Test Crosses

Question 1

for punnet squares…

Answer 1

it is important to know the genotype of each parent. also, you need pure breeding strains of individuals if you make controlled crosses. AA or aa

Question 2

why is it hard to determine the genotype of a dominant individual

Answer 2

because you don’t know if it is homozygous dominant or heterozygous

Question 3

Test cross

Answer 3

when you cross the genotype of one parent with a double recessive to find the genotype of an individual with a dominant trait. The unknown is Ax times aa.

Question 4

if there is a single individual with the recessive trait…

Answer 4

you know that the unknown parent was heterozygous.

Question 5

Probability of 7

Answer 5

We can never be 100% sure that the unknown parent is AA, because dominant individuals could, by chance, all 

come from the first column. However, if we can get at 

least 7 straight individuals with dominant phenotypes with no 

recessive phenotypes, we can be better than 99% sure - 

(probability of Aa = 1/27 = 1/128).

Question 6

when can you use test crosses

Answer 6

• when the organism produces more than once and many offspring in a lifetime
• organism MUST be pure
• if there is a test cross with all dominant traits, you can assume it is homozygous (tentatively)
• if there are any recessive traits, then the organism is not homozygous