Mendel was not the first person to study how traits were passed between generations, however, he was the first to take a careful and quantitative look at the situation. In doing so, he made a number of inferences that continue to have a profound influence on how we think about the transmission of information between generations. He didn't know about DNA, genes, or chromosomes, but his insightful studies laid the groundwork for their discovery.
Mendel quantitatively analyzed how specific traits behaved between generations and concluded that all offspring have two factors for a given character: one was received from the father and one was received from the mother. For a given character (e.g., flower color), one trait was always expressed (i.e., purple was dominant), whereas the other trait might recede ("disappear") between generations (i.e., white was recessive). In other words, recessive traits are only expressed if the two factors are the same (in the homozygous state), whereas a dominant trait will be expressed even if only one factor is present (in the heterozygous state).
Mendel proposed that the discrete units for a given character would segregate away from one another during gamete production (e.g., individuals that were heterozygous with purple flowers could produce gametes with either a white or purple factor, but not both). This is often referred to as the Law of Segregation. Mendel looked at seven different characters and found that each character behaved independent of the other. He concluded that the factors associated with different characters assorted independently (in a random fashion) from one another. This is often referred to as the Law of Independent Assortment. You should be able to relate both the Law of Segregation and the Law of Independent Assortment to the movement of chromosomes during Meiosis I.
It has been about 150 years since Mendel made his observations and inferences. In the years since, geneticists have validated his basic conclusions and we now know they describe the fundamentals of genetics. While we now refer to "factors" as genes, Mendel was the first to predict their presence and behavior between generations. His basic conclusions are as correct today as they were when he first proposed them, and his work spurred the work of thousands of geneticists.
The image below illustrates the relationship between genotypes and phenotypes, with flower color used as an example. An organism's genotype is its genetic makeup for a particular trait; that is, the alleles it has for a particular gene. The phenotype is the organism's appearance; that is, the trait produced by the alleles the individual has. How does the genotype for a trait lead to the phenotype? The different alleles at a gene differ in their DNA sequences, thereby producing different enzymes when transcribed into RNA and translated into a protein. These enzymes can have different levels of activity; therefore, the organism's phenotype will differ depending on the enzymes present. The manner in which gene products interact to determine the phenotype is often complex.
In peas, the character "seed shape" has two traits: the dominant round seed and the recessive wrinkled seed. Pea seeds contain starch, a polymer of sugar molecules. When the seed is developing, sugar in the seed is converted to starch by enzymes. The dominant allele R codes for a functional enzyme that plays a role in starch biosynthesis. The recessive allele r , however, has a defect in its DNA sequence and is nonfunctional.
In individuals with two copies of the recessive allele ( rr ), sugar accumulates in the developing seed because starch synthesis is incomplete. This accumulation of sugar results in the seeds taking up more water than "normal" seeds. When the seeds dry out at the end of their development, rr seeds lose more water than RR or Rr seeds, resulting in a wrinkled appearance. Heterozygous ( Rr ) individuals have one functional copy of the starch synthesis gene and resemble the dominant homozygote because they are able to convert sugar to starch within their seeds.
Mendel and Modern Genetics Part 1 VoiceThread Transcript
Traits with simple dominant/recessive expression patterns, where one trait is completely dominant over another, such as those we have discussed so far, are called Mendelian traits . Mendelian traits are found in humans, and include non-disease examples such as attached versus unattached earlobes, mid-digital hair, bent thumbs, and a few other traits. These traits are useful for understanding the basic principles of genetics, but as you will learn in upcoming tutorials, very few traits are actually inherited in this simple manner.