In dog genetics, every gene has two copies, one from each parent. If an individual has two different version of a particular gene, they can interact in two basic ways. A dominant gene will mask the presence of a recessive one. As a result, recessive traits (traits caused by recessive genes) are only expressed in individuals with two identical recessive genes.
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Simple Mendelian Traits
Many traits follow the simple dominant and recessive pattern of inheritance. These are called Mendelian traits, after the monk who first noticed these traits. In dogs, Mendelian dominant traits include a correct bite, darker eyes, the presence of a dewlap (loose skin on the neck), long ears, upright ears and a short coat. Mendelian recessive traits include over- or underbites, lighter eye colours, the lack of a dewlap, short ears, floppy ears and a long coat.
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Labrador Retriever Coat Color
Labrador Retriever coat colours are determined by two independent sets of genes, called the B- and E-loci. The B-locus determines the presence or absence of black pigment and is dominant over a lack of black pigment. If the dominant gene is present, the Labrador Retriever will be black. If there are two copies of the recessive gene, the dog will be chocolate or yellow, depending on the E-locus. In the E-locus, the gene for chocolate Labrador Retrievers is dominant over the gene for yellow Labrador Retrievers.
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Some diseases involve the sex chromosomes, the X and Y chromosomes. Since the Y-chromosome only has a few genes on it, most of these genes act differently in male dogs. In females, these genes act like regular dominant or recessive genes. But in males, only one gene is present. This means that even if there is only one recessive allele, it acts like a dominant allele in males. Disorders that follow this pattern include progressive retinal atrophy in Huskies and X-linked severe combined immunodeficiency in Basset Hounds and Corgis.
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Temperament in dogs is a complex trait. More than two genes control a dog's personality, and environment influences it, so it does not always follow a simple pattern. However, there are some generalisation that are generally accepted. For example, it appears that intelligence is genetically dominant over unintelligent genes. Also, an even-keeled temperament is recessive to both a shy temperament and a vicious temperament.