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Lecture 4 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.

Dominant and recessive are operational definitions.

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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

Some genes make enzymes

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Incomplete dominance-

Now lets look at something different:

Although straightforward dominance/recessive relationships are the rule,

there are a number of variations on this pattern of inheritance of PHENOTYPES

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:

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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:

P: Pure white x Pure red

F1: All Pink

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

How are these results explained?

How do we relate genotype to phenotype

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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

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The classic example of this is the colors of carnations.

R1R2 x R1R2

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

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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

LM and LN

These two alleles represent two different forms of the protein on the cell surface.

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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.

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

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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.

Genotype Phenotype= Phenotype=

Precipitation by antigen on RBC surface

- LN - LM

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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 rules 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 in a heterozygote.

Genotype phenotype

Tt tall? Short? In between?

With respect to blood in a LN LM individual and a LM LM

individual

Looking at RBC by the naked eye- the heterozygote will be

like the homozygote.

With regards to the M and N blood groups (by

Immunoprecipitation) the phenotype is different in the two

individuals.

We have discussed

pea shape,

flower and eye color,

Morphology

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.

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Phenotypes

We have discussed

pea shape,

flower and eye color,

Morphology

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 results from a mutation in the gene coding for globin.

Hemoglobin, major constituent of RBCs is globin+ heme.

HbA: an allele that codes for the normal globin protein

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

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)

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

HbS/HbS

HBS/HbA

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

Recessive/ incomplete co-dominantDominant dominant

-

+ HbA

/HbA

HbS/H

bS

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 than HbA/HbA individuals. 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/HbSHbS/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

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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 _____ rather

than the Mendelian ______ 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

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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.

The expected genotypes and phenotypes are as follows:

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

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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The C gene in rabbits

· C- full color

· cch- chinchilla (light gray)

· ch- Himalayan (albino, black extremities) · c- albino

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

Dominant ---------------> Recessive

C ---> Cch ---> Ch ---->c

The dominance relationship is relative to alleles being tested

CC Ccch Cch Cc full color

cchcch cchch cchc chinchila

chch chc Himalayan

cc albino

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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 An on RBC Ab in blood

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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 transfusions

There is a reciprocal relationship between antigens in the RBC surface and antibodies present in the serum. If an individual has A antigens in their RBCs, they will have B antibodies in their sera. The biological significance of this reciprocal relationship and the presence of the antibodies are not clear.This relationship has important implications for blood transfusions:

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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.