a group B individual can receive blood
only from individuals of groups B or O (with B being preferable),
and can donate blood to individuals with type B or AB.
O Blood group O (or blood group zero in some countries)
individuals do not have either A or B antigens on the surface of
their RBCs, and their blood serum contains IgM anti-A and anti-
B antibodies against the A and B blood group antigens.
Therefore, a group O individual can receive blood only from a
group O individual, but can donate blood to individuals of any
ABO blood group (i.e., A, B, O or AB). (Universal donors)
Crosses involving Multiple Alleles
O Ex. A woman with blood type A marries a
man with blood type AB. What are the
possible blood types for the offspring?
O Recall: Type A has 2 genotypes; IAIA or IAi, so
two separate crosses need to be done
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Crosses involving Multiple Alleles
O Cross 1: IAIA x IAIB
IA IA
IA IAIA IAIA
IB IAIB IAIB
Genotypic ratio: ½ IAIA : ½ IAIB
Phenotypic ratio: ½ type A: ½ Type AB
Crosses involving Multiple Alleles
O Cross 2: IAi x IAIB
IA i
IA IAIA IAi
IB IAIB IBi
Genotypic ratio: ¼ IAIA : ¼ IAi : ¼ IAIB : ¼ IBi
Phenotypic ratio: ½ Type A: ¼ Type AB: ¼ Type B
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Chromosome Theory of Inheritance
O Proposed by Walter Sutton and Theodor Boveri
(1902)
O Studied chromosomes during the various phases of
meiosis
O Findings:
O Chromosomes occur in pairs
O Chromosome pairs segregate (separate) during
anaphase I (this backs up Mendel’s claims of the
Law of Segregation
O Chromosomes align themselves independently
along the equator (this backs up Mendel’s idea of
independent assortment
Chromosome Theory of Inheritance
O What the theory says
O Mendel’s factors or genes are carried on
chromosomes
O The segregation and independent
assortment of chromosomes during meiosis
is what accounts for inheritance
O Note: The work of Sutton and Boveri
confirmed Mendel’s ideas about genes
(factors) and how they are inherited
(segregation and independent assortment)
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Thomas Morgan and Gene Linkage
O Studied fruit flies (Drosophila melanogaster)
O Did crosses involving eye colour
O Results:
O Found that chromosomes in fruit flies are the
same except one pair
O He called the dissimilar pair sex chromosomes
because he believed they determined the sex of
the fly
O found that certain traits such as eye colour in fruit
flies are found on the X chromosome. This is
what he called “sex-linkage”. Today we called this
“sex-linked” traits or genes
Sex-linked traits
O Sex-linked trait: a trait that is carried on one of the
sex chromosomes (X or Y)
O Morgan also found that certain genes on the same
chromosome are called “linked genes”
O He said that linked genes get inherited together and
not separately as Mendel had proposed (They do not
obey Mendel’s Law of Independent Assortment). This
would account for some differences in ratios of
crosses.
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Law of Independent Assortment in Modern TermsO Morgan also found that genes on the same
chromosome that are separated by a great
distance will separate as a result of crossing
over
O If crossing over does not occur, genes that
are located on the same chromosome will
be inherited together while those on
separate chromosomes will sort
independently.
Sex linked traits in humans
O Sex-linked traits are traits that are carried on sex
chromosomes (X and Y)
O Most often the traits are carried on the X
chromosome (because of the size difference)
O Most of the traits are recessive and sometimes may
be lethal
O Sex-linked traits affect males more often than
female. Since they are recessive, females would
have to have 2 defective X chromosomes. Males only
have 1.
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Sex linked traits in humans
O Examples
O Red-green colour blindness
O Male pattern baldness
O Hemophilia
O Duchenne Muscular Dystrophy
Sex-linked crosses
O Ex. A woman with normal colour vision and
with no instances of colour-blindness in her
family history marries a man who is colour-
blind. What are the possible ratios for their
offspring?
O Parents: XNXN x XnY
O Note: for sex-linked crosses, X and Y
chromosomes must be involved to
distinguish males and female offspring
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Sex-linked crosses
XN XN
Xn XNXn XNXn
Y XNY XNY
Genotypic ratio: ½ XNXn : ½ XNY
Phenotypic ratio: ½ carrier female: ½ normal male
Note: Carriers for recessive sex-linked disorders do not have
the disorder, but do have the ability to pass it on to their
offspring. Only females can be unaffected carriers
Sex-linked crosses
O Ex. 2 A woman who is a carrier for male
pattern baldness marries a male who is not
bald. What are the possible genotypic and
phenotypic ratios of their offspring?
O Parents: XBXb x XBY
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Sex-linked crosses
XB Xb
XB XBXB XBXb
Y XBY Xb Y
Genotypic ratio: ¼ XBXB : ¼ XBXb : ¼ XBY: ¼ Xb Y
Phenotypic ratio: ¼ normal female: ¼ carrier female: ¼
normal male: ¼ bald male
Note: out of the females, ½ were normal and ½ carriers but
physically they would look the same.
Sex-linked traits: important points
O 1. sex-linked traits are recessive (small
letters)
O 2. sex-linked traits are carried on the X
chromosome, normally
O 3. if a person has a big allele and a small
allele, they are a carrier of the trait. Only
females can be carriers. Males cannot be
carriers (only one X chromosome).
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Polygenic Inheritance
O Polygenic inheritance: where traits are affected by
more than one gene.
O Because of this, you get a range of phenotypes
instead of just two or three (ex. Not just tall or short
but some in the middle).
O Continuous variation: variation among individuals in
a population where there is a gradient of phenotypes
O Ex. Length of an ear of corn
O Human skin colour
O Human height
Polygenic Inheritance
O How does this work?
O Two or more genes work at the same time for the
same trait. This leads to variations in the expression
of the trait.
O Ex. In corn, the shortest lengths occur when the
alleles for both genes (four alleles in total) are
homozygous recessive. The longest lengths occur
when all the alleles are homozygous dominant.
All other lengths occur by other allele
combinations.
O CUT OFF FOR GENETICS #1 TEST