Traits, Genes, and Alleles Section 6.4 Traits, Genes, and Alleles

Objectives SWBAT explain how there can be many versions of one gene. SWBAT describe how genes influence the development of traits.

Vocabulary Gene Allele Homozygous Heterozygous Genome Genotype Phenotype Dominant Recessive

Key Concept Genes encode for proteins that produce a diverse range of traits. So, what is a gene? A gene is a piece of DNA that provides a set of instructions to a cell to make a certain protein. This definition is for “genes that make stuff” – the 2% of our DNA. Does not apply to our body plan (regulatory) genes or the DNA switches.

A Gene’s Locus Each gene has a specific locus (location) on a pair of homologous chromosomes. You can think of a Locus as a gene’s address on a chromosome.

Alleles Most genes exist in many forms called alleles. An allele is any of the alternative forms of a gene that may occur at a specific locus. our cells have two alleles for each gene, one from each parent.

Alleles The two alleles from your parents may be the same (homozygous) or different (heterozygous). Homozygous – describes two of the same alleles at a specific locus. Heterozygous – describes two different alleles at a specific locus.

Question How are the terms locus and allele related? An allele is an alternative form of a gene, which codes for a different form of the same trait. Alleles are found at the same locus on homologous chromosomes.

Genes Influence Traits A genome is all of an organism’s genetic material. Every individual, unless they have an identical twin, has a unique genome that, when “combined” with the organisms developmental environment, results in your traits. Some traits can be seen, like eye color, while other traits, like the chemical make up of the eyeball, cannot readily be seen.

Genes Influence Traits: Genotype and Phenotype A genotype is the genetic makeup of a specific set of genes. This includes all of an individual’s genes whether they are masked or not (ex. the white flower gene was present but not observable). A phenotype is the physical expression of a trait in an individual. A “hidden” gene (like the white flower) does not matter to the phenotype.

Dominant and Recessive Alleles In general, alleles can be thought of as dominant or recessive. A dominant allele is the allele that is expressed when two different alleles or two dominant alleles are present. Dominant alleles are written on paper using an upper case letter. Rr rr

Dominant and Recessive Alleles A recessive allele is the allele that is only expressed when two copies are present. Recessive alleles are represented on paper as lowercase letter. Rr rr

Dominant and Recessive Alleles Dominant allele does not necessarily mean that it is better or stronger than a recessive allele. It only means that when it is present in a heterozygote that it is expressed and the other allele is not. A dominant allele may not be the most common allele in a population.

Question If a recessive allele helps an organism reproduce, but the dominant allele hinders reproduction, which will be more common in a population? The recessive allele will be more common.

Traits occur in a range While Mendel used “either-or” traits, most traits occur in a range – there are factors (environmental factors) that influence traits and need to be accounted for (ex. diet and its effects on development). Other examples, a lack of sunshine or nutrients can stunt the growth of plants. Inheritance is much more complex than demonstrated by Mendel’s experiments. Ex. co-dominants, poly genes, etc.