Ch_5_Lecture on Inheritance part 2

Chapter Overview

Title: Inheritance

Source: Biology: The Core Third Edition

Publisher: Pearson Education, Inc.

Key Concepts

Definitions and Historical Background

• Heredity: The biological process through which traits, attributes, and genetic characteristics are transmitted from one generation to another through the mechanisms of reproduction.

• Genetics: The scientific study of heredity and the variation of inherited characteristics, including the roles of DNA, genes, and chromosomes in heredity.

• Gregor Mendel: Often referred to as the father of genetics, Mendel was an Austrian monk who, in the mid-1800s, conducted groundbreaking experiments with pea plants. He discovered fundamental laws of inheritance, establishing the foundation for modern genetics. His principles were widely recognized after the structure of DNA was elucidated in 1953.

Traits and Phenotypes

Dominant and Recessive Traits

• Phenotype: The observable physical characteristics of an individual, such as eye color, height, or specific traits like the ability to roll one’s tongue.

• Genotype: The genetic constitution of an individual, representing the alleles present on their chromosomes that affect the traits observed in the phenotype.

Common Traits

• Dominant Traits: Examples include bent little finger, tongue rolling, presence of dimples in the chin, free earlobes, left thumb crossing over the right, hitchhiker’s thumb, presence of hair on fingers, dimpled cheeks, curly hair, freckles, the ability to raise eyebrows, widow’s peak, and the ability to wiggle ears. These traits can mask the expression of recessive traits when present.

Alleles and Genes

Concepts

• Alleles: Different forms of a gene that may result in varying traits; individual alleles are labeled with the same letter to signify their relationship (e.g., A for dominant and a for recessive).

• Individuals inherit one allele from each parent, leading to two types of genotypes:

  • Homozygous: When both alleles in the pair are the same (e.g., AA or aa).

  • Heterozygous: When the alleles in the pair are different (e.g., Aa).

• Dominant alleles will be expressed in the phenotype, while recessive alleles require two copies to manifest the associated traits.

Determining Phenotype

Dominant vs. Recessive Alleles

• In cases where an organism is heterozygous for a trait, the presence of a dominant allele will dictate the phenotype, while recessive alleles only manifest phenotypic expression in homozygous conditions.

• Example: In pea plants, purple flowers, represented by the allele (P), are dominant over the white flowers (p), which are recessive. Thus, a plant with genotype PP or Pp exhibits purple flowers, while only a genotype of pp yields white flowers.

Monohybrid Crosses

Basics

• A Monohybrid Cross focuses on the inheritance of a single trait, usually involving a contrast between dominant and recessive alleles.

• Example: Crossing a pea plant with purple flowers (PP) with a plant with white flowers (pp) results in all offspring (F1 generation) exhibiting purple flowers, and they will all have a heterozygous genotype (Pp).

Punnett Squares

• Using Punnett Squares: This predictive tool helps determine the probability of offspring genotypes based on parental alleles.

• Example with homozygous dominant (BB) and homozygous recessive (bb): This pair will produce all heterozygous offspring (Bb).

• Example with heterozygous (Bb) and homozygous recessive (bb): This results in a 50% chance of producing heterozygous offspring (Bb) and 50% homozygous recessive offspring (bb).

Complex Patterns of Inheritance

• Recessive Genes: Despite their phenotypic invisibility in heterozygous individuals, recessive genes can be prevalent in the population, leading to conditions such as dwarfism and polydactyly.

• Human Genetic Disorders: Many genetic disorders in humans are recessive, and individuals may carry these alleles without showing symptoms. To confirm carrier status or identify genetic conditions, genetic testing is essential.

Non-Mendelian Genetics

• Incomplete Dominance: In cases of incomplete dominance, heterozygous individuals display an intermediate phenotype rather than showing a dominant trait completely.

• Example: In snapdragon plants, crossing a red-flowered parent (RR) with a white-flowered parent (rr) results in pink offspring (Rr).

• Codominance: Here, both alleles are expressed equally in the phenotype of the heterozygous offspring.

• Example: Hybrid flowers can have both black and white petals (Rr).

Multiple Alleles and Polygenic Traits

• Some traits are influenced by more than two alleles, resulting in broader genetic diversity (e.g., human blood types: A, B, AB, and O).

• Multiple Allele Examples:

  • Blood type A is defined by genotypes IAIA or IAi

  • Blood type B is defined by genotypes IBIB or IBi

  • Blood type AB is defined as IAIB

  • Blood type O is defined as ii

• Polygenic Inheritance: Certain traits, such as height and skin color, are influenced by multiple genes, allowing for a continuous range of phenotypic expressions rather than discrete categories.

Sex-Linked Traits

• Human Genetics: Males have one X chromosome and one Y chromosome, while females have two X chromosomes. This chromosomal difference affects the inheritance of certain traits.

• X-linked Traits: Traits linked to the X chromosome may display different patterns of inheritance and expression between genders, such as hemophilia.

• Males are generally more susceptible to expressing X-linked recessive traits due to their single X chromosome, meaning any recessive allele on this chromosome is expressed phenotypically.

Epigenetics

• Environmental Influence: Epigenetics refers to the study of how external environmental factors can modify gene expression, which may lead to variations in phenotype without altering the underlying DNA sequence.

• Examples: The coat color in certain cats can change based on environmental conditions, such as temperature.

• Transgenerational Effects: Influences affecting parental gametes might be passed down through generations, creating phenotypic changes in descendants due to environmental factors experienced by ancestors.