Claybrook Farm
~~~~~~~~~~~~~~~~~~~~~~~~
Mendelian genetic inheritance simplified for a single gene trait.
Genetics is incredibly complicated microbiology, and this is an extremely oversimplified introduction to very basic genetic concepts. Please keep that in mind as you read through this page. Many people have dedicated their entire careers to the study of genetics, and much is being learned on a daily basis in the continuing field of genetic research.
Genes, the unit of heredity in cells, are inherited from one's parents. They are made up of pairs of alleles in the individual. One gene allele is inherited from the mother (via the egg), and the other one from the father (via the sperm).
Capital letters usually indicate a dominant gene allele, or one that is "stronger". Only one dominant allele of a gene pair needs to be present for that allele to be expressed (be evident). Dominant alleles mask, or hide, the recessive ("weaker") allele. The recessive allele must be inherited from both parents in order to be expressed. In other words they must be present as a pair to be shown. Recessives are indicated with a lower case letter.
Most visible traits are produced by many genes interacting in the individual. These are called polygenic traits. But some traits are controlled by a single gene. For our theoretical simplified example, we will use a single gene trait.
A = dominant allele a = recessive allele
If A indicates short hair in a dog, for this example, and a indicates longhair, then:
AA would be a shorthaired dog with no longhair recessive.
Aa would be a shorthaired dog with a longhaired recessive, but you would not know it was carrying the longhaired trait by looking at the dog, since the dominant allele, A, masked the recessive allele, a.
aa would be a longhaired dog , since the recessive alleles must be present in a pair to be expressed, however please see * Note below regarding the quantity of hair likely to be present in such a dog that had an Aa parent. As can clearly be seen, it is not possible for the aa dog to carry or pass on the dominant trait, which for this example, is shorthair..
For our example below, both of the parents for this square are shorthaired dogs that carry the longhair recessive - Aa. This Mendelian square calculates the percentage of offspring that should statistically be produced. Of course, real life eggs and sperm do not follow statistics for every litter, so actual results in a given litter may vary widely. But, if a large number of such breedings were done, they would statistically average out according to the Punnett's square, below.
The theoretical puppies produced in this
litter:
25% AA (shorthaired, no longhaired recessive)
50% Aa ( shorthaired, with longhaired recessive)
25% aa (longhaired recessive)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For the example below, the father for this square is a shorthaired dog that carries the longhair recessive - Aa, while the mother is a longhair - aa.
The theoretical puppies produced in this
litter:
50% Aa ( shorthaired, with longhaired recessive)
50% aa (longhaired recessive)
* Note: While the presence or absence of longhair is indeed controlled by a single gene, true longhaired, with a quantity of longhair, is a polygenic trait. Therefore, if the dog does not have all the longhair genes, it will have just slightly longer hairs, and these may be only in limited areas on the dog, like on the back of the legs or on the tail. These barely longhaired dogs can practically pass for smooths even in a totally ungroomed condition. These almost shorthaired "longhaired" dogs are produced by breeding an Aa dog, from our examples above, to either another Aa or an aa dog. The resulting aa puppies will nearly always not have much longhair. To get profuse longhair, aa dogs must be bred together for many generations. This is seen in many breeds, like Dachshunds, for example. © Claybrook Farm
For
more information contact:
Claybrook Farm -- Michelle Henninger -- Shippensburg, PA 17257
717-263-0932
Copyright Ó Claybrook Farm
All rights reserved.