Lecture 24: Extensions of Mendelian Genetics

Question 1

What is the probability of getting a ssyy offspring from a SsYy x SsYy cross?

Answer 1

• How to achieve ss?

Product rule, there’s half a chance of each parent supplying one s. 1/2 x 2 = 1/4

• How to achieve yy?

Product rule, there’s half a chance of each parent supplying one y. 1/2 x 2 = 1/4

ssyy = 1/4 x 1/4 = 1/8

Question 2

What is the probability of the offspring from the cross below? Aa BB Cc x Aa Bb cc = Aa Bb cc

Answer 2

How can we achieve Aa?

P of parent 1 supplying A = 1/2
x
P of parent 2 supplying a = 1/2

P of parent 1 supplying a = 1/2
x
P of parent 2 supplying A = 1/2

(1/2 x 1/2 = 1/4) + (1/2 x 1/2 = 1/4) = 1/2

Question 3

Lethal alleles change the ratio from 3:1 to _____ because a combination isn’t viable

Answer 3

• 2:1

Question 4

Polymorphic: _____ gene, _____ alleles

Answer 4

• One

- Many

Question 5

In snapdragons (flowers), RR x rr = all Rr, Rr x Rr = RR + Rr + Rr + rr which is a _____ ratio. Blending inheritance appears to be proven with the parents but with the F1 offspring, they appear pink because less red pigment is being made.

Answer 5

• 1 : 2 : 1

Question 6

Incomplete Dominance: red + white = pink

Answer 6

• Heterozygote is halfway in between

Question 7

Co-Dominance is where both phenotypes exist….

Answer 7

• Side by side within the organism

Question 8

The most famous example of co-dominance is the ABO blood antigen system. Where if you get a big I^A and a big I^B in terms of antigen you see….

Answer 8

• Both A & B antigens being produced

Question 9

What is epistasis? Give an example

Answer 9

• Where a gene(s) affect the action of another
• Gene A determines whether colour is produced (A) or not (a)
• Gene B determines whether coat colour is brown (B) or black (b)
• White ones are homozygous for a (aa)
• Gene B is said to be under epistatic control of gene A

Question 10

What are polygenic traits? Give an example

Answer 10

• A phenotype controlled by many genes that have an additive effect (many genes contribute to how smart you are)
• Character appear continuous or quanititative
• Skin colour (environment also blurs the line between phenotypes)

Question 11

The environment also affects phenotype (e.g nutrition / height / weight / life expectancy). Give an example:

Answer 11

• Hydrangea is pink in alkaline soils but blue in acidic soils

Question 12

Environment smooths differences among phenotypes e.g. size of wheat ears

Answer 12

• Genetically identical, identical environment (blocky)
• Vary environment slightly (less blocky more slopey)
• Completely change environment (very slopey)

Question 13

Pleiotropy is where ____ gene has ____ effects. Give an example:

Answer 13

• One
• Many
• Two copies of sickle cell allele