MULTIPLE ALLELES

Ms. Gunjan M. Chaudhari

Characters of Multiple Alleles

The most important and distinguishing features of multiple alleles are

summarized below:

1. Multiple alleles of a series always occupy the same locus in the

chromosome.

2. Because, all the alleles of multiple series occupy the same locus in

chromosome, therefore, no crossing-over occurs within the alleles of a

same multiple allele series.

3. Multiple alleles always influence the same character.

4. The wild type allele is nearly always dominant, while the other mutant

alleles in the series may show dominance or there may be an intermediate

phenotypic effect.

5. When any two of the mutant multiple alleles are crossed, the phenotype

is mutant type and not the wild type.

Symbolism for Multiple Alleles

The dominance hierarchy is defined at the beginning of each problem involving

multiple alleles.

A capital letter is commonly used to designate the allele which is dominant to all other

alleles in the series.

The corresponding lowercase letter designates the allele which is recessive to all

others in the series.

Other alleles which are intermediate in their degree of dominance between these two

extremes, are usually assigned the lower case letter with some suitable super script.

Ex. Coat Colour in Rabbits

Full Colour (Agouti) - CC

Chinchilla - CchCch

Himalayan - ChCh

Albino – cc

Wild type (CC): Chinchilla (Cch Cch):

Himalayan (ch ch): Albino (cc):

The albino allele is recessive. The Himalayan allele is dominant to albino, but

recessive to everything else. Chinchilla is partially dominant to Himalayan and

albino, but recessive to wild type. Wild type is dominant to all of the other alleles.

This can be shown in the following way:

CC >Cch Cch > Ch Ch > cc

The ABO blood system

• This is a controlled by a tri-allelic gene

• It can generate 6 genotypes

• The alleles control the production of antigenson the surface of the red blood cells

• Two of the alleles are codominant to one another and both are dominant over the third

• Allele IA produces antigen A

• Allele IB produces antigen B

• Allele i produces no antigen© 2007 Paul Billiet ODWS

10/4/2017 6

Blood Types

Blood types and transfusions

• Blood types vary and your immune system recognises your own blood type as being self

• Other blood types are recognised as non-self

• If a blood which is incompatible with your body is transfused it will result in the agglutination of the foreign red blood cells

© 2007 Paul Billiet ODWS

Blood types and transfusions

• People who are Type A blood produce antibodies to agglutinate cells which carry Type B antigensThey recognise them as non-self

• The opposite is true for people who are Type B

• Neither of these people will agglutinate blood cells which are Type OType O cells do not carry any antigens for the ABO systemType O cells pass incognito

• What about type AB people?© 2007 Paul Billiet ODWS

Donor-recipient compatibility

Recipien

t

Type A B AB O

A

Donor B

AB

O

= Agglutination

= Safe

transfusion

Note:

• Type O blood may be transfused into all the other types = the universal donor.

• Type AB blood can receive blood from all the other blood types = the universal recipient.

© 2007 Paul Billiet ODWS

What is Crossing Over?

The exchange of chromosomal segments between two non-

sister chromatids

Ref: http://gnn.tigr.org/whats_a_genome/Chp3_2.shtml

Crossing Over

When Does it Happen?

During Prophase I of Meiosis

Remember the differences between Mitosis and Meiosis?

Meiosis increases genetic diversity in a speciesMitosis creates genetically identical daughter cells

Where does Crossing Over Occur?

Genetic swapping occurs between paired homologous

chromosomes in our sex cells—The Egg and Sperm

Homologous Chromosomes Exchanging

DNA

by Crossing Over

From: http://www.ultranet.com/~jkimball/BiologyPages/M/Meiosis.html#crossing_over

Why Does Crossing Over Occur?

To provide genetic variation during meiosis

Its Why You and I Don’t Look Alike

Crossing Over ensures a combination of the maternal and

paternal genes we inherited

BOTTOM LINE

Crossing Over Basics

• Occurs at One or More Points Along Adjacent Homologues during Synapsis

• Points contact each other

• DNA is Exchanged

http://waynesword.palomar.edu/images/cross3.jpg

Crossing Over Basics

• Gene Mapping– Tracking crossing over helps determine

where genes are located on the chromosome

– Genes that are far apart have a GREATERchance of crossing over

– Genes that are closer have a LESS LIKELYchance of crossing over

Steps of Crossing- over

• Synapsis of homologous chromosomes – Zygotene

• Tetrad formation- Pachytene

• Crossing over- Pachytene

• Disjunction

FACTORS AFFECTING

CROSS OVER

1. Temperature

2. X-Rays

3. Chemicals

4. Age

5. Interference

6. Sex

Significance of Crossing-over

• Produces new combinations of traits.

• Forms raw material for evolution.

• Establishes concept of linear arrangement

of genes.

• Helps to determine loci of genes in the

chromosomes.

References

•www.microbiologyprocedure.com2

•http://www.saburchill.com/IBbiology/chapters03/images/04MULTIPLE_A

LLELES.ppt#258,3,Combinations

•http://www.clfs.umd.edu/grad/mlfsc/res/CrossingOver.ppt

•http://www.ssapunjab.org/sub%20pages/edusat/bio4.pdf

•http://www.cst.cmich.edu/users/schis1j/bio326-02/lectures/9-

23.ppt#279,8,Slide 8

•http://woodwardsworld.org/uploads/Genetic_Fundamentals___Gregor_

Mendel.ppt#319,63,Blood Types

Single Crossovers: Non-crossover

(Parental) and Crossover (Recombinant)

Gametes

What is the maximum % recombination?

Double Crossovers

• Each synapsis is

made up of 2 pairs

of sister

chromatids

• This matched set

of 4 chromatids is

called a tetrad

MEIOSIS AND CROSSING

OVERChromosomes are matched in

homologous pairs

Chromosomes

Centromere

Sister chromatids Figure 8.12