S .Y.B.Sc Semester III Botany Paper II Unit II :Cytogenetics

1. S.Y.B.Sc Semester IIIBotany Paper IIUnit II :Cytogenetics Chromosomal aberrationsBy Mrs. Mandakini R Ingle Department of Botany Satish Pradhan Dnyanasadhana College, Thane (w)

2. Basic definitions • Chromosomes, DNA and genes • Chromatides, and centromere • Arms of a chromosome (p and q) • Karyotype • Autosomes and sex chromosomes • Genotype and phenotype

3. Human Chromosomes • The chromosome carries the genetic information. • composed of deoxyribonucieic acid (DNA) on framework of protein . • Segments of DNA molecules comprise the genes; the units of heredity.

4. Chromosomes • During cell division, the chromosome can be seen to consist of 2 parallel strands; the chromatids, held together at one point, the centromere.

5. Human Chromosomes

6. Karyotype • It is the set of chromosomes of an individual. • It is the systematized arrangement of the chromosomes of a single cell. • In the human cell, there are 46 chromosomes or 23 pairs (diploid number); of these 23 pairs, 22 are similar in both sexes and are called the autosomes.The remaining pair is called sex chromosomes: XX in the female cells and XY in the male cells . • Chromosomes are arranged in groups A to G according to their shape & size.

7. Karyotype of a normal female

8. Karyotype of a normal male

9. Chromosomal Abnormalities

10. Chromosomal Abnormalities • Chromosomal abnormalities are either numerical or structural. • They are a very common cause of early spontaneous miscarriage. • Usually, but not always, cause multiple congenital anomalies and learning difficulties.

11. Chromosomal Aberrations (abnormalities) • Structural Aberrations • Deletion • Duplication • Inversion • Translocation • Numerical Aberrations (abnormalities) • Polyploidy: Multiple of the haploid (> Diploid) • Aneuploidy: Abnormal number

12. Structural abnormalities • Deletion : loss of a portion of a chromosome • Duplication : extra piece of a chromosome.. • Inversion : fragmentation of a chromosome followed by reconstitution with a section inverted. • Translocation : the transfer of a chromosome or a segment of it to a non-homologous chromosome

13. Chromosomal Deletions • a deletion results in a lost portion of a chromosome, • Deletion Causative Agents: Heat, radiation, viruses, chemicals, errors in recombination. result in partial monosomy, • the organism is monosomic for the portion of the chromosome that is deleted, • as in monosomy, most segmental deletions are deleterious.

14. Deletion

15. Intercalary Terminal Recognizing Deletions

16. Intercalary Terminal Homologous Pairs? Hemizygous Hemizygous: gene is present in a single dose. Psuedodominance: hemizygous genes are expressed.

17. Mental retardation Slow motor skill development Low birth weight and slow growth Small head (microcephaly) Partial webbing of fingers or toes Wide-set eyes (hypertelorism) High-pitched cry Cri-du-chat Syndrome

18. Chromosomal Duplication ...an event that results in the increase in the number of copies of a particular chromosomal region,

19. Duplication Causes: • duplications often result from unequal crossing over, • can occur via errors in replication during S-Phase. Effects: • results in gene redundancy, • produces phenotypic variation, • may provide an important source for genetic variability during evolution.

20. Unequal Crossing Over Produces both duplications and deletions!

23. Duplication in Evolution essential genes do not tolerate mutation, duplications of essential genes, then subsequent mutations, confers adaptive potential to the organism, new gene family members are ‘recruited’ to perform new functions.

24. Chromosomal Inversions inversion: aberration in which a portion of the chromosome is turned around 180o.

25. A B C B A Paracentric Inversion ...an inversion in which the centomere is not included, A B C ...a paracentric inversion does not change arm length ratio.

27. A B C A C B Pericentric Inversion ...an inversion in which the centromere is included, ...a pericentric inversion results in a change in chromosome arm length.

29. Inversion Loopno crossing over

30. Paracentric Produces haploid gamete.

31. Paracentric Produces gamete with inversion.

32. Paracentric Produces a chromosome with two centromeres. Nonviable gametes.

33. Paracentric Outcomes 1 Normal Gamete, 1 Inversion Gamete, No Crossover Classes Recombination is not inhibited, but recombinant gametes are selected against.

34. Dicentric/Ascentric …results only when the crossing over occurs within the region of the paracentric inversion,

35. Dicentric ...a chromosome having two centromeres;

36. Non-Viable (gametes) Segregate

37. Acentric …a chromosome having no centromeres, …segregates to daughter cells randomly, or is lost during cell division, …deletions impart partial monosomy.

38. Pericentric

39. Recombination and Inversions • Paracentric and Pericentric; • 1 Normal Gamete, • 1 Inverted Gamete, • No Crossover Classes = No Recombination, Inversions select against recombinant gametes, thus preserves co-segregation of specific alleles.

40. Inversions and Evolution • Inversions ‘lock’ specific alleles together, • all offspring get the alleles from either a wild-type, or inverted chromosome, • If the ‘set of alleles’ is advantageous, the set can be maintained in the population.

41. Terminal Translocation

42. Reciprocal Translocation

43. Robertsonian Translocations …the fusion of long arms of acrocentric chromosomes,

44. Down Syndrome • 95% of Down Syndrome individuals are a result of Trisomy 21, • the probability of having a second Down Syndrome child is usually similar to the population at large, • However, there is second cause of Down Syndrome caused by a Robertsonian translocations that is heritable.

45. Structural abnormalities Robertsonian Translocation Reciprocal Translocation

46. Reciprocal translocations • An exchange of material between two different chromosomes is called a reciprocal translocation. When this exchange involves no loss or gain of chromosomal material, the translocation is 'balanced' and has no phenotypic effect. • Balanced reciprocal translocations are relatively common, occurring in 1 in 500 of the general population.

47. Reciprocal translocations • Finding a balanced translocation in one parent indicates a recurrence risk for future pregnancies and antenatal diagnosis by chorionic villus sampling or amniocentesis should be offered as well as testing of relatives.

48. Reciprocal Translocation

49. Reciprocal Translocation Robertsonian Translocation

50. Robertsonian Translocation