Joblings Flashcards
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Lecture 1
Sex Determination.
What is the two-fold cost of sex?
Only half of the offspring are able to bear children. Two individuals are needed to reproduce.
Describe the origin of asexual species.
Usually of recent evolutionary origin.
Give 2 examples of asexual organisms.
Aphids. Desert Grassland Whiptail.
What are the benefits of sex?
Increase of fitness through sexual selection and biparental care. Increased rate of evolution via mixing of alleles and sexual selection. Increased genetic diversity. Reduced sibling competition. Having homo/heterozygotes reduces the effect of cyclical environment changes.
Describe the sex chromosome configurations of Drosophila.
3 autosomes in one haploid set. Males XY; Females XX.
What is the function of Y in sex determination in Drosophila?
Not involved.
What is the initial step in Drosophila sex determination?
Evaluation of the X:A ratio: 1:2 in males, 2:2 (1:1) in females.
Describe the female determining pathway of Drosophila.
X:A = 1; Sex-lethal gene on (Sxl); Activate tra gene; dsx RNA spliced to female; Repression of male structural genes.
Describe the male determining pathway of Drosophila.
X:A =0.5; Sxl off; tra off; dsx default (male); DSX-M protein formed; Repression of female genes.
How are chromosomes counted to determine the X:A ratio?
X subunits (numerator) combine with other X and autosome subunits (denominator) to form dimers. XX homodimers are active, XA are inactive heterodimers and AA homodimers are also inactive. Sex pathway depends on concentration of active homodimers.
What is the role of Numerator-Denominator heterodimers?
Helix-loop-helix transcription factors.
Describe the Sxl switch.
In females, high conc of XX activates transcription at P(E) (early promoter) of Sxl; Early type SXL-protein forms; Early SXL blocks male Sxl exon when the P(L) (late promoter) transcribes the gene; Late female SXL-protein forms. The Sxl prtotein loop acts as memory to maintain female splicing pattern. In males, P(E) is not activated; Exon not blocked; Sxl transcribed by P(L) with all exons.
What effect would a null mutation in Sxl/tra/sis-b (numerator gene) have on the final phenotype?
Male produced.
How do gymnadomorphs arise?
Null mutation in dsx gives male + female features due to lack of repression of either. Every cell determines own sex, mosaicism possible; Feminisation of brain structures can change sexual orientation - expression of female specific tra in olfactory bulbs may cause recognition of opposite pheromones.
Describe the sex chromosome configurations in C. elegans.
No Y. Hermaphroditic XX; Male XO. Primary switch involves X:A ratio.
Describe the male determining pathway of C. elegans.
1:2 ratio; XOL-1 gene activated; Represses sdc (sex determination and dosage compensation) gene; No SDC-1/-2/-3 protein; Her-1 activity not repressed; Cascade activates; Repression tra-1; Male phenotype.
Describe the hermaphroditic determining pathway of C. elegans.
2:2 ratio; XOL-1 off; sdc on; SDC-1/-2/-3 made; Her-1 repressed; Cascade; Tra-1 activated; Female phenotype.
How is sex determined in mammals?
Pathway triggered by a primary genetic switch. Secondary sex determined by hormones and not cell-autonomous responses.
Describe the differentiation of gonads in mammals.
Female and male gonads stem from an indifferent gonad at ~7 weeks. If Y present - Mullerian duct degenerates. Wolffian duct forms Vas deferens. If Y absent - Wolffian duct degenerates. Mullerian duct forms fellopian tube.
What is the evidence for the Y chromosome being dominantly sex-determining in mammals?
Polyploid genotypes with multiple Y are all male. This may explain the skewed male:female ratio in populations.
Describe the structure of the Y chromosome.
60Mb compared to 160 of X. 26Mb of euchromatin.
Describe the genetic content involved in sex determination, of the Y chromosome.
SRY - Sex-determining region 1. Consists of TDF - Testis Determining Factor - DNA binding protein.