Mitosis and meiosis

Meiosis has the job of creating random assortments of chromosomes in haploid cells (1N). To do this the cells go through two successive divisions. At the first homologous chromosomes separate. This separation is random, so humans can have over 8 million possible types. In addition, during the first Prophase recombination between chromosomes alters the linkage between maternal and paternal versions of genes

So the genetic diversity in a population partly arises simply by mixing up the kinds of genes (mutational types) by meiosis

In this section we will look at the ways that this process affects distribution of traits by looking at the genetic outcome of meiosis as first studied by Gregor Mendel. The science which deals with this issue is called genetics.

The term "genetics" was actually introduced in 1905 by William Bateson who was one of the rediscoverers of Mendels work (he was the first to show that Mendels concepts could be applied to an animal-chickens).

Mendel was a monk in a monastery in Brünn nin what is now the Czech Republic (more specifically, Moravia) but was then, in the mid-19th century part of Austria. He became interested in the nature of inheritance, and performed experiments with the system which was easiest for him to use, the garden pea.

These experiments were eventually presented to scientific societies and published in 1866. They were not really appreciated for several decades, and were rediscovered by Bateson, among others, who published a book describing his results in 1909.

Mendel was not a member of the scientific elite of his day, though they were aware of his work through its publication. It's likely that they were not able to fully appreciate the work. Bateson, who was a professor at Cambridge University, was a member of that elite, and was able to popularize his concepts and provide additional support for them

Mendel did experiments in what was termed "hybridization" at the time

He studied what happened when "true breeding" plants were crossed to each other. True breeding plants are plants that always produce offspring that look the same, e.g. plants with yellow peas producing more plants with yellow peas

People thought at the time that hereditary information from each parent were mixed with each other in their offspring. Mendel's leap was to imagine that heredity consisted of units

Hereditary units could be associated with particular observable traits (e.g, those yellow peas). Each parent contributed one unit to the offspring, so there were two of each-one maternal and one paternal

How did Mendel come to this hypothesis?
It is hard to know since the abbot that succeeded Mendel was a lifelong rival who opposed Mendel's scientific experiments. The new abbot destroyed all of Mendel's notebooks on his death! One conjecture is that Mendel noted that there was one obvious phenotype which had an "either/or" nature: gender

Offspring of any animal, including humans, are either male or female. They didn't show a "mixture" of sexual characteristics. In large populations of animals the ratio of male to female is 1:1. This suggests a simple model of sex determination in which one of the sexes carries a single unit which determines maleness (or femaleness). A child receiving that unit was that gender, one not receiving it was the other

Mendel wanted to demonstrate the existence of such a unit. His work didn't directly involve gender since pea plants do not have a gender (their flowers have both male and female aspects). Instead, he studied other observable traits: flower color, plant height, flower position, pod or pea color, pea shape, etc. With these simple observable traits he was able to demonstrate that genetic units existed, and that they were inherited in pairs, one from each parent.

Some genetic nomenclature

To discuss Mendel's results it helps to use the genetic nomenclature (also largely invented by Bateson)

• Gene: a unit of hereditary information; each is at a unique location on a chromosome, also called a locus.
• Allele: genes can come in various forms which carry distinct information-each distinct form is called an allele.
  • A gene concerned with pea color might specify yellow versus green
  • A particular form of the gene specifying green would be called an allele
  • There can be multiple forms which have give the same observable trait; each is a unique allele
• Phenotype vs. genotype: the observable effect of an allele is its phenotype; genotype is just the nature of the genes carried by an individual (for example, identifying what alleles he carries).
• Homozygous vs. heterozygous: since each individual carries two of these alleles, they can either be identical (homozygous) or different (heterozygous)
  • "Homo" means same while "hetero" means different
• Dominant vs. recessive: if an individual carries two alleles with different phenotypes (e.g., yellow peas versus green peas) he can not express both of them-they are mutually exclusive
  • Most of the time one is expressed to the exlusion of the other; the one whose phenotype is expressed is dominant (e.g., when a yellow and green allele are present the peas appear yellow-yellow is dominant).
  • The allele whose phenotype is not expressed is recessive (green is recessive)

In addition to these genetic terms there are some common genetic symbols:

• Dominant and recessive genes are represented by uppercase and lowercase letters, respectively
  • The dominant allele might be referred to as "A" while the recessive is symbolized by an "a"
  • A dominant homozygote would be AA, a recessive homozygote is aa; the heterozygote is Aa
  • Another gene could be symbolized as B and b
• To track inheritance the generations of a genetic cross are termed:
  • P parental generation
  • F1 first-generation offspring
  • F2 second-generation offspring