Genes, Traits and Mendel's Law of Segregation

Mendelian inheritance of colour of flower in the culinary pea, 1912.
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How are traits passed from parents to offspring? The answer is by gene transmission. Genes are located on chromosomes and consist of DNA. These are passed from parents to their offspring through reproduction.

The principles that govern heredity were discovered by a monk named Gregor Mendel in the 1860s. One of these principles is now called Mendel's law of segregation, which states that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization.

There are four main concepts related to this principle:

  1. A gene can exist in more than one form or allele.
  2. Organisms inherit two alleles for each trait.
  3. When sex cells are produced by meiosis, allele pairs separate leaving each cell with a single allele for each trait.
  4. When the two alleles of a pair are different, one is dominant and the other is recessive.

Mendel's Experiments With Pea Plants

Cross pollination of peas diagram
Steve Berg

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.

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, the F1 generation, were green.

The F2 Generation

F1 Plant Self-Pollination
Steve Berg

Mendel 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. From these experiments, Mendel formulated what is now known as Mendel's law of segregation.

The Four Concepts in the Law of Segregation

F1 Plants
Steve Berg

As mentioned, Mendel's law of segregation states that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization. While we briefly mentioned the four primary concepts involved in this idea, let's explore them in greater detail.

#1: A Gene Can Have Multiple Forms

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: Organisms Inherit Two Alleles for Each Trait

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.

The Law of Segregation Concepts Continued

Dominant and Recessive Traits
Steve Berg

#3: Allele Pairs Can Separate Into Single Alleles

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 complement 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: The Different Alleles in a Pair Are Either Dominant or Recessive

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. This is known as complete dominance.

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).

Genotype and Phenotype

Genetics Cross
(Figure A) Genetics Cross Between True-Breeding Green and Yellow Pea Pods. Credit: Steve Berg

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 heterozygous.

The F1 generation plants (Figure A) are all heterozygous for the pod color trait. Their genetic makeup or genotype is (Gg). Their phenotype (expressed physical trait) is green pod color.

The F2 generation pea plants (Figure D) show two different phenotypes (green or yellow) and three different genotypes (GG, Gg, or gg). The genotype determines which phenotype is expressed.

The F2 plants that have a genotype of either (GG) or (Gg) are green. The F2 plants that have a genotype of (gg) are 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).