Athro, Limited Biology Genetics

It's Genetics...

Is is common to encounter news accounts of some new discovery that some trait is "Genetic" or that it is explained by genetics. Well what does it mean to say that something is caused by genetics, that it has a genetic basis? When we say that some trait has a genetic basis, we may mean any one of four very different things, that is, any of four different levels of understanding; anywhere from the very simple sense that there is some involvement of heredity, that the trait is inherited in some way, to a detailed understanding of the molecular basis of the trait.

"Genetic" means lots of things

1. Genetic = some portion of the trait is inherited in some way.
2. Genetic = the trait is controlled by some combination of named Mendelian genes, which may be may be identified as being located on a particular chromosome, and near some other genes.
3. Genetic = The length of DNA of the gene responsible for a trait has been identified and its sequence determined.
4. Genetic = the gene responsible for a trait has been sequenced, and we know the molecular biology of how the gene product works and how mutations in it produce different results. Only at this level do we fully understand the genetics of the trait.

Heredity and Heritability

Children tend to look like their parents, but not exactly, and not like a mixture halfway between each parent. Children often inherit distinct characteristics, some from one parent, some from another, and some from a grandparent. Yet in other characteristics, children seem distinct. A phenotype is the appearance of an individual (what it looks like) whereas the genotype represents the genes carried by that individual. Genetics is the study of how those characteristics (both phenotype and genotype) are passed from parents to their children. The phenotype of an individual is the result of the interaction between the individuals genotype and its environment.

Heritability is the extent to which the phenotype is determined by the genetic makeup of the individual. Some traits are purely genetic, and these traits have a high heritability. Other traits are strongly influenced by the individual's environment, and their heritability is low. Many traits are produced by a complex mixture of genetics and environment.

Mendelian Genetics and Gregor Mendel

Part of the study of genetics involves being able to predict the appearance (Phenotype) and genetics (Genotype) of the offspring of animals or people. Sometimes this is just for fun such as breeding dogs to get a certain color, but sometimes it is very serious, as when dealing with diseases that can be passed (that is, inherited) from parents to children.

Without any understanding of the molecular basis of how some gene works, we can still identify that some trait is genetic. Indeed, genetics had its origins in the late 1800s, well before any concept of the molecular basis of heredity.

The basic unit of inheritance is the Gene. A gene is a length of DNA on a chromosome that does something particular for an organism. A gene can come in more than one form. These flavors of genes are called Alleles. An allele represents one or more DNA sequences in a gene that produces a particular phenotypic result. Several different mutations might cause a gene to produce a defective protein. All of these mutations might be lumped into a single allele - the gene in a form that produces a defective protein. There isn't necessarily a one to one mapping between mutations to the DNA sequence of a gene and different alleles for that gene. Alleles are just broad variants of a gene that can produce different results.

Genes come in two copies per individual.

The different alleles of a gene may be dominant and recessive with respect to each other.

Examples of Genes

It is rare that an aspect of an organism will be controlled by a single gene that has just two alleles. Heredity is seldom this simple. Most real world phenotypes are produced by complex interactions between sets of genes.