Terminology for Genetics Help
A phenotype is any measurable characteristic or distinctive trait possessed by an organism. The trait may be visible to the eye, such as the color of a flower or the texture of hair, or it may require special tests for its identification, as in a serological test for blood type. The phenotype is the result of gene products that interact together in a given environment.
EXAMPLE 2.1 Rabbits of the Himalayan breed will, in the usual range of environments, develop black pigment (a phenotype) at the tips of the nose, tail, feet, and ears. If raised at very high temperatures, an all-white rabbit is produced. The gene for Himalayan color pattern specifies a temperature-sensitive enzyme that is inactivated at high temperature, resulting in a loss of pigmentation.
The kinds of traits that we shall encounter in the study of simple Mendelian inheritance, i.e., those that follow Mendel's two laws (Mendel's Law), will be considered to be relatively unaffected by the normal range of environmental conditions in which the organism is found.
All of the alleles possessed by an individual constitute its genotype.
- Homozygous. The union of gametes carrying identical alleles produces a homozygous genotype. A homozygote contains the same alleles at a single locus and produces only one kind of gamete.
- Pure Line. A group of individuals with similar genetic background (breeding) is often referred to as a line or strain or variety or breed. Self-fertilization or mating closely related individuals for many generations (inbreeding) usually produces a population that is homozygous at nearly all loci. Matings between the homozygous individuals of a pure line produce only homozygous offspring like the parents. Thus, we say that a pure line "breeds true."
- Heterozygous. The union of gametes carrying different alleles produces a heterozygous genotype. A heterozygote contains two different alleles at a single locus and produces different kinds of gametes.
- Hybrid. The term hybrid, as used in the problems of this book, is synonymous with the heterozygous condition. It can also refer to the progeny of inter-species crosses. Monohybrids involve a single factor hybrid. Later in this chapter we will consider heterozygosity at two (dihybrids) or more loci (polyhybrids).
Practice problems for these concepts can be found at:
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