Definition: A gene is a segment of DNA that codes for a particular protein.

Example: Different gene sequences for eye color result in blue or brown eyes.

Vocabulary: DNA (Deoxyribonucleic Acid) is the molecule that carries genetic instructions for the development, functioning, growth, and reproduction of all known organisms.

Definition: Alleles are alternate forms of a gene that occupy the same corresponding position on homologous chromosomes.

Example: For the eye color gene, a person might inherit a blue eye allele from their mother and a brown eye allele from their father.

Highlight: Homologous chromosomes carry the same genes but may have different alleles.

Definition: Dominant alleles are represented by capital letters and mask the effects of recessive alleles, while recessive alleles are represented by lowercase letters and are usually hidden.

Example: B represents the dominant allele for brown eyes, while b represents the recessive allele for blue eyes.

Highlight: The combination of alleles received from parents determines an individual's genotype and phenotype.

Definition: Genotype is the genetic makeup of an organism determined by the alleles received, while phenotype is the physical appearance of a trait determined by the genotype.

Example: A person with the genotype BB or Bb will have brown eyes (phenotype), while someone with bb will have blue eyes.

Highlight: The relationship between genotype and phenotype is crucial for understanding how genetic information is expressed in an organism's characteristics.

Definition: Homozygous genotypes have two identical alleles (BB or bb), while heterozygous genotypes have two different alleles (Bb).

Example: BB is homozygous dominant, bb is homozygous recessive, and Bb is heterozygous.

Highlight: Understanding homozygous and heterozygous genotypes is essential for predicting the inheritance of traits in offspring.

Definition: Simple dominance occurs when a dominant allele completely hides the recessive allele, requiring only one dominant allele to express the dominant trait.

Example: Brown eyes (B) are dominant over blue eyes $b$, so individuals with BB or Bb genotypes will have brown eyes.

Highlight: Many common human traits, such as cleft chin, widow's peak, and freckles, follow the pattern of simple dominance.

Definition: Fraternal twins result from two separate eggs fertilized by two separate sperm, while identical twins come from a single fertilized egg that splits.

Example: Semi-identical twins, a rare occurrence, result from two sperm fertilizing one egg.

Highlight: Understanding the genetic differences between types of twins is important for studying inheritance patterns and genetic variation.

Definition: Alleles are variations of genes carried on homologous chromosomes received from parents.

Example: The combination of alleles for eye color determines whether an individual will have brown or blue eyes.

Highlight: This review sets the stage for understanding more complex inheritance patterns and prediction methods.

Definition: A Punnett square is a diagram that predicts the possible genotypes and phenotypes of offspring from a genetic cross.

Example: A Punnett square for a cross between two heterozygous parents (Bb x Bb) shows the probability of offspring genotypes and phenotypes.

Highlight: Punnett squares are essential tools in genetics for predicting inheritance patterns and probabilities.

Definition: Genotypic ratio represents how often each genotype appears in the offspring, while phenotypic ratio represents the expected physical traits.

Example: In a cross between two heterozygous parents (Bb x Bb), the genotypic ratio is 1:2:1 (BB:Bb:bb), and the phenotypic ratio is 3:1 (dominant:recessive).

Highlight: Understanding these ratios is crucial for predicting the likelihood of specific traits appearing in offspring.