Human Biology Laboratory
Biology 1410

Introduction to Genetics

Vocabulary continued Mendel's experiments led him to believe that there were two copies of every gene. With our current understanding of chromosomes this makes sense but it was puzzling to his contemporaries. This next group of terms share in common that we need to think about two copies of each gene.

Alleles are variations of a gene. For this week's lab we start with a consideration of the coat color gene in cattle. We will discuss two variations of the coat color gene: red and black. The red allele is responsible for red coat color and the black allele causes black coat color. We mostly use letters as an abbreviation for alleles, B = black, b = red.

An individual that contains two copies of the same allele for a specific gene is homozygous.
An individual that contains two different alleles of a gene is heterozygous.
A cow that has two alleles for a red coat is homozygous (bb).
A bull with two alleles for a black coat is homozygous (BB).
A calf that has one allele for red coat color and one allele for black coat color is heterozygous (Bb).

Heterozygous individuals contain two different alleles which results in conflicting instructions. One gene says "black coat" while the other says "red coat". What will the result be?
Dominant allele
The allele that is seen (or expressed) in a heterozygote is said to be dominant. We usually use a capital letter to indicate dominance (B).
Recessive allele
The allele that is hidden (or suppressed) in a heterozygote is said to be recessive. We usually use a lower case letter to indicate recessiveness (b).
A heterozygous calf has one red allele and one black allele (Bb). These animals are always black. This tells you that the black allele is dominant and the red allele is recessive because the black coloration is hiding the red. We use "B" to indicate the dominant black allele and "b" for the recessive red allele.

If you happen upon a stray calf in the middle of the road it is pretty easy to figure out the color the animal but can you determine the alleles that it carries?
The appearance of an animal (or any other form of life) is considered its phenotype (think ph = physical).
An animal's genotype is its genetic make-up (think gen = genetic).
Black is the phenotype of a calf with at least one black (B) allele.
Red is the phenotype of a calf with two red (bb) alleles.

Two calves may be the same phenotype and yet be different genotypes. Cattle that are BB are black but so are animals that are Bb: Two different genotypes that have the same phenotype.
What is the genotype of a calf that has a red phenotype?

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©2007 Michael T. Dixon