1. Zoo-352 Principles of genetics Lecture 8 Testing the Law of Segregation

2. The law of Mendel in genetics
In Mendel experiments on pea plant, the F2 generation would have a phenotypic ratio of 3:1, a standard Mendelian ratio for a monohybrid cross.
We would also expect a genotypic ratio of 1:2:1 in the F2 generation and twice are heterozygotes.
The challenge is to demonstrate that this genotypic ratio exists in the F2 offspring, when we can only observe phenotypes.

3. How could Mendel determine the F2 genotype as either homozygous dominant or heterozygous?
The simplest way to test the hypothesis is to: 1) self-fertilize the F2 individuals to produce an F3 generation (Figure 1).
Another way to test the segregation law is to use a 2) testcross, which crosses any organism with a recessive homozygote.
Another type of cross is a 3) backcross, which crosses offspring with a parent or an individual with the parental phenotype.
When the parent has the homozygous recessive phenotype, a backcross is also a testcross.

4. Figure 1: Mendel self-fertilized F2 tall and dwarf plants

5. Test Cross
A test cross is the process by which we cross (or mate) an organism of known phenotype but unknown genotype with an organism which we know its genotype and phenotype to determine what the unknown genotype is.
For instance, consider that you are given a plant whose phenotype you know is tall but you do not know its genotype. You do know however that it is either heterozygous tall or homozygous tall. How do you know this? Well only these two genotypes can produce a tall phenotype.
Before you actually do the cross, you can use the Punnett square to determine all the possible genotypes and corresponding phenotype proportions. Then by test crossing the unknown tall plant with a short plant (that you know must be homozygous recessive), you can use your offspring results to determine what the genotype of the unknown plant was.

6. 50% heterozygous (Tall) and 50% homozygous recessive (short).
Phenotype
Genotype
Punnett square
Tall
TT or Tt t T Tt
100% heterozygous (Tall)
Dwarf tt t T Tt tt
50% heterozygous (Tall) and 50% homozygous recessive (short).

7. DD X dd D d Dd Testcross 1: F phenotype: A homozygous tall A dwarf
F genotype: D d
Gametes: Dd
F1:
Tall (100%)

8. Dd X dd D d d Dd dd Testcross 2: P phenotype: A heterozygous tall
A dwarf Dd X dd
P genotype:
Gametes: D d d Dd dd
F1:
(Tall phenotype)
(Dwarf phenotype)

9. BB X bb Bb X bb B b b B b Bb bb Bb
Example: In Guinea pigs, black color is dominant over white. How would you find out whether a black is homozygous (BB) or heterozygous (Bb)?
By Testcross: BB × bb and Bb × bb BB X bb Bb X bb
F genotype: B b b B b
Gametes: Bb bb
(Black)
(White) Bb
F generation=
Black (100%)

10. Monohybrid Crosses Practice Exercises
Question 1: Curly hair is dominant to straight hair in humans. Show a cross between a heterozygous curly haired parent with a pure homozygous straight haired parent.
P phenotype:
A heterozygous curly
A straight Cc X cc
P genotype:
Gametes: C c c Cc cc
F1:
Curly phenotype (50%)
Straight phenotype (50%)

11. Question 2: A homozygous round seeded plant is crossed with a homozygous wrinkled seeded plant.
What are the genotypes of the parents?
What percentage of the offspring will also be homozygous?
RR, rr
0 %
F phenotype:
A homozygous round
A wrinkled RR X rr
F genotype: R r
Gametes: Rr
F1:

12. Question 3: A brown mouse is mated to a white mouse and all of their offspring are brown.
Which allele is dominant?
What are the genotypes of the mice that were crossed?
If two of the F1 brown offspring were mated together, what percentage of the F2 mice would be brown?
Brown
BB, bb
75% BB X bb Bb X Bb
F genotype: B b B b B b
Gametes: B b BB
(Brown) Bb bb
(White) Bb
F1=
(Brown)
F2=

13. Imagine that you are a rabbit breeder with two purebred rabbits, a male with black fur and a female with tan fur. When you cross your rabbits, all of the F1 kits (baby rabbits) have tan fur.
Which trait is dominant, and which is recessive?
Black fur is dominant, while tan fur is recessive.
Tan fur is dominant, while black fur is recessive.
Both tan fur and black fur are dominant. √
Which statement best describes the relationship between genotype and phenotype?
Genotype always completely determines phenotype.
Phenotype always completely determines genotype.
Genotype determines phenotype, but with influence from the environment.
Phenotype determines genotype, but with influence from the environment. √