Intro to Mendelian genetics

Most students of biology have heard the phrase “Mendelian genetics,” owing to Gregor Mendel’s famous pea plant breeding experiments in the mid-19th century. In 1915, Mendel’s principles of heredity were first applied to the fruit fly Drosophila melanogaster when “The Mechanism of Mendelian Heredity” was published by the original dream team of fly researchers (partly covered in a previous post). This post is a primer on the central principles of Mendelian genetics and their application to D. melanogaster — it can serve as a helpful guide for an undergraduate or graduate level biology student.

Mendelian genes and the laws of inheritance

Almost a century before the discovery of DNA, Mendel had worked out a rudimentary theory of the principles of heredity and the “units” of inheritance (what we today call genes) through his experiments on pea plants. This theory of inheritance consisted of two laws.

  • The Law of Segregation: This law refers to the fact that the two alleles of a gene will randomly segregate during meiosis, meaning that each resulting gamete will end up with one copy of each gene (assuming this is a diploid organism).
the law of segregation
The Law of Segregation of genes refers to the segregation of the two alleles of each gene during meiosis, when gametes are created. Image Source.
  • The Law of Independent Assortment: This law refers to the fact that alleles of a gene segregate randomly, with the segregation of one allele of a gene having no effect on the segregation of a different allele of a different gene. (Note: This is not technically true all the time. For example, linked genes (genes residing close together on the same chromosome) often segregate together due to their physical proximity. But this rule still holds true often enough to justify its being called a “law.”)
laws of Mendelian genetics
During meiosis in diploid organisms, the two alleles of a gene undergo both segregation and independent assortment, leading to a (theoretically) random distribution of alleles in the gametes that are produced. Image source.

Dominant vs Recessive alleles

The concepts of “dominance” and “recessivity” are used to describe the way that alleles of a particular gene get translated into phenotypes.1 Roughly speaking, for a given gene, if an individual possesses one dominant allele and one recessive allele, the dominant allele will manifest at the phenotype level; if the individual possesses two recessive alleles, then the recessive allele will manifest at the phenotype level. “Dominant” alleles always dominate “recessive” ones.

For example, let’s suppose that eye color is determined by one gene that has two different alleles, Brown or blue. If Bob possesses one dominant Brown allele and one recessive blue allele, he will have brown eyes.

Punnett Squares are used to diagram Mendelian genetic crosses

Reginald C. Punnett, a British geneticist, developed the Punnett square diagram in 1905 as a simple method for illustrating the phenotypic outcomes of a simple genetic cross.2

In the example below, a heterozygous green pea plant (bearing one dominant Green allele, G, and one recessive yellow allele, g) is crossed to a homozygous yellow pea plant (bearing two recessive yellow alleles). The outcome of this cross is 50% green pea plants and 50% yellow pea plants, owing to the random segregation of the dominant and recessive alleles.

simple Punnett square
Punnett square depicting a simple heterozygous dominant X homozygous recessive genetic cross. Image source.

The above example is simple and tracks just two different alleles of one gene, but in theory Punnett squares can be expanded to include as many gene or allele combinations as desired. Below is a Punnett square tracking two different alleles of three different genes (S, Y, and A), leading to a more complex range of possible phenotypes in the offspring.

complex Punnett square
A complex triple heterozygote X triple heterozygote genetic cross. Image source.

Mendelian genetics in Drosophila

So what does all of the above have to do with D. melanogaster? Due to the fly’s ease of maintenance and genetic manipulation, Mendel’s principles of inheritance were quickly uncovered and applied in Drosophila. Many of the newly generated fly mutations followed the same rules of random segregation and independent assortment as those described by Mendel in his pea plants, but there were a few exceptions. Below is a nice example of one such exception discovered by Thomas Hunt Morgan: the case of sex linkage.

The white gene, responsible for synthesizing the red pigment found in the compound eye, is located on the X chromosome. When a homozygous mutant white-eyed female (carrying a mutation in the white gene) is crossed to a red-eyed male (who only has one X chromosome, which carries a wild-type white gene), the resulting offspring phenotypes are perfectly correlated with sex: all females have wild-type red eyes, and all males have mutant white eyes. This is a classic example of a sex-linked trait that you’ll find in any course covering Mendelian genetics.

sex linkage in drosophila
Because all the male offspring inherited their X chromosome from their homozygous mutant white-eyed mother, they all have white eyes. Image Source.

References

  1. What are dominant and recessive? Available at: https://learn.genetics.utah.edu/content/basics/patterns
  2. Punnett, Reginald Crundall (1907). Mendelism (2 ed.). London, UK: Macmillan.

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