Overview
This lecture explains how DNA, genes, and alleles determine human traits and describes inheritance patterns using cystic fibrosis as an example.
DNA, Genes, and Chromosomes
- DNA is arranged on chromosomes inside cells.
- Chromosomes are long strings of DNA containing instructions for traits.
- A gene is an individual set of instructions on DNA that determines a specific characteristic.
- All humans have the same set of genes, with minor differences between sexes.
- Differences in people arise from different versions of genes, called alleles.
Alleles and Traits
- An allele is a version of a gene that determines a specific trait (e.g., eye or hair color).
- Different alleles lead to different features, even for the same gene.
- Humans have 23 pairs of chromosomes, receiving 23 from each parent.
- Each person inherits two alleles for each gene, one from each parent.
Inheritance and Reproductive Cells
- Reproductive cells (egg and sperm) contain 23 chromosomes each.
- When egg and sperm combine, the new cell has 46 chromosomes.
- Which chromosome (and thus allele) a parent passes on is random.
Example: Cystic Fibrosis Inheritance
- Cystic fibrosis is caused by a gene on chromosome 7.
- The normal allele is represented by uppercase F; the cystic fibrosis allele by lowercase f.
- People can have combinations: FF (normal), Ff (carrier), or ff (has cystic fibrosis).
- If both parents are carriers (Ff), there is a 25% chance the child will have cystic fibrosis.
- Dominant alleles mask the effect of recessive alleles; only the dominant trait is expressed when both are present.
Key Terms & Definitions
- Gene — a DNA segment that determines a specific trait or function.
- Allele — a different version of a gene.
- Chromosome — a DNA structure carrying genetic information.
- Dominant allele — an allele that is expressed over another (uppercase letter).
- Recessive allele — an allele whose effect is hidden when paired with a dominant allele (lowercase letter).
- Carrier — a person with one normal and one disease allele who does not show symptoms.
Action Items / Next Steps
- Learn how to draw genetic diagrams to predict inheritance probabilities.
- Review vocabulary related to genetics for understanding future lessons.