Section 6.4

1. Section 6.4 Traits, Genes, and Alleles

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

3. Vocabulary • Gene • Allele • Homozygous • Heterozygous • Genome • Genotype • Phenotype • Dominant • Recessive

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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

12. 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

13. 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.

14. 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.

15. 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.