Patterns of Inheritance

Introduction to Heredity and Genetics

Heredity is the transmission of traits from one generation to the next, while genetics is the scientific study of how these traits are inherited. The foundational principles of inheritance were first described by Gregor Mendel, whose experiments with pea plants revealed predictable patterns in the way traits are passed down. Modern genetics has expanded on Mendel’s work, showing that genes, alleles, chromosomes, and environmental factors all contribute to an organism’s phenotype.

Mendel’s Experiments and the Logic of Inheritance

Mendel’s Experimental Approach

Mendel used controlled crosses between pea plants to study how traits are inherited. He focused on characters (heritable features, such as flower color) and their traits (variants, such as purple or white flowers). By crossing true-breeding plants (which consistently produce the same trait) and hybrids (offspring of different varieties), Mendel observed how traits appeared and disappeared across generations.

• P generation: Parental plants

• F1 generation: First filial generation, offspring of the P generation

• F2 generation: Offspring from crosses among F1 individuals

• Discrete inheritance: Traits can disappear in one generation and reappear in the next, indicating that alleles retain their identity rather than blending permanently.

Genes, Alleles, and Homologous Chromosomes

Genes are segments of DNA located at specific loci on chromosomes. Each gene can exist in different forms called alleles. In diploid organisms, each individual inherits one allele from each parent for every gene.

• Homozygous: Both alleles are identical (e.g., PP or pp).

• Heterozygous: The two alleles are different (e.g., Pp).

• Genotype: The genetic makeup (allele combination) of an organism.

• Phenotype: The observable trait or characteristic.

• Complete dominance: The heterozygote displays the dominant phenotype.

Mendel’s Laws

Law of Segregation and Monohybrid Crosses

The law of segregation states that the two alleles for a gene separate during gamete formation, so each gamete receives only one allele. A monohybrid cross (e.g., Pp × Pp) demonstrates this principle, producing a 3:1 ratio of dominant to recessive phenotypes in the F2 generation, even though the genotypic ratio is 1:2:1.

• Punnett square: A diagram used to predict the outcome of a genetic cross.

• Genotype ratio: 1 PP : 2 Pp : 1 pp

• Phenotype ratio: 3 purple : 1 white (for complete dominance)

• Probability: Predicts expected outcomes over many offspring, not exact results in every family.

Testcross: Determining an Unknown Genotype

A testcross is used to determine the genotype of an individual with a dominant phenotype. This individual is crossed with a homozygous recessive individual. The resulting offspring phenotypes reveal whether the tested parent is homozygous dominant or heterozygous.

• BB × bb: All offspring are Bb (display dominant trait).

• Bb × bb: Offspring are 1 Bb (dominant) : 1 bb (recessive).

• Inference: The appearance of a recessive offspring indicates the tested parent is heterozygous.

Law of Independent Assortment and Dihybrid Crosses

The law of independent assortment states that alleles of different genes segregate independently during gamete formation, provided the genes are on different chromosomes or far apart on the same chromosome. A dihybrid cross (e.g., RrYy × RrYy) produces a 9:3:3:1 phenotypic ratio in the F2 generation if both genes assort independently.

• Gametes from RrYy: RY, Ry, rY, ry (each with equal probability)

• Phenotype ratio: 9 (both dominant) : 3 (one dominant, one recessive) : 3 (one recessive, one dominant) : 1 (both recessive)

• Linked genes: Genes located close together on the same chromosome may not assort independently.

Chromosome Theory of Inheritance

The chromosome theory of inheritance states that genes are located at specific loci on chromosomes, and the behavior of chromosomes during meiosis explains Mendel’s laws. Segregation of alleles corresponds to the separation of homologous chromosomes in meiosis I, while independent assortment is explained by the random orientation of chromosome pairs at metaphase I.

• Segregation: Each gamete receives one allele per gene.

• Independent assortment: Chromosome pairs align independently, leading to genetic variation.

Beyond Simple Dominance

Incomplete Dominance

Incomplete dominance occurs when the heterozygote’s phenotype is intermediate between the two homozygous phenotypes. For example, in snapdragons, crossing red (RR) and white (rr) flowers produces pink (Rr) offspring. The F2 generation from pink × pink yields a 1:2:1 ratio of red:pink:white flowers.

• Alleles still segregate: Red and white phenotypes reappear in the F2 generation, showing that alleles do not blend permanently.

• Complete dominance: The heterozygote has the same phenotype as one of the homozygotes, unlike incomplete dominance.

Genetic Ancestry and Sex-Linked Inheritance

Tracing Lineages with Genetic Markers

Genetic markers on the Y chromosome and mitochondrial DNA are used to trace paternal and maternal lineages, respectively. The Y chromosome is passed from father to son, while mitochondrial DNA is inherited from the mother. Shared genetic markers can indicate common ancestry, but identifying a specific historical ancestor requires more evidence than just a shared marker.

• Y-chromosome: Tracks paternal lineage.

• Mitochondrial DNA: Tracks maternal lineage.

• Limitations: Shared markers suggest common ancestry but do not prove identity of a specific ancestor.

Summary Table: Key Terms in Mendelian Genetics