Chromosomes and Reproduction Chapter 13

What you need to know! The differences between asexual and sexual reproduction The role of meiosis and fertilization in sexually reproducing organisms The importance of homologous chromosomes to meiosis

Karyotype Visual display of an organism’s chromosome number Mitotic cells in late prophase are stained, photographed, cut out, and sorted into homologous pairs Used to detect genetic abnormalities in organisms (i.e. amniocentesis – prenatal screening of the amniotic fluid)

Chromosome Numbers Fertility depends on even numbers of chromosomes for meiosis to take place: –Human Chromosome Set: 2 X 23 = 46 Infertility is a result of uneven chromosome numbers (47 Down’s Syndrome, 45 Turner’s Syndrome) Mules are infertile with 33 chromosomes Paired chromosomes are called homologous chromosomes –Diploid cells 2n

Types of chromosomes Sex chromosomes consist of XX or XY All mammals require at least one X chromosome –If there is a second (in women) it is inactivated (Barr Bodies) Sex is determined by the pretense or absence of the Y chromosome (different in non mammals) 22 pairs (44) autosomes: all other chromosomes (not sex)

Somatic Cells Body cells with 2n chromosomes Use mitosis to produce 2 identical diploid cells

Gametes Sex cells with n chromosomes Use meiosis to produce 4 unique haploid cells –Egg/sperm Fertilization is the fusion of sperm and egg n + n  2n

Asexual Reproduction Oldest form of reproduction by means of mitosis –Mostly lower order organisms Adv: Easy and fast DisAdv: No genetic variation Types: Bacteria – binary fission Plants: vegetative propagation Budding: anemonies Fragmentation: seastars Sporulation: mosses & ferns

Sexual Reproduction Evolutionarily novel process of meiosis (evolved from mitosis) Virtually all higher order organisms engage in sexual reproduction –Many can alternate sex depending on resources Adv: unlimited number of new genotypes DisAdv: complicated, time-consuming, dangerous, costly Types: Fusion of gametes Parthenogenesis: development of females from unfertilized eggs (bees and ants)