Friday, March 13, 2009
: The first step in sexual reproduction is the production of
gametes, each of which has half the chromosomes of the parent. Chromosomes
assort independent into gametes (Mendel’s second law).
Crossing over occurs
within chromosomes, creating chromosomes that differ from chromosomes in the
creates individuals that have genotypes different from either of their
Siblings differ in genotype from one another.
Bacteria and viruses:
Although sexual reproduction occurs only in eukaryotes,
bacteria and viruses have other mechanisms that result in new combinations of
Sexual reproduction is an evolved trait
There is great variety in modes of sexual reproduction
Isogamy vs. aniogamy:
isogamous species produce gametes of the same size and
form; anisogamous species produce gametes that differ in size and form.
fungi and algae are isogamous.
All other sexually reproducing species are
The sex that produces the larger gamete is defined to be the female.
Hermaphroditic individuals vs. separate males and females
most plants are
hermaphroditic; most animals have separate sexes.
Some hermaphrodites, for
example peas, can self-fertilize; some, for example slugs, cannot.
Genetic vs. environmental sex determination
: in humans, as in many other
species with separate sexes, sex is determined by genotype.
In mammals, XX
individuals are female; XY individuals are male.
In birds, males are ZZ and
females are ZW.
Other genetic mechanisms are found in other groups. For
example, in bees, ants and wasps, males are haploid and females are diploid.
some species of plants and animals, sex is determined by the environment.
example, in most species of turtles and in all crocodilians, the sex of an individual
depends on the temperature of the egg during a critical period of development.
Usually males are produced when eggs are incubated at low temperatures and
females are produced when eggs are incubated at higher temperatures.
Capable or incapable of asexual reproduction
: Some plants and animals can
produce new genetically identical individuals by budding or fission.
can produce seeds asexually (apomixis).
Some animals can produce diploid eggs
offspring that are genetically identical to their mother.
Some species never
engage in sexual reproduction.
For example, several species of whiptail lizards,
including the endangered California species,
comprised of females that reproduce only by parthenogenesis.
Many species, including aphids, can reproduce both sexually and asexually