physical basis of heridity

Heredity or Hereditary is the process of passing the traits and characteristics from parents to offsprings.
The offspring cells get their features and characteristics aka genetic information from their mother and father.

1. PHYSICAL BASIS OF HERIDITY SANDESH,G.M

2. heridity Heredity or Hereditary is the process of passing the traits and characteristics from parents to offsprings . The offspring cells get their features and characteristics aka genetic information from their mother and father. Heredity and genetics are the reason you look so much like your parents

3. Physical basis of heridity When Gregor Mendal formulated his laws of heredity, he postulated a particulate nature for the units of inheritance . What exactly these particles were he did not know. The unraveling of the physical basis of heredity led to understand location of genes and their transfer. Today scientists understand not only the physical location of hereditary units (i.e., the genes ) but their molecular composition as well.

4. INTRODUCTION In the year 1900, Mendel’s paper was rediscovered by Correns, de Vries and Tshermack . This paper of Mendel outlined the essiential features of the behavior of genes during inheritance. In the same year, the second edition of “The Cell in Development and Inheritance” by Wilson was published. At that time it was well established that. Each species has a constant chromosome number Each chromosome divides longitudinally during mitosis.

5. Contd.., It was generally supposed that genes were located on chromosomes, but a critical assessment of the supposition was lacking. It was not recognized that chromosome occurred in pairs and that different chromosomes of the haploid set were distinct from each other.

6. When, why and how..??? 1902 -Correns proposed that genes are located on chromosomes. The idea of chromosomal basis of segregation and independent assortment was put forth by Sutton. 1903 - The hypothesis that genes are located in chromosomes was designated as Sutton- Boveri hypothesis. Chromosomal theory of Inheritance 1865 - Mendels work was published. Hypothetical units of inheritance - Factors 1900 - Reconstitution of mendels work was done. W. Sutton T.Boveri

7. Chromosomal theory of inheritance Mendel Factors are in pairs Factors separate at the time of gamete formation. Alleles or factors assort independently Sutton and Boveri Chromosomes which have these genes (factors) are in pair (Homologous) Homologous chromosomes separate at the time of gamete formation. Chromosome assort independently

8. Salient features of theory 1. Both chromosomes as well as genes occur in pairs in the somatic or diploid cells.

9. Contd.., 2. A gamete contains only one chromosome of a type and only one of the two alleles of a character.

10. Contd.., 3. The paired condition of both chromosomes as well as Mendelian factors is restored during fertilization.

11. Parallelism between chromosomes and genes Chromosomes are also transferred from one generation to the next as in the case of genes (Mendelian factors). The number of chromosomes is fixed in each living species. These are found as homologous pairs in diploid cells. One chromosome from father and the other contributed by the mother constitute a homologous pair.

12. Contd.., Before cell division, each chromosome as a whole and the alleles of genes get replicated and are separated during mitotic division. Meiosis takes place during gamete formation. Homologous chromosomes form synapses during prophase-I stage which in later course get separated and transferred to daughter cells. Each gamete or a haploid cell has only one allele of each gene present in the chromosome. A characteristic diploid number is again established by the union of the two haploid gametes. Both chromosomes and the alleles (Mendelian factors) behave in accordance to Mendel's law of segregation.

13. Evidences for the theory Sex chromosomes. White eye gene of drosophila (sex linkage). Non-disjunction of X chromosomes (sex linkage). Attached X chromosomes. Bar eye gene (A duplication) Y-linked inheritance Structural and numerical chromosome changes Linkage and chromosome maps. Genetic material In situ hybridization Transgenic organisms

14. Sex chromosomes Sex Determination by chromosomes : Those chromosomes which are involved in the determination of sex of an individual are called sex chromosomes while the other chromosomes are called autosomes. 22 + X 22 + XX (b) The X-0 system (c) The Z-W system 76 + ZW 76 + ZZ 32 (Diploid) 16 (Haploid) (d) The haplo-diploid system

15. Contd.., In 1902, Mc Clung discovered the accessory or X chromosome of grasshopper and postulated that chromosome was involved in sex determination. This was the first demonstration that a character was associated with a specific chromosome. Sex is also governed by genes. Since a specific chromosome is involved in sex development, the genes governing this trait is located in the X chromosome.

16. White eye gene of drosophila (sex linkage) In 1910, Morgan presented a more direct evidence supporting chromosome theory of heredity. Morgan first observed and noted Wild type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type are called mutant phenotypes. In males, only one copy of a sex-linked gene is required in order for them to express a trait, such as white eyes. Two copies are required in females.

17. Contd.., Morgan mated a wild-type (red-eyed) female with a mutant white-eyed male. The F 1 offspring all had red eyes The F 2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However, no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes, and half had red eyes. Morgan then bred an F 1 red-eyed female to an F 1 red-eyed male to produce the F 2 generation.

18. Contd.., Since all F 1 offspring had red eyes, the mutant white-eye trait ( w ) must be recessive to the wild-type red-eye trait ( w + ). Since the recessive trait—white eyes—was expressed only in males in the F 2 generation, Morgan hypothesized that the eye-color gene ( w ) is located on the X chromosome and that there is no corresponding locus on the Y chromosome. This explains chromosome theory of inheritance. P Generation F 1 Generation F 2 Generation Ova (eggs) Ova (eggs) Sperm Sperm X X X X Y W W + W + W W + W + W + W + W + W + W + W + W W + W W W

19. Non-disjunction of x chromosomes From 1913 to 1916, Calvin Bridges performed experiments that indicated genes are found on chromosomes. His experiments like unusual transmission of a sex-linked gene in drosophila were a part of his doctoral thesis advised by Thomas Hunt Morgan. Nondisjunction , as described by Bridges, caused sperm or egg cells to contain abnormal amounts of chromosomes . In some cases, that caused the offspring produced by the sperm or eggs to display traits that they would typically not have. Failure of homologues to separate in Anaphase I His research on nondisjunction provided evidence that chromosomes carry genetic traits , including those that determine the sex of an organism

20. Contd.., Non disjunction gives rise to some cytological expectations. Those having vermilion eye should have XXY chromosome constitution. This was later confirmed by cytological studies. This was a concrete proof in favor of chromosomal theory of inheritance .

21. Attached x chromosome Attached X chromosome ( X^X) consists of two full lenth X chromosome sharing common centromere so that thay are always inherited together. In 1922, L.V Morghan discovered a female fly, which regularly produced unusual transmission of a sex linked gene (Yellow body colour ). She concluded two X chromosomes of the fly attached together. Cytological studies confirmed presence of the sex linked genes are located in the X chromosome. Hence its also a basis for chromosomal theory of inheritance.

22. The bar eye locus of drosophila Bar eyes is a trait in which flies have a reduced number of facets. Ultra-bar (double bar) is a trait in which flies have even fewer facets than the normal bar homozygote. Both traits are X-linked and show complete dominance.

23. Contd.., Sturtevant demonstrated that phenotype of flies with two bar genes on one chromosome and none on the other (double bar heterozygous), was different from those with one bar gene on each of the two chromosomes. This indicates that position of gene with respect to adjacent regions also influences its expression. This was called position effect. Later C.B. Bridges and his group, through a study of salivary gland chromosomes demonstrated cytologically that Bar character was associated with two such repeats of 16A region in X-chromosome and that ultrabar (double bar) had three such repeats in the same region. Thus confirmed that genetic effect was localized to a distinctly recognizable small segment of a chromosome.

24. Y-linked inheritance Some genes like H-Y genes in humans are located in the short arm of Y chromosome (Y s ). Such genes are transmitted from father to son and absent in females. Such a characters are known as holandric characters since they are confined to males only. This clearly indicates H-Y genes are located on Y chromosome , thus heredity to such genes through Y chromosome proves the physical basis of heredity.

25. Structural and Numerical changes in chromosomes Structural changes: If genes are located in chromosomes, structural changes should lead to a corresponding change in linkage relationship of concerned genes. Ex: when a segment is inverted, the inversion heterozygote shows a inverted gene sequence to corresponding inverted segment. Similarly in reciprocal translocation homozygotes, a corresponding change in the two linkage groups is obtained. Since structural changes in chromosomes are always accompanied by corresponding changes in the concerned linkage groups genes must be located on the chromosomes.

26. Contd.., Numerical changes: It also furnish evidence in favour of chromosome theory of heredity. In Monosomic plants for a specific chromosomes, genes located in the chromosome should be of single copy. So they show a deviation from normal inheritance pattern. The same is the case of trisomics where they carry three copies of the concerned genes than two. This property of monosomics and trisomics is exploited in assigning the different linkage groups to individual chromosomes of the haploid set. Thus genes on present on the chromosomes that favours heredity can be understood.

27. Linkage and chromosome maps In 1910, Morgan described the phenomenon f linkage and recombination between two sex linked genes in drosophila. Proposed that strength of linkage between any genes depends on the distance between them in the chromosome. Based on these Sturtevant proposed preparation of chromosome map based on frequencies of recombination between linked genes. The number of different linkage groups is the same as the number of chromosomes in the haploid set. There are no single case is known where it differs. This is exactly what would be expected if the genes are located in chromosomes.

28. Contd.., In 1931, Stern demonstrated that recombination between the linked genes indeed involved the exchange of chromatin material between homologous chromosomes. This was rather direct demonstration that genes are located in chromosomes . Linkage maps of bacteria and viruses were demonstrated to be circular. This led to expectation of chromosomes of both bacterial and viral would also be circular. The isolation of same proved it. Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes Long aristae (appendages on head) Gray body Red eyes Normal wings Red eyes Wild-type phenotypes II Y I X IV III 48.5 57.5 67.0 104.5

29. Genetic material It is accepted beyond any reasonable doubt that DNA is the genetic material in eukaryotes and most prokaryotes. By far the vast majority of DNA occurs in chromosomes of eukaryotes and prokaryotes. Therefore it is self evident that the genetic information or genes must be an integral part of chromosomes

30. In situ hybridization Gene expression occurs as follows: RNA polymerase produces RNA copies (mRNA) of one strand of the gene. 2. The mRNA direct synthesis of specific protein. 3. The protein participates in cellular activities to produce the phenotype specified by the gene. Radioactive labelled DNA copies of different mRNA molecules have been shown to base-pair with DNA in different but specific location of chromosomes in case of IN SITU hybridization. This demonstrates beyond any doubt that genes are present in chromosomes and they are the physical basis of heredity.

31. Transgenic organisms The mRNA produced by gene can be used to obtain copies of the concerned genes as follows. 1. The enzyme reverse transcriptase can be used t produce a DNA copy (cDNA) of the mRNA using latter as a template. 2. Alternatively, the genomic DNA of the organism may be isolated and the genomic DNA segment complementary to the mRNA may be obtained . Genes obtained by either technique have been transferred on to a different organisms, where they are expressed. Copies of gene obtained by both the approaches produce identical proteins. This clearly shows that the genes are located in chromosomes .

32. CONCLUSION The foregoing considerations clearly demonstrate that the genes are located on chromosomes and the chromosomes that carry genes to off springs are the physical basis of heredity .

33. Thank you..,