2/16/16
BIOLOGY 207 - Dr.Locke
Lecture#13 - Independent Assortment (Mendel’s second Law)
Required readings and problems:
Reading: Open Genetics, Chapter 3, 6
Problems: Chapter 3, 6
Optional
Griffiths (2008) 9th Ed. Readings: pp 89-109
Problems: 9th Ed. Ch. 3: 1-20 (not #6 or #9)
Campbell (2008) 8th Ed. Readings: Concept 14.3, 15.3
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.
Biol207 Dr. Locke section
Lecture#13
Fall'11
page 1
2/16/16
Review:
Mendel's First Law - Segregation of Alleles
Each unit of heredity is discrete (particulate) -> 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
Gametes R
r
Rr
R
RR Rr Progeny
r
Rr rr
Genotype Ratio Phenotype ratio
RR
1
3
1/3 breed true
Rr
2
2/3 don't
rr
1
1
breed true
This offers the simple test:
Test for single locus Mendelian Inheritance
1) Choose pure 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, with dominant and recessive alleles
Biol207 Dr. Locke section
Lecture#13
Fall'11
page 2
2/16/16
Mendel's 2nd Law -> Independent Assortment - dihybrid cross
Two distinct loci - each with single locus, with dominant and recessive alleles
Example:
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
RR yy x
rr YY
Gametes:
Ry
rY
RrYy
F1
Round Yellow
Rr Yy
x
Rr Yy
P2
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
RY
or
Ry
ry
rY
50%
50%
Biol207 Dr. Locke section
Lecture#13
Fall'11
page 3
2/16/16
Meiosis Result: gametes of four types in equal proportions
RY
ry
Ry
rY
four types of gametes in the F2 progeny
Called a “Punnett” Square
Haploid (1N) gametes fuse to form the zygote (2N)
If male has 4 types and female 4 types-> 16 possible combinations of genotypes
but only 4 possible phenotypes
Note:
Round/wrinkled
Yellow/green
9 Round, Yellow
12 (3)
12(3)
3 Round, green
3 wrinkled, Yellow
4 (1)
4(1)
1 wrinkled, green
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
- no interaction between the two genes that favors the formation of particular gamete
combinations
- 9:3:3:1 ratio is expected for genes loci that assort independently
Gene loci that assort independently may be
1) on different chromosomes, or 2) far apart on the same chromosome.
NOT located near each other on the same chromosome.
Biol207 Dr. Locke section
Lecture#13
Fall'11
page 4
2/16/16
What if the parents are:
Round, Yellow x wrinkled, green
(RRYY)
x
(rryy)
The cross:
RRYY
x
rryy
Round, Yellow
wrinkled, green
RY
ry
RrYy
Round, Yellow
P1
gametes
progeny (F1)
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 underlying the dominant phenotypes then a
test cross should be done.
Biol207 Dr. Locke section
Lecture#13
Fall'11
page 5
2/16/16
Test cross
- a cross between
1 - an individual of unknown genotype, and
2 - one of known genotype
- tests the unknown genotype with the known to 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.
Biol207 Dr. Locke section
Lecture#13
Fall'11
page 6
2/16/16
Example of a test cross with two loci
R? Y? ( use the RrYy as an unknown example to be tested)
“Unknown”
Round, Yellow
RrYy
RY ry Ry rY
(4 types)
RY
Ry
rY
ry
(tester individual)
wrinkled, green
x
rryy
ry
(one type)
ry
Rr Yy
Rr yy
rrYy
rr yy
P1
gametes
phenotype
Ratio Not
Round, Yellow
1
9
Round, green
1
3
wrinkled, Yellow
1
3
wrinkled, green
1
1
**The frequency of progeny reveals the RrYy double heterozygote nature of the
“unknown” genotype
Biol207 Dr. Locke section
Lecture#13
Fall'11
page 7
2/16/16
Suppose the unknown individual to be tested was RRYy instead
Unknown
(tester individual)
Round, Yellow
wrinkled, green
RRYy
x
rryy
P1
RY Ry
ry
gametes
(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
Or RrYy Round, Yellow 1
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
RrYy =
RrYY =
RRYy =
Biol207 Dr. Locke section
Lecture#13
Fall'11
RRYY =
page 8
2/16/16
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
(Fig)
- 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 are close on the same chromosome.
These two loci would not be expected to show independent assortment.
Instead they show linkage. (next lecture)
Biol207 Dr. Locke section
Lecture#13
Fall'11
page 9