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Summary of chaper 2 of 'Genetics' VU amsterdam
- Instelling
- Vrije Universiteit Amsterdam (VU)
This summary covers complete chaper 2 of genetics.
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Chapter 2- Pangenesis: the first attempt to explain the transmission of hereditary traits from generation to
generation.
Mendel chose pea plants as his experimental organism. When two distinct individuals with different
characteristics are bred to each other, the experiment is called a cross, or a hybridization, and the offspring are
referred to as hybrids. Mendel was intrigued by the consistency with which offspring of subsequent
generations showed characteristics of one or the other parent. The pea plants that he used had several
properties that were advantageous for studying plant hybridization.
1. The species were available in several varieties, which varied in height and in the appearance of the
flowers, seeds and pods.
2. Another advantage was the ease of making crosses.
The term ‘gamete’ is used to describe haploid reproductive cells that fuse to form a zygote. In flowering plants,
reproduction occurs by a pollination event. Male gametes (sperm cells) are produced within pollen grains that
form in the anthers. The female gametes (egg cells) are produced within the ovules that form in the ovaries.
For fertilization to occur, a pollen grain lands on the stigma, which stimulates growth of a pollen tube. This tube
enables sperm cells to enter the stigma and migrate toward the ovule where the egg cells are made.
Fertilization takes place when a sperm enters the micropyle, an opening in the ovule wall, and fuses with the
egg cell.
In some experiments, Mendel allowed plants to reproduce by self-fertilization, which means that the pollen
and eggs are derived from the same plant. In peas, two modified petals are fused to form a keel that encloses
the reproductive structures of a flower. Because of this covering, pea plants naturally reproduce by self-
fertilization. Usually, pollination occurs even before the flower opens.
However, Mendel wanted to make crosses between different plants. Fortunately, pea plants contain large
flowers that are easy to manipulate, making it possible to cross two particular plants and study the outcomes –
cross-fertilization. This process requires that the pollen from one plant need to be placed on the stigma of
another plant. Mendel was able to pry open immature flowers and remove the anthers before they produced
pollen. Therefore, these flowers could not self-fertilize. He then obtained pollen from another plant by gently
touching its mature anthers with a paintbrush. Mendel applied this pollen to the stigma of the flowers that
already had the anthers removed. In this way, he was able to cross-fertilize his pea plants, thereby creating any
hybrid he wanted.
Mendel obtained several varieties of peas that were considered to be distinct. These plants had many different
morphological characteristics.
- Characters: these are the general characteristics of an organism.
- Trait (variant): this term is used to describe specific properties of a character.
For example, her eye colour (character) is brown (trait).
Breeding true means that a trait does not vary in appearance from generation to generation. For example, if
the seeds from a pea plant were yellow, the seeds from pea plants in the next generations would be yellow as
well. A variety that continues to produce the same trait after several generations of self-fertilization is called
true-breeding line, or true-breeding strain. Characters that Mendel studied were: height, flower colour, flower
position, seed colour, seed shape, pod colour and pod shape.
- Empirical approach: an evidence-based approach to a study and interpretation of information.
Empiricism believes that you gain knowledge and information by using your 5 senses and
experimenting. Mendel did this.
Mendel began with true-breeding plants that differed in a single character – parental generation (P
generation). Crossing true-breeding parents to each other, called a P cross, produces offspring (F1 generation)
that show the trait of one parent but not from the other parent. This prompted Mendel to follow the
transmission of this character for one additional generation (F2 generation). The plants from the F1 generation
were allows to self-fertilize to produce F2 generation plants.
For each of the seven characters, Mendel cross-fertilized two different true-breeding strains. They differ only
from one of the seven characters that Mendel studied. The F1 seeds were collected and they were allowed to
grow and self-fertilize. This produces plants from the F2 generation. The F2 seeds were collected and were
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