WHY SEX? -SEXUAL REPRODUCTION-TECHNICAL POINT OF VIEW FOR DISCUSSION - LATEST BIOMEDICAL ENGINEERING UPDATES,JOBS,BOOKS,WORKSHOPS,LECTURES,RESEARCH,NOTES,NEWS,TRAININGS,SEMINARS,CONFERENCE,SCIENCES,TECHNOLOGY

Sex is Very Costly

Large amounts of energy required to find a mate and do the mating: specialized structures and behavior required
Intimate contact provides route for infection by parasites (AIDS, syphillis, etc.)
Genetic costs: in sex you pass on only half your genes to your children
Males are an expensive luxury- in most species they contribute little to rearing offspring

But There are Some Advantages

More genetic diversity: more potential for survival of species when environmental conditions change
- Shuffling of genes in meiosis
- Crossing-over in meiosis
- Fertilization: combines genes from 2 separate individuals
DNA back-up and repair
- Asexual organisms don’t have back-up copies of genes, sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged
- Sexual mechanisms , especially recombination, are used to repair damaged DNA- the undamged chromosome acts as a template and eventually both chromosomes end up with the correct gene

Reproduction Without Sex and Sex Without Reproduction Both Occur in Nature

Sex is the transfer of genes from one cell to another and in microorganisms this often occurs without cell division, so that there is no reproduction
Many species can reproduce without sex
- Most single-cell organisms
- Some multcellular organisms

There Are Many Successful Asexual Species

Single cells reproduce whenever they divide
Some multicellular organisms reproduce by budding or branching from parent (Hydra, sponges, sea anenomes)
Some higher organisms produce eggs that develop into new individuals without fertilization ; called parthenogenesis (virgin birth)
- Aphids in summer (they become sexual in the fall)
- Whiptail lizards
- Some salamanders
Offspring of asexual organisms are clones of the parent (genetically identical)

Sexual Reproduction Has Been Adopted by Most Higher Organisms

Almost all organisms with eukaryotic cells undergo sexual reproduction
The sexual lifestyle:
- Diploid cells: 2 sets of chromosomes
- Meiosis: a type of cell division that produces reproductive cells that are haploid (1 set of chromosomes); usually there are 2 types of reproductive cells, sperms and eggs
- Fertilization: combination of a sperm and egg to produce a new diploid cell (zygote)
- Development of the zygote into a new individual

There Are a Large Number of Natural Sexual Strategies

Hermaphrodites have both sexes on same individual (many flowers, earthworms, snails, some fish); in earthworms and snails when 2 individuals mate each fertilizes the other
Alternate sexual and asexual stages (gall wasps, aphids)
Fertilization may be internal or external
Development of embryo may be internal or external

Sex Determination Can be Genetic or Environmental

In many species sex is determine solely by the chromosomes- birds and mammals
In other species environmental factors are important
- In turtles sex is determined by temperature of egg development
- In some species environmental factors change the sex at different stages of life
  - Some marine worms change sex (from male to female) when they get larger
  - Coral reef fishes (wrasse) : if the male fish dies the largest female in the group changes into a new male

In Humans (and Other Mammals) Sex is Determined Genetically

We have 46 chromosomes, 23 pairs
22 pairs are called somatic chromosomes
The 23rd pair consists of two chromosomes, the X and Y, that are somewhat different from each other
The X and Y determine sex: a person who is XX is female; a person who is XY is male
- The X chromosome is required for life, the Y is not
- The sex of a child is always determined by the father- he can make both X and Y sperm
- The Y chromosome is small and degenerate but it has a gene controlling the production of testosterone- if this hormone is present the embryo develops into a male

Sex Organs Develop from a Unisex Gonad

At 5-6 weeks the developing embryo, whether male or female has a pair of unspecialized gonads and 2 paired ducts, the Wolffian and Mullerian, which connect to the future urethra
Differentiation into male or female starts at about the 7th week
If the embryo has a Y chromosome testosterone is produced and male gonads develop
If there is no Y chromosome the embryo develops into a female
Male development:
- Wolffian duct -> vas deferens and epididymis
- Mullerian duct -> degenerates
- Unisex gonads -> testes
- Labioscrotal swellings -> scrotum
- Genital tubercle -> penis
- Testes descend into scrotum in 7th month
Female development
- Mullerian duct -> oviduct and uterus
- Wolffian duct -> degenerates
- Unisex gonads -> ovaries
- Labioscrotal swellings -> labia majora
- Genital tubercle -> clitoris

Phenotypic Sex May Differ from Genetic and Gonadal Sex

Hermaphrodites have both male and female sex organs
- Double fertilization can produce some XY cells in an XX individual
- An XY person who is insensitive to testosterone (receptor defect) will develop some female characteristics
- Mutation may block production of testosterone in XY individual
Some individuals have unusual numbers of X and Y chromosomes: usually male if there is at least one Y chromosome

Both Sexes Form Gametes by Meiosis

Some definitions:
- Haploid cell = cell with 1 set of chromosomes
- Diploid cell = cell with 2 sets of chromosomes (1 from each parent)
- Gamete = haploid reproductive cell (sperm or egg)
- Zygote = diploid cell produced by fusion of an egg and sperm during fertilization

Production of Haploid Gametes in Meiosis:

Note that the first deivision separates the homologues and that the second division separates the sister chromatids