L3-5: Gene Interaction

Question 1

What are three principles of Mendelian Inheritance?

Answer 1

1. Law of segregation (when gametes form, alleles are separated so that each gamete carries only one allele for each gene.
2. Law of Independent Assortment
3. Principle of Dominance

Question 2

What are the differences between germline and somatic mutation?

Question 3

What is incomplete dominance and codominance?

Answer 3

Incomplete dominance = phenotypes combine to create a range of possible phenotypes (e.g. Red and white flower bred to form some shade of pink)

Codominance = two equally dominant alleles are both expressed.

Question 4

What is pleiotropy?

Answer 4

One gene affects many phenotypic characteristics.
E.g. Sickle cell genetic makeup.

Many symptoms can be traced back to one defective allele.

Question 5

What is X-inactivation?

Answer 5

Human females ar functionally mosaic.
One X chromosome is silenced in each cell, and the one that is silenced is random.

All descendants keep the same pattern.

Question 6

What is mosaicism?

Answer 6

Post-zygotic mutations:

• EARLY in development = descendants in many tissues
• LATER in development = more localised

Question 7

Describe Turner Syndrome.

Answer 7

Occurs in females that inherit an X chromosome that has a part/ all of it missing.

Results in disease that correlates with the relative proportion of 45,X0 cells compared to normal 46,XX cells. (In cells that have the OTHER X chromosome inactivated through X inactivation)

Question 8

Describe the three features that give mitochondrial mutations a distinct pattern of inheritance.

Answer 8

1. Replicative segregation (random segregation gives range to a wide range of cells with differentp proportions of mutant:normal phenotype)
2. Homoplasmy and heteroplasmy (Mitochondria can all have same genome (homo) or diff (hetero))
3. Maternal inheritance (Sperm mitochondria are eliminated from embryo, mtDNA is inherited from MOTHER. Males with mutant mtDNA can’t pass it on, females WILL)

Question 9

What is skewed X chromosome inactivation?

Answer 9

Skewed mosaicism needs >70% of the cells to have inactivated the same chromosome.

Some cells can have mutant genes that may cause preferential inactivation.

Question 10

Describe penetrance and expressivity.

Answer 10

Penetrance is the proportion of individuals with a mutation that actually develop the phenotype.

E.g. 50 people have the allele for polydactyl, but only 41 show it (82% penetrance)

Expressivity is the degree to which the phenotype is expressed (severity of disease).

Question 11

Describe epistasis.

Answer 11

The action of genes at one loci modify the expression of genes at a nother loci.

Epistatic gene = gene that does the masking

Hypostatic gene = gene that gets masked

Question 12

Describe the process of recessive epistasis for human ABO blood groups.

Answer 12

ABO blood ground antigens differ in their terminal sugar (A, B)

I(A) and I(B) code for transferases that add terminal sugars to substrate (H).

H gene codes for a different transferase that makes the substrate.

• HH and Hh individuals express functional fucosyl transferase
• hh individuals don’t, can’t make substrate H and thus the A/B transferases can’t add sugars.

On top of this, the i allele codes for non-functional transferase.

• ii individuals cannot add A or B antigens, and thus they are blood group O.

Thus, expression of ABO locus alleles depends on genotype at H locus.

• ABO is hypostatic to recessive h allele.