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Lecture 18; 2007 Biology 207; Section B2; Good Lecture#18 - Sex chromosomes and sex linkage Readings: Griffiths et al (2004) 8th Edition: pp 29-54 7th Edition: Ch. pp 37-40; 51; 73-76; 146-147; 681-684 Assigned Problems: 8th Ch. 2; 25-27, 29-33, 51-55: Ch. 3: 28 7th Ch. 2: 33-39, 41-49, 52-54; Ch. 3: 30 Concepts: How do sex chromosomes behave in meiosis and heredity? 1. In many species the sex of an individual is determined by the sex chromosomes. 2. Morphologically and genetically dissimilar sex chromosomes act as a homologous pair during meiosis. 3. Chromosomal sex determination gives a non-Mendelian pattern of inheritance. In many animals sex is determined by sex chromosomes Plants: Most have both male (stamens) and female (pistil) reproductive organs (called a hermaphrodite) and therefore we do not need to consider the determination of sex in plants Animals: Often, sex determination is due to, a pair of "sex" chromosomes. Note: 1- Not all species use chromosomes to determine sex. In some species sex is determined by environmental conditions (eg. temperatures; sex-ratio in the population) 2- Some species are parthenogenic - females lay fertile eggs - no males required Sex chromosomes are usually dissimilar In many animals (most mammals - including humans and model systems) the males have one pair of chromosomes that appear morphologically different --> heteromorphic pair - referred to as X-chromosome Y-chromosome Human Female has (X X) + 22 autosome pairs Human Male has (Y X) + 22 autosome pairs Autosomes = non-sex chromosomes Sex chromosomes Fig. 2-22 (8th) 2-13 (7th) Morphologically dissimilar -- size, structure Genetically dissimilar -- carry different genes X-chromosome contains most of the genes associated with the sex-chromosomes. Y-chromosome contains relatively few genes - most of which are on one arm. There is one important Y-chromosome gene, called: TDF = Testis Determining Factor on the Y-pg. 54 (8th) 682(7th), which is responsible for the person developing as a male. Note: This is the same as SRY gene in Fig. 2-22 If mutant XY -> develops as a female; although she has both X and Y chromosomes. X and Y chromosomes act as a homologous pair in meiosis Although morphologically and genetically dissimilar - the X and Y act as a homologous chromosome pair in meiosis - they do share some regions of sequence (gene) similarity. - region of similarity --> aids in pairing of chromosomes during meiosis. -> Pseudoautosomal region 1 Lecture 18; 2007 Biology 207; Section B2; Good . They segregate in meiosis. The X/Y meiocytes yield 4 gametes, each with one of the two chromosomes Therefore, in XX XY species: X Y heterogametic sex (different gametes) -> X Y Female gametes X X Male gametes X or Y > XX (Female) or XY Male progeny 2 XX and 2 XY --> 2 : 2 ratio of hetero- and homogametic progeny again Apparent Non-Mendelian Inheritance of gene on sex chromosomes - due to genetic dissimilarities. Y-chromosome + Y-linked genes Genes located on the Y-chromosome exhibit Y-linkage. Father to son transmission only. eg. - TDF --> sex determination - - hairy ear rim --> Fig 2-39 (8th) 2-33 (7th) X-chromosome + X-linked genes Genes and alleles on the X-chromosome show X-linkage. 1) Fathers X --> always goes to daughters only Y --> always goes to sons only 2) Mothers X --> equally to both daughters and sons Reciprocal Crosses Autosomal genes: the exact same pattern of inheritance is seen in reciprocal crosses. Sex linked genes: on the X chromosome produce reciprocal crosses do not result in the same pattern. Genetic test for sex-linkage vs. autosomal location Example of white gene locus in Drosophila. T.H. Morgan in 1910 correlated the hereditary behavior of a gene with that of the X-chromosome. Supported the idea that genes were on chromosomes - Sutton & Boveri (1903) - >Chromosome theory of heredity Dominant wild type allele (w +) -> has red colour eyes. Recessive mutant allele (w -) -> has white eyes. The red colour is due to the presence of red pigments, which are absent in the white mutant. Morgan observed an inheritance pattern consistent with the white gene locus being on the X chromosome. The reciprocal crosses resulted in different outcomes: see Fig 2-24 (8th) 2-15 (7th) Outcomes: Reciprocal crosses are not equivalent 1) First cross: red female X white male F1 -->all red females and males F2 --> get 3:1 red:white but all females= red, males= 1/2 red 1/2 white, (all white flies are male) 2) Second cross: white female X red male F1 --> red females and white males F2 --> get 2:2 red:white (1/2 white 1/2 red female) (1/2 red 1/2 white males) Non-X/Y Sex Chromosomes Alternative systems: 2 Lecture 18; 2007 Biology 207; Section B2; Good A) ZZ ZW System ZZ-males (homogametic) ZW-females (heterogametic) - found in birds and moths Fig 3-6 (8th) 3-11 (7th) - reminder here X and Y sex linkage pattern is backwards 1) Z in males and females 2) W-female only B) X/O system XX-female X/O male where O is an absence of a chromosome - found frequently in insects Question: But how can X/O be male if, as in humans, sex determining gene (eg. TDF) is on the Y chromosome? Sex determination mechanisms A) Single Gene specified Y-chromosome gene (eg. TDF) induces gonadal cells to differentiate into testis. Male hormones then induce the body to develop into a male. B) Chromosome ratios - specifically X : Autosome ratio 2N individuals = (Autosome set x 2) + (XX) = female 2A:2X (Autosome set x 2) + (X -) = male 2A:1X This system is found in most insects including Drosophila (which has XX XY sex chromosomes) C) Environmental Factors - Rearing temperature - alligator - Sex ratio, etc – tropical reef fish 3 Lecture 18; 2007 Biology 207; Section B2; Good 4