Lab One- Recognizing Mendelian Traits

Genotype

The genetic makeup of an organism; in other words, it describes an organism's complete set of genes. In a more narrow sense, the term can be used to refer to the alleles, or variant forms of a gene, that are carried by an organism.

Gene

a segment of DNA that codes for a trait that is expressed in the phenotype. For example, a gene that controls for earlobe shape, earwax, height, ect.

Dominant allele

an allele that masks the effect of another allele (a recessive allele). For example, the allele that codes for free hanging earlobes is dominant. Only one of these alleles are needed for the trait to be expressed in the phenotype thus the dominant allele will always be expressed in the phenotype even if a recessive allele is present as well. These alleles are also denoted by a capital letter.

Recessive allele

an allele whose phenotypic effect is masked by another allele (the dominant allele). At least two of these alleles are needed to express the trait in the phenotype. The allele that codes for attached earlobes is recessive thus, if you have attached earlobes you must have two copies of the recessive allele. Recessive alleles are denoted by lowercase letters.

Trait

any observable characteristic or feature, e.g., hair, height, earwax type, earlobe shape. Traits are coded for by segments of DNA called a gene.

Variant

any variation of a trait. For example, hair is a trait with many variants: blond, red, black, ect. Another example variants of flower color in pea plants include purple and white. Variants of a trait are controlled for by variants of a gene, which are called alleles.

Discrete traits

are those that do not have a continuous distribution, they are either present or absent such as dimples or no dimples. These are also referred to as Mendelian traits. Mendelian traits are under simple genetic control meaning they are controlled for by only one gene with two alleles. Additionally, they follow Mendelian principles of inheritance. All of the traits discussed in today's lab are ___ traits.

Continuous Traits

are those that have a continuous distribution, such as height and skin pigmentation. They are also considered complex and polygenic meaning they are controlled by multiple genes and are expressed along a continuous range of variation.

Allele

is a variant form of a gene. Genes can be coded for by a single allele or multiple alleles, but you will always inherit at least one allele of a gene from your mother and one allele of that same gene from your father. The alleles may be the same or they may be different but are located at the same position on a chromosome. For example, earlobe shape is a discrete trait that is coded for by a single gene with two alleles; one allele codes for an attached lobe and the other allele codes for an unattached lobe. Similarly, blood type can be coded for by multiple alleles: A, B, and O. If you have type AB blood, you will have inherited an A allele from one parent and a B allele from your other parent. If you have type AA blood, you will have inherited an A allele from your mom and an A allele from your father.

Heterozygous

refers to the presence of two different alleles in the genotype with at least one dominant allele and one recessive allele. Because of the presence of a single dominant allele, these individuals will express the dominant variant in their phenotype.

Homozygous

refers to the presence of two of the same alleles in the genotype. If an individual has two dominant alleles in their genotype, they are considered homozygous dominant and will express the dominant variant in their phenotype. If an individual has two recessive alleles in their genotype they are considered homozygous recessive and will express the recessive variant in their phenotype.

Phenotype

the observable physical properties of an organism; these include the organism's appearance, development, and behavior. An organism's phenotype is determined by its genotype.