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Non-Mendelian
Inheritance
Complex Patterns of Inheritance
• Many things can happen to Mendel’s
factors during the process of meiosis
– Mutations
– cross-over between homologues
– non-disjunction.
Complex Patterns of Inheritance
(cont’d)
• There are also exceptions to the postulate
that factors occur in pairs and the law of
dominance, as well as the law of
independent assortment—it is now knows
many genes are linked on the same
chromosome.
• Linked traits are genes that are located on
the same chromosome.
Increasing Genetic Variability
Recall: Cross-over in Meiosis
• During Meiosis I, the chromatids in a
tetrad pair are so tightly aligned together
that the non-sister chromatids from
homologous chromosomes actually
exchange genetic material in a process
known as crossing over.
• This further shuffles the ancestral genes
so that a single chromosome in a gamete
may have genes from both the maternal
and paternal ancestors.
• Crossing over can occur at any location on
a chromosome, and it can occur at several
locations at the same time.
• It is estimated that during meiosis in
humans, there is an average of two to
three crossovers for each pair of
homologous chromosomes.
• The chromosome pieces further away
from the centromere cross over most
frequently; the frequency diminishes as
the centromere is approached.
• There can also be multiple cross-overs.
• They may occur because the genetic code
on each section of the chromatid is similar.
Sex-Linked Traits (p 318-320)
• Some patterns of inheritance seem to
weigh more heavily in males
– I.e. more males than females have
hemophilia, red-green colour blindness,
Duchenne muscular dystrophy and others.
How this was discovered:
• In 1910 T. H. Morgan studied the
Drosophila fly and found a mutant male fly,
which expressed the trait of white eyes
instead of the normal red eyes.
• This trait was very unusual in that species
and Morgan wanted to see if the trait
would be passed on to its offspring.
• He experimented
to find if this
strange trait would
be inherited
according to
Mendel's research.
• First he crossed
the mutant male fly
with a normal
female with red
eyes, to observe
whether the white
or red eyes were
dominant.
• The F1 generation all had red eyes, which
made Morgan conclude that red eyes were
dominant over white. (See Fig 12.11 p 319)
• He continued the steps of Mendel's
experiment by crossing two flies from the
F1 generation with each other.
• Out of 4252 flies in his F2 generation, 782
had white eyes but surprisingly all the flies
with white eyes were also male.
• This strange observation puzzled Morgan
to wonder why there weren't any females
with white eyes.
• He then crossed flies from the F1
generation with the original male fly with
white eyes.
• This cross resulted in white-eyed and redeyed males and females, making a 1:1:1:1
ratio.
• We see this pattern in humans—in
hemophilia, and red-green
colourblindness.
• Why? How does it specifically affect
males more often than females?
Sex-Linked Trait
• If a gene is found only on the X
chromosome and not the Y chromosome,
it is said to be a sex-linked trait.
• Because the gene controlling the trait is
located on the sex chromosome, sex
linkage is linked to the gender of the
individual.
• Usually such genes are found on the X
chromosome. The Y chromosome may
be missing such genes (See Diagram
above.).
• The result is that females will have two
copies of the sex-linked gene while males
will only have one copy of this gene.
• If the gene is recessive, then males
only need one such recessive gene to
have a sex-linked trait rather than the
customary two recessive genes for traits
that are not sex-linked.
• This is why males exhibit some traits more
frequently than females.
Showing sex-linked Punnet squares,
Red-Green Colourblindness:
• Because the allele is linked to the X
chromosome, we show it as a superscript
on the X:
Eg.
XC
Xc
normal vision gene
colour blind gene
• Children who inherit
this trait have
difficulties with green
hues, usually seeing
them towards a “red”
spectrum.
• Reds tend to be
seen darker, and in
low light colour
differentiation is
difficult—both
appearing black to
the person.
Eg. In humans, red-green colourblindness
is a recessive trait located on the X
chromosome.
• The Y chromosome does not carry this
trait at all.
• A normal-vision mother whose father was
colour blind has a child with a normalvision man.
• What is the probability the couple will have
a child with colour-blindness?
• What is the probability a son born to them
will be colour blind?
XCXc
XC
XC
Xc
XCY
x
Y
XC XC
XCY
XC Xc
XcY
Genotypic Ratios:
1:1:1:1
Phenotypic Ratios:
3:1
• Notice, Y doesn’t have this gene!
Hemophilia
• Hemophilia is a
term that covers a
wide variety of
clotting disorders—
some clotting
factor(s) are
missing or are
defective, resulting
sometimes in
uncontrolled
bleeding.
• Hemophilia is a recessive sex-linked trait
on the X chromosome.
• What are the chances a couple will have a
daughter with hemophilia if the mother is a
probable carrier and the dad has a form of
hemophilia?
• What is the chance they would have a
child with hemophilia?
XHXh
x
Xh
XhY
Y
XH
Xh
Genotypic Ratios:
Phenotypic Ratios:
• Notice, Y doesn’t have this gene!
SEX-INFLUENCED TRAIT
• These are not true sex-linked because it
is not on the X nor Y chromosomes, but
because of hormones or other such
differences between genders, these traits
show up and “look” like sex-linked traits.
• For example, in male-pattern-baldness is a
dominant trait in males but recessive in
females.
Male Pattern Baldness
• For example, male-pattern-baldness is a
dominant trait in males but recessive in
females.
– Male heterozygotes will go bald.
– Female heterozygotes will not go bald.
• A female would need to inherit the trait
from both parents to lose her hair.
• We would do a standard punnett square
for a dihybrid cross:
• A female with a lot of hair, is a carrier for
baldness has children with a man who is
losing his hair, yet is a carrier (dad was not
bald).
• What will be the phenotypes and
genotypes of their children?
XXBb
XB
x
XYBb
Yb
XB
Xb
Genotypic Ratios:
Phenotypic Ratios:
• REMEMBER the Bb has different meanings depending on gender!!!
TYPES OF DOMINANCE:
EXCEPTIONS to MENDEL’s
Law of Dominance
Incomplete Dominance
• In Mendel’s law of dominance,
heterozygotes exhibited the dominant
phenotype.
• When two alleles of a gene appear to be
blended in the phenotype, the alleles are
said to show incomplete dominance – they
don’t look like either parent.
An example is snapdragon colour
• Red snapdragons
crossed with white
snap dragons
yield pink snap
dragons—a
complete
BLENDING of the
alleles.
• In the example,
note how the
allele is shown.
• Another way the alleles are shown is to
use RR for the incompletely dominant red
colour, and R’R’ for the white colour.
THUS pink flowers would be RR’
• If two pink snap dragons are crossed,
what will be the expected phenotypes and
genotypes?
__________
?
?
?
Genotypic Ratios:
Phenotypic Ratios:
X _________
?
Curly Hair
• In humans, curly hair is incompletely
dominant to straight hair.
• Children who are heterozygous will have
wavy hair.
• What will be the phenotypes and
genotypes of the children from a female
with curly hair and man with wavy hair?
• C represents curly hair OR HC
• C’ represents straight hair or HS
__________
?
?
?
Genotypic Ratios:
Phenotypic Ratios:
X _________
?
Co-Dominance Inheritance
• It may seem as if incomplete dominance
and co-dominance are the same, but they
are not.
• When two alleles of a gene are clearly
expressed in the phenotype, the alleles
are said to be co-dominant.
• This results in two distinct and detectable
gene products
Blended: At a distance, the cattle appear
roan coloured and mottled:
• The individual hairs are either red or they are white.
• In doing the PUNNET square we use both
capital letters for the trait—R for red and
W for white.
• Eg. A roan bull (RW) mates with a white
cow.
• What will be the phenotypes and
genotypes of their offspring?
RW
X
WW
Genotypic Ratios:
Phenotypic Ratios:
• 50% will be roan and 50% will be white
Appaloosa Horses
• 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).
• Cross a white horse with an appaloosa
horse
__________
?
?
?
Genotypic Ratios:
Phenotypic Ratios:
X _________
?
Blood Typing
• Blood is typed according to what type(s) of
antigen (a cellular product that induces
antibody formation in a foreign host) are
found on the surface of the red blood cells.
• Blood type is determined by reacting the
blood with antibody against the antigens.
• Typical blood types are the ABO bloodgroups. (Text P 325 & 978)
Type AB Blood
• The AB blood type in humans is the result
of an individual carrying both the IA and the
IB alleles.
MN Blood Group system in Humans
• In humans, our M and N blood groups are
co-dominant.
• Our blood cells exhibit both antigens but
on separate blood cells.
• The MN blood group system is under the
control of an autosomal locus found on
chromosome 4, with two alleles
designated LM and LN.
• The blood type is due to a glycoprotein
present on the surface of red blood cells.
The table below is an old one but it shows
the frequencies of MM blood, MN blood, and
NN blood.
Blood Type and Geographic
Location
• HowStuffWorks Videos "Why Tell Me Why:
Different Blood Types"
INCREASING THE
GENE-POOL
MULTIPLE ALLELE INHERITANCE
• Mendel never knew that some traits occur
in more than pairs.
• This is called multiple allele inheritance
because more than two alleles are
possible for one trait—but only two alleles
are inherited and involved.
• In humans, our major blood type system is
a classic example.
ABO Blood Grouping
• We commonly call it the ABO system.
• As a multiple allele, we write it like we did
for co-dominance—that is because both
the A and B are equally strong.
• The exception is the type O which is a
recessive condition:
IA - for type A antigens
IB - for type B antigens
i - for the recessive O condition (which
produces neither the A nor B antigens and
does not interfere with type A or B blood).
• The following allele combinations are
possible:
IA IA
IB IB
IA IB
ii
IA i
type A blood (notice A is
dominant to O)
IB i type B blood
type AB blood—both
antigens are present
type O blood—no A nor B
antigens are present
• A woman is homozygous for type B blood
(IBIB)has a child with a man who is
heterozygous for type A blood.
• This means he is IAi.
• What will be the genotypes and
phenotypes of their children?
IAi
IA
IB
IB
Genotypic Ratios:
Phenotypic Ratios:
X IBIB
i
• What is the probability a couple whose
blood types are AB and O will have a child
with type A blood?
__________
?
?
?
Genotypic Ratios:
Phenotypic Ratios:
X _________
?
POLYGENIC INHERITANCE
• Many characters cannot vary in a
population across a continuum (gradient).
• For example, skin color in humans is a
quantitative character – this means the
character is controlled by more than one
gene at the same time (polygenic
inheritance) – that is, the trait depends on
several chromosomal locations at the
same time.
• This is different from multiple allele
inheritance where only TWO alleles are
passed on but in the population there are
several types of alleles for one trait—try
not to confuse these!.