MULTIPLE ALLELES. Ms. Gunjan M. Chaudhari

Transcription

1 MULTIPLE ALLELES Ms. Gunjan M. Chaudhari

2 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.

3 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 - C ch C ch Himalayan - C h C h Albino cc

4 Wild type (CC): Chinchilla (C ch C ch ): Himalayan (c h c h ): 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 >C ch C ch > C h C h > cc

5 The ABO blood system This is a controlled by a tri-allelic gene It can generate 6 genotypes The alleles control the production of antigens on the surface of the red blood cells Two of the alleles are codominant to one another and both are dominant over the third Allele I A produces antigen A Allele I B produces antigen B Allele i produces no antigen 2007 Paul Billiet ODWS

6 Blood Types 10/4/2017 6

7 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

8 Blood types and transfusions People who are Type A blood produce antibodies to agglutinate cells which carry Type B antigens They 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 O Type O cells do not carry any antigens for the ABO system Type O cells pass incognito What about type AB people? 2007 Paul Billiet ODWS

9 Donor-recipient compatibility Donor Recipien t Type A B AB O A B AB O = Agglutination = Safe transfusion 2007 Paul Billiet ODWS 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.

10 What is Crossing Over? The exchange of chromosomal segments between two nonsister chromatids

11 Crossing Over Ref:

12 When Does it Happen? During Prophase I of Meiosis Remember the differences between Mitosis and Meiosis? Meiosis increases genetic diversity in a species Mitosis creates genetically identical daughter cells

13 Where does Crossing Over Occur? Genetic swapping occurs between paired homologous chromosomes in our sex cells The Egg and Sperm

14 Homologous Chromosomes Exchanging DNA by Crossing Over From:

15 Why Does Crossing Over Occur? To provide genetic variation during meiosis

16 BOTTOM LINE Its Why You and I Don t Look Alike Crossing Over ensures a combination of the maternal and paternal genes we inherited

17 Crossing Over Basics Occurs at One or More Points Along Adjacent Homologues during Synapsis Points contact each other DNA is Exchanged

18 Crossing Over Basics Gene Mapping Tracking crossing over helps determine where genes are located on the chromosome Genes that are far apart have a GREATER chance of crossing over Genes that are closer have a LESS LIKELY chance of crossing over

19 Steps of Crossing- over Synapsis of homologous chromosomes Zygotene Tetrad formation- Pachytene Crossing over- Pachytene Disjunction

20 FACTORS AFFECTING CROSS OVER 1. Temperature 2. X-Rays 3. Chemicals 4. Age 5. Interference 6. Sex

21 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.

22 References LLELES.ppt#258,3,Combinations Gregor_ Mendel.ppt#319,63,Blood Types

23 Single Crossovers: Non-crossover (Parental) and Crossover (Recombinant) Gametes What is the maximum % recombination?

24 Double Crossovers

25 Chromosomes MEIOSIS AND are CROSSING matched in homologous OVER pairs Each synapsis is made up of 2 pairs of sister chromatids This matched set of 4 chromatids is called a tetrad Chromosomes Centromere Sister chromatids Figure 8.12