Patterns of Inheritance

Genetics and Heredity

Genetics is the scientific study of heredity, which is the transmission of traits from one generation to the next. Gregor Mendel, working in the 1860s, discovered that parents pass discrete genes (heritable factors) to their offspring, and these genes retain their individual identities across generations.

• Heredity: The passing of traits from parents to offspring.

• Genetics: The study of heredity and variation in organisms.

• Gene: A unit of heredity responsible for a specific trait.

• Example: Mendel’s experiments with pea plants established foundational principles of inheritance.

Mendel’s Experiments and Terminology

Mendel used garden peas for their ease of cultivation and distinct varieties. He controlled reproduction by hand pollination, creating purebred varieties and crossing them to produce hybrids.

• Character: A heritable feature (e.g., flower color).

• Trait: A variant of a character (e.g., purple or white flowers).

• Hybrid: Offspring of two different purebred varieties.

• P generation: Parental plants.

• F1 generation: First filial generation, hybrid offspring.

• F2 generation: Second filial generation, offspring of F1 hybrids.

Mendel’s Law of Segregation

Mendel’s law of segregation states that pairs of alleles segregate during gamete formation, and fusion of gametes at fertilization restores allele pairs. This explains why traits can disappear in one generation and reappear in the next.

• Allele: Alternative version of a gene.

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

• Dominant allele: Determines phenotype in heterozygotes.

• Recessive allele: No noticeable effect in heterozygotes.

• Punnett square: Diagram showing possible genetic outcomes of a cross.

• Phenotype: Physical appearance.

• Genotype: Genetic makeup.

Relationship Between Alleles and Homologous Chromosomes

Alleles reside at the same locus on homologous chromosomes. These chromosomes may carry identical or different alleles at any locus.

• Gene locus: Specific location of a gene on a chromosome.

• Homologous chromosomes: Chromosome pairs carrying alleles of the same genes.

Mendel’s Law of Independent Assortment

This law states that each pair of alleles segregates independently during gamete formation. The inheritance of one character does not affect the inheritance of another.

• Monohybrid cross: Cross between individuals heterozygous for one character.

• Dihybrid cross: Cross between individuals heterozygous for two characters.

• Independent assortment: Seen in the inheritance of multiple characters, such as coat color and vision in Labrador retrievers.

Testcrosses and Probability

A testcross determines the genotype of an individual with a dominant phenotype by mating it with a homozygous recessive individual. Genetic crosses obey the rules of probability, such as the rule of multiplication.

• Testcross: Mating to reveal unknown genotype.

• Rule of multiplication: Probability of compound event is the product of individual probabilities.

Family Pedigrees

Pedigrees are family trees used to analyze inheritance patterns. Mendel’s laws help deduce genotypes from observed phenotypes.

• Pedigree: Diagram showing inheritance of traits in a family.

• Wild-type trait: Most common trait in nature, not necessarily dominant.

Human Traits Controlled by a Single Gene

Some human traits, such as freckles or a widow’s peak, are determined by a single gene and follow Mendelian inheritance patterns.

• Example: Inherited disorders like cystic fibrosis (recessive) and achondroplasia (dominant).

Table: Autosomal Disorders in People

Table 9.1 summarizes autosomal disorders inherited as dominant or recessive traits.

Variations on Mendel’s Laws

Not all inheritance patterns strictly follow Mendel’s laws. Complex patterns include incomplete dominance, codominance, pleiotropy, polygenic inheritance, and environmental effects.

• Incomplete dominance: Heterozygotes have intermediate phenotype (e.g., hypercholesterolemia).

• Codominance: Both alleles are expressed (e.g., AB blood type).

• Multiple alleles: More than two alleles for a gene (e.g., ABO blood groups).

• Pleiotropy: One gene affects multiple traits (e.g., sickle-cell disease).

• Polygenic inheritance: Multiple genes affect a single trait (e.g., height, skin color).

Epigenetics and Environmental Influence

Phenotypic characters often result from both genetic and environmental factors. Epigenetic inheritance involves transmission of traits by mechanisms not directly involving DNA sequence, such as chemical modifications to chromosomes.

• Epigenetic inheritance: Traits transmitted by chromosome modifications.

• Environmental influence: Non-genetic factors affecting phenotype.

Chromosomal Basis of Inheritance

The chromosome theory of inheritance states that genes are located at specific loci on chromosomes, and their behavior during meiosis and fertilization accounts for inheritance patterns.

• Segregation and independent assortment: Chromosomes undergo these processes during meiosis.

Linked Genes

Linked genes are located near each other on the same chromosome and tend to be inherited together, often not following independent assortment.

• Linked genes: Genes inherited as a set due to proximity on chromosome.

Sex Determination and Sex-Linked Genes

Sex chromosomes (X and Y) determine sex in humans. Most sex-linked genes are found on the X chromosome and exhibit unique inheritance patterns.

• Sex-linked gene: Gene located on a sex chromosome.

• Examples: Red-green colorblindness, hemophilia.

• Inheritance: Males are more frequently affected by X-linked recessive disorders.

Evolutionary Connections

Selective breeding and genetic analysis have led to the diversity of dog breeds. Genome sequencing enables construction of evolutionary trees, providing insights into genetic and evolutionary questions.

• Evolutionary tree: Diagram showing relationships among breeds based on genetic data.

Key Equations

• Rule of multiplication:

• Punnett square ratios: for genotype, for phenotype in monohybrid crosses.

Summary Table: Mendelian vs. Non-Mendelian Inheritance

Additional info:

• Dog breeding and genetic disorders in dogs are used as examples to illustrate principles of inheritance and evolution.

• Pedigrees and genetic testing are important tools in human genetics for predicting and diagnosing inherited disorders.