9 ­ GENETICS Incomplete, Codominance and Polygenic Inheritance.notebook April 08, 2011
Beyond Mendel's Observations Scientists now realize that patterns of inheritance are more complicated than Mendel proposed.
Multiple Alleles
Many traits are expressed by genes that have more than two alleles (even though any one diploid individual can only have at most two alleles for any gene). An example of a trait that has multiple alleles is the ABO blood groups in humans.
Drosophila melanogaster – the Fruit Fly
•
•
Many different eye colours are possible
Red is most common, but apricot, honey and white eyes are also present
Capital letters are used for such questions with superscripts to indicate the specific alleles involved
ð E1
ð E2
ð E3
ð E4
E.g. Red Apricot
Honey
White
AND
E1 > E2 > E3 > E4
Dominance Hierarchy and Symbols for Eye Colour in Drosophila
dominant over:
Wild Type (Red) ­ E1E1, E1E2, E1E3, E1E4
Apricot, Honey, White
Apricot ­ E2E2, E2E3, E2E4
Honey, White
3
3
3
Honey ­ E E , E E
4
White ­ E E
4
4
White
None
9 ­ GENETICS Incomplete, Codominance and Polygenic Inheritance.notebook April 08, 2011
Incomplete Dominance
Alleles are two alleles that when mixed produce a trait that is a blend of both.
Example:
For a specific species of rose. if a red flowered rose plant is crossed with a white flowered rose, the offspring will all produce pink roses. The ratios of the offspring are 1 red, 2 pink, and 1 white. Incomplete Dominant Alleles are symbolized by a capital letter and a superscript letter or prime signal (i.e. Fw or R' ) Incomplete Dominance Inheritance
Therefore, the possible alleles for the trait "rose colour" are ...
FW ­ this is the allele for the gene to produce white pigment
FR ­ this is the allele for the gene to produce red pigment
The following genotypes and corresponding phenotypes are possible:
Genotype Phenotype
F F produces white rose flowers
F F produces red rose flowers
F F produces pink rose flowers
W
W R
R W
R 9 ­ GENETICS Incomplete, Codominance and Polygenic Inheritance.notebook April 08, 2011
Incomplete Dominance Inheritance
ex 1: Cross a white rose with a red rose that show incomplete dominance
FW is the allele for the gene to produce white pigment.
FR is the allele for the gene to produce red pigment. Parents: Incomplete Dominance Inheritance
Genotypes:
FW FW F F F F W
R
R
R
Phenotypes:
White roses
Pink roses Red roses W
F
F
R
FW FR
9 ­ GENETICS Incomplete, Codominance and Polygenic Inheritance.notebook April 08, 2011
Incomplete Dominance Inheritance
ex 2: Cross a pink rose with a pink rose that show incomplete dominance
FW is the allele for the gene to produce white pigment.
FR is the allele for the gene to produce red pigment. Parents: Incomplete Dominance Inheritance
Genotypes:
FW FW F F F F W
R
R
R
Phenotypes:
White roses
Pink roses Red roses W
F
F
R
FW FR
9 ­ GENETICS Incomplete, Codominance and Polygenic Inheritance.notebook April 08, 2011
Incomplete Dominance Inheritance
Codominant Alleles Are two alleles that are both fully expressed. This means that the phenotype clearly shows both variations of that trait. a) Codominant alleles are symbolized by a capital letter with a superscript. b) Codominant alleles express both allele phenotypes. LCLC T T
L L LT LC 9 ­ GENETICS Incomplete, Codominance and Polygenic Inheritance.notebook April 08, 2011
Codominance Inheritance
An example of codominace in human is ABO blood types, the heterozygote AB type manufactures antibodies to both A and B types. The possible alleles for this gene are IA, IB, and i. There are actually multiple alleles in the population, but each individual only has two alleles. The following genotypes and corresponding phenotypes are possible:
The human ABO blood types are based on the proteins that are attached to red blood cells.
Protein "A", Protein "B", both Proteins, no Proteins
9 ­ GENETICS Incomplete, Codominance and Polygenic Inheritance.notebook April 08, 2011
Codominance Inheritance
ex 3: Mr Andress has type AB blood and Ms Andress has heterzygous type A blood. List the possible genotype and phenotype ratios for their children?
IA
IB
i
Genotypes:
IAIA ______% IBIB ______%
IAIB ______% IAi ______% IBi ______%
ii ______%
Phenotypes:
Type "A" ______% Type "B" ______%
Type "AB" ______%
Type "O" ______%
IA
IB
i
9 ­ GENETICS Incomplete, Codominance and Polygenic Inheritance.notebook April 08, 2011
ex 2: If a man has type O blood and a woman type B blood, what possible blood types could their children have? If this couple has six children, all with type B blood, what could you infer about the woman's genotype?
IB i
Transfusions and Blood Groups
9 ­ GENETICS Incomplete, Codominance and Polygenic Inheritance.notebook April 08, 2011
A medical problem:
some blood transfusions produce lethal clumping of cells.
Codominance Inheritance
Sickle Cell Anemia is one of the most thoroughly studied genetic disorders. ­ normal red blood cells are flat and disk­shaped ­ sickle­shaped cells are elongated and C shaped ­ misshaped red blood cells do not transport oxygen effectively because they cannot pass through small blood vessels. ­ this leads to blockages and tissue damage A A Hb Hb ­ no sickle cell anemia
HbAHbS ­ have some normal and some sickle cells, but
they rarely experience any symptoms
S S Hb Hb
­ has sickle cell anemia
An individual being heterozygous with the the sickle cell trait are resistant to malaria. In certain parts of Africa this could be an advantage.
9 ­ GENETICS Incomplete, Codominance and Polygenic Inheritance.notebook April 08, 2011
POLYGENETIC INHERITANCE
Some traits (phenotypes) are controlled by more than one gene. This is known as polygenic inheritance.
Human eye colour is controlled by at least two genes:
a) one set of alleles which codes for brown vs. blue b) another set of alleles which codes for green vs. blue
A man and a woman, each heterozygous for both genes, could have children with five different eye colors, ranging from light blue (no dominant alleles) through light brown (two dominants) to almost black (all four alleles dominant).
Human skin colour is also a classic example of a polygenic trait. It is known that at least three or four genes control skin colour, and for each of those genes, dark pigment has incomplete dominance with light pigment.
Assume three genes (for simplicity), and to avoid confusion among them, let’s arbitrarily call them genes A, B, and C. Then, someone who is AABBCC would have very dark skin colour and someone who is aabbcc would have very light skin colour. If they would get married and have children, their children would all be AaBbCc.