Genetics is the study of inheritance. Inheritance involves the transfer of information from parents to offspring. This information is in the form of genes which are coded segments of deoxyribonucleic acid (DNA). DNA is found in all nuclei and is organized into chromosomes.

With one exception, which will be mentioned later, human beings have two complete sets of genes. We inherit one set of twenty three chromosomes from each of our parents. Thus each cell of our bodies (except sex cells) contain 23 pairs or 46 chromosomes. Body cells are said to be diploid, while sex cells are haploid.

Not all genes for a particular trait are identical. When two genes for the same trait are slightly different they are said to be alleles. For instance, the gene for hair color may be an allele for blonde, or an allele for black hair. Not all alleles are equally effective at expressing themselves. Some alleles are dominant while others are recessive. Traditionally dominant alleles are abbreviated with upper case letters while recessive alleles of the same gene are abbreviated with the same letter, only lower case.

A genotype is a list or description of the exact genes which are present in an organism. The phenotype is a description only of the traits which are visible or measurable. For example, a genotype might be "one dominant allele for black hair and one recessive allele for blonde hair", but since black hair is dominant over blonde hair, only the black-hair allele would be visible. The phenotype is simply "black hair*.

Two words are commonly used to describe genotypes more easily. Heterozygous means that the two alleles are different. In our example of hair color the genotype could be simply stated as "heterozygous". The genotype could also be written "Bb" where "B" stands for the dominant allele and "b" for the recessive allele.

Homozygous indicates that the two alleles are identical. The genotype could be "homozygous dominant" (two dominant alleles, e.g. BB) or "homozygous recessive" (two recessive alleles, e.g. bb).

Notice that there are only two alleles for any given trait, because one allele was inherited from each parent.

With this background in vocabulary you are ready to work some problems in genetics. We will work many problems with animals and not with humans, because genetic problems often involve the need for many offspring as well as matings between siblings, or between offspring and parents.

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