Sexual Reproduction

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Sexual Reproduction

• Reproduction with two parental inputs

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Remember

• Asexual reproduction requires one parent, who
  reproduces offspring that are genetically
  identical to itself.
• Some examples of asexual reproduction
• Binary fission
• Budding
• Binary fusion
• Rhizomes

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Asexual vs. sexual reproduction
Asexual Sexual
Requires one parent Requires two parental inputs
Offspring are all clones Offspring are genetically different
Quick process Slower process
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Sexual reproduction

• Sexual reproduction is the production of
  offspring of two parents using gametes.
• A gamete is HAPLOID, while the resulting
  offspring are DIPLOID.
• Haploid Has only one set of chromosomes (23 in a
  human, including one sex chromosome)
• Diploid Has two sets of chromosomes (46 in
  human, including two sex chromosomes)

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Gametes

• In mammals, the gametes involved in sexual
  reproduction are the sperm cell and the egg cell.
• Each has 23 chromosomes, including a sex
  chromosome (X or Y). They fuse together to form a
  single cell with 46 chromosomes, including two
  sex chromosomes (XX or XY)

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Meiosis

• Meiosis is the form of cell division that forms
  gametes.
• Steps, in brief
• DNA replication
• Division one
• Division two
• Four daughter cells are formed

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Meiosis cont

• The cell that divides is DIPLOID (also known as
  2n), and all the daughter cells are HAPLOID (also
  known as n)
• Sometimes parts of the chromosomes are rearranged
  during meiosis, which gives rise to new
  combinations of genes (called GENETIC
  RECOMBINATION). This makes it very unlikely you
  will ever be genetically identical to anyone

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Mitosis vs. Meiosis (an extra division)
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Glossary words so far

• Gamete, meiosis, diploid, haploid

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Heterogamy

• The size of the gametes in animals is different.
  This is called HETEROGAMY
• The female gamete (egg) is large, while the male
  gamete (sperm) is much smaller

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Female gamete
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Male gamete
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Fertilisation
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Fertilisation

• Fertilisation is the moment the two gametes fuse
  together to create one diploid cell, called a
  ZYGOTE
• This cell then divides over and over by mitosis
  to create a bundle of cells, then distinct organs
  and structures and finally a complete baby

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The controversy

• There is continued debate as to the point in
  which a zygote becomes a FETUS, and whether the
  change into a fetus is the beginning of life, or
  whether life only begins at birth.
• This debate is especially significant
  politically, in the discussion of legal and moral
  abortion rights

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External fertilisation

• Some animals release their gametes into the
  environment, so fertilisation occurs outside the
  body
• Sperm need a watery environment to swim to the
  eggs, so this is limited to aquatic environments

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Examples of external fertilisation

• Fish and amphibians (frogs) use a similar
  mechanism.
• Eggs are jellylike and are released and
  fertilised in the environment.
• Coral polyps release eggs and sperm into the
  environment and they mix and look like a cloud

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Internal fertilisation

• This occurs WITHIN the breeding female.
• Can occur in marine environment (eg. Sharks,
  octopi), or on land (eg. Mammals, reptiles,
  insects, birds)

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Some examples of internal fertilisation

• Insects have genital structures at the end of
  their abdomen

The male bird does not have a penis, but sperm is
deposited in the female via close genital
contact. An egg forms inside the female, forms a
hard shell and is laid
Reptiles were the first to evolve a penis, for
easy delivery of sperm to the female. They were
also the first to have an egg with a protective
covering
Most mammals retain the fertilised egg within the
female, where the offspring grows. Monotremes
(echidna and platypus) lay fertilised eggs
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Glossary words

• Heterogamy, ova, ovules, sperm, zygote, fetus,
  external fertilisation, internal fertilisation

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The benefits of sex

• Mixing genetic pools means that poor genes (such
  as disease genes) can be bred out of a
  population
• This mixing also means that good genes (eg. That
  increase strength) can be selected for, and
  increase in the population
• Mixing also allows for ADAPTATION to environments

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Are there any drawbacks to sexual reproduction?

• Effort required to find a suitable mate is
  considerable
• Only half of your genetic information is passed
  on to the next generation, thus diluting your own
  contribution to the species as time goes on.

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Glossary

• Adaptation, genetic recombination, mixing genetic
  pools.

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Life cycles

• Different organisms have different life cycles

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Human Life Cycle
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Generalised plant and animal Life Cycles
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Life cycles of plants

• Some plant life cycles (eg. Ferns) have two
  stages
• The normal diploid stage of the main plant
  (SPOROPHYTE)
• The plant develops parts (GAMETOPHYTE) where both
  male and female gametes (in most plant species),
  which fuse together inside the flower/other plant
  structure. This creates the seed, which will give
  rise to the seedling.

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Sexual structures of flowering plants
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Sex in flowering plants

• Most plants have both pollen (male) and egg
  (female) producing organs. This means that they
  can SELF POLLINATE, if needed.
• In self pollination, pollen falls into ovary
• BUT no mixing of the gene pool
• They also may be fertilised by other flowers
  pollen via movement from wind or insects

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Sex in flowering plants cont.

• Some flowers do not make both pollen and eggs.
  They must be fertilised by a different flower.
• Sometimes male and female flowers exist on the
  same plant, and can fertilise each other. This is
  called MONOECIOUS. Eg. pumpkin
• Sometimes the male and female flowers grow on
  different plants. They are DIOECIOUS. Eg.
  kiwifruit

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Glossary

• Life cycle, sporophyte, gametophyte, stigma,
  ovary, stamen, monoecious, dioecious, self
  pollination.

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Determining sex in animals

• Firstly an important distinction SEX is
  biological, while GENDER is social
• If an animal can produce both eggs and sperm,
  they are called HERMAPHRODITES. We often call
  humans with indeterminate sex this as well, but
  its incorrect.

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Simultaneous Hermaphrodites

• These animals are producing both sperm and eggs
  at the same time.
• Examples snails, earthworms

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Sequential Hermaphrodites

• These animals have the potential to produce
  either sperm or eggs, dependent on the situation.
• For example, some species of Angelfish live in
  harems of many females with a single mating
  male. If that male dies or becomes infertile for
  any reason, one of the large females turns into a
  male.
• Some anemone fish do the same, but start out as
  large groups of males with one female

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Determining sex in mammals

• In mammals, sex is determined by the presence of
  the Y chromosome. If the Y chromosome is present,
  the animal is MALE.
• This causes some problems occasionally when some
  people are born with an extra chromosome. If this
  extra chromosome makes their sex XXY, the person
  may show some characteristics of both males and
  females, but they are legally male because of the
  presence of the Y chromosome.

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Sex chromosomes

• Mammals have an XX/XY sex chromosome system, but
  other animals have some slightly different set
  ups
• Bees males are haploid, while females are
  diploid
• Birds ZW is female, while ZZ is male
• Cockroaches females have XX, but males have XO,
  which means they have only one sex chromosomes
  and it is X