Download Lecture#16 Page 1 BIOLOGY 207

BIOLOGY 207 - Dr.McDermid
Lecture#16 Independent Assortment
Readings: Griffiths et al, 7th Edition:
Ch 2, 34-37, Ch 5, 144-145 + Figure 5-7
Problems: Griffiths et al, 7th Edition:
Tier 1: Ch 3, #18, 24, 25, 28, 29, 32 Ch 4, #3 Tier 2: Ch 3, #26, 27, 30 Ch 4, #14
Concepts:
Mendel’s second law
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.
Mendel's First Law - Segregation of Alleles
Example: Simple monohybrid cross with dominant and recessive alleles:
A simple test for 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
Mendel's 2nd Law
Independent Assortment
Example: Figure 2-9 and 2-10
Locus 1
- Round (R) vs. wrinkled (r)
Locus 2
- Yellow (Y) vs. green (y)
Parents: - true breeding (pure) lines
Round; green
x
Lecture#16
wrinkled; yellow
Page 1
x
gametes
F1
P2
Chromosome alignment at anaphase I has two equally frequent possibilities
Meiosis Result:
gametes of four types in equal proportions
4 possible phenotypes
Round, Yellow
Round, green
wrinkled, Yellow
!
wrinkled, green
Result:
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
What if the parents are:
Round, Yellow
Lecture#16
x
wrinkled, green
x
Page 2
The cross:
Progeny:
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.
Test cross
-> The idea here is to reveal the genotype of the tested unknown parent (AA or Aa) by
crossing it with the tester parent.
Example of a test cross
R? Y?
Rr Yy x
( use the RrYy as an example to be tested)
rryy
(tester individual)
(one type)
(4 types)
Lecture#16
Ratio
NOT Ratio
Page 3
Suppose the individual to be tested was RRYy instead
RR Yy
x
rryy
(one type)
(2 types)
Ratio
***Test cross reveals
- the genotype of the tested individuals
- if the two gene loci are assorting independently
Independent assortment
______________________________________________________________________________________________
Lecture notes: Copyright © 2002 Heather McDermid and the Department of Biological Sciences, University of Alberta
Images are Copyright©2000 by W.H. Freeman & Co. in Griffiths et al, Introduction to Genetic Analysis
Lecture#16
Page 4