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Genetics

Part 1

A: Suppose, the purple parent pea carries Pp, the white may be only pp. The cells of the white peas produce only gametes type p and the purple both P and p. The table shows how they cross:

 

 

 

 

parents

P

p

p

Pp

pp

 

 

 

 

The offspring is 50% white and 50% purple, as given in this task (445 / 445 + 437 = 50%, 437 / 445 + 437 = 50%).

B: In this example, there were a few hypotheses to describe. First, certain features (purple vs white) in a given sign (color) dominate over the other. The dominant signs will not disappear in at least some percentage of the offspring. Second, a homozygous cell produces only one type of alleles, while a heterozygous – both dominant and recessive. Third, we detect how the sign is splitted. The second Mendel’s Law would be more evident if we had both parents heterozygous, but even in homo- and heterozygous cross experiment it is evident that here the genetic material of the purple Pp is splitted into two patterns – the potentially white p and definitely purple P.

Part 2

The purple flowers are Pp. These cells will give P and p gametes. The table shows what they produce after crossing:

 

 

 

 

parents

P

p

P

PP

Pp

p

Pp

pp

 

 

 

 

The offspring F2 genetically will be PP, Pp, Pp and pp, or 1 PP, 2 Pp, 1 pp. This is a genetically split ratio 1 : 2: 1. Both PP and Pp are purple, because if there is at least one dominant gene, no matter which is the second, the phenotype must correspond to the dominant one. Therefore, 1 PP is purple, 2 Pp are purple too, 1 pp is white. The number of purple is 1 + 2 = 3, the number of white is 1. Phenotypically, there will be a 3 : 1 split.

Part 3

A: P – dominant color (purple), p – recessive color (white), T – dominant height (tall), t – recessive height (dwarf). A heterozygous purple tall plant is PpTt and will give gametes PT, Pt, pt, pT. A white dwarf is recessive for both characteristics – pptt and will give gametes pt only. The table shows how they cross:

 

 

 

 

Parents

PT

Pt

pt

pT

pt

PpTt

Pptt

pptt

ppTt

 

 

 

 

The result is the following: 4 types of plants – tall purple, dwarf purple, dwarf white and tall white. The ratio is 1 : 1 : 1 : 1. The probability of a white dwarfed pea is 1 / 1 + 1 + 1 + 1 = 1 / 4 = 0.25 or 25%.

B: The hypothesis of independent assortment states that pairs of signs pass to offspring independently. The mentioned signs (color and height) are encoded in different chromosomes. This means that no matter what color gene the plant has, the height alleles of Tt will distribute as 1 : 1. No matter how tall the plant is, the color alleles PP give only P, pp give only p, Pp give both P and p. During crossing, no matter how tall and what color the plant is, the chance of getting P or p from Pp is 1 : 1. This means that after meiosis a brand new combination is possible, independently of what genotype the parent cells had.