Download Biology 30 Patterns and Probabilities

Biology 30 Notes December 10, 2014 – Biology 30 Patterns and Probabilities
Watch the probability video.
https://www.youtube.com/watch?v=Qcmdb25Rnyo
Chromosomal Theory of Inheritance – genes are carried on chromosomes and chromosomes
segregate independently into gametes during meiosis. Genes exist at specific sites arranged in a
linear manner along chromosomes.
Linked Genes – Genes that are found on the same chromosome are sometime called linked.
Alleles of two genes on the same chromosome can be found in different gametes. How??
CROSSING OVER.
Review: When does crossing over occur, when homologous chromosomes synapses and form
tetrads during prophase I of meiosis. Crossing over is when non-sister chromatid in a tetrad
exchange pieces of chromosomes. This is a random event and can occur anywhere along a
sister chromatid, except near the centromere.
Suppose you are studying two genes and crossing over occurs between them then the alleles
will end up on separate chromosomes and will therefore migrate into different gametes.
Crossing over is more likely to occur between genes that are farther apart on a chromosome
than between genes that are closer together.
Gene Linkage & Crossing Over
- since we have only 23 pairs of chromosomes and thousands of genes, some genes must
coexist on each chromosome
- Mendel was just lucky during his studies that of the 7 traits he chose to follow, they existed
on 7 different chromosomes of the pea plant
When crosses involving two or more traits don’t yield the expected phenotypic results – due to
the linkage effect of genes on the same chromosome
ex: wing shape and body colour don’t seem to sort independently in fruit flies. When
curved wings/black body colour flies are crossed with straight wings/normal body colour,
instead of 9:3:3:1 phenotypic ratio, there is the 3:1 ratio found when only a monohybrid
cross is done
** genes on the same chromosome tend to segregate together – Linked genes
BUT! Crossing-over will sometimes cause linked genes to be split up
- The closer two genes are on the same chromosome, the more likely that they will stick
together
- The farther apart two genes are on the same chromosome, the more likely they will
segregate separately due to crossing over.
Chromosome Mapping
The concept of crossing over is used to determine the relative positions of genes on
chromosomes, this is known as chromosome mapping.
Experiment: You cross a homozygous recessive purple-eyed, vestigial winged fly (ppvv) with a
heterozygous dominant normal-eyed normal-winged fly (PpVv).
pv
PV
PpVv
Normal eyedNormal winged
Pv
Ppvv
Normal eyed
Vestigial winged
pV
ppVv
Purple eyed
Normal winged
pv
ppvv
Purple eyed
Vestigial winged
Except the phenotype ratio to be 1:1
But you find that 45% (PpVv) and 45% (ppvv) and 5% (Ppvv) and 5% (ppVv)
Since the chromosomes of these F1 flies have a different combination of alleles than the
chromosomes of the P generation these flies are called recombinant types or recombinants.
The F1 flies with the phenotypes normal eyes and normal wings or purple eyes and vestigial
wings are called parental types because their chromosome are identical to those of the P
generation.
The percentage of recombinant types in the F1 generation is directly proportional to the distance
between the genes.
The percentage of flies that are recombinant types corresponds to the recombination
frequency – that is the percentage of times that a crossover occurred as P gametes formed.
Experiment: Cross produces 1000 offspring.
- 450 are normal eyes and normal wings (PpVv) – parental type
- 450 are purple eyes and vestigial wings (ppvv) – parental type
- 50 are normal eyes and vestigial wings (Ppvv) – recombinant type
- 50 are purple eyes and normal wings (ppVv) – recombinant type
The recombination frequency – 50+50/1000 x 100% = 10%
10% of the offspring of the F1 generation were recombinant types. Because the recombination
frequency is directly proportional to map distance 1% = 1 map unit, you can use this value to
create a chromosome map showing the relative distance between the linked genes. In this
example the genes for eye colour and wing type are 10 map units apart.
The position of the genes on the chromosome can be mapped using the crossover %
- the higher the % of crossing over, the farther apart the genes are on the chromosome
- 1% in crossover = 1 map unit away from each other
- using many crossover frequencies, genes can be mapped on each of the chromosomes – all
map distances are additive, so many genes can be mapped
- ex: if crossover between A & C is 3%, A and B is 8% and B and C is 5%, locate the
genes along the chromosome
A
C
B
3
5
8
Watch Video - https://www.youtube.com/watch?v=TU44tR0hJ8A
Example: Find the following:
- the distance between the genes for eye colour (A) and wing type (B) is 10 map units
- the distance between the genes for eye colour (A) and body colour (C) is 4 map units.
- The distance between the genes for body colour (C) and wing type (B) is 6 map units.
10
B
A
4
C
6