Chapter 4-Sec. 1: (Mendel's Work)
The student will be able to…
1. …define the terms heredity and genetics.
2. …understand how Mendel's work helped scientists discover genetics.
First, write down the vocab words. Then, click below to learn more about cell reproduction.
the passing of physical characteristics to offspring
different forms of characteristic
the scientific study of heredity
the factors that control a trait
different forms of a gene
its trait always shows up in an organism
its trait is hidden if the dominant allele is there
In the mid nineteenth century, a priest named Gregor Mendel tended a garden in a central European monastery. Mendel’s experiments in that peaceful garden would one day revolutionize the study of heredity. Heredity is the passing of physical characteristics from parents to offspring.
Mendel wondered why different pea plants had different characteristics. Some pea plants grew tall, while others were short. Some plants produced green seeds, while others had yellow seeds. Each different form of a characteristic, such as stem height or seed color, is called a trait. Mendel observed that the pea plants’ traits were often similar to those of their parents. Sometimes, however, the plants had different traits from those of their parents.
Mendel experimented with thousands of pea plants to understand the process of heredity. Today, Mendel’s discoveries form the foundation of genetics, the scientific study of heredity.
Dominant and Recessive Alleles
Mendel reached several conclusions on the basis of his experimental results. He reasoned that individual factors, or sets of genetic “information,” must control the inheritance of traits in peas. The factors that control each trait exist in pairs. The female parent contributes one factor, while the male parent contributes the other factor. Finally, one factor in a pair can mask, or hide, the other factor. The tallness factor, for example, masked the shortness factor.
Genes and Alleles
Today, scientists use the word gene for the factors that control a trait. Alleles (uh leelz) are the different forms of a gene. The gene that controls stem height in peas, for example, has one allele for tall stems and one allele for short stems. Each pea plant inherits two alleles from its parents—one allele from the egg and the other from the sperm. A pea plant may inherit two alleles for tall stems, two alleles for short stems, or one of each.
An organism’s traits are controlled by the alleles it inherits from its parents. Some alleles are dominant, while other alleles are recessive. A dominant allele is onewhose trait always shows up in the organism when the allele is present. A recessive allele, on the other hand, is hidden whenever the dominant allele is present. A trait controlled by a recessive allele will only show up if the organism does not have the dominant allele.
In pea plants, the allele for tall stems is dominant over the allele for short stems. Pea plants with one allele for tall stems and one allele for short stems will be tall. The allele for tall stems masks the allele for short stems. Only pea plants that inherit two recessive alleles for short stems will be short.
Alleles in Mendel’s Crosses
In Mendel’s cross for stem height, the purebred tall plants in the P generation had two alleles for tall stems. The purebred short plants had two alleles for short stems. The F1 plants each inherited an allele for tall stems from the tall parent and an allele for short stems from the short parent. Therefore, each F1 plant had one allele for tall stems and one for short stems. The F1 plants are called hybrids. A hybrid (hy brid) organism has two different alleles for a trait. All the F1 plants are tall because the dominant allele for tall stems masks the recessive allele for short stems.
When Mendel crossed the F1 plants, some of the offspring in the F2 generation inherited two dominant alleles for tall stems. These plants were tall. Other F2 plants inherited one dominant allele for tall stems and one recessive allele for short stems. These plants were also tall. The rest of the F2 plants inherited two recessive alleles for short stems. These plants were short.
In fruit flies, long wings are dominant over short wings. A scientist crossed a purebred long-winged male fruit fly with a purebred short-winged female. Predict the wing length of the F1 offspring. If the scientist crossed a hybrid male F1 fruit fly with a hybrid F1 female, what would their offspring probably be like?
Symbols for Alleles
Geneticists use letters to represent alleles. A dominant allele is represented by a capital letter. For example, the allele for tall stems is represented by T. A recessive allele is represented by the lowercase version of the letter. So, the allele for short stems would be represented by t. When a plant inherits two dominant alleles for tall stems, its alleles are written as TT. When a plant inherits two recessive alleles for short stems, its alleles are written as tt. When a plant inherits one allele for tall stems and one allele for short stems, its alleles are written as Tt.
Significance of Mendel’s Contribution
Mendel’s discovery of genes and alleles eventually changed scientists’ ideas about heredity. Before Mendel, most people thought that the traits of an individual organism were simply a blend of their parents’ characteristics. According to this idea, if a tall plant and a short plant were crossed, the offspring would all have medium height.
However, when Mendel crossed purebred tall and purebred short pea plants, the offspring were all tall. Mendel’s experiments demonstrated that parents’ traits do not simply blend in the offspring. Instead, traits are determined by individual, separate alleles inherited from each parent. Some of these alleles, such as the allele for short height in pea plants, are recessive. If a trait is determined by a recessive allele, the trait can seem to disappear in the offspring.
Unfortunately, the importance of Mendel’s discovery was not recognized during his lifetime. Then, in 1900, three different scientists rediscovered Mendel’s work. These scientists quickly recognized the importance of Mendel’s ideas. Because of his work, Mendel is often called the Father of Genetics.
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