Lecture 2.3: The Modern Synthesis

Terms

• Gene
  • A term which is hard to define, especially as you learn more about genetics
  • For our purposes it can be thought of as something that is responsible for particular traits or characteristics of an organism.
• Chromosomes
  • Large structures that carry many genes
  • In many organisms they are found in pairs in the nucleus of the cell.
    • These pairs of chromosomes are called “homologous” because they are similar (the word “homo” means “same” or “similar”).
    • It is important to realize that these homologous pairs of chromosomes are similar but are not identical!

Unknown source, probably a textbook

• Locus (plural loci)
  • The place on the chromosome where the gene is located
  • Latin for “place”: from which we get our word location!
• Allele
  • Genes come in different “flavors.” These different “flavors” are the alleles.
  • Example: The gene for flower color may have two alleles—pink (represented by the letter A) and white (represented by the letter a).
• Genotype
  • The genetic makeup of an organism
  • It is defined specifically as “the collection of different alleles found at different gene loci.”
  • Example:
    • The genotype of flower #1 is AA.
    • The genotype of flower #2 is Aa.
    • The genotype of flower #3 is aa.

Unknown source

• Phenotype
  • The manifestation of the genotype
  • From the Greek word phanein: to show
    • Other words with the same root: phantom, epiphany, theophany, diaphanous.
  • Usually thought of as what you “see,” like pink flowers, white flowers, blue eyes, brown eyes, tall people, short people, etc.
    • However, it can also be the physiological manifestation of the genotype, like lactose intolerance, a tendency to have high cholesterol, etc.
  • Example: since the A allele is dominant over the a allele…
    • Flower #1: genotype is AA → phenotype is pink flowers
    • Flower #2: genotype is Aa → phenotype is pink flowers
    • Flower #3: genotype is aa → phenotype is white flowers

Top of Figure 23.3, page 431, Campbell's Biology 5th Edition

Summary

Genotype, phenotype, alleles, loci, chromosomes, etc.

• The genotype is the collection of alleles found at a particular locus on a pair of homologous chromosomes.
  • Warning: not all organisms have homologous pairs of chromosomes, but most plants and animals do. We will get into these details in the genetics unit.
• Alleles in an organism come in pairs because chromosomes come in pairs.
• Alleles can be dominant or recessive
  • Warning: not all alleles are either dominant or recessive! But for most beginning discussions of genetics, we assume this is the case.
• You cannot know what the genotype is by observing the phenotype (that is, by “just looking at the organism”).
  • In a sense, the alleles are playing “hide and seek” with us!

About the Warnings

• Don't worry about the warnings right now if they confuse you.
  • They are included for absolute accuracy and completeness.
  • They are included for those students who might benefit from them.
• If you assume that all chromosomes come in homologous pairs, and that alleles are either dominant or recessive, you will be fine for now.
  • In fact it is better to keep your thinking rather “simple” now so you can fully grasp the basic concepts.
  • Genetics can become very confusing, very fast!
  • Later in the course we will talk about the exceptions. I just didn’t want to leave you with any wrong impressions.