Mendel's Experiments (3)

Question 1

Describe Mendel’s experiment.

Answer 1

There were a large number of pure-breeding no plants available to Mendel in the monastery garden and, between the years 1856 and 1863, he raised and tested over 28,000 of them, carefully analyzing seven pairs of seed and plant characteristics such as plant height, pod shape, pod colour, flower position, seed colour, seed shape and flower colour.

Question 2

What was Mendel trying to establish?

Answer 2

He was trying to establish what happens to the alternative forms of traits, e.g. purple and white colour in flowers when they are combined together to form a hybrid.

Question 3

An example of Mendel’s experiment: (6)

Answer 3

• Mendel transferred pollen grains from the stamen of a pure-breeding tall pea plant and dusted them onto the stigma of the pistil of a pure-breeding short pea plant (having first removed the anthers of these flowers to prevent self -pollination). pure-breeding plants = plants that always give rise to offspring that are similar to themselves. •
• He then collected and sowed the seeds. •
• The resulting F1 generation all grew into tall plants. It seemed as if the short characteristic had completely disappeared in this generation of plants. •
• The F, generation was then allowed to self-pollinate, creating an F2 generation.
• When Mendel counted the plants in the F2 generation, he found that three-quarters of them were tall and one quarter was short. The ratio of tall plants to short plants was 3: 1. He counted thousands of plants before he came to these proportions.
• It seemed that the characteristic for shortness had not completely disappeared in the F1 generation, as it reappeared in the F2 generation. In this way, Mendel’s work showed that characteristics are passed on from one generation to another.

Question 4

In Mendel’s experiment on crossing tall and short pea plants, as described, complete dominance was shown, what is complete dominance?

Answer 4

A characteristic that is fully expressed in the phenotype of a heterozygous organism (i.e. tallness) is the dominant allele.

Question 5

When a dominant and a recessive allele are present together, only the ____ allele has an effect on the _____.

Answer 5

dominant

phenotype

Question 6

The _____ phenotype is the same as the _____ phenotype with dominant alleles, i.e. genotypes TT and Tt, both plants will be tall.

Answer 6

heterozygous

homozygous

Question 7

In the time since Mendel carried out his experiments on pea plants, new technologies and methods of research have allowed scientists to refine and expand on his work. Although he never stated his discoveries as ‘Laws’, the essence of his basic principles of ____ is still correct and can be summarized as follows: (3)

Answer 7

Inheritance

1. Mendel’s law of segregation
2. Mendel’s law of dominance
3. Mendel’s law of independent assortment

Question 8

What is Mendel’s law of segregation?

Answer 8

During meiosis, allele pairs separate (segregate) so that the gametes have a single allele for each characteristic.

Question 9

What is Mendel’s law of dominance?

Answer 9

In a cross of parents that are pure for contrasting traits, only the dominant trait will appear in the phenotype. Recessive alleles will always be masked by dominant alleles.

Question 10

What is Mendel’s independent assortment?

Answer 10

The alleles of different genes (e.g. height of plant and colour of flowers) segregate randomly and independently of one another during gamete formation.

Question 11

What is a test cross?

Answer 11

If an organism shows the dominant phenotype, how do we know if it is homozygous or heterozygous for the dominant trait? The only way to find out is to cross an unknown dominant organism several times with an individual showing the recessive phenotype.

Question 12

If the offspring all show the dominant characteristic the organism must have been ____.

Answer 12

homozygous

Question 13

If the cross yields any recessive offspring, the organism must be ____.

Answer 13

heterozygous