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Ch. 11: Introduction to GeneticsMendel
11-1 The Work of Gregor Mendel
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Traits are characteristics that living things have. They are used to identify living things and to group them. Scientists put living things having the same group traits in the same group.
For example, a large mammal that had a trunk would be identified as an elephant. All living things that had these similar traits – large size, being a mammal, having a trunk – would also be identified as elephants.
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Organisms within a group share traits but no two are exactly alike. All organisms have individual differences or individual traits.
All elephants are large, but some may be larger than others.
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Many of an organism’s traits, whether they are group or individual traits, are inherited. That is, they are determined by factors that are passed from parents to offspring.
The study of genetics, which is the study of heredity or the delivery of characteristics from parent to offspring, will help us to explain HOW traits are passed from generation to generation.
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11-1 The Work of Gregor Mendel
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Gregor Mendel’s Peas
Gregor Mendel was an Austrian monk whose work was important to the understanding of heredity.
Mendel carried out his work with ordinary garden peas. Peas are small and easy to grow. A single pea plant can produce hundreds of offspring.
Today we call peas a “model system.”
Gregor Mendel’s Peas
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11-1 The Work of Gregor Mendel
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Gregor Mendel’s Peas
Mendel knew that
• the male part of each flower produces pollen, (containing sperm).
• the female part of the flower produces egg cells.
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Male parts: stamen Female parts: pistil
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Gregor Mendel’s Peas
During sexual reproduction, sperm and egg cells join in a process called fertilization.
Fertilization produces a new cell. In peas, this new cell develops into a tiny embryo encased within a seed.
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Gregor Mendel’s Peas
Pea flowers are self-pollinating.
Sperm cells in pollen fertilize the egg cells in the same flower.
The seeds that are produced by self-pollination inherit all of their characteristics from the single plant that bore them.
Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce offspring identical to themselves.
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Gregor Mendel’s Peas
Mendel wanted to “cross” his stocks of true-breeding plants by joining male and female reproductive cells from two different plants.
He cut away the pollen-bearing male parts of the plant and dusted the plant’s flower with pollen from another plant.
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11-1 The Work of Gregor Mendel
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Gregor Mendel’s Peas
This process is called cross-pollination.
Mendel was able to produce seeds that had two different parents. The offspring, then, would have traits different from its parents.
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Genes and Dominance
Genes and Dominance
Mendel studied seven pea plant traits, each with two contrasting characters or forms.
For example, he studied plant height (the trait), which had 2 different characters, either short or tall.
One by one, Mendel crossed plants with contrasting characters for each of the seven traits. He then studied their offspring.
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Genes and Dominance
Each original pair of plants is said to be the P (parental) generation.
The offspring are called the F1, or “first filial,” generation.
The offspring of crosses between parents with different traits are called hybrids.
Mendel found that the F1 hybrid plants of his crosses all had the character of only one of the parents.
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Genes and Dominance
Mendel’s F1 Crosses on Pea Plants
From p. 264
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Genes and Dominance
Mendel’s Seven F1 Crosses on Pea PlantsMendel’s F1 Crosses on Pea Plants
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Genes and Dominance
Mendel's first conclusion was that inheritance is determined by “factors” that are passed from one generation to the next.
Today, scientists call the “factors” that determine traits genes.
Mendel’s Conclusions
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A gene is a portion of a chromosome that determines a certain trait.
Remember, chromosomes are found in the cell nucleus and are made up of DNA and proteins.
Each chromosome contains many genes.
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Genes and Dominance
Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait.
Mendel was very lucky in the traits that he chose to study because most traits are determined by several genes working together. In humans, for example, the shape of the eyes or ears is polygenic, controlled by many genes. Also, for each gene, there are usually more than 2 contrasting forms.
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In any case, the different forms of a gene are called alleles.
In Mendel’s pea plants, each true-breeding parent contributed one allele (one form of the gene) at fertilization. The offspring, therefore, received a total of 2 alleles, one of each form (or character).
The tall plant, for example, contributed a tall allele and the short plant contributed a short allele. The offspring thus had one tall and one short allele.
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Mendel’s second conclusion resulted from his observations regarding the offspring of parents with different alleles. It is called the principle of dominance.
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Genes and Dominance
The principle of dominance states that some alleles are dominant and others are recessive.
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Genes and Dominance
Mendel observed that the offspring of his crosses had the character of only one of the parents. In the tall x short cross, for example, the offspring were all tall. He concluded that the tall trait was dominant and the short one was recessive.
He concluded that an organism with a dominant allele for a trait will always exhibit that form of the trait and that an organism with the recessive allele for a trait will exhibit that form only when the dominant allele for that trait is not present.
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All of the offspring in Mendel’s F1 generation received one dominant allele and one recessive allele. But, because the recessive trait is not exhibited due to the presence of the dominant allele, the plants exhibited only the dominant traits.
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As you will note in the previous slide, in genetics, dominant alleles are represented by an upper case letter (T) and recessive alleles are represented by a lower case one (t).
Since each individual has a total of 2 alleles (one from each parent), their genetic makeup for a trait is represented by two letters. For the Mendel’s F1 tall plants that would be Tt.
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Segregation
Segregation
Next Mendel crossed the F1 generation with itself to produce the F2 (second filial) generation.
The traits controlled by recessive alleles reappeared in one fourth of the F2 plants.
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Mendel's F2 Generation
P GenerationF1 Generation
Tall Tall Tall Tall Tall TallShort Short
F2 Generation
Segregation
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Segregation
Mendel had assumed that a dominant allele had masked the corresponding recessive allele in the F1 generation.
But how could he explain that the trait controlled by the recessive allele showed up in some of the F2 plants?
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Segregation
The reappearance of the trait controlled by the recessive allele indicated that at some point the allele for shortness had been separated, or segregated, from the allele for tallness.
That is, when the F1 tall plants were crossed, Tt x Tt, the short alleles (t) had somehow segregated from the tall alleles (T) and found each other (tt) to produce a short plant.
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Segregation
Mendel suggested that the alleles for tallness and shortness in the F1 plants segregated from each other during the formation of the sex cells, or gametes.
This is called the principle of segregation.
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Segregation
The alleles separated during gamete formation.
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Segregation
When each F1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene.
Therefore, each F1 plant produces two types of gametes—those with the allele for tallness, and those with the allele for shortness.
The F2 generation had new combinations of the alleles.