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Sex-linked colours | Linkage between different loci | Mendel's Chart

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          Chromosomes are long molecules that contain the genetic information passed to us by our parents. More or less small segments of chromosomes can be called genes. Each gene is responsible for producing a certain protein that, in turn, will have a specific function in the body.

          A chromosome carries each gene at a specific length. The exact length or position of a gene on a chromosome is called the locus. When referring to more then one locus the word loci is used.

          A wild type gene and its mutant form counterparts are called alleles or allelomorphs. All allelomorphs are genes for the same trait that share the same locus.

          Organisms produced by two parents receive from them a full set of chromosomes. From generation to generation the number of chromosomes must remain constant. Nature has developed a mechanism called meiosis that ensures that each parent will pass a copy of exactly half of its genes to the offspring. Each parent contributes with the same number of chromosomes. When both halves combine, each new set contains the original number of chromosomes and genes. Genes for the same trait exist at the

same length or locus on the specific paternal and maternal chromosomes.

          Genes, however, may change. Sometimes, during meiosis, genes are not perfectly copied and small imperfections occur. These imperfections are known as mutations. Some mutations produce a new allele with a new function on the body. The original gene from which the mutation is produced is called the wild-type.

          The specific combination of genes of an organism is called the genotype. The visible or measurable effects of genes is the phenotype.

          Chromosomes are also responsible for sex differentiation. A bird will be a cock if he possesses two X chromosomes. These are both of the same length and carry the same number of genes. Hens, on the other hand, possess one X chromosome and one Y chromosome, which is smaller and contains less genes than the X chromosome. Because of this, the hen is said to be hemizygous.

          Because for each trait, an individual has two matching genes, one from each parent (except for the sexual chromosomes), it becomes necessary to understand the relations between these genes. An individual is said to be homozygous (double factor) for a certain trait, if the gene for that trait exists in double quantity. If, on the other hand, an individual has a wild-type gene and the correspondent mutation, this individual is said to be heterozygous (single factor).

          In heterozygous state, one of the genes may be dominant over the other. This means the dominant gene expresses itself on the phenotype while the recessive gene does not. A dominant gene produces the same effect in single and double quantity. A recessive gene will A cock, heterozygous for opaline, has two genes for this trait, the wild-type gene produces the normal colour and the mutation is responsible for the opaline colour. Because the wild-type is dominant over the mutation, the cock will not show the opaline colour. An individual is said to be split for the recessive gene that exists on the genotype but does not express itself on the phenotype.

          When, in double quantity, a certain gene produces an effect different from the effect it produces in single quantity, the gene is said to be incomplete dominant.

          One can only judge the relations between genes if they are allelomorphs, that is, if they occupy the same locus.


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Home | Introduction | Notions on genetics | Two distinct loci and the colour pigments | Colour depth | The violet factor
Sex-linked colours | Linkage between different loci | Mendel's Chart
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