The paired chromosome X or Z in the homogametic sex is generally well conserved, large, and gene-rich, whereas the sex-specific Y or W is usually small, heterochromatic, and almost devoid of active genes.
Transitions are particularly likely when the ancestral sex chromosome exhibits little genetic differentiation, since WW or YY combinations are then chromosomal sex determination pptx in Cambridge likely to be lethal Figure 5. ESD is favored over GSD when specific environments are more beneficial to one sex selecting for sex-determining mechanisms that match each sex to its best environment.
Vertebrates: Mammalia placental, marsupial, and monotreme mammalsAves birdsReptilia turtles, snakes, crocodiles, lizardsAmphibia frogs, toads, salamandersand Teleostei bony fishes. Evolution 63 : —
When separate sexes are favored, the transition can occur via several evolutionary pathways. Figure 2. If separate sexes evolve by gradual increase in sexual investment from a hermaphrodite, sex determination may also be due to polygenic inheritance. Box 2. Heteromorphic sex chromosomes: sex chromosomes that are morphologically distinct.
Some disorders caused by recessive chromosomal sex determination pptx in Cambridge on the X chromosome in humans -Color blindness -Duchenne muscular dystrophy -Hemophilia 30 No Transcript 31? In some species of reptiles, including alligatorssome turtlesand the tuatarasex is determined by the temperature at which the egg is incubated during a temperature-sensitive period.
The X and Y sex chromosomes are different in shape and size from each other, unlike the rest of the chromosomes autosomesand are sometimes called allosomes. Sex Determination and Sex Chromosomes Dr. From Wikipedia, the free encyclopedia.
Evolution 66 : — While biologists are generally aware of other modes of sex determination such as female heterogamety in birds, temperature-dependent sex determination in reptiles, or development of males from unfertilized eggs in bees , these alternatives are often viewed as strange and aberrant .
For example, in vertebrates other than mammals, dmrt1 a DM family gene and its paralogs act as the primary sex determination signal in African clawed frog Xenopus laevis  , chicken Gallus gallus  , medaka fish Oryzias latipes  ,  , and possibly the smooth tongue sole Cynoglossus semilaevis .
Here we survey our current understanding of how and why sex determination evolves in animals and plants and identify important gaps in our knowledge that present exciting research opportunities to characterize the evolutionary forces and molecular pathways underlying the evolution of sex determination.
Nanda, I. The Myths Deconstructed These myths do not survive a survey of sex determination systems across the tree of life.