Summary Notes designed for the International Baccalaureate Diploma Programme. These notes are detailed yet concise, with all the information to achieve a 7 in IB Biology HL or SL. These notes were made using information from the IB syllabus, Oxford IB Diploma HL Biology Textbook, Bioninja and the B...
Unit 10.1
• Meiosis undergoes division twice to produce 4 haploid
daughter cells with 23 chromosomes each.
• Chromosomes replicate during the S phase of interphase.
• Prophase I: Chromosomes condense, nuclear
membrane dissolves, homologous chromosomes
form bivalents, crossing over occurs resulting in
genetic variation.
• Metaphase I: Spindle fibres attach to centromeres
of bivalents and align them along the middle of the
cell randomly.
• Anaphase I: Spindle fibres contract and
homologous chromosomes move to opposite
sides.
• Telophase I: Nuclear envelopes form and sister
chromatids decondense, cytokinesis occurs
resulting in 2 diploid cells.
• The same process occurs again known as Meiosis II, except this separates the sister
chromatids, producing 4 haploid daughter cells.
• Independent assortment of genes is due to the random orientation of pairs of homologous
chromosomes in metaphase I.
• Bivalents can form a chiasmata between non-sister chromatids which results in the crossing
over of genes in prophase I. This allows the exchange of genetic information and form
recombinant chromatids.
• Trisomy can occur in anaphase I or II where non-disjunction occurs and either the bivalent or
sister chromatid does not split. This causes Downs Syndrome.
Unit 10.2
• Dihybrid crosses are used to determine the genotype
and phenotype of offspring for two particular genes
that are unlinked.
• When given the genotype of both genes, the possible
gametes must be found using the FOIL method. Then,
these gametes can be crossed.
• Heterozygous genotypes produce a 9:3:3:1 ratio within
a dihybrid cross.
• When performing a test cross to determine a certain
genotype, the unknown must always be crossed with a
homozygous recessive genotype.
• Linked genes are represented as vertical pairs:
• They tend to be inherited together because they have a similar loci on the same chromosome
and therefore do not follow independent assortment.
• In the example given above, the only gametes produced are AB and ab as the genes are linked.
• These gametes are then crossed normally using a punnet grid.
• Genes can also be sex-linked if they are located on the X or Y chromosome.
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