Science at a Distance
a check up
Use this department to check up on the accuracy of your lecture notes. Make sure that you have written down the following definitions, explanations and important concepts in your notes.
Mendel - Part One
A phenotypic trait is a small, discrete part of the outward, physical manifestation of an organism.
- Mendel chose varieties of pea plants whose purity was certain, and which had bred consistently for many generations under very stringent conditions.
- Mendel chose varieties of pea plants for his experiments that differed from one another in a limited number of characteristics.
- Mendel picked those traits that could be distinguished "with sharp and certain separation" from each other. For example the color of the seeds in one type of plant were green, and in a second plant the color of the seeds was yellow.
- Seed color was, therefore considered a "sharp" phenotypic trait that could be used in his experiments.
The collection and configuration of all the genes coding for all the phenotypic traits of an organism.
- Each of the traits studied by Mendel are controlled by two genes.
- Some of Mendel's plants had red flowers, some had white flowers, in each case the color of the flower was controlled by two genes.
- The simple binary nature of this phenomenon made it possible for Mendel to represent the form of these two genes using letters; an uppercase letter (R, A, T, etc.) denoted the usual form of the gene, whereas a lowercase letter (r, a, t, etc.) denoted an unusual form of the gene.
- Today we commonly call the usual form of the gene the "wild type" and the unusual form of the gene the "mutant".
In simple Mendelian traits, paired genes occur in three distinct combinations; e.g. RR, Rr and rr.
- When an organism contains two genes that are in the same configuration, e.g. RR, or rr, this is said to be a homozygous state.
- When an organism contains two genes for the same trait, but in different combinations, e.g. Rr, this is said to be a heterozygous state.
Dominant and Recessive
In some situations, the products of one form of the gene mask the effects of the alternate form of the gene when they are in a heterozygous combination.
- If the "R" gene codes for a "red" flower, and the "r" gene is a mutant form of this gene which cannot produce the red pigment, a combination of these two gene would be heterozygous; e.g. Rr.
- If the plant having this combination of genes also had red flowers then Mendel said that the "R" form of the gene was dominant and the "r" form of the gene was recessive.
- Because of this phenomenon, a red-flowered plant could have the genotype RR or Rr. It would be impossible from external observation to determine the correct genotype.
- Only plants with the genotype rr would have white flowers under these circumstances.
Patterns of Inheritance
Mendel was successful in deducing the nature of a plant's genotype because he understood how the patterns of inheritance could be explained using a binary system of controlling genes (or "elementes" as he called them).
- Mendel may not have been the first to see that phenotypic traits show "patterns of inheritance", but he was the first to interpret them correctly.
- He studied what happened to a trait such as flower color as it was inherited through at least three generations; parents to children to grandchildren.
- He conducted his experiments using large numbers of pea plants in each generation, and he counted the number of offspring that showed a particular trait in each generation.
- He expressed his results as ratios; i.e. the ratio between the offspring that showed red flowers to those that showed white flowers. When the total number of offspring was large, these ratios came close to whole numbers (e.g. 3:1).
- The ratios Mendel saw in the patterns of inheritance could be interpreted using a simple model, where each trait was controlled by two "elementes" (genes) that had two forms, (R and r).
Diploid and Haploid
Cells or organisms that contain two sets of chromosomes, DNA and genes are said to be Diploid whereas cells or organisms that only contain a single set of chromosomes, DNA and genes are said to be Haploid
- The adult forms of Mendel's pea plants contain two sets of genes for each trait; they are diploid.
- The sex organs of the plant are in the flowers.
- Male sex organs, called anthers, produce the male sex gametes which are produced in very large numbers and housed in pollen grains.
- The female sex organ, called the ovary, holds the female gametes or ovules (egg cells).
- After meiosis, each of the sex gametes (male and female) only contain one set of genes for each trait; they are haploid.
- When a male gamete fuses with a female gamete at the time of fertilization, the zygote is a combination of the two sets of genes from each parent; it is once again diploid.
Science at a Distance
© 1997, Professor John Blamire