Reproduction and Development - Asexual and Sexual...

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Asexual and Sexual Reproduction In asexual reproduction , the offspring are genetically identical to one parent The process begins with mitosis Protists typically divide by fission Cnidarians typically divide by budding Asexual reproduction in Euglena
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Asexual and Sexual Reproduction In sexual reproduction , a new individual is formed by the union of two gametes (egg and sperm) A zygote is formed Develops by mitosis into a multicellular organism Haploid gametes are produced in the gonads
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Parthenogenesis Offspring are produced from unfertilized eggs Common among arthropods Some are exclusive Others switch! Common also in some lizard species Different Approaches to Sex
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Hermaphroditism One individual has both testes and ovaries Tapeworms and earthworms Hermaphroditism may be sequential Individuals change sex Bluehead wrasse Switch sexes! Hamlet bass Switch sexual roles! Protogyny From female to male Protandry From male to female
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In some reptiles, sex is determined by environmental changes In mammals, it is determined early in embryonic development Embryonic gonads are indifferent Y chromosome converts them to testes Responsible gene is SRY » S ex-determining r egion of the Y chromosome Sex Determination
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Sex determination
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Evolution of Reproduction Among the Vertebrates Vertebrate sexual reproduction evolved in the ocean before vertebrates colonized land Most marine bony fish use external fertilization Male and female gametes are released into the water where fertilization occurs Most other vertebrates use internal fertilization Male gametes are introduced into the female reproductive tract
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There are three strategies for internal fertilization 1. Oviparity Fertilized eggs are deposited outside mother’s body to complete their development 2. Ovoviviparity Fertilized eggs are retained within the mother to complete their development Young obtain nourishment from egg yolk 3. Viviparity Fertilized eggs are retained within the mother to complete their development Young obtain nourishment from mother’s blood
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The eggs of most bony fish are fertilized externally Eggs contain little yolk Young fish must seek its food from the water surrounding it Thousands of eggs are fertilized, but only a few of resulting individuals reach maturity Fertilization in most cartilaginous fish is internal Development of young is viviparous Fish and Amphibians
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Fertilization is external in most amphibians Eggs of most species develop in water With some interesting exceptions Development is divided into embryonic, larval and adult stages Poison arrow frog Male!
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