Understanding Homologous Chromosomes and Alleles: Dominant and Recessive Traits, Slides of Ecology and Environment

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• A chromosome is a long strand of DNA molecules. On this chromosome are smaller “packages” of information that code for a particular protein. Proteins then control the expression of a certain trait.
• For example, if the enzyme tyrosinase (a protein) is not produced, then the skin is white because the pigment melanin is absent.

Every human somatic (body) cell has 23 sets of homologous chromosomes. One pair (in males), the sex chromosomes, are not homologous chromosomes since they are different one from the other.

These alternate forms of a gene are called ALLELES. They occupy the same location on homologous chromosomes. The one allele that always expresses itself is called the dominant allele. The other allele, that does not express itself unless paired with another of its kind , is called recessive.Docsity.com

• We use letters to represent the alleles, with upper case letters representing dominant alleles and lower case representing recessive alleles.
• Ex.

E = free earlobes e = attached lobes