Genes are the hereditary units. in next few slides we'll try to understand the concept of genes with suitable examples.

1. Concept of Gene
Presented by:
Sanjay Kumar Vishwakarma

2. Allele
• Alleles are alternative form of a gene
• Alleles are of two types, i.e., either dominant and recessive
• They occupy the same locus on a particular chromosome

3. • A haploid cell has single copy of an allele, diploid two and polyploid
more than two for a character
• An individual may have identical alleles at the corresponding locus
of homologous chromosomes (homozygous) or two different alleles
(heterozygous)

4. Multiple alleles
• Two different forms of a given gene exist in species are referred to
as multiple alleles
• Individuals within the population are only able to hold two of them
• It can only be studied in population

5. • Some well-known examples of multiple alleles are
• (1) Fur color in rabbits
• (2) Wing type in Drosophila
• (3) Eye color in Drosophila
• (4) Self-incompatibility alleles in plants
• (5) ABO blood group in humans

6. • One of the most firmly established series of multiple alleles in
humans involves the genetic locus controlling the blood types
• There are four blood types present in human i.e., blood types
A, B, AB and O

7. ABO Blood Types
• 3 alleles (IA, IB and IO) for the gene that determines blood type
Allele Codes
IA Type A
IB Type B
IO Type O

8. • The ABO locus controls the type of glycolipids found on the surface
of erythrocytes, apparently by specifying the type of glycosyl
transferases synthesized in the RBC
• The specific types of glycolipids on the red cell surface provide the
antigenic determinants that react with specific antibodies present in
the blood serum

9. Blood group Terminal sugars
of antigens
present
Antibodies
present
Red cell types
agglutinated
Transfusions
accepted from
A A (galactosamine) Anti-B B, AB A or O
B B (galactose) Anti-A A, AB B or O
AB A (galactosamine)
and B (galactose)
None None A, B, AB or O
O None Anti-A and
Anti-B
A, AB and AB O
Blood transfusions compatibilities for the ABO blood groups

10. Pseudoallele
• Pseudoalleles are closely linked and functionally related genes
i.e., they governs different expressions of the same character
• A cluster of pseudoalleles is known as pseudoallele series or a
complex locus or a complex region
• Pseudoalleles are considered to occupy a complex locus which
is divided into sub loci

11. • They exhibit low frequency of genetic recombination by crossing over
• They exhibit cis-trans position effect
• In trans heterozygotes such mutants produce mutant phenotype, but in
cis-heterozygotes they produce a wild phenotype
• The well-known examples of pseudoalleles are lozenge gene and star
asteroid in Drosophila

12. Lozenge Eye in Drosophila
• Green and Green (1949) studied lozenge locus in Drosophila
• The mutant gene produces eye with glossy smooth surface
• Several alleles of lozenge gene were identified and all mapped at one locus
• All heterozygotes carrying two different mutants were lozenge in phenotype
• But progeny of such heterozygotes produced wild type recombinants at a
frequency much higher than expected spontaneous mutation. This indicated that
lz1 and lz2 were pseudoalleles

13. • The recombination between pseudoalleles is very rare
• lz1 +/ lz2 + Trans-heterozygote (mutant phenotype)
• lz1lz2 /+ + Cis-heterozygote (wild phenotype)

14. Isoalleles
• An allele which is similar in its phenotypic expression to that of
other independently occurring allele is known as isoallele
• Isoalleles are of two types
• 1. Mutant Isoalleles
• 2. Normal Isoalleles

15. • Mutant Isoalleles: Such alleles act within the phenotypic range of a
mutant character.
• Normal Isoalleles: Such alleles act within the phenotypic range of a
wild character.

16. Multiple Choice Question

17. 1. An allele is dominant if it is expressed in
a) Both homozygous and heterozygous states
b) Second generation
c) Heterozygous combination
d) Homozygous combination
Answer: a

18. 2. If there are multiple alleles for a gene which of the following is true?
a) All the alleles expressed at the same time
b) Among the multiple copies present only the most dominant one is expressed
c) Only two alleles are present at a time
d) Only one allele is dominant rest all are recessive
Answer: c

19. 3. When two genetic loci produce identical phenotypes in cis and trans position, they are
considered to be
a) multiple alleles
b) the parts of same gene
c) pseudoalleles
d) different genes
Answer: c

20. 4. Isoalleles are
a) Similar alleles with different expression
b) Alleles with similar phenotype
c) Different alleles with different expression
d) All of the above
Answer: d

21. Thank You