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Lecture 4

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Dominance relationships

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What is the biochemical explanation for dominance?

The genetic definition of dominance is when an allele expresses its phenotype in the heterozygous condition.

By saying A is dominant over a, we are saying AA and Aa have the same phenotype.

Conversely the genetic definition of recessive is when allele does not express its phenotype in the heterozygous condition.

For example a gene responsible for height in the pea plant has a

dominant allele, T.

T/T= 6ft T/t= 6ft t/t=2ft

By definition T is dominant to t. And t is recessive to T.

Now if the short phenotype is observed in the heterozygote, then t is dominant and short is dominant to tall.

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Many, but not all genes, are responsible for the production of specific enzymes.

***** Remember enzymes catalyze biochemical reactions.

Substrate ---------> product

EnzymeA ^ |

GeneA

Wild-type= phenotype observed most of the time in nature

Genes make enzymes

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Incomplete dominance-

Although straightforward dominance/recessive relationships are the rule, there are a number of variations on this pattern of inheritance.

One of these variations is called Incomplete dominance

Incomplete dominance is the occurrence of an intermediate phenotype in the heterozygote.

The heterozygote exhibits a phenotype intermediate between the two homozygotes

A good example of this is in four o'clock plants:

How are these results explained genetically?

How do we relate genotype to phenotype

By applying Mendel's laws can you relate the phenotypic classes to the genotypic classes?

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The following explanation readily explains the

phenotypic outcome:

P: Pure white x Pure red

F1: All Pink

F2: 1/4Red 1/2Pink 1/4White

Do not use C and c to denote the two alleles- Use C1 and C2

In practice incomplete dominance can lie anywhere on the

phenotypic scale

The phenotype of the heterozygous individual is the key towards

determining whether an allele behaves as a recessive, dominant,

or incomplete dominance

If there is incomplete dominance, then

Phenotype ratio= Genotype ratio

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The classic example of this is the colors of carnations.

R1 R2

R1 R1R1 R1R2R2 R1R2 R2R2

R1 is the allele for red pigment. R2 is the allele for no pigment.

Thus, R1R1 offspring make a lot of red pigment and appear red. R2R2 offspring make no red pigment and appear white. R1R2 and R2R1 offspring make a little bit of red pigment and therefore appear pink.

Often in biological systems, substrate is limiting leading to incomplete dominance phenotypes.

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Co-dominance:

The biochemical basis of co-dominance is understood for the blood groups M and N

The surface of a red blood cell carries molecules known as antigens.

More than 20 different blood group systems are recognized. the best known are the ABO system and the Rh system.

The MN blood group system is of little medical importance.

In this system there are two antigens, M and N.

The L gene in humans codes for a protein present on the surface of the red blood cells.

There exist two allelic forms of this gene

These two alleles represent two different forms of the protein.

So with respect to the red blood cells, the genotype and phenotype relationships are as follows:

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We are used to phenotypes as flower color, height, hair length, shape etc.

The blood group phenotype is at a much finer level- that of the cell and is harder to observe.

*****

Remember the phenotype chosen is what the geneticists happens to notice. In this respect it can be somewhat subjective and depend on how observant the geneticists happens to be.

Both proteins are being expressed in the heterozygote.

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To determine the phenotype of the LM and LN blood cells

a very specific set of antibodies is required. The

anti- LM antibodies specifically recognize the LM

blood-cell surface proteins and the anti LN antibodies

specifically recognize the LN surface proteins.

In practice, specific recognition by each antibody results in precipitation of the red-blood cells. This is because each antibody actually has two functional binding sites enabling extensive cross-linking to occur.

So with the anti- LM and anti- LN antibodies, one

can determine which form of the L gene (LM or LN) is being expressed in each individual

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In this case, the heterozygote is expressing both proteins.

Therefore, with respect to RBC expression of LM

and LN protein these alleles are co-dominant

Genotype Phenotype RBC surfaceAntigen expressed

Precipitation by - LN - LM

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Blood groups

In this case, the heterozygote is expressing both proteins.

Therefore, with respect to RBC expression of LM

and LN protein these alleles are co-dominant

P: LM LM x LN LN

F1: LM LN

F2: LM LM: LM LN : LN LN

1 2 1

Genotype Phenotype RBC surfaceAntigen expressed

Precipitation by - LN - LM

LM LM No Yes LM

LN LN Yes No LN

LN LM Yes Yes LM and LN

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Paternity issues:

Paternity issues:

The M and N blood typing can be used to disprove that an individual was the biological father of a child. For example if the mother expressed only the

M antigen, she could be only of one genotype- LMLM.

If the child was of the genotype LMLN, we know the

biological father must possess at least one LN allele.

Mother's genotype Father's genotype

LMLM x LMLN

or

LNLN

This technique only rule out potential fathers. It cannot prove that an individual is the father. As you will learn later in the course, DNA fingerprints can actually be used to identify the individual father.

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Phenotypes

When examining a dominance relationship between two

alleles, we compare genotype to phenotype.

Specifically we look at the genotype of the

heterozygote. With respect to the M and N blood group

the phenotype is different than that of which we are

used to. We have discussed pea shape, flower color,

morphology and behavior as phenotypes. These are all

properties that are easily visualized. However with

specialized tools, microscopes and specific probes

such as antibodies we can detect less easily

visualized phenotypes.This indicates that the

phenotype has subjective nature to it. It depends on

the way the observer chooses to define it. This in

turn depends on the individual's powers of observation

and the tools available. For example, shown below are

a normal and a mutant Drosophila wing. What is the

difference?

Wild-type Mutant

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Sickle cell anemia

Sickle cell anemia is a good example of the variance in dominance relationships.

Sickle cell is an inherited disorder that results from a mutation in the gene coding for the protein globin.

Hemoglobin is a major constituent of the red blood cells and is involved in O2 transport.

HbA: an allele that codes for the normal hemoglobin protein

HbS: an allele that codes for an abnormal form of hemoglobin

We will examine the phenotype of the two homozygotes and the heterozygote at three levels:

the individual,

the cell

the protein.

Normal O2 levels Low O2 levels

(Sea level) (High altitude)

Depending on the O2 levels, the HBS allele (and the HBA allele) behaves as a dominant or recessive.

Remember, the phenotype of the heterozygote is the key to understanding whether a gene behaves as a dominant or recessive.

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Genes and their products

Genes and their products (primarily proteins) do not function in isolation, they interact with the environment.

What is the cellular phenotype with respect to these genotypes

The HBS allelic form of the protein causes the red blood cells to sickle.

Cell shape

HbA/HbA Normal shape

HbS/HbS Sickled

HBS/HbA Partially sickled***

At this level the alleles HbA and HBs are incompletely dominant

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Phenotype at the level of the protein.

So the HbS allele is classified differently depending on the level the phenotype is analyzed:

Individual Cell Protein

-

+HbA/HbA

HbS/HbS

HbA/HbS

With respect to the proteins HbS and HbA are co-dominant

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Fitness

Individuals homozygous for HbS/HbS often die in childhood. Yet, the frequency of the HbS allele is quite high in some regions of the world. In parts of Africa frequencies of 20% to 40% are often found for the HbS allele.

It was found however that in areas in which there was a high HbS allelic frequency, that there was also a corresponding high frequency of mosquitoes infected with the protozoan parasite, plasmodium. This parasite causes Malaria in humans. It was proposed and later proven that heterozygous HbA/HbS individuals are more resistant to the mosquito born parasite. Consequently this allele in maintained in the population in spite of its deleterious consequences in the homozygous state.

This condition in which the heterozygote is more fit than either of the two homozygotes is known as a balanced polymorphism (over dominance, heterozygote advantage)

HbA/HbA HbA/HbS HbS/HbS

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Lethal alleles-

Most of the mutations that we have discussed do not affect the viability of the individual. For example the mutations that produce white eyes in Drosophila or wrinkled yellow cotyledons in the plant do not disrupt viability. This means that the mutated gene is specifically involved in determining eye color and is not involved in processes central to viability of the fly.

What would be the genetic consequences if we isolated a mutation that disrupted an enzyme that was critical for the viability of the fly?

For example in Drosophila, Cy is a dominant mutation that produces Cy wings in the heterozygous condition but also behaves as a recessive lethal.

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Curly

When a heterozygous Cy male is crossed to a heterozygous Cy female, Cy to non-Cy progeny are produced in a 2:1___ rather than the Mendelian 3:1___ ratio

+ = normal or wild type genecy= dominant Cy mutation

One explanation for the ___ rather than the expected ___ ratio is that Cy behaves as a recessive lethal mutation and cy/cy individuals die prior to reaching adulthood

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How would you test this hypothesis?

Take the progeny and perform a test cross with the homozygous recessive parent (+/+ wild-type fly)

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Unlike cy, most recessive lethal alleles do not have an additional dominant visible phenotype.

For example let say a gene codes for an essential enzyme.

GeneA (normal enzyme)Genea (mutant enzyme)The expected genotypes and phenotypes are as follows:

genotype: A/A A/a a/A a/aphenotype: alive alive alive die

Phenotype of survival is 3:1

Lethal mutations arise in many different genes.

These mutations remain “silent” except in rare cases of homozygosity.

A mutation produces an allele that prevents production of a crucial moleculeHomozygous individuals would not make any of this molecule and would not survive.

Heterozygotes with one normal allele and one mutant allele would produce 50% of wild-type molecule which is sufficient to sustain normal cellular processes- life goes on.

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Lethal stocks

It is difficult to keep a stock that is a recessive lethal and has no other phenotype. In each generation some of the lethal alleles are eliminated

Gene A encodes an essential enzyme: · A = normal allele that encodes functional enzyme · a = mutant allele that encodes a non-functional enzyme and is recessive lethal (lethal when homozygous)

Genetics helps solve this:In order to maintain a lethal allele geneticists use "marker" mutations such as cy

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Take a heterozygous fly for both the A gene and curly

P:

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Multiple alleles

We have described a gene as exiting in one of two states: normal or mutant.

Each of these states is called an allele of that gene.

However it is possible and common for a gene to have more than two forms. Many genes exist in three or more forms (we say there exists three or more alleles of that gene)

Such a gene is said to have multiple alleles

******It is important to remember that even though a given gene may have many forms, each individual possesses only two forms of that gene. Diploids contain two copies of each gene.

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For example in Drosophila, many alleles exist for the white gene: 1) The normal (wild-type) allele W or w+ gives red eyes

2) white allele w has white eyes

3) white apricot wa gives apricot colored eyes

As of 1996, there exist over 150 alleles of the white gene

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3 alleles

How many genotypes are possible given three alleles at the white gene?

With 3 alleles, there are six possible pair-wise combinations

W+/w W+/W+ W+/wa w/w wa/wa wa/w

In a given protein the number of potential alleles=

(No of amino acids in protein)19

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The C gene in rabbits

·

These represent different alleles of the c locus with the following dominance relationship:

The dominance relationship is relative to alleles being tested

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ABO blood groups

The Human A,B,O blood group is the result of multiple allelism They were discovered in 1900 by Dr. Landsteiner.

The 4 blood types were defined on the basis of a clumping reaction. Serum (the liquid part of the blood Ab) from one individual is mixed with red blood cells (erythrocytes) from another individual. If they belong to different groups they will clump. This reaction is similar to the M and N groups discussed earlier. The clumping is due to the presence of antibodies in the serum.

Blood group Genotype Ab in blood An on RBC

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The ABO gene has three alleles

IA synthesizes an enzyme that adds sugar A to RBC surfaceIB synthesizes an enzyme that adds sugar B to RBC surfacei does not produce an enzyme

A phenotype arises from two genotypes

B blood type is due to two genotypes

AB blood type is due to a single genotype

O Blood type is due to a single genotype

Three alleles give you six genotypes but only four phenotypesEach phenotype is determined by two alleles

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Blood transfusionsThis relationship has important implications for blood transfusions:

· If O individuals are transfused with A blood, the anti-A antibodies will react with the A cells resulting in clumping.· If O individuals are transfused with B or AB blood, clumping also occurs· O individuals can only receive O blood, but they can donate Red blood cells to A,B, AB, and O individuals- they are universal donors.· Since AB individuals have no antibodies they can receive RBC from A,B,AB, or O individuals. They are universal recipients· With respect to dominant relationships we say IA and IB are dominant to i and that IA and IB are co-dominant

BA

A

AB

B

A B

O

BA A

A

B

B

AB

Can get blood from anybodyCannot get blood from anyone except O individuals but can donate RBC to anyone

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Multiple alleles at the human HLA loci

The HLA locus is the basis of tissue incompatibility in humans. That is, when an organ transplant or tissue graft is required, the success of the procedure depends on host donor genotypes. If the two are mismatched, graft rejection occurs. Whether a tissue is rejected depends primarily on the genotypes at two important loci known as the HLA loci:

HLA-A------------ 23 recognized allelesHLA-B------------ 47 recognized alleles

The HLA and HLB genes code for proteins that are located on the surface of the cells. For a successful transplant, the donor and recipient must have matching alleles at the HLA-A and HLA-B genes or risk graft rejection.

If the alleles are different however, graft rejection will occur in some but not all of the transplant combinations

Why this multiple allele system evolved at the HLA locus in unclear. It probably involved the tagging of the system as self or non-self (foreign). Cancer cells often express foreign antigens and are recognized by the immune system as foreign and destroyed.

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Polymorphism

Polymorphism is the existence of two or more allelic forms of a gene segregating in a population. Often these allelic forms have different phenotypic consequences.