Cycle 5 - Genetics 1

Question 1

State Mendel’s hypotheses

Answer 1

• Variations in traits are due to different alleles.
• Alleles separate randomly into gametes.
• Organisms inherit two alleles for each trait.
• Appearance of heterozygotes determined by dominant alleles

Question 2

Explain the product rule of probability

Answer 2

When 2+ events are independent, the probability that they will both occur is their individual probabilities multiplied

Ex., rolling 2 4s on a die = 1/6 x 1/6 = 1/36

Question 3

Explain the sum rule of probability

Answer 3

The probability that either event A or event B or event C will occur is the probability of each event summed

Ex., rolling a 7 –> 6, 5, 4 … 3, 2, 1 (rainbow). There are 6 possible combinations here. To roll, for example, a 6 and 1 consecutively, that is a 1/6 x 1/6 = 1/36 chance. Thus, since there are 6 outcomes, 1/36 x 6 (sum it 6 times) the chance to roll a sum of 7 is 1/6.

Question 4

Compare codominance and incomplete dominance

Answer 4

Incomplete (insufficient dominance) - blend of two phenotypes (ex., red flower and white flower = pink; neither red or white is being fully expressed)

Codominance (no dominance) - both alleles expressed (ex., blood type AB)

Question 5

Explain X chromosome inactivation

Answer 5

Dosage Compensation: mammalian females turn off one of their X chromosomes by transitioning into a condensed form of chromosomes, called a Barr body. Occurs during embryonic development and which X of the two

However, consider XHXh –> the decativation of the left most chromosome will not trigger the disease hemophilia because the dominant H allele is active in enough critical cells (recall there is one set of Xs in each cell). But this is not always the case.

Question 6

Explain epistasis

Answer 6

Epistasis is when one gene affects the expression of another. This can cause an inhibiting effect.

Ex., Labs. BB or Bb produces black fur due to intense pigment, bb produces brown fur due to less pigment. However, another gene at a different locus determines whether pigment is produced at all (E gene.). If ee is inherited, the dog will appear yellow regardless of its B and b combinations.

Sex-linkage inactivation can create epistasic effects (ex., calico cats)

Question 7

Explain the production of skin pigments and how they exist on the membrane of cells

Answer 7

• Skin pigments are made by melanocytes (skin cells), and the pigment is melanin. Melanocytes package the pigment into melanosomes.
  
  • 2 types: black (eumelanin) and red (pheomelanin)
  • These are exported to the skin cells. Exportation is mediated by MC1R (can be switched on or off by proteins)
    
    • Turned on = black melanin production
    • Turned off = red melanin production
      
      • 4 skin alleles (4 slots, 2 combos, ex., SW or WW):
      • W allele - sometimes on or off (brown)
      • B allele - always on (black)
      • R allele - always off (red)
      • S allele - always off initially but can be mutated very easily to start to produce black pigment - causes spots

Question 8

Explain how a WB skin cell might funcyion

Answer 8

Example of a WB cell:

• Both W and B are transcribed and then translated by the rough ER of ribosomes
• Next they are packaged into vesicles and moved to the golgi where they are sent to the cell membrane
• Both proteins (ex., W and B here) rest on the membrane where the signalling proteins can interact with each one separately to create a “blend” (in this case black would triump)