Work Of Gregor Mendel Flashcards
Outline the experiments carried out by Gregor Mendel
Gregor Mendel, in the 1860s, formulated the principles of genetics by careful and methodical experimentation with garden peas, Pisum sativum. Mendel chose garden peas because they were easy to grow, produced new generations quickly and had easily distinguishable characteristics. He was also able to strictly control the breeding patterns of his peas.
Mendel examined the following seven characteristics found in peas:
Flower colour, purple or white
Flower position, axial or terminal
Seed colour, yellow or green
Seed shape, round or wrinkled
Pod shape, inflated or constricted
Pod colour, green or yellow
Stem height, tall or short.
Mendel needed to control fertilisation. Self-fertilisation was ensured by placing a bag over the flowers to make sure pollen from the stamens lands on the carpel of the same flower. Cross-fertilisation was ensured by cutting off stamens from a flower before pollen was produced, then dusting the carpel of the flower with pollen from another plant. To ensure reliability, Mendel used thousands of plants in each experiment.
Mendel worked with true-breeding plants: self-fertilised plants which produced all offspring identical to the parents.
Mendel first cross-fertilised two true-breeding plants for one characteristic, for example tall plants were crossed with short plants (Mendel called these plants a P1 parent generation).
The offspring produced are called F1 (1st filial) generation.
The F1 generation were then self-fertilised or cross-fertilised to produce a second generation, F2.
Each of the seven traits that Mendel studied had a dominant and a recessive factor. When two true-breeding plants were crossed, only the dominant factor appeared in the first generation. The recessive factor appeared in the second generation in a 3:1 (dominant : recessive) relationship.
Describe the aspects of the experimental techniques used by Mendel that led to his success.
Mendel was successful because he:
used peas, which were easily grown and produced successive generations rapidly
selected easily observable characteristics
strictly controlled the fertilisation process
used mathematics rigorously to analyse his results
used large numbers of plants
studied traits that had two easily identified factors.
Describe outcomes of mono hybrid crosses involving simple dominance, using Mendel’s explanations.
Monohybrid crosses involve one factor only. For example, a cross might involve a true breeding (homozygous) tall plant crossed with a true breeding (homozygous) short plant. This produces a first generation where all of the plants are tall. Mendel explained the first generation trait as the dominant factor.
Distinguish between homozygous and heterozygous genotypes in monohybrid crosses.
Alternative factors for the same characteristics are termed alleles, e.g. tall and short are alleles for height; purple and white are the alleles for flower colour.
For each characteristic there are at least two factors controlling the phenotype. Gametes (sperm and egg) from each parent contain only one factor. When gametes are formed the pair of factors segregate (separate).
Factors that are the same are termed homozygous, e.g. TT and tt.
Factors that are different are termed heterozygous, e.g. Tt.
In the heterozygous condition the factor that is fully expressed is termed dominant and the factor that has no noticeable effect is called the recessive.
Distinguish between the terms allele and gene, using examples.
An allele is an alternative for a particular inheritable characteristic, e.g. tall (T) and short (t) are two alleles for the characteristic of height in some plants. In humans, straight (S) and widow’s peak (s) are two alleles for hairline. With the development of modern genetics, we now identify these factors as genes. A gene is a section of DNA coding for proteins that expresses itself as the phenotype of an organism. Alleles are alternative forms of a gene. In many sources you will find the terms allele and gene used to mean the same thing.
Explain the relationship between dominant and recessive genes and phenotype using examples.
Phenotype is the outward appearance of an organism. The genotype is the actual alleles that are present on the chromosomes of the organism. A homozygous tall plant would have two identical alleles for height (TT) and would appear tall.
A heterozygous tall plant would have the phenotype of a tall plant but would have non-identical alleles (Tt). In this case, tall is dominant and short is recessive and is not expressed. The following diagram shows the results of crossing two heterozygous plants.
Outline the reasons why the importance of Mendel’s work was not recognised until some time after it was published.
Mendel began his work in 1858 and published the results of his experiments in 1866, but his work lay undiscovered until 1900 when others performed similar experiments. It was only then that the importance of his work was realized. It is unclear why such original work went unnoticed, perhaps:
Mendel was not a recognized, high profile member of the scientific community
he presented his paper to only a few people at an insignificant, local, scientific meeting
other scientists did not understand the work or its significance.
