lesson 3: heredity and variation
Heredity is the study of how traits are passed on from the parents to the offspring. Genetics is the study of heredity and variation. An understanding of the field will help you understand why you have similarity with your parents.
Do you know the first person who worked on the topic inheritance? His name is Gregor Mendel, an Austrian by nationality and a monk by vocation. He worked and lived in a monastery with large gardens planted with different kinds of peas. Are you familiar with peas (chicharo)? These are how peas look like.
Why do you think Mendel chose green peas for his experiments? Well, peas have traits that are easily noticeable. They are easy to breed and grow, which make it easy to reproduce new generations of peas in a short period of time and in large numbers.
It has been observed that pea plant normally pollinates itself because it has perfect flowers, or flowers with both the male and female reproductive organs. The pollen grains from the same flower can pollinate the eggs cells or ovules from the same flower. However, to cross-pollinate, Mendel tried to remove the stamens while still young. Then, when the pistil matured, he dusted it with the pollen grains from another pea plant.
Mendel’s hypothesis based on his experiments
Based on his first experiment Mendel was able to hypothesize that there must be a factor in plants that controls the appearance of a trait. Recall that in the cross between pure breed round and wrinkled seeds, only the round trait came out. The wrinkled trait was hidden. This factor is now called a gene. Have you ever encountered this word before? Since there are two alternative expressions of a trait, he reasoned further that traits are controlled not only by one but a pair of factors now called alleles. This resulted to the formulation of his hypothesis which states that: In every organism, there is a pair of factors that control the appearance of a particular trait.
In Genetics, the two factors mentioned by Mendel are called alleles. These alleles could be dominant or recessive. These alleles maybe represented this way: a capital letter for the dominant trait, and a small letter for the recessive trait. We can use the first letter of either the dominant or recessive trait to represent a particular trait. For example, R for round seed and r for wrinkled seed. Or it could be regarding the color of pod where you can represent G for green pod and g for yellow pod. Do not confuse yourself with the use of the first letter. Remember the same letter will be used for both the dominant and recessive traits. They will only differ as to whether it will be capital letter or small letter.
The alleles making up a pair maybe identical or not. For example, for round it is RR and this type of allele is homozygous or homozygote. If the allele is Rr like the F1 generation, it is heterozygous or heterozygote. An individual maybe heterozygous for some traits and homozygous for others.
Today, the alleles which are represented by paired letters are referred to as the individual genotype. The genotype therefore, refers to the genetic composition of the person, while phenotype is the physical appearance or feature of the person. For example, RR allele is homozygous round in genotype and the phenotype is round. Is the explanation clear? I hope so.
You might ask, “Do factors or genes change as they are transferred from one generation to the next?”
Using Mendel’s experiment, the answer is no. “In successive generations, each factor is transmitted unchanged.” Remember that the factor for wrinkled in P1 generation is the same as that found in F1 generation. This means that individual factors do not mix as they are transmitted from generation to generation.
Another hypothesis formulated by Mendel is known as “The Law of Segregation”, which states that: “During gamete formation, the pair of factors segregate or separate from each other.” These factors are located in the chromosomes and are transmitted to the offspring through the egg cell and the sperm cell.
OBJECTIVES OF THE LESSON:
At the end of this lesson, learners are expected to demonstrate an understanding of: (1) the Mendelian Genetics;(2) Monohybrid and Dihybrid crosses; (3) Non- Mendelian Genetics; and (4) Sex-related traits.