Course Overview. Changes in chromosome number. Calvin Bridges cross. Drosophila. Sex determination. February 1. Drosophila

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1 Course Overview Changes in chromosome number February 1 Outline Week Topic Chapter 1 Course objectives and Introduction to genetics Ch. 1 & Ch. 2 2 Human Pedigrees Ch. 2 3 Patterns of Inheritance: sex-linkage Ch. 2 4 Chromosomal basis of inheritance Ch. 3 5 Changes in chromosome number Ch Gene Mapping Ch. 4 (Ch. 16) 7 Gene to Phenotype Ch. 6 8 Modified Mendelian ratios Ch. 6 9 Model organisms and mutants Ch. 6 (Ch. 16) 10 Genetics of Plant Development (Arabidopsis) Ch Genetics of Animal Development (Drosophila) Ch Behaviour Genetics/Quantitative genetics Ch papers Drosophila melanogaster has 4 chromosomes: 1 pair of sex chromosomes and 3 pairs of autosomes semi-colons are used to indicate when genes are on different chromosomes Eg: w;+;+;+ Drosophila 1;2;3; Calvin Bridges cross What is the expected ratio of progeny from the cross: w/w X +/Y White-eyed females X red-eyed males? X + X w X w X w X + (red ) X w Y (white ) X w X w X w X + (red ) X w Y (white ) Y Sex determination In Drosophila sex is determined by the X:A ratio 1X:2A is male 2X:2A is female 2X:3A is intersex In Drosophila melanogaster dosage compensation is by hyperactivation of the X in males Text p. 79

2 Text p. 491 Aneuploidy Aneuploid: an individual organism whose chromosome number that differs from wildtype by part of a chromosome set E.g. Drosophila nullo X (X0) sterile males differ from wildtype males by the loss of Y Text p Stages in the production of mature male and female gametes 2n Spermatogonium 2n Oogonium 2n Primary Spermatocyte 2n Primary Oocyte Meiosis I 2n Secondary Spermatocytes 2n Secondary Oocyte 2nFirst Polar body n n n Spermatids n Meiosis II n Ootid n n n Three polar bodies Text p. 86 Spermatozoa (sperm) n n n n Ovum (egg) n

3 Monosomic (2n-1) The absence of one copy of a chromosome from a pair E.g. Turner syndrome (45 X) Phenotype is female Trisomy (2n+1) Trisomics have an extra copy of one chromosome Autosomal trisomies: trisomy 13 Patau syndrome trisomy 18 Edwards syndrome trisomy 21 Down syndrome Sex chromosome trisomies: Klinefelter syndrome (XXY) XYY XXX Trisomic: e.g. 47 XXY Klinefelter syndrome 47 XXY karyotype Phenotype male but Barr bodies present Down syndrome (Trisomy 21) have an extra copy of chromosome 21 Trisomy 21 Text p.494

4 Text p.493 Trisomic Dosage compensation Gametic ratio : e.g. for gene A on a trivalent chromosome 1/6 A 2/6 a 2/6 A/a 1/6 a/a X-chromosome inactivation: Mitosis Mitosis Many Mitoses In early development of female mammals one X-chromosome becomes in activated and this inactivation persists throughout all subsequent mitotic divisions Barr bodies: an inactivated X- chromosome highly condensed and visible as a dark staining spot Cell line with inactive maternal X Cell line with inactive paternal X X- inactivation X- inactivation E.g. Calico cats In early development of female mammals one X-chromosome becomes in activated and this inactivation persists throughout all subsequent mitotic divisions Text p.495 and p.324 Text p.495 and p.324

5 12 possible trisomics The effects of one extra chromosome In humans trisomics form almost half of chromosomally abnormal spontaneous abortuses. For example trisomy 16 is most common in abortions and not seen among live births In Datura plants the effects an extra chromosome can be seen from the shape of the fruit The jimsonweed plant (Datura stramonium) Text p.77 Euploidy Euploid: having multiples of the basic chromosome set Text p.486 Text p.486

6 Chromosome structure Duplication: the replication of a segment of chromosome A. B C D E becomes A. B CC D E Inversion: a chromosomal rearrangement in which the chromosome is broken twice and flipped 180 degrees then rejoined A. B C D E becomes A. C B D E Translocation: the segment of one chromosome moved to another chromosome A. B C D E becomes A. B C H I Deletion the loss of a segment within one chromosome and the juxtaposition of the two segments on either side of the deleted segment A. B C D E becomes A. B E

7 Inversion: a chromosomal rearrangement in which the chromosome is broken twice and flipped 180 degrees then rejoined The loops in these polytene chromosomes reveal the breakpoints of the inversions Inversions Translocation: the segment of one chromosome moved to another chromosome Translocation Inversion picture from Text p.618 Deletion the loss of a segment within one chromosome and the juxtaposition of the two segments on either side of the deleted segment E.g. cri du chat syndrome (5p15.2 and 15.3 are deleted) Deletions

8 Naming chromosomal aberrations Drosophila melanogaster : Duplication -Dp(3L) (i.e. a duplication in the left arm of chromosome-3) Inversion- In(1) (i.e. Inversion in chromosome-1) Translocation- T(3R) (i.e. translocation in the right arm of chromosome-3) Transposition-Tp(2L) (i.e. transposition in the left arm of chromosome-2) Deficiency- Df(3R) (i.e. deficiency in the right arm of chromosome-3) More details can be found at: Human sex chromosomes SRY Introduction to genetic analysis Griffiths, A., Wessler, S.R., Lewontin,R.C., Gelbart, W.M.,Suzuki, D.T. and Miller, J.H. Eighth Edition, W.H. Freeman and Company NY Part I Transmission genetic analysis Chapter 1: all questions p Chapter 2: all the questions p Chapter 3: questions #1-12,18,19, 22, 25-27, 29, 30, 32, Part IV The nature of heritable change Chapter 15: sections 15.1 and 15.3; questions #1-3,11-13,19,21,22, 32, 38, 52.