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Lecture 15; 2007
Biology 207; Section B2; Good
Independent Assortment
Readings:
Griffiths et al (2004)
8th Edition: pp 36-40; 102; 115-123; 7th Edition: Ch. pp 76-78; 144-146
Assigned Problems:
8th Ch. 3: 19, 22-29, 40; 7th Ch. 3: 18, 24-32
Concepts:
How do genes on different chromosomes behave in meiosis?
1. Mendel observed allele pairs, at different gene loci, segregate independently
(independent assortment).
2. When independent assortment occurs it results in predictable frequencies of F2
progeny.
3. Test crosses are more effective than F2 cross's to study several genes
simultaneously in one cross.
Review Mendel's First Law - Segregation of Alleles
Each unit of heredity is discrete -> an allele.
The segregation of alleles ensures that one allele in the pair is present in a gamete and
in equal numbers.
Example: Simple monohybrid (one-heterozygote) cross with dominant and recessive
alleles: Rr x Rr.
Rr
Gametes
Rr R
r
Genotype
RR
R
RR
Rr
r
Rr Progeny Rr
rr
rr
Ratio Phenotype
ratio
1
3
2
1
1
1/3 breed
true
2/3 don't
breed true
This offers the simple test for single locus Mendelian Inheritance
1) Choose pure (true breeding) lines that show a character difference P1 (R,r)
2) Cross the line --> F1 progeny
3) Cross the F1 individuals (P2) --> F2 progeny
F1 are Round therefore Round is dominant to wrinkled (assume single gene character)
F2 are 3/4 Round and 1/4 wrinkled - segregating as a single locus
Mendel's 2nd Law - Independent Assortment
Two distinct loci - dihybrid cross
- each with dominant and recessive alleles
Example: Figure 2-10
Locus 1 - Round (R) vs. wrinkled (r)
Locus 2 - Yellow (Y) vs. green (y)
Parents: - true breeding (pure) lines
cross Round; green with wrinkled; Yellow
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Lecture 15; 2007
Biology 207; Section B2; Good
RR yy x rr YY
gametes R y r Y
RrYy
F1
Round Yellow
Rr Yy x Rr Yy
P2
gametes
These two gene loci are located on different chromosomes so they will segregate with
their chromosomes and independently of each other.
Chromosome alignment at anaphase I has two equally frequent possibilities
R Y or R y; r y or r Y
Meiosis Result: gametes of four types in equal proportions
RY ry Ry rY use these four types of gametes in the F2 progeny
See Figure 2-10 - called a Punnett Square
Haploid (1N) gametes fuse to form the zygote (2N)
If male has 4 types and female 4 types then there are 16 possible combinations of
genotypes but only 4 possible phenotypes
Note:Round/wrinkled Yellow/green; 9 Round, Yellow; 3 Round, green; 3 wrinkled,
Yellow; 1 wrinkled, green
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Lecture 15; 2007
Biology 207; Section B2; Good
Result: get a characteristic 9:3:3:1 ratio of the F2 progeny
Note: - each gene locus acts the same as in a mono-hybrid cross - but no interaction
between the two genes that favors the formation of particular gametes
A 9:3:3:1 ratio is expected for genes loci that assort independently
Gene loci on different chromosomes assort independently
Gene loci that assort independently may be on different chromosomes (or far apart on
the same chromosome.
What if the parents are:
Round, Yellow x wrinkled, green
(RRYY) x (rryy)
The cross:
RRYY x rryy P1
Round, Yellow wrinkled, green
RY ry gametes
RrYy progeny (F1)
Round, Yellow
We expect the same results in the F2 if the F1 were crossed to themselves.
Examination of the progeny would show the 9:3:3:1 ratio
Note: the above crosses are called F2 crosses
If one wants to focus on the genotypes under lying the dominant phenotypes then a test
cross should be done.
Test cross
- cross between an individual of unknown genotype
- tests for the unknown genotype - so one can determine the unknown genotype
A/? (not sure if AA or Aa)
- must show the dominant phenotype for one or more gene loci
- use a tester individual, which is known to carry only the recessive alleles of the
genes in question a/a
-> The idea here is to reveal the genotype of the tested parent (AA or Aa) by crossing it
with the tester parent.
Example of a test cross
R? Y? ( use the RrYy as an example to be tested)
Unknown (tester individual)
Round, Yellow wrinkled, green
RrYy x rryy P1 RY ry Ry rY ry gametes
(one type)
(4 types)
ry
phenotype
Ratio
Not
RY
Rr Yy
Round, Yellow
1
9
Ry
Rr yy
Round, green
1
3
rY
rrYy
wrinkled, Yellow
1
3
ry
rr yy
wrinkled, green
1
1
These progeny reveal the RrYy double heterozygote nature of the genotype
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Lecture 15; 2007
Biology 207; Section B2; Good
Suppose the individual to be tested was RRYy instead
(one type)
(2 types)
ry
phenotype
Ratio
RY
Rr Yy
Round, Yellow
1
RY
Rr Yy
Round, Yellow
1
Ry
Rryy
Round, green
1
Ry
Rryy
Round, green
1
all Round therefore must have been RR not Rr
1:1 Yellow:green --> must have been Yy not YY
***Test cross reveals
- the genotype of the tested individuals
- if the two gene loci are assorting independently
Independent assortment
- Expected for gene loci on different chromosomes
- In the F2 cross it predicts a 9:3:3:1 ratio in the 4 phenotypes of the progeny (Figure 210)
- In the test cross one expects a 1:1:1:1 ratio when the F1 are test crossed to the
double recessive homozygote. But what about two gene loci on the same chromosome.
These two loci would not be expected to show independent assortment.
Instead they show linkage.
Different Segregation Ratios:
There are a number of cases where the gene segregation ratio seems unusual, not
because there isn’t random segregation, but because the genetics of the trait is more
complex than a simple on/off phenotype.
Problems:
In the plant Arabidopsis, the recessive allele hyg confers seed resistance to the drug hygromycin,
and her, a recessive allele of a different gene, confers seed resistance to herbicide. A plant that
was homozygous hyg/hyg · her/her was crossed to wild type, and the F1 was selfed. Seeds
resulting from the F1 self were placed on petri dishes containing hygromycin and herbicide.
a. If the two genes are unlinked, what percentage of seeds are expected to grow?
b. In fact, 13 percent of the seeds grew. Does this percentage support the hypothesis of no
linkage? Explain. If not, calculate the number of map units between the loci.
c. Under your hypothesis, if the F1 is testcrossed, what proportion of seeds will grow on the
medium containing hygromycin and herbicide?
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