Download 15.1 and 15.2 notes: -Law of segregation – Homologous

15.1 and 15.2 notes:
-Law of segregation – Homologous chromosomes and segregation of alleles of
a single trait to different gametes.
-Law of Independent assortment – Non-homologous chromosomes and
independent assortment of alleles on different chromosomes for 2 or more
genes.
-Fig. 15.2 – Occurs at Anaphase of Meiosis 1.
-Look at Table of historical events in DNA discoveries.
-Late 1800’s improved microscopy led to understanding of meiosis and
mitosis.
- 1900 Cytology and genetics rediscovery of Mendel’s papers and similarities
between Mendel’s “factors” and behavior of chromosomes.
- 1902 Walter Sutton and others came up with Chromosomes Theory of
Inheritance. That is, Mendelian genes have specific loci (locations), on
chromosomes. This is how segregation and independent assortment can
occur. Fig. 15.2.
Idea of specific gene associated with specific chromosome came form
Thomas Morgan – skeptical of Mendel’s ideas and chromosome theory.
- Showed chromosomes are the location for Mendel’s heritable factors.
- Key thing is he selected the fruit fly as his model organism;
a) Breed fast and often.
b) Lots of offspring (100’s) in a single mating.
c) Easy to keep.
d) Only 4 chromosomes and easily seen under light microscope. 3 pairs
autosomal and 1 pair sex (XX female and XY male).
BUT NO SUPPLIER OF MUTANTS!
So, Morgan crossed his own after he irradiated the wild type. After many
generations he got a mutant males with white eyes (w). The wild type had red
eyes (w+). Fig. 15.3 in textbook.
Fig. 15.4.
- P generation
(male) ww x w+w+ (female)
- F1 generation
ww+
x ww+
- F2 generation 3:1 Ratio and All females red eyes and ½ males red and ½
males white eyed.
Therefore, eye color linked to sex of fly. Otherwise would have got ½
females with white eyes and ½ males with white eyes.
This supported the chromosome theory that:
a) Specific gene on specific chromosome
b) Sex-linked traits have unique inheritance pattern.
XwXw+
x
XwY
Xw+Xw XwXw+ Xw+Y
Xw+Xw
x
Xw+Y
Xw+Y
Xw+Xw+ Xw+Xw Xw+Y
Xw Y
Linked genes means that genes are located on the same chromosome and so
are inherited together.
Ratios deviate from Mendel’s Law of Independent Assortment.
Morgan also looked at other traits such as body color and wing size.
NOT SEX LINKED.
b+ = gray bodies
b = black bodies
vg+ = normal wing size
vg = vestigial wings
Fig. 15.5.
P gen:
b+b+vg+vg+
F1 gen:
F2 gen:
x
b b vg vg
b+ b vg+ vg (heterozygous wild type phenotype)
b+ b vg+ vg
x
b b vg vg (test cross) Male.
Got more parental phenotypes than if independent assortment (ratios would
be fairly equal between parental phenotypes and non-parental phenotypes),
so Morgan concluded that genes linked. But did get small number of nonparental phenotypes so recombination taking place and exchange of geneic
material (crossing over), in prophase of Meiosis I.
Can use this recombination data to construct a genetic map.
Further apart two genes are, the higher the probability of crossing over and
the recombination frequency higher.
Mendel also saw recombinants in his crosses:
YyRr x yyrr (test cross)
Gave: ½ parental type and ½ recombinants
So, 50% recombinants = 50% frequency of recombination.
50% frequency of recombination seen for any two genes located on
different chromosomes. This is due to the random orientation of
homologous chromosomes at metaphase of meiosis I and separation during
Anaphase I resulting in independent assortment of alleles (Fig. 15.2).
Morgan’s student Alfred Sturtevant came up with:
Recombination frequency = # recombinants x 100 =
Total # offspring
%=
map units.
See Fig. 15.7.
If 50% recombination frequency then physically linked genes BUT
genetically unlinked (act as if on different chromosomes).
Can only approximate one gene in relation to another, NOT actual distances.
Compare to cytogenic map (locate genes using chromosomal features such as
staining bands and looking at them under microscope). Get correct order but
spacing not same in both maps.
Pedigree Q pg. 270 Textbook:
Beth and Tom both have siblings with cystic fibrosis (CF).
But, both sets of parents as well as Beth and Tom do not have CF.
CF is caused by a recessive allele.
a) What is the probability of having a child with CF?
b) What is the probability of a) if Tom is a carrier and Beth is NOT?
a) CCf x
CCf
CC CCf CCf CfCf
Neither have CF so 2/3 of being carrier.
Then ¼ of a CfCf baby so: 2/3 x 2/3 x ¼ = 4/36 = 1/9.
c) CC x CfC means that 0 probability of getting child with CF.