Before Mendel, the basic rules of heredity were not understood. For example, it was known that green-seeded pea plants occasionally produced offspring that had yellow seeds; but were the hereditary factors that controlled seed color somehow changing from one generation to the next, or were certain factors disappearing and reappearing? And did the same factors that controlled seed color also control things like plant height?
- 3.1: Mendel’s First Law
- Mendel’s First Law, also called The Law of Equal Segregation, states that during gamete formation, the two alleles at a gene locus segregate from each other; each gamete has an equal probability of containing either allele. More than one allele of a gene can be present in an individual since most eukaryotic organisms have at least two sets of homologous chromosomes. For organisms that are predominantly diploid, chromosomes exist as pairs, with one homolog inherited from each parent.
- 3.2: Relationships Between Genes, Genotypes and Phenotypes
- A specific position along a chromosome is called a locus and each gene occupies a specific locus; each locus will have an allelic form. The complete set of alleles (at all loci of interest) in an individual is its genotype. The visible or detectable effect of these alleles on the structure or function of that individual is called its phenotype
- 3.3: Biochemical Basis of Dominance
- For the majority of genes studied, the normal (i.e. wild-type) alleles are haplosufficient. So in diploids, even with a mutation that causes a complete loss of function in one allele, the other allele, a wild-type allele, will provide sufficient normal biochemical activity to yield a wild type phenotype and thus be dominant and dictate the heterozygote phenotype.
- 3.4: Crossing Techniques Used in Classical Genetics
- Mendel also invented several testing and analysis techniques still used today. Classical genetics is the science of solving biological questions using controlled matings of model organisms. It began with Mendel in 1865 but did not take off until Thomas Morgan began working with fruit flies in 1908. Later, starting with Watson and Crick’s structure of DNA in 1953, classical genetics was joined by molecular genetics, the science of solving biological questions using DNA, RNA, and proteins isolated
- 3.5: Sex-Linkage- An Exception to Mendel’s First Law
- In the previous chapter we introduced sex chromosomes and autosomes. For loci on autosomes, the alleles follow the normal Mendelian pattern of inheritance. However, for loci on the sex chromosomes this is mostly not true, because most of the loci on the typical X-chromosome are absent from the Y-chromosome, even though they act as a homologous pair during meiosis. Instead, they will follow a sex-linked pattern of inheritance.
- 3.6: Phenotypes May Not Be As Expected from the Genotype
- The phenotypes described thus far have a nearly perfect correlation with their associated genotypes; in other words an individual with a particular genotype always has the expected phenotype. However, many phenotypes are not determined entirely by genotype alone. They are instead determined by an interaction between genotype and non-genetic, environmental factors.
- 3.7: Phenotypic Ratios May Not Be As Expected
- For a variety of reasons, the phenotypic ratios observed from real crosses rarely match the exact ratios expected based on a Punnett Square or other prediction techniques. There are many possible explanations for deviations from expected ratios. Sometimes these deviations are due to sampling effects, in other words, the random selection of a non-representative subset of individuals for observation. On the other hand, it may be because certain genotypes have a less than 100% survival rate.
Thumbnail: Pea plants were used by Gregor Mendel to discover some fundamental laws of genetics. (Flicker-Christian Guthier-CC:A)