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Senin, 26 Januari 2009

Mendel's Law of Segregation

How are traits passed from parents to offspring? The answer is by gene transmission. Genes are located on chromosomes and consist of DNA. They are passed from parents to their offspring through reproduction. The principles that govern heredity were discovered by a monk named Gregor Mendel in the 1860's. One of these principles is now called Mendel's law of segregation.


Mendel worked with pea plants and selected seven traits to study that each occurred in two different forms. For instance, one trait he studied was pod color. Some pea plants have green pods and others have yellow pods. Since pea plants are capable of self fertilization, Mendel was able to produce true-breeding plants. A true-breeding yellow-pod plant for example would only produce yellow-pod offspring. Mendel then began to experiment to find out what would happen if he cross-pollinated a true-breeding yellow pod plant with a true-breeding green pod plant. He referred to the two parental plants as the parental generation (P generation) and the resulting offspring were called the first filial or F1 generation.

Law of Segregation

When Mendel performed cross-pollination between a true-breeding yellow pod plant and a true-breeding green pod plant, he noticed that all of the resulting offspring, F1 generation, were green.

(Figure A) Image Credit: Steve Berg, used with permission.

He then allowed all of the green F1 plants to self-pollinate. He referred to these offspring as the F2 generation. Mendel noticed a 3:1 ratio in pod color. About 3/4 of the F2 plants had green pods and about 1/4 had yellow pods.

(Figure B) Image Credit: Steve Berg, used with permission.

From these experiments Mendel formulated what is now known as Mendel's law of segregation. This law states that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization. There are four main concepts involved in this idea. They are:

1. There are alternative forms for genes. This means that a gene can exist in more than one form. For example, the gene that determines pod color can either be (G) for green pod color or (g) for yellow pod color.

2. For each characteristic or trait organisms inherit two alternative forms of that gene, one from each parent. These alternative forms of a gene are called alleles. The F1 plants in Mendel's experiment each received one allele from the green pod parent plant and one allele from the yellow pod parent plant. True-breeding green pod plants have (GG) alleles for pod color, true-breeding yellow pod plants have (gg) alleles, and the resulting F1 plants have (Gg) alleles.

(Figure C) Image Credit: Steve Berg, used with permission.

3. When gametes (sex cells) are produced, allele pairs separate or segregate leaving them with a single allele for each trait. This means that sex cells contain only half the compliment of genes. When gametes join during fertilization the resulting offspring contain two sets of alleles, one allele from each parent. For example, the sex cell for the green pod plant had a single (G) allele and the sex cell for the yellow pod plant had a single (g) allele. After fertilization the resulting F1 plants had two alleles (Gg).

4. When the two alleles of a pair are different, one is dominant and the other is recessive. This means that one trait is expressed or shown, while the other is hidden. For example, the F1 plants (Gg) were all green because the allele for green pod color (G) was dominant over the allele for yellow pod color (g). When the F1 plants were allowed to self-pollinate, 1/4 of the F2 generation plant pods were yellow. This trait had been masked because it is recessive. The alleles for green pod color are (GG) and (Gg). The alleles for yellow pod color are (gg).

(Figure D) Image Credit: Steve Berg, used with permission
Genotype and Phenotype
From Mendel's law of segregation we see that the alleles for a trait separate when gametes are formed (through a type of cell division called meiosis). These allele pairs are then randomly united at fertilization. If a pair of alleles for a trait are the same they are called homozygous. If they are different they are called heterozygous. In the first example (Figure A), the F1 plants were all heterozygous for the pod color trait. Their genetic makeup or genotype was (Gg). Their phenotype or expressed physical trait was green pod color.
The F2 generation pea plants showed two different phenotypes (green or yellow) and three different genotypes (GG, Gg, or gg). The genotype determines the phenotype that is expressed. The F2 plants that had a genotype of either (GG) or (Gg) were green. The F2 plants that had a genotype of (gg) were yellow. The phenotypic ratio that Mendel observed was 3:1, 3/4 green plants to 1/4 yellow plants. The genotypic ratio however was 1:2:1. The genotypes for the F2 plants were 1/4 homozygous (GG), 2/4 heterozygous (Gg), and 1/4 homozygous (gg).

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