Genetics and Chromosomes

Chromosomes and Genes

• A chromosome is a DNA thread wrapped in protein that carries inherited information from one generation to the next.
• A gene is a portion/segment of DNA that controls one particular characteristic, e.g., the gene for eye color.

Alleles

• An allele is an alternative form of a given gene.
• Each sexually-reproducing organism has two alleles for each of its characteristics, one inherited from each parent.
• Alleles can be similar or different.
• Multiple alleles are alleles with more than two alternative forms, e.g., the gene for blood group in humans has three alleles (I­A, I­B, and I­O).

Genotype and Phenotype

• Genotype is the combination of alleles an organism has for a given characteristic, written using a pair of letters (e.g., AA, Aa, and aa).
• Phenotype is the outward expression of the genotype in the form of physical characteristics of an organism.

Dominance and Recessiveness

• A dominant allele is one that gets expressed in the phenotype to the exclusion of another, represented by a capital letter (e.g., A, B, T).
• A recessive allele is one that is overshadowed in the phenotype by the presence of a dominant allele, represented by a small/lowercase letter (e.g., a, b, t).

Codominance and Homozygosity

• Codominance occurs when two different alleles are equally expressed in the phenotype when present together (e.g., I­A and I­B for blood group).
• Homozygous refers to a genotype where both alleles are the same (e.g., AA, aa, BB, bb), resulting in homozygotes or pure breeds.

Heterozygosity

• Heterozygous refers to a genotype where the two alleles are different, such as one allele is dominant while the other is recessive (e.g., Aa, Tt, Bb), resulting in heterozygotes or hybrids.

Learn about the basics of genetics, including chromosomes, genes, and inheritance. Understand how chromosomes carry inherited information and genes control characteristics.