Genetics Primer
Our knowledge of how genetics works is mostly based on the discoveries of Mendel1. He noted that when tall beans and dwarf beans crosspollinated with each other the offspring produced was a mix of the two, and the ratio of the mix could be calculated by probability. This is where genes come in. Genes are how information is passed from one generation to the next. Each trait displayed, such as being a tall bean or a dwarf bean, is described by two single genes. Each parent passes one single gene to their offspring each, and when these two genes are combined they determine the traits the child exhibits. This is known as gene expression - if a particular trait is visible, such as blue eyes, then we say that the blue eyed gene is being expressed.
1Gregor Mendel (1822 - 1884)Mendel was a german scientist and priest. His research sadly was not accepted in his lifetime, but is now the cornerstone of our modern understanding. He published one paper on his research, and an english version of this can be found at http://www.mendelweb.org/Mendel.html |
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Genes can either be dominant or recessive. A dominant gene will always be expressed. A recessive gene is one where the gene is only expressed if the individual has two copies of the gene. For example, let the tall gene in beans be 'H', a dominant gene, and the dwarf gene be 'h', a recessive gene. If a bean has either one or two copies of the 'H' gene, then it will definitely be a tall bean, e.g. 'HH', 'Hh'. It will only be a dwarf bean if it has two copies of the 'h' gene, e.g., 'hh'.
Note: Dominant genes are always written with a capital letter, and recessive genes are always written using a lower case letter.
Consider the two possible types of tall beans - one with two copies of the dominant gene 'HH', and the other with a copy of the recessive gene instead 'Hh'. These two plants would look the same, so we say they have the same phenotype, but would produce different offspring than each other. The first type could pass on only the 'H' gene, wereas the second plant could pass on either the 'H' gene or the 'h' gene. This difference is genes is important - since the first plant always passes on a dominant gene, it will always produce tall beans, regardless of what type of bean the other parent is. The second bean could possibly have dwarf offspring. The full genetics is known as the genotype.
| Quick Overview | |
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| Dominant gene | Capital letter used, if present this characteristic will be expressed. |
| Recessive gene | Lower case letter used. Only expressed if dominant gene is not present. |
| Phenotype | What genes are expressed, e.g., HH and Hh phenotype is dominant characteristic , hh phenotype is recessive characteristic. |
| Genetype | Complete genetic code - what is expressed and what is carried. |
Discovering Genotypes
The phenotype of any plant or animal is simply what genes are being expressed, or what the individual looks like. Thus simple examination of the individual can determine this. The genotype is the full genetic code, which is some cases is impossible to be certain of.
- Case 1: The individual is expressing a recessive gene. In this case both the phenotype and the genotype of the individual is known. Consider the dwarf bean. A bean will only be a dwarf if it has two copies of the recessive gene, 'hh'. Thus its phenotype is dwarf, and its genotype is 'hh'.
- Case 2: The individual is expressing the dominant gene, but one of its parents was expressing the recessive gene. Since the individual is expressing the dominant gene, it must have at least one copy of that. Since the known parent only carries the recessive gene, this also must have been passed to the individual. For the bean, if one of the parents was a dwarf then the only gene it could have passed on to its offspring is the recessive gene 'h'. Thus the phenotype is tall, and the genotype is 'Hh'.
- Case 3: The individual is expressing the dominant gene, and neither of the parents were expressing the recessive gene. This is the case that is impossible to determine the genotype. We know the individual must have one copy of the recessive gene, but what is the other gene inherited from the parents. If both parents are expressing the dominant gene, then they could be either 'HH' or 'Hh', thus they could pass either the dominant or the recessive gene to their offspring and there is no way of telling by looking. In this case the phenotype is tall, and the genotype is either 'HH' or 'Hh'. We write this uncertainty by 'H-'. The only way to be certain the individual is carrying the recessive gene is if their offspring expresses that gene.