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9.3 Blueprint of life: 2. Gregor Mendel's
experiments
Extract from Biology Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW
[Edit: 6 Oct05]
Prior learning: Preliminary module 8.3 (subsection 7)
outline the experiments carried out by Gregor Mendel
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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.
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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:
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used peas, which were easily grown and produced successive generations rapidly
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selected easily observable characteristics
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strictly controlled the fertilisation process
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used mathematics rigorously to analyse his results
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used large numbers of plants
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studied traits that had two easily identified factors.
describe outcomes of monohybrid 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.
Thus, Mendel was able to explain his observed ratios, i.e.:
F1 - all tall
F2 - 3 tall : 1 short
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.
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.
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.
solve problems involving monohybrid crosses using Punnett squares or other appropriate techniques
A monohybrid cross involves the inheritance of one characteristic. All problems apply Mendel's basic laws of inheritance. The following is typical of a problem that uses Punnett squares to solve problems involving monohybrid crosses.
Worked example
In peas, the gene for the characteristic tall (T) is dominant over the gene for a short plant (t). If a homozygous tall plant (TT) is crossed with a heterozygous tall plant (Tt), what will be the possible phenotypes of the offspring?
A Punnett square is a diagrammatic method of indicating the possible offspring produced from a particular cross.
In the sample problem, a homozygous tall plant (TT) is crossed with a heterozygous tall plant (Tt), By filling in the squares, it is possible to work out all of the combinations that are likely to occur.
When you analyse the information in this case, you can predict that 100% of the offspring will be tall plants: 50% are homozygous tall (TT); 50% will be heterozygous tall plants (Tt).
In gerbils, agouti (light brown) coat colour is dominant to black coat colour. If a gerbil, which is heterozygous for agouti coat-colour, is crossed with a black-coated mouse, what will be the possible coat colour of the offspring?
Here are some other useful sites that describe how to solve monohybrid crosses using Punnett squares or other appropriate techniques:
Examples of Punnett squares 
Access
Excellence>
Mendel's experiments Access
Excellence
Mendel's discoveries Bill
Kendrick
perform an investigation to construct pedigrees or family trees, trace the inheritance of selected characteristics and discuss their current use
Background
A pedigree is a family tree showing a line of descent. It can be used to trace the occurrence of inherited traits in parents and offspring through a number of generations.
By convention, circles represent females and squares, males. A line between a square and a circle represents a union and a line down indicates offspring from the union.Filled in symbols represent individuals displaying the phenotype being studied. For example:
In pattern 1, the son and father are both affected. This is a reasonable indication that the characteristic is dominant. An affected offspring must have at least one affected parent if the phenotype is dominant. Other features of pedigrees of a dominant trait are:
- Heterozygous individuals will be affected
- Two affected parents can produce an unaffected child (both parents would be heterozygous)
In pattern 2, the daughter is affected but neither parent is. This can only happen if the characteristic is recessive and the offspring are homozygous, e.g. bb. Both parent must be heterozygous, Bb. Other features of pedigrees of a recessive trait are:
- Heterozygous parents will be unaffected
- Two affected parents will always have an affected child.
Pedigrees are valuable tools in genetic counselling. It allows a pattern of inheritance to be traced throughout generations of a family. This can allow identification of the genetic disease and advice can be made available on the probability of a couple having an affected child. Cystic fibrosis is an example of a recessive genetic disease. Huntington's chorea is an example of a dominant genetic disease.
- Perform a first-hand investigation using your own family to draw a family tree. Then look at some inherited characteristics such as those listed below.
| Characteristic | Dominant form | Recessive form |
|---|---|---|
| ear lobes | free | attached |
| hair | straight | curly |
| dimples | present | absent |
| tooth gap | no gap | gap |
| mid digital hair | hair present | hair absent |
| widow's peak | present | absent |
| eyelashes | long | short |
process information from secondary sources to identify and describe an example of hybridisation within a species and explain the purpose of this hybridisation
Background
Hybridisation means the breeding of two different types of plants or animals. For example, a mule is the result of the union between a horse and a donkey, two different species. The resulting animal has desirable characteristics from both parents but all mules are sterile and cannot produce any offspring. Hybridisation also occurs between different varieties or breeds within a species, such as dog, cattle or sheep breeds. Many, probably most agricultural animals and plants are the result of hybridisation. This results in offspring with desirable characteristics e.g. cross breeding cattle to produce better meat or to be tick resistant and Triticale a grain that is a cross between wheat and rye, two different species. Hybridisation is a good way of producing new commercial plants and animals.
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Process information from secondary sources, such as Biology text-books or the Internet, to identify and describe an example of hybridisation within a species and explain the purpose of the hybridisation.
Here are some good starting points.Why Borders?
Australian Border Leicester Association
Cattle breeds: Braford Agriculture, NSW Department of Primary Industries
Silky Terrier
The Dog Pages
outline the reasons why the importance of Mendel's work was not recognized 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.
Answer:
In gerbils, agouti (light brown) coat colour is dominant to black coat colour. If a gerbil, which is heterozygous for agouti coat-colour, is crossed with a black-coated mouse, what will be the possible coat colour of the offspring?
Call the black coated mouse bb and the agouti mouse Bb where B stands for the agouti colour (light brown) and b stands for black coat. Draw a Punnett square containing the alleles for coat colour.
Now fill in the squares.
The result would be 50% agouti and 50% black coated gerbils.