Overview
This lecture introduces the foundations of genetics, focusing on Mendel's three laws, basic terminology, and how traits are inherited through generations.
Mendel and the Foundations of Genetics
- Gregor Mendel is known as the father of genetics and conducted inheritance experiments with pea plants in the 1800s.
- Mendel worked before the discovery of DNA and chromosomes, predicting genetic principles without knowledge of molecular biology.
DNA, Genes, and Chromosomes
- DNA contains genetic information, usually dispersed as chromatin in the cell nucleus, and condenses into chromosomes during cell division.
- A gene is a segment of DNA that codes for a specific protein, which determines a particular trait.
- Each person has two copies of each chromosome (homologous chromosomes): one from each parent.
Alleles, Genotype, and Phenotype
- Alleles are versions of a gene located at the same position on homologous chromosomes, coding for the same trait.
- Genotype is the genetic makeup or combination of alleles for a trait (e.g., brown eye allele + blue eye allele).
- Phenotype is the observable trait (e.g., brown or blue eyes), which may not reveal all alleles present in the genotype.
Mendel's Three Laws
First Law: Law of Dominance
- Crossing two homozygous individuals (e.g., black-black and white-white) produces offspring expressing only the dominant trait (e.g., all black).
- Dominant alleles mask the presence of recessive alleles in heterozygous individuals.
Second Law: Law of Segregation
- Alleles for a trait separate during gamete formation, so offspring receive only one allele from each parent per gene.
- Recessive traits can reappear in subsequent generations if two recessive alleles are inherited.
Third Law: Law of Independent Assortment
- Genes for different traits are inherited independently, so combinations of traits in offspring can differ from those found in parents.
- Traits (e.g., hair color and eye color) do not have to be inherited together.
Key Terms & Definitions
- Gene — segment of DNA coding for a protein that determines a trait.
- Allele — alternative version of a gene found at the same locus on homologous chromosomes.
- Genotype — set of alleles present in an individual for a given trait.
- Phenotype — physically expressed trait resulting from the genotype.
- Homozygous — having two identical alleles for a specific gene.
- Heterozygous — having two different alleles for a specific gene.
- Dominant allele — allele that is expressed over a recessive allele in the phenotype.
- Recessive allele — allele whose effect is masked by a dominant allele.
Action Items / Next Steps
- Review lesson 5 on DNA and lesson 10 on meiosis for background understanding.
- Prepare for the next lecture on exceptions to Mendel's laws.