Transcript Beyond Mendel’s Laws… Incomplete Dominance

Beyond Mendel’s Laws…
Incomplete Dominance
• There is still a dominant and a recessive trait,
however the heterozygous individual will show an
intermediate form of the two traits!
• ”Blending”
• Another way of saying this is that the dominant
allele is not completely dominant, which is why
we say it is incomplete.
•
• Example: Flower colour in snap dragons
•
• Red is dominant (R)
• White is recessive (R’) – note for incomplete
dominance we do not write a lower case
“r”…why do you think this is?
• In snap dragons, flower colour is determined
by incomplete dominance. Red flowers (RR),
white flowers are (R’R’), and pink flowers are
(R, R’).
• If red and pink flowered plants are crossed,
what are the chances of producing a red
flower?
• Let RR be red
• Let RR’ be pink
• Let R’R’ be white
RR
RR’
RR
R
RR’
R
RR
X
RR’
R
R’
R
R
RR
X
R
R
RR
RR’
R’
R
RR
X
R
R
R
RR
RR
RR’
R’
RR
X
R
R
R
RR
RR
R’
RR’
RR’
RR
X
R
R
R
RR
RR
R’
RR’
RR’
RR’
Results
• ¼ RR = 25% chance Red
• 2/4 RR’
• ¼ R’R’
Sample Incomplete Dominance problems
•
• If a pink flower is crossed with a white flower, what is the chance of
producing a pink flower?
• If 2 pink flowers are crossed, what is the chance of producing a pink
flower? What about for producing a white flower?
• If a pink flower is crossed with a white flower, what is the chance of
producing a red flower?
Co-Dominance
• There are even situations where the heterozygous
individual will show both forms of the two traits!
•
• In other words, both traits are dominant, or “codominant”
•
• Example: Feather colour in chickens
•
• Black feathers are dominant (B)
• White feathers are also dominant (W) Note! We write both
dominant alleles as upper case
• Barred feathers (BW) is the heterozygous expression
• In chickens, feather colour is determined by
codominance. The allele for black feathers (B)
is codominant to the allete for white feathers
(W). The heterozygous chicken (BW) has
some black and some white feathers – often
referred to as “barred” feathers/plumage.
•
• If a “barred” chicken is mated with a black
feathered rooster, what is the chance of
producing barred chickens?
• Let BB be black
• Let WW be white
• Let BW be barred
• ½ BB
• ½ BW, therefore we have a 50% chance of
producing barred chickens
Sample Codominance questions
• If a black chicken is crossed with a white rooster,
what is the chance of producing a white chicken?
• If a barred chicken is crossed with a barred
rooster, what is the chance of producing a black
chicken?
Human Blood Types (Multiple
Alleles)
• Every trait that we’ve looked at so far has
involved only two possible alleles per trait: one
dominant and one recessive. (eg. T for tall, t for
short)
•
• We’ve seen instances where there are two
dominant alleles (codominance) or where the
recessive gene partially shows through in
heterozygotes (incomplete dominance)…but it
was always only two alleles.
• Some traits have more than two alleles! They still
exist in pairs only (one from mom and one from
dad) but human blood types have three alleles
on the same gene (chromosome 9):
•
• IA
• IB
• i
•
• There are FOUR phenotypes and SIX possible genotype
combinations:
•
• Type A
Homozygous
IA IA
•
Heterozygous
IA i
•
• Type B
Homozygous
IB IB
•
Heterozygous
IB i
•
• Type AB
IA IB
•
• Type O
ii
•
•
• There are two dominant alleles, IA and IB and one recessive allele, i.
• The presence of IA means that the red blood cells
will have the A-type glycoprotein on their
surfaces
•
• IB means that the B-type glycoprotein will be on
the surface
•
• If someone has both IA and IB they will have both
glycoproteins (Type AB blood)
•
• i means that there will be no glycoprotein on the
surface and is called Type O (very important fact!)
Human Blood sample problems
1) A man who has homozygous type A blood marries a woman
with type AB blood. What blood types could their children
have?
•
2) A woman with type O blood marries a man with
homozygous type B blood. What genotypes and
phenotypes could show up in the children?
•
3) What will the blood types be for children whose mother
has heterozygous B blood and father has type O?
• A lawyer is defending a man with type B blood. A woman
who is known to have type A blood, has a child with type O
blood and is claiming that this man in the father. Using a
Punnett Square, show how you would determine whether
or not he is the father!
•
• The identification of two newborn babies were
accidentally mixed up in the hospital! You have been
given the blood types of the babies and parents and been
asked to sort out which baby belongs to which parent:
Baby 1: type O
Mr. Brown: type A
Baby 2: type A
Mrs. Brown: type AB
Mr. Smith: type B
Mrs. Smith: type B
Sex Linked Genetics
•
• Human beings have 23 pairs of chromosomes, 22 of them
are common to both men and women and are referred to
as the AUTOSOMAL CHROMOSOMES.
•
• The 23rd pair of chromosomes is called the SEX
CHROMOSOMES because these determine what sex (male
or female) the person will be. Each sex chromosome is
called either X or Y and looks like this:
•
• Woman XX
• Man
XY
• Many genes are carried on the X chromosome,
while very few are on the Y (approximately 1% of
the genes compared to X).
• Since so few genes are on the Y chromosome, we
basically treat it as if there is no chromosome
there at all. Men have only one copy of the X
chromosome so this means that SEX-LINKED
genes on the X chromosome are expressed in
males whether they are recessive or dominant!
Let’s look at an example: A woman is colour blind, which
we know to be a sex-linked recessive trait (XbXb). She
marries a man with normal vision (XBY). What are the
chances of the children having normal vision?
• Let b = colour blindedness
•
• Let B = normal colour vision
• We write the alleles as
superscripts on top of the X
to indicate that it is sexlinked.