Mendel's Law of Independent Assortment

The principles that govern heredity were discovered by a monk named Gregor Mendel in the 1860's. One of these principles, now called Mendel's law of segregation, states that the alleles for a trait separate when gametes are formed. These allele pairs are then randomly united at fertilization. Mendel arrived at this conclusion by performing monohybrid crosses. These were cross-pollination experiments with pea plants that differed in one trait, for example pod color.

Mendel began to wonder what would happen if he studied plants that differed in two traits. Would both traits be transmitted to the offspring together or would one trait be transmitted independently of the other? From his experiments Mendel developed the principle now known as Mendel's law of independent assortment.


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Mendel's Law of Independent Assortment


Mendel performed dihybrid crosses (mating of parent plants that differ in two traits) in plants that were true-breeding for two traits. For example, a plant that had green pod color and yellow seed color was cross-pollinated with a plant that had yellow pod color and green seeds. In this cross, the traits for green pod color (GG) and yellow seed color (YY) are dominant. Yellow pod color (gg) and green seed color (yy) are recessive.


The resulting offspring or F1 generation were all heterozygous for green pod color and yellow seeds (GgYy).

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

Mendel then allowed all of the F1 plants to self-pollinate. He referred to these offspring as the F2 generation. Mendel noticed a 9:3:3:1 ratio. About 9 of the F2 plants had green pods and yellow seeds, 3 had green pods and green seeds, 3 had yellow pods and yellow seeds and 1 had a yellow pod and green seeds.

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

Genotype and Phenotype

In Mendel's experiment with pod color and seed color (Figure A) we see that the genotype or genetic makeup of the F1 plants is GgYy. The phenotypes or expressed physical traits are green pod color and yellow seed color. Both of these traits are dominant.

The F2 generation pea plants (Figure B) show two different phenotypes for each trait. Pod color is either green or yellow and seed color is either yellow or green. There are nine different genotypes:

F2 Genotypes (F2 Phenotypes)
GGYY, GGYy, GgYY, GgYy
(Green pod, Yellow seeds)
GGyy, Ggyy
(Green pod, Green seeds)
ggYY, ggYy
(Yellow pod, Yellow seeds)
ggyy
(Yellow pod, Green seeds)

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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.

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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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