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An Introduction to Genetics: The Work of Gregor Mendel CHAPTER 11
Who was Mendel? Genetics is the scientific study of heredity. Gregor Mendel was an Austrian monk. His work was important to the understanding of heredity. Mendel carried out his work with ordinary garden peas.
Mendel knew that the male part of each flower produces pollen, (containing sperm). the female part of the flower produces eggs During sexual reproduction, sperm and egg cells join in a process called fertilization.
What work was done on pea plants? Pea flowers are self-pollinating: Sperm cells in pollen fertilize the egg cells in the same flower. Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce offspring identical to themselves. Mendel wanted to produce seeds 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.
This process is called cross-pollination. Mendel was able to produce seeds that had two different parents.
What is a trait? A trait is a specific characteristic that varies from one individual to another. Mendel studied seven pea plant traits, each with two contrasting characters. Mendel’s F 1 Crosses on Pea Plants
Each original pair of plants is the P (parental) generation. The offspring are called the F 1, or “first filial,” generation. The F 2 generation was the offspring of the F 1 generation.
What are Alleles? Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait. The different forms of a gene are called alleles. Since organisms get half of their genes from their mother and half from their father, there are two alleles for every trait.
An Example of Alleles Take a trait such as eye color. We will use red eyes in fruit flies. The alleles that will code for eye color will be: Red eyes: represented by R White eyes: represented by r The two possible alleles for eye color are R and r. Red eyes is the dominant trait (since it is uppercase) and white eyes is the recessive trait (since it is lowercase).
An Example of Alleles What are the three possible combinations of alleles? RR, Rr, rr If an organism has the alleles RR, what eye color will it have? red eyes If an organism has the alleles rr, what eye color will it have? white eyes If an organism has the alleles Rr, what eye color will it have? red eyes
An Example of Alleles Organisms that have two identical alleles for a particular trait (RR or rr in our example) are said to be homozygous. Organisms that have two different alleles for the same trait (Rr in our example) are said to be heterozygous.
An Example of Alleles Going back to our eye color example. The physical characteristic, such as red eyes or white eyes, is called the phenotype. The phenotype is what is actually visible. The genetic makeup (or what the alleles actually are), such as RR or rr, is the genotype. So, is it possible that two different flies can have the same phenotype but different genotypes??? Yes, one fly can be RR and the other fly Rr but both have red eyes.
What are the three basic “laws” of Mendel? There are three basic “laws” or conclusions from Mendel’s work on peas: 1.The Law of Dominance 2.The Law of Segregation 3.The Law of Independent Assortment.
The Law of Dominance The principle of dominance states that some alleles are dominant and others are recessive. An organism with a dominant allele for a trait will always exhibit that form of the trait. An organism with the recessive allele for a trait will exhibit that form only when the dominant allele for that trait is not present.
The Law of Segregation The law of segregation states that gene pairs segregate when gametes form. This means that, during meiosis, each pair of genes separates and half of an organisms gametes contain one gene and half of the gametes carry the other gene.
Segregation Alleles separate during gamete formation.
The Law of Independent Assortment The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. It means that the separation of genes is random and independent of each other. Independent assortment helps account for the many genetic variations observed in plants, animals and other organisms.
Review Questions 1.Describe how Mendel cross-pollinated plants. 2.How many alleles are there for every trait? Where do organisms get these alleles from? 3.What happens to alleles during segregation? 4.A female fruit fly that is heterozygous for eye color breeds with a male fruit fly that is homozygous dominant for eye color. What alleles do the male and female have?