Course book: Chapter 1 "The mendelian world"

Question 1

What is “the principle of independent segregation”?

Answer 1

It is a mendelian principle that describes how alleles (chromatids) are individually proliferating. Mendel discovered this when crossing peas of varying characteristics and finding that the recessive trait is visible in 25% of offspring (when crossing heterozygotes)

Question 2

Is the following statement true?

In a heterozygous allele, the expressed phenotype is always the dominant allele.

Answer 2

No, not necessarily. A heterozygotic phenotype can sometimes be a mix between the two alleles.

Example: Mendel crossed two tulips, one hosting the genotype aa (red), the other hosting the genotype AA (white). Some offspring (50%) ends up being pink.

Question 3

What is “the principle of independent assortment”?

Answer 3

The principle describes how alleles can recombine to generate new phenotypes. This is possible because:

(1) Parental generation consists of different genotypes (see below).

P1 = RRYY, P2 = rryy
- There’s only one possible gamete per genotype (P1 = RY, P2 = ry).

(2) The resulting diploid genome is described as RrYy.

(3) RrYy can generate four different gametes (see below)

RY, Ry, rY, ry.

(4) When the different genotypes are inbred, it’s possible for previously unseen phenotypes to appear. The alleles recombine according to the table below. If you’re confused, there’s a figure in the document “Chapter 1”.

RY x RY = RRYY
2RY x Ry = RRYy
2RY x rY = RrYY
2RY x ry = RrYy

Ry x Ry = RRyy
2Ry x rY = RrYy
2Ry x ry = Rryy

rY x rY = rrYY
2rY x ry = rrYy

ry x ry = rryy

Question 4

What does it infer that two genes are linked?

Answer 4

If two genes are linked, they are proliferated together 50-100% of the time.

Question 5

During what stage of the meiosis cell cycle does the “crossing” of genes occur?

Answer 5

In the very beginning of meiosis, chromatids are lined up in order to segregate them. By this point, the diploid genome has been replicated in order to produce four gametes. At this point, the chromatids coil around each other, sometimes, the tension will be too high, causing dsDNA break (blunt ends). Blunt end repair follows, sometimes ligating diferent chromatids together.