Posts Tagged: recessive genes
Luckily Gregor Mendel tested his theories on the garden pea that has a relatively simple genetic structure. He crossed yellow peas with green peas and tall plants with short plants to discover the fundamental laws of inheritance.
When he crossed yellow peas with green peas he often got only yellow peas. But when he crossed the second generation together he got a few green peas mixed in with three times as many yellow peas. When he crossed green peas together, he got only green peas.
Mendel theorized that each parent contributed the “elementen” (one gene) for any given trait so the offspring had a pairing of those two genes and that you could not “draw from the external resemblances conclusions as to internal nature.”
What you see on the outside doesn’t always tell you the genetic makeup.
In today’s terms, we would say the gene for yellow is dominant over the gene for green which is recessive. When he crossed yellow with yellow and got only yellow, at least one of the parent plants was homozygous for yellow – meaning that parent plant carried only the yellow gene. But when he crossed yellow to yellow and got some green peas, *both* of the parent plants carried the recessive green gene. When he crossed green to green he could not get yellow because *neither* parent plant carried the yellow gene.
Before breeding, the DNA strands carrying “… chromosome pairs are split apart and distributed into cells called gametes. Each gamete contains a single copy of every chromosome, and each chromosome contains one allele for every gene.”
Which variation of a gene winds up on which DNA strand and which strand from the father combines with which strand from the mother is due to chance thus making breeding so very interesting.
Genetic testing is helping to make breeding decisions a bit easier. Most of the genetic tests we have available for our dogs are for a simple recessive gene, like yellow or green in the garden pea.
What this means is that there is one gene controlling the trait with two or more possible variations (“alleles”) – a dominant allele and recessive alleles. The recessive allele will only express itself if both parents contributed recessive alleles. When there is only one recessive allele the dominant allele it will ‘cover up’ the recessive.
Using genetic testing
If your puppy’s parents have been tested or if your puppy shows a genetic trait, such as yellow or chocolate color, you can make an educated guess about his genetic make up.
If he is yellow, then his parents are either yellow or carry yellow as a hidden gene. Both must carry at least one copy of the yellow gene.
If he is black, but has a yellow parent, then he carries one copy of the gene for yellow. And it works the same for chocolate.
Because yellow and chocolate are controlled by different genes you can’t know whether he carries the gene for the other color based on his color. With one exception. Yellow Labradors usually have black noses and eye rims. When a yellow Lab has a chocolate nose and eye rims, he is homozygous for both chocolate and for yellow. Although it is a natural color in the breed, it is a disqualification in the show ring.
In a graph, the dominant trait (such as black in Labradors) is capitalized and the recessive trait (chocolate) is lower-case, thus:
BB = homozygous black in capital letters
Bb = heterozygous black (hidden chocolate in lower case)
bb = homozygous chocolate
Therefore the yellow puppy with chocolate points, would be shown as bbee with “e” signifying yellow. The dominant allele – “E” – means “not yellow.”
Early in the Labrador’s history, yellow and chocolate puppies would appear occasionally. But because those colors were not popular, they were rarely bred (and sometimes not even allowed to live). With no genetic testing available and limited knowledge of inheritance, breeders didn’t know that those recessive colors lurked in their dogs’ genetics and would appear when the dog was bred to another dog who carried the same recessive color.
It is possible for a recessive gene to remain hidden for many generations. For example, Sandylands Mark, born in 1965, was black carrying chocolate despite 19 generations of blacks and yellows in his pedigree.
As a side note, because chocolate is a recessive gene, ads proclaiming “dominant chocolate” stud dogs are incorrect. Chocolate is not only recessive to black, but is a separate gene from yellow. The ads probably mean “pure for chocolate” instead.
For traits you can’t see, you should test unless both parents are clear for the trait.
EIC, CNM and PRA are examples of simple recessives. Here are some charts showing potential outcomes for a hypothetical trait “X” where “X” is the dominant allele and “x” is the recessive allele.
These examples hold true for any trait that is a simple recessive.
Note: statistics only hold true with very large samples, except for breeding clear to clear or affected to affected where all puppies will be like their parents. However a carrier to carrier breeding could have all clear puppies or could have all affected puppies.