(Solved) Genetics Help

tee

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12 years ago

Genetics Help

Could someone please help me with these question I have done a little bit for each. Slight Smile

Question 1.
Pea pod shape is controlled by a one gene - two allele genetic system. Full pod shape is completely dominant to constricted pea pod shape. Pea plant size is also controlled by a one gene – two allele system with tall size being completely dominant to dwarf size. Given a plant was heterozygous for pod shape and homozygous recessive for plant size:

a.   What is the phenotype of this plant?
b.   If this plant was crossed to a dihybrid (heterozygote), what would be the phenotypic ratio of pod shape among the progeny?
c.   What would be the phenotypic ratio of plant size?
d.   What would be the phenotypic ratio of pod shape and plant size together?

Question 2.
A pure breeding, tall pea plant white flowers is crossed with a pure-breeding short plant with red flowers. The F1 plants are tall with red flowers. When allowed to self-fertilise themselves, these F1 plants produce the following F2: 326 tall, red; 104 tall, white; 117 short red and 29 short white. Explain these data; indicating genotypes of the parents and the different F2 phenotypes. What is the expected numbers in the various F2 classes? Test the goodness of fit between the data and your hypothesis using ?2.

Question 3.
In any breeding program (i.e. for cattle, horses, dogs guinea pigs), which is more easily eliminated from a population; a trait expressed by a completely dominant allele, or a trait expressed by a completely recessive allele?

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tee Author

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12 years ago

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Bio_World100

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12 years ago Edited: 12 years ago, Bio_World100

So, this is what's given in the question:

Pea pod shape: Full is completely dominant to Constricted
Pea plant size: Tall is completely dominant to Dwarf

So, lets define our alleles:

Pea pod shape

Full = F (upper case since dominant and NOT recessive)
Constricted = f (lower case since recessive and NOT dominant)

Pea plant size

Tall = T (upper case since dominant and NOT recessive)
Dwarf = (lower case since recessive and NOT dominant)

Question 1

a) Since the plant is heterozygous for pod shape, there MUST be a dominant and a recessive allele. So, the genotype of the plant would be Ff, and therefore the phenotype would be Tall.

Since the plant is also homozygous recessive for plant size, there MUST be two recessive alleles. So, the genotype of the plant would be tt, and therefore the phenotype would be Dwarf.

Therefore, the overall phenotype would be Tall, Dwarf.

b) If this plant was crossed to a dihybrid (heterozygote), the cross would then be:

Fftt (from question 1a) x FfTt (heterozygote)

The possible gametes for the genotype Fftt would be Ft and ft.
The possible gametes for the genotype FfTt would be FT, Ft, fT, and ft,

So, what you would have to do then is draw a Punnett Square and figure out the possible phenotypes. You should end up getting 8 possibilities. Since they're ONLY asking for the phenotypic ratio for pod shape, you would only take into consideration the following genotypes: FF, Ff, or ff. So, the ratio I got for Full:Constricted was 3:1. There are 6/8 (3/4) Full and 2/8 (1/4) Constricted.

c) Since they're ONLY asking for the pehnotypic ratio for pod size, you would only take into consideration the following genotypes: TT, Tt, or tt. So, the ratio I got for Tall:Dwarf was 1:1. There are 4/8 (1/2) Tall and 4/8 (1/2) Dwarf.

d) Now, since they're talking about both traits put together (pod shape and pod size), the possible genotypes would be FFTt, FFtt, FfTt, Fftt, FfTt, Fftt, ffTt, and fftt. So, you have 3/8 Full & Tall, 3/8 Full & Dwarf, 1/8 Constricted & Tall, and 1/8 Constricted & Dwarf. So, your phenotypic ratio would then be 3:3:1:1.

I will post question 2 next.

Hope this helps! Smiling Face with Open Mouth

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mockereo

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12 years ago

Question 3:

The completely dominant allele is easier to remove from the population, because any organism that has the dominant allele will show the dominant phenotype. Since you can distinguish which individuals have the dominant allele, you can remove them from the population easily.

A recessive allele would be more difficult, because heterozygous individuals would appear the same as homozygous dominant individuals. You would have a difficult time completely getting rid of all the carriers that posses the recessive allele and also have the dominant one to mask its presence.

Example: if short hair (S) was dominant to long hair (s), all individuals with SS or Ss genotype would have short hair. Only individuals without the short hair S allele - in other words genotype ss - would be long-haired, and so by taking out all short haired individuals from your breeding population, only long hair genes remain.

If you were trying to select short hair in this situation, you could remove all the ss/long-hair individuals, but many in your population would be heterozygous, still carrying the recessive long-hair allele while showing the dominant short-hair phenotype. That way the recessive long-hair allele is still in your population despite removing all long-haired individuals.

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Bio_World100

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12 years ago

Question 2

So, this question deals with two pairs of traits like in the last question: size of pea plant (tall or dwarf) and colour or pea plant (white or red).

Since the question states that the F1 Generation is Tall and Red, these two traits MUST be the dominant ones.

So, let's define our alleles:

Size of Pea Plant

Tall = T (upper case since dominant and NOT recessive)
Dwarf = t (lower case since recessive and NOT dominant)

Colour of Pea Plant

Red = W (upper case since dominant and NOT recessive)
White = w (lower case since recessive and NOT dominant)

P1 Generation Cross

So, this situation involves a dihybrid cross since you're dealing with two pairs of traits.

The question states that the Parental (P1) generation is PUREBRED. That would mean the genotypes of the parental generation would be homozygous for all four traits: Tall, Dwarf, Red, and White.

So, your cross would then be:

P1 Generation: TTww (tall, white) x ttWW (short, red)
The genotypes of the parents are TTww and ttWW

The gamete for the TTww genotype would be Tw.
The gamete for the ttWW genotype would be tW.

When you combine these gametes, you get the genotype TtWw. This is the genotype for the F1 Generation.

Self-fertilization of the F1 Plants

So, your cross would then be:

F1 x F1: TtWw x TtWw

The gametes for the TtWw genotype would be TW, Tw, tW, and tw.

When you combine these gametes, you get 16 possible genotypes. These are the genotypes for the F2 Generation.

The following is a list of all the F2 phenotypes:

9/16 Tall, Red
3/16 Tall, White
3/16 Dwarf, Red
1/16 Dwarf, White

I will calculate the expected numbers in the various F2 classes and post it next!

Hope this helps! Smiling Face with Open Mouth

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Bio_World100

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12 years ago

So, when the F1 plants were self-fertilized, the following was produced in the F2 generation:

326 tall, red
104 tall, white
117 short, red
29 short, white

The total number of obervations is 576, so based on this, we can calculate the expected values of each outcome:

The possible phenotyes that we got from the Punnet Square were 9/16 tall & red, 3/16 tall & white, 3/16 short & red, and 1/16 short & white. So, the expected values will then be the following:

Tall, red: 576 (9/16) = 324
Tall, white: 576 (3/16) = 108
Short, red: 576 (3/16) = 108
Short, white: 576 (1/16) = 36

Hope this helps! Smiling Face with Open Mouth

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duddy

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