Chromosome types are distinguished by centromere location.
Genes located on the same chromosome tend to be inherited together
as a group, but these groups can be broken apart by "crossing over".
Chromosome morphology:
Eucaryotic chromosomes contain proteins (histones
and nonhistones) plus DNA. Some DNA winds around
clumps of histone to form nucleosomes, protecting that
DNA from transcription. Chromosome arms attach to a
dense region called the centromere. Chromosome types are
distinguished as follows:
- Metacentric: centromere centrally located; two long arms
- Sub-metacentric: centromere off-center; arms a bit unequal
- Acrocentric: centromere near one end, second arm short
- Telocentric: centromere at one end; no second arm
Chromosomal theory of inheritance: Genes are located
on chromosomes.
- The location of each gene is called its locus.
- Linkage: Genes on the same chromosome are linked
(unless they are very far apart) and are inherited together as a
linkage group. The number of linkage groups always equals
the haploid number (N).
- Crossing over: Sometimes, in Prophase I of meiosis,
chromosome arms may break and rejoin in switched order, rearranging
linked genes.
- Chromosome maps: The probability of crossing over
between linked genes varies with the distance between them.
Recombination frequencies can thus be used to make chromosome
maps (linkage maps).
- Proof of the chromosomal theory: Experiments show that
crossing over of chromosomes always accompanies the recombination
of linked genes.
Genetic crosses involving linked genes:
- AABB x aabb is no longer the same as AAbb
x aaBB
- F1 genotypes are heterozygous at each locus.
During formation of F1 gametes, parental combinations
of alleles tend to stay together.
- Each F1 individual is crossed to an organism recessive
for all genes being studied (a test cross). Phenotypes in
the offspring of such a cross correspond to genotypes in the
F1 gametes.
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