Transcript Lesson 6. Beyond Mendel - Blyth-Biology11

BEYOND MENDEL’S LAWS
Mendel studied traits that
• Incomplete
dominance
were
purely dominant
or
recessive.
Unfortunately,
• Codominance
in
exists
• nature,
Multiplethere
alleles
more than simply two
• Polygenic inheritance
choices for some traits.
• Linked genes
• Sex-linked inheritance
Snapdragons can be white,
red, pink, yellow, or
orange.
Lesson 2.
INCOMPLETE DOMINANCE
Neither trait is dominant or recessive.
A heterozygous individual is a blend of the two traits.
Ex. Snapdragons
Instead of using R and r, we use FR and FW to show
incomplete dominance.
Red = FR FR
Pink = FR FW
White = FW FW
INCOMPLETE DOMINANCE
Cross a Red Flower
with a White flower:
If we crossed two plants
from F1, we would get:
CO-DOMINANCE
BOTH alleles for a trait are
dominant, a heterozygous
individual expresses both traits.
Ex. Black rooster FBFB x white hen
FWFW.
The F1 are all ‘barred’ (FBFw
black and white feathers)
EXAMPLE
In four o’clock plants, red flowers are
incompletely dominant over white flowers.
Heterozygous flowers are pink. If a red flower
is crossed with a white flower what is the colour
of:
a)The F1 generation?
b)The F1 generation crossed with the red parent?
c)The F1 generation crossed with the white
parent?
EXAMPLE
A cross between a yellow snake & a blue
snake produces offspring that are all
green:
a)What are the genotypes of the parent
generation (P)?
b)What are the genotypes of the F1
generation?
c)What would be the phenotypic ratios of
offspring produced by two green snakes?
What is Sickle Cell Anemia (SCA)?
• First described in Chicago in
1910 by James Herrick as an
inherited condition that results
in a decrease in the ability of
red blood cells to carry
oxygen throughout the body
– Sickle red blood cells become hard and irregularly
shaped (resembling a sickle)
– Become clogged in the small blood vessels and
therefore do not deliver oxygen to the tissues.
– Lack of tissue oxygenation can cause excruciating
pain, damage to body organs and even death.
Some Genetic History
 The error in the hemoglobin gene results from a genetic
mutation that occurred many thousands of years ago in people
in parts of Africa, the Mediterranean basin, the Middle East,
and India.
 A deadly form of malaria was very common at that time
 Malaria epidemics caused the death of many
 In areas where malaria was a problem, children who
inherited one sickle hemoglobin gene and who, therefore,
carried the sickle cell trait - had a survival advantage.
 Unlike the children who had normal hemoglobin genes, they
survived the malaria epidemics they grew up, had their own
children, and passed on the gene- for sickle hemoglobin.
Genetics
 2 copies of the gene
for Hb (each parent)
 HbS –Recessive
 S=Sickle
 A=Normal
Multiple Alleles: Human Blood Groups
• In humans a single gene controls a person’s
ABO blood type
• This gene determines what type of an antigen
protein is attached to membrane of a red blood
cell
• Three alleles A,B and O
• Combination of three alleles make 4 different
blood types ___________
• A human blood
group TYPES
or “type” is a group
BLOOD
of red blood cells that have the same
antigen protein markers on the cell
surface
• There are four human blood groups:
• A, B, AB, O
Type A produces
antibodies against
Type B antigens
Type B produces
antibodies against
Type A antigens
Type AB produces
no antibodies
against blood
antigens
Type O produces
antibodies against both
Type A &
Type B antigens
Blood Type
% Amongst
Canadians
Can
Receive
From
Can Donate
To
A
42
A, O
A, AB
B
9
B, O
B, AB
AB
3
A, B, AB, O
AB
O
46
O
A, B, AB, O
BLOOD TYPES
• AB = Universal recipient (can receive from anyone)
• O = Universal donor (can donate to anyone)
• Each blood type has + and – versions (Rh factor)
• + = Universal recipient (can receive from anyone)
• – = Universal donor (can donate to anyone)
EXAMPLE
If a woman has AB blood, and a man has
type A blood, what are the possible blood
types of their offspring?
Sickle Cell Trait

Sickle haemoglobin (S) + Normal haemoglobin (A) in RBC
 Adequate amount of normal Hb (A) in red blood cells
 RBC remain flexible
 Carrier
 Do Not have the symptoms of the sickle cell
 Heterozygous advantage (HbA HbS)
Polygenic Inheritance
• Display continuous
variation, in which
phenotypes vary
gradually from one
extreme to another
– Height, skin colour, eye
colour
• Variation is controlled by
more than one gene;
polygenic trait
Polygenic inheritance explains this…
Or this…
Inheritance of linked genes
Lack of independent assortment
with linked genes
Only way for Pl and pL is crossing over –
3% of the time
SEX-LINKED INHERITANCE
• Using fruit flies as
test subjects, Thomas
Morgan studied eye
colour using simple
monohybrid crosses.
Red eyes (R) are
dominant over white
eyes (r).
SEX-LINKED INHERITANCE
• When he crossed
purebred white-eyed
males with red-eyed
females, he was unable to
produce a female with
white eyes.
• He concluded that the
gene must be located on
the X chromosome.
SEX-LINKED INHERITANCE
• Some traits are
located on the sex
chromosomes, so the
inheritance of these
traits depends on the
sex of the parent
carrying the trait.
SEX-LINKED INHERITANCE
• Most known sexlinked traits are Xlinked (carried on the
X chromosome). This
is probably because
the X chromosome is
much larger than the
Y chromosome
SEX-LINKED DISORDERS
• Some sex-linked traits are associated with
disorders.
• Most are found on the X chromosome, Y-linked
disorders are rare.
• Males are at a much greater risk for inheriting
sex-disorders because they only inherit one X,
so if the X has the allele for the disorder, they
will suffer from the disorder.
• Recessive lethal X-linked traits result in death.
EXAMPLES OF SEX-LINKED
TRAITS and DISORDERS
• Red-green colour vision deficiency
(CVD) , hemophilia, X-linked severe
combined immunodeficiency (SCID)
Hemophilia
• Condition that affects
body’s ability to produce
proteins involved in blood
clotting
• X-linked recessive
• “Royal Disease”
Symptoms
• Uncontrolled bleeding “bleeds”
• Most bleeding is internal and
often into joint spaces
• Prior to 1960, life expectancy
was 11 years.
• Today, life expectancy only 10
years shorter than an
unaffected male
SEX-LINKED INHERITANCE
• Punnett squares are used to
predict the outcome of sexlinked inheritance.
• Assume the trait is X-linked
unless told otherwise!
• Most disorders are recessive,
some are dominant, the
question will tell you.
• A “carrier” is a female who is
heterozygous for the trait.
EXAMPLE
• Hemophilia is a recessive X-linked trait.
What is the probability of a couple having
a hemophiliac child if the man does not
have hemophilia and the woman is a
carrier?
Determining sex-link patterns in a
pedigree
• This pedigree shows the
inheritance of red-green
CVD in a family. Identify the
genotype of each family
member.
• How does the inheritance
pattern support X-linked
inheritance?
• Hint Start with a legend
Barr Bodies: Inactive X Chromosomes
• In females every cell only has one
functioning X chromosome
• Early in embryo development one X
chromosome is condensed tightly into
a Barr body
• Occurs randomly in each cell
• Explains calico and tortoiseshell cats
(50% of cells have active X with allele
for black, and 50% have and active X
allele for orange)
• All calicos are female, male calicos
are sterile. Why?