Summary
IB Biology: Genetics Summary HL
Genetics (unit 10) summary
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10.1 MEIOSISInterphase is an active period that precedes meiosis and involves key events needed
to prepare the cell for successful division. DNA is replicated during the S phase of
interphase, resulting in chromosomes that contain two identical DNA strands. These
genetically identical strands are called sister chromatids and are held together by a
central region called the centromere. These chromatids separate during meiosis II,
becoming independent chromosomes each made of a single DNA strand.
If DNA replication did not occur prior to meiosis there would be no need for a 2nd
meiotic division (meiosis I = diploid → haploid). The fact that DNA replication does
occur suggests that meiosis evolved from mitosis (where initial DNA replication is
necessary). One benefit of the duplication of chromatids is that it increases the
potential for genetic recombination to occur (more variation).
The first meiotic division is a reduction division (diploid → haploid) in which
homologous chromosomes are separated
★ P-I: Chromosomes condense, nuclear membrane dissolves, homologous
chromosomes form bivalents, crossing over occurs
★ M-I: Spindle fibres from opposing centrosomes connect to bivalents (at
centromeres) and align them along the middle of the cell
★ A-I: Spindle fibres contract and split the bivalent, homologous chromosomes
move to opposite poles of the cell
★ T-I: Chromosomes decondense, nuclear membrane may reform, cell divides
(cytokinesis) to form two haploid daughter cells
The second division separates sister chromatids (these chromatids may not be
identical due to crossing over in prophase I)
★ P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes
move to opposite poles (perpendicular to before)
★ M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at
centromere) and align them along the cell equator
★ A-II: Spindle fibres contract and separate the sister chromatids, chromatids
(now called chromosomes) move to opposite poles
★ T-II: Chromosomes decondense, nuclear membrane reforms, cells divide
(cytokinesis) to form four haploid daughter cells
Independent assortment describes how pairs of alleles separate independently from
one another during gamete formation. According to independent assortment, the
inheritance of one gene/trait is independent to the inheritance of any other gene/trait.
Independent assortment is due to the random orientation of pairs of homologous
chromosomes in meiosis I.
, The orientation of each homologous pair is random and is not affected by the
orientation of any other homologous pair. This means an allele on one chromosome
has an equal chance of being paired with, or separated from, any allele on another
chromosome (their inheritance is independent of one another). Independent
assortment will not occur if two genes are located on the same chromosome (linked
genes).
During prophase I of meiosis, homologous chromosomes become connected in a
process known as synapsis. The connected homologues are known as a bivalent (bi
= two chromosomes) or a tetrad (tetra = four chromatids). The chromosomes are
connected by a protein-RNA complex called the synaptonemal complex. While
autosomes always undergo synapsis during meiosis, sex chromosomes often remain
unpaired.
While in synapsis, non-sister chromatids may break and recombine with their
homologous partner (crossing over) . These non-sister chromatids remain physically
connected at these points of exchange – regions called chiasmata. Chiasmata
(singular = chiasma) hold the homologous chromosomes together as a bivalent until
anaphase I. Chiasmata formation between non-sister chromatids can result in the
exchange of alleles.
When chiasmata form between bivalents in prophase I, DNA can be exchanged
between non-sister homologous chromatids. This exchange of genetic material is
called crossing over and produces new allele combinations on the chromosomes.
These chromosomes that consist of genetic material from both homologues are
called recombinant chromosomes. Crossing over results in new combinations of
alleles in haploid cells and thus increases the genetic diversity of potential offspring.
10.2 INHERITANCE
According to the law of independent assortment, pairs of alleles are inherited
independently of one another if their gene loci are on separate chromosomes –
these genes are said to be unlinked. This is due to the random orientation of
homologous pairs during metaphase I of meiosis.
The independent segregation of unlinked genes results in a greater number of
potential gamete combinations, as well as a greater variety of possible phenotypes.
This also results in more complex inheritance patterns (e.g. monohybrid versus
dihybrid crosses).
A dihybrid cross determines the genotypic and phenotypic combinations of offspring
for two particular genes that are unlinked. Because there are two genes, each with
two alleles, there can be up to four different gamete combinations.
A linkage group is a group of genes whose loci are on the same chromosome and
hence don’t independently assort. Linked genes will tend to be inherited together and
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