 Asexual reproduction yields two identical organisms (with exception of mutations)  Sexual reproduction merges the genes of the two parents  Result: two versions of each chromosome ◦ Called homologous chromosomes  Organism is diploid (2n=46)  All somatic cells are diploid

 All can be lined up based on size, creates a karyotype

 First 22 are autosomes  Chromosome 23 is the sex chromosome  Haploid (n=23) sex cells merge to create a zygote in sexual reproduction  Meiosis creates these sex cells (gametes)  The overall process is the sexual life cycle

Homologous chromosomes Chromosomes replicate Meiosis I Meiosis II Homologous chromosomes separate Sister chromatids separate Interphase I of Meiosis

 Similar to mitosis, chromatin replicates  Centrioles replicate, proteins form around to make the centrosomes ◦ Also called the microtubule organizing centre ◦ Collectively, with microtubules, is called mitotic spindle

 Longest part of meiosis  Chromatin condenses, chromosomes become visible ◦ Homologous chromosomes come pair up, forming a tetrad – called synapsis ◦ The chromatids can overlap, called a chiasmata  All other features are similar to mitosis

 Almost identical to metaphase in mitosis, with the tetrad exception

 Sister chromatids remain attached, tetrads separate (cells are now haploid)

 Homologous chromosomes are at opposite poles of cell ◦ Each end is now haploid  Cytokinesis takes place simultaneously  Some cells proceed to interphase II, others directly to prophase II  No cells replicate chromatin again

 Spindle forms again, chromosomes move towards equator

 Closely resembling mitosis, metaphase plate is formed

 Sister chromatids finally separate

 Mitotic spindle breaks down, nucleus reforms around two daughter nuclei

 Some of the processes can go awry  If anaphase I or II does not proceed correctly, the chromosomes may not be shared evenly  This is called nondisjunction  More of a problem if it occurs during meiosis I  Can take place in mitosis, but not as much of a concern

 An abnormal chromosome number is called aneuploidy  If faulty gamete succeeds in fertilization, the offspring with have health consequences  If a gamate with an extra chromosome fertilizes, the zygote is trisomic (2n+1)  If the gamete has a chromosome missing, the zygote is monosomic (2n-1)  Ex. Extra chromosome 21 – Down syndrome

 If the extra chromosome is an X in a male (XXY), called Klinefelter syndrome  The individual will have male sex organs, but the testes will be abnormally small, and he will be sterile  As well, femine characteristics such as breast enlargement is common

 Final example is Turner syndrome  Example of monosomy X  Individuals are classified X0, and are female  However, sex organs do not mature at adolescence and secondary sex characteristics fail to develop

 Also possible for anaphase I or II to completely fail and all chromosomes move to one cell  Referred to as polyploidy  Could have three chromosome sets (triploidy, 3n) or four (tetraploidy, 4n)