9 - GENETICS Incomplete, Codominance and Polygenic Inheritance

9 GENETICS Incomplete, Codominance and Polygenic Inheritance.notebookApril 08, 2011

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.

Beyond Mendel's Observations Scientists now realize that patterns of inheritance are more complicated than Mendel proposed.

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

E.g. Red ð E1 AND E1 > E2 > E3 > E4

Apricot ð E2

Honey ð E3

White ð E4

Dominance Hierarchy and Symbols for Eye Colour in Drosophila

dominant over:

Wild Type (Red) E1E1, E1E2, E1E3, E1E4 Apricot, Honey, WhiteApricot E2E2, E2E3, E2E4 Honey, WhiteHoney E3E3, E3E4 WhiteWhite E4E4 None


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' )

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

Incomplete Dominance Inheritance

The following genotypes and corresponding phenotypes are possible:

Genotype Phenotype

FWFW produces white rose flowersFRFR produces red rose flowersFWFR produces pink rose flowers



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:

FW FR FWFR


Genotypes:FW FW FW FR FR FR

Phenotypes:White rosesPink roses Red roses



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:

FW FR FWFR


Genotypes:FW FW FW FR FR FR

Phenotypes:White rosesPink roses Red roses



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.

LTLT LCLC

LT LC


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

Codominance Inheritance


Codominance Inheritanceex 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?

IB iIA

Genotypes:IAIA ______% IBIB ______%IAIB ______% IAi ______% IBi ______%ii ______%

Phenotypes:Type "A" ______% Type "B" ______%Type "AB" ______%Type "O" ______%

IB iIA


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?

iIB

Transfusions and Blood Groups


A medical problem:

some blood transfusions produce lethal clumping of cells.

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

HbAHbA no sickle cell anemia HbAHbS have some normal and some sickle cells, but they rarely experience any symptoms HbSHbS has sickle cell anemia

Codominance Inheritance

An individual being heterozygous with the the sickle cell trait are resistant to malaria. In certain parts of Africa this could be an advantage.


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.

POLYGENETIC INHERITANCE

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.

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9 - GENETICS Incomplete, Codominance and Polygenic Inheritance