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By the end of this topic, you should be able to:
Understanding genetics starts with knowing the basic vocabulary. These terms form the foundation for everything else you'll learn in this topic.
A gene is a section of DNA that contains the instructions for making a specific protein or polypeptide. Think of it as a recipe in a cookbook - each gene is a recipe for a particular protein.
The locus is the specific position where a gene is located on a chromosome. It's like the page number where you'd find a particular recipe in a cookbook. Every organism has the same genes at the same loci on matching chromosomes.
Alleles are different versions of the same gene. For example, a gene for eye color might have a blue allele and a brown allele. Alleles exist because the DNA sequence can vary slightly between individuals, though the gene is still in the same location (locus) on the chromosome.
The genotype is the genetic makeup of an organism - which alleles it possesses. For example, if we use B for a brown eye allele and b for a blue eye allele, a person could have genotype BB, Bb, or bb.
The phenotype is the observable characteristics of an organism that result from its genotype interacting with the environment. Using our eye color example, the phenotype would be the actual color of the person's eyes that you can see.
An organism is homozygous for a gene when it has two identical alleles. For example, BB or bb. The organism is "pure-breeding" for that characteristic.
An organism is heterozygous for a gene when it has two different alleles. For example, Bb. The organism has one copy of each version of the gene.
A dominant allele is expressed in the phenotype even when only one copy is present. We represent dominant alleles with capital letters (like B). If you have genotype BB or Bb, the dominant allele will show in your phenotype.
A recessive allele is only expressed in the phenotype when two copies are present (homozygous). We represent recessive alleles with lowercase letters (like b). You need genotype bb to see a recessive characteristic.
Codominant alleles are both expressed equally in the phenotype when present together. Neither dominates the other. For example, in snapdragon flowers, a red allele (C^R) and a white allele (C^W) are codominant - a plant with both (C^R C^W) has pink flowers showing both red and white pigments.
Linkage occurs when two or more genes are located on the same chromosome. Because they're physically connected on the same DNA molecule, linked genes tend to be inherited together rather than assorting independently.
A test cross is a breeding experiment used to determine the unknown genotype of an organism showing a dominant phenotype. The organism is crossed with one that has the recessive phenotype (which has a known homozygous recessive genotype). The offspring ratios reveal whether the unknown parent was homozygous dominant or heterozygous.
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