Mendelian Genetics

Question 1

What was Mendel’s First and Second Law?

Answer 1

1. The Principle of Segregation
2. The Principle of Independent Segregation

Question 2

Outline the man steps in Mendel’s experiments.

Answer 2

1. Identified two different phenotypes/traits
2. Cross bred the P generation to get the F₁ generation
3. Bred F₁ x F₁ to get the F₂ generation
4. Observed phenotypes/traits in F₂ generation

Question 3

Give examples of the traits Mendel observed in his pea plants.

Answer 3

1. Seed coat colour/flower colour
2. Seed colour
3. Seed shape
4. Pod colour
5. Pod shape
6. Stem height
7. Flower position

Question 4

What do the P and F stand for in terms of breeding generations?

Answer 4

P: Parental generation

F: Filial generation

Question 5

How did Mendel explain these results? What did he postulate? [5]

Answer 5

1. Factors (genes) come in pairs, meaning each cell or individual has two alternate forms (alleles) of each gene.
2. For each pair of genes, the alleles may be identical (homozygous) or different (heterozygous).
3. Each reproductive cell (gamete) produced by an individual contains only one allele of each gene.
4. In the formation of gametes, any particular gamete is equally likely to include either allele.
5. The fusion of male and female gametes is a random process that reunites alleles in pairs.

Question 6

What is a Punnett square?

Answer 6

A visual representation of allele inheritance from random fertilisation. Shows the probability of each genotype and phenotype being formed.

Question 7

What ratios did Mendel work out for inheritance of traits in his peas?

Answer 7

A 3:1 ratio of inheritance.

Question 8

Define the Principle of Segregation.

Answer 8

In the formation of gametes, the paired hereditary determinants separate (segregate) in such a way that each gamete is equally likely to contain either member of the pair.

Question 9

How did Mendel test his hypothesisand confirm the Principle of Segregation?

Answer 9

He realised it could be tested via self-fertilisation of the F₂​ generation plants.

• Plants grown from homozygous genotyoes would be true-breeding (e.g. WW smooth seeds and ww wrinkled seeds)
• Those with heterozygous genotypes would yield a smooth and wrinkled seeds in the ratio of 3:1.
  
  • Of these seeds 1/3 gave smooth F₃ seeds, and 2/3 gave plants with pods containing both smooth and wrinkled seeds in a ratio of 3:1.
  • Therefore 1/4 of all his F₂ seeds were WW, 1/2 were Ww, and 1/4 were ww. The phenotype being 3/4 smooth and 1/4 wrinkled.

Question 10

How can you determine the genotype of an F₂ generation pea plant?

Answer 10

Use a testcross to cross a heterozygous plant with a homozygous one.

• WW genotype will show 100% smooth phenotype.
• Ww will show 50% wrinkled and 50% smooth.
• ww will show 100% wrinkled if crossed with another ww, or 100% smooth if crossed with WW.

Question 11

Define the Principle of Independent Assortment.

Answer 11

Segregation of the members of any pair of alleles is independent of the segregation of other pairs in the formation of reproductive cells (gametes).

Question 12

How did Mendel come up with his Second Law?

Answer 12

1. He found that the F₂ generation of dihybrid peas gave a 3:1 ratio for each trait independantly.
2. There are 4 combinations of gametes, e.g. WG, Wg, wG, and wg.
3. He did a testcross of all these combinations against a wg genotype, giving a ratio of 1:1:1:1.
   
   • Round yellow, round green, wrinkled yellow, wrinkled green.

Question 13

What are the ratios seen in phenotypes for a mono-, di-, and trihybrid cross?

Answer 13

1. 3:1
2. 9:3:3:1
3. 27:9:9:9:3:3:3:1

Question 14

What are the types of dominance? [3]

Answer 14

• Dominant
• Incomplete dominance
• Co-dominance

Question 15

Define codominance.

Answer 15

Both alleles of the heterozygote are expressed, e.g. ABO blood groups.

Question 16

Define incomplete dominance.

Answer 16

The phenotype of the heterozygous genotype is intermediate between phenotypes of homozygous genotypes.

E.g. the colour of snap dragon flowers can be red or white, or crossed to make pink.

Question 17

Define complementation.

Answer 17

The phenomenon in which two recessive mutations with similar phenotypes result in a wildtype phenotype when both are heterozygous in the same genotype; indication that the mutations are in different genes.

Question 18

What are the principles of a complementation test?

Answer 18

Brings two mutant genes together in the same cell or organism.

If this cell or organism is nonmutant, the mutations are said to complement one another and it means that the parental strains have mutations in different genes. If the cell or organism is mutant, the mutations fail to complement one another, and it means that the parental mutations are in the same gene.

Question 19

Give an example of a complementation test.

Answer 19

Using heterokaryons (fungi).

• Mutant 1 has a mutation on gene 1, mutant 2 on gene 2, and mutant 3 on gene 2 (dif. locus).
• Breeding them to form heterokaryons (F₂).
  
  • Mut1 x Mut2 gives complementation: one functional copy of each gene.
  • Mut2 x Mut3 gives noncomplementation: no functional copy of gene 2.