MULTIPLE ALLELES SMG

Multiple Alleles - Introduction- Examples: 1. ABO blood group system in man 2. Self sterility in Nicotiana

1. MULTIPLE ALLELES Dr Saji Mariam George Associate Professor (Retired) Assumption College Autonomous Changanacherry

2. MULTIPLE ALLELES Most of the genes exist in two alternative forms having different phenotypic effects called alleles ( allelomorphs ) which occupy the same locus in the homologous chromosomes. Usually, a gene is represented by a most common allele called the wild type and an alternate allele , the mutant. However, there are several genes that exist in more than two alternative forms called multiple alleles , which control alternative forms of the same character and they occupy the same locus in a pair of homologous chromosomes . Multiple alleles controlling a particular trait may be present in a population of individuals, but only two alleles are present in a diploid individual. Examples :1. ABO blood group system in man 2. Self sterility in Nicotiana (Tobacco) .

3. 1. ABO blood group system in man Karl Landsteiner (1900 – 1901) had observed that there are differences in human blood. Landsteiner found two antigens ( agglutinogens ) viz. A and B on the membranes of red blood corpuscles (RBCs). The presence or absence of these antigens determine a person’s blood type in the ABO blood group system. Karl Landsteiner Human red blood cells © 2020 The Franklin Institute

4. ABO blood group system in man Antigens present Blood type A A B B No antigens O When only the A antigen is present on the membrane of RBC , the blood is type A . When only the B antigen is present on the membrane of RBC, the blood is type B . When the membrane of RBC contain no antigens , the blood is type O .

5. Landsteiner also found that the plasma contains two types of antibodies (agglutinins) viz. anti - A ( α ) and anti - B ( β ) . If the membranes of RBCs of a person contain antigen A (i.e., A blood group), the corresponding antibody anti - A ( α ) is absent in the plasma. But , the plasma will carry pre-formed anti - B ( β ) antibodies. If the membranes of RBCs of a person contain antigen B (i.e., B blood group), the corresponding antibody, anti - B ( β ) is absent in the plasma. But, the plasma will carry pre-formed anti A ( α ) antibodies.

6. If both A and B antigens are absent on RBC membranes(i.e., O blood group) , the corresponding antibodies anti - A ( α ) and anti - B ( β ) are present in the plasma. Landsteiner’s findings enabled safe blood transfusions between people with compatible blood groups. He received Nobel prize in Physiology and Medicine in 1930 for the discovery of ABO blood group system .

7. In 1902, Landsteiner’s colleagues, Alfred von Decastello and Adriano Sturli , identified a fourth blood type – the AB group . In AB blood type, both antigens A and B are present in the membranes of RBCs and no antibodies in the plasma. Antigens present Blood type Antibody present A and B AB No antibodies

8. Four blood groups of man in ABO Blood group system: A , B , AB and O . Antigens present Blood type Antibody present A A anti B ( β ) B B anti A ( α ) A and B AB No antibodies No antigens O anti A ( α ) , anti B ( β )

9. Inheritance of multiple alleles that control the ABO blood group system in man. Felix Bernstein (1924), a German mathematician had made a statistical analysis of the available data on ABO blood groups and proposed a pattern for the inheritance of multiple alleles that control the ABO blood group system in man. The ABO blood group system is controlled by a gene I , located in the 9 th chromosome. It has three alleles I A , I B , and I O or i (Multiple alleles). A person carries only two of these alleles, one from each parent. Felix Bernstein

10. The I A allele control the production of A antigen. So, persons with the genotype I A I A (homozygous condition)or I A I O or I A i ( heterozygous condition) have A blood group . I B allele control the production of B antigen. So, persons with the genotype I B I B (homozygous condition)or I B I O or I B i (heterozygous condition) have B blood group . In this system, the I A and I B , alleles are co-dominant , since each allele is expressed equally in the I A I B heterozygotes . Hence persons with the genotype I A I B will have both A and B antigens on the membranes of the RBCs and have the AB blood group . The I O or i allele is recessive to both the I A and I B alleles. There is no production of A or B antigens . So persons with the genotype I O I O or ii have no antigens on the membranes of RBCs and have the O blood group .

11. Multiple alleles of ABO blood group system in man Genotype Antigens present Blood type I A I A , I A I O or I A i A A I B I B , I B I O or I B i B B I A I B A and B AB I O I O or ii No antigens O

12. Since O blood group contain no antigens, the RBCs are not clumped (agglutinated) by the serum of any blood group. So, the persons of blood group O can give blood to all. Hence they are called as the universal donors . Since AB blood group contain no antibodies in the plasma, the serum from individuals of blood type AB does not cause clumping (agglutination) of RBCs of any blood type. So, individuals having AB blood group can receive blood from persons of all the blood groups. So, persons with AB blood group can be considered as universal recipients .

13. 2. Self sterility in Nicotiana (Tobacco) In plants with bisexual flowers, both male and female reproductive units are formed in the same flower. In many such plants self pollination takes place and seeds are formed. But in certain other plants, self pollination will not take place. In some of these plants, this may be due to self sterility or self incompatibility , which is the inability of the pollen grains from the same plant to effect fertilization. That means, self sterility or self incompatibility ensures cross pollination or out crossing.

14. Joseph Gottlieb Kolreuter (1764, a German botanist) first described self sterility in Nicotiana plant. https://collections.nlm.nih.gov Joseph Gottlieb Kolreuter by  Bug_Girl   https://davesgarden.com Nicotiana

15. East and Mangelsdorf (1925) reported that self sterility in Nicotiana is due to a gene S showing multiple allelism . In Nicotiana , there are 15 self sterility alleles such as S 1 , S 2 , S 3 , S 4 , S 5 , S 6 , S 7 , S 8 , S 9 , S 10 , S 11 , S 12 , S 13 , S 14 and S 15 . (i.e., S 1 , S 2 , S 3 , S 4 , S 5 …………S 15 , E ast and Yarnell , 1928) . However, only two of these alleles are present in the cells of a diploid plant. Edward Murray East Pau l Christoph Mangelsdorf

16. There is a gene controlled reaction between the diploid tissue of the female sporophyte and the haploid gametophytic tissue of the pollen tube. In this reaction, each allele acts separately. The alleles that determine self sterility produce their effect by controlling the rate of pollen tube growth. That is, an allele in the female parent’s tissue prevents the growth of pollen grains that carry the same allele. The products of the S- alleles expressed in female tissues of Nicotiana are ribonucleases , the S- RNases . The arrest of growth of pollen tubes in styles in incompatible crosses may be due to the entry of the S- RNase into the pollen tube and degradation of pollen tube RNA. In incompatible crosses, the pollen tubes grow slowly so that before the gametes reach the ovule, the flowers will be withered.

17. Example: If a plant S 1 S 2 is pollinated by its own pollen or by the pollen of another S 1 S 2 plant, no pollen grains will reach the ovules in time to effect fertilization. Thus a plant S 1 S 2 discriminates against both S 1 and S 2 pollen tubes. That is, pollen grains containing a certain allele, for example, S 1 fail to grow properly in the style of a plant that carries the same allele. Such alleles are called oppositional alleles. (CC BY-SA 3.0)  via  Commons Wikimedia

18. In compatible combination, the pollen tubes grow rapidly and effect fertilization. Example : A cross between a S 1 S 2 ♀ and a S 3 S 4 ♂ will produce offspring in all four possible combinations such as S 1 S 3 , S 1 S 4 , S 2 S 3 and S 2 S 4 . (CC BY-SA 3.0)  via  Commons Wikimedia

19. Multiple alleles - control self sterility in Nicotiana Credit : Suzuki et. al ., 1986 The presence of an allele in the female parent’s tissue prevents the growth of pollen tubes that carry the same allele.

20. THANK YOU