Patterns of Inheritance

Lecture Objectives

  • Discuss Gregor Mendel’s discoveries, experiments, and laws.

  • Describe traditional forms of inheritance that Mendel discovered.

  • Describe concepts such as incomplete dominance, codominance, multiple alleles, pleiotropy, polygenic inheritance, linked genes, and sex-linked genes.

  • Complete monohybrid and dihybrid crosses.

  • Interpret a pedigree.

Mendel's Uncovering of Basic Laws of Inheritance

  • Gregor Mendel utilized pea plants to conduct his studies on heredity.

  • Reason for Choosing Pea Plants:

    • Pea plants presented easily observable and discrete “heritable factors” such as flower color and seed shape, which included the following traits:

    • Flower color: Purple, White

    • Seed shape: Round, Wrinkled

    • Seed color: Yellow, Green

    • Flower position: Axial, Terminal

    • Pod color: Green, Yellow

    • Pod shape: Inflated, Constricted

    • Stem length: Tall, Dwarf

  • Definitions:

    • Character: A heritable feature that varies among individuals.

    • Trait: A variant of a character.

    • Mendel studied characters that occurred in two distinct traits.

Structure of Flowers in Pea Plants

  • Carpels: Female reproductive parts (produce eggs).

  • Stamens: Male reproductive parts (produce pollen).

  • Mendel controlled self-pollination and cross-pollination to perform his experiments.

Mendel’s Experimental Procedure

  • Removed stamens from a purple flower and transferred pollen from stamens of a white flower to the carpel of the purple flower.

  • Parents in the experiments are referred to as the parental (P) generation.

  • Their hybrid offspring are called the F1 generation, and a cross of the F1 plants results in the F2 generation.

Monohybrid Cross

  • Definition: A cross between purebred parent plants that differs in only one character.

  • Hybrids: Offspring produced from two different purebred varieties.

  • Generational Labels:

    • P generation (parental)

    • F1 generation (first offspring)

    • F2 generation (second offspring; typically displayed in a 3:1 ratio for dominant to recessive traits)

Mendel's Principles of Inheritance

  • Alternative versions of genes are referred to as alleles (e.g., purple and white for flower color).

  • Genotype: The specific combination of alleles an organism carries.

  • Phenotype: The physical characteristics expressed as a result of the genotype.

  • An organism gets two alleles for each inherited character (one from each parent).

    • Organism is homozygous for a gene if both alleles are identical.

    • Organism is heterozygous for a gene if the alleles differ.

    • Dominant Allele: Determines appearance if present.

    • Recessive Allele: Has no noticeable effect on appearance when dominant allele is present.

Examples of Seed Color Inheritance

  • Yellow seed color is dominant (Y) and green seed color is recessive (y).

    • Genotypes and their corresponding phenotypes:

    • Homozygous dominant (YY) → Yellow

    • Heterozygous (Yy) → Yellow

    • Homozygous recessive (yy) → Green

Law of Segregation

  • States that the two alleles for a gene segregate from one another during gamete production, meaning that each gamete carries only one allele for each character.

  • This law is explained by the behavior of homologous chromosomes during meiosis.

Punnett Squares

  • Purpose: To visualize and predict the offspring genotypes and phenotypes from parental crosses.

  • Example: A monohybrid cross between two heterozygous parents (Yy x Yy).

    • Produces offspring in a 3:1 phenotype ratio (3 yellow, 1 green) and a 1:2:1 genotype ratio (1 YY : 2 Yy : 1 yy).

Test Cross

  • Definition: A method to determine an organism's unknown genotype by mating it with a homozygous recessive individual.

    • Example: A black lab (with unknown genotype) crossed with a chocolate lab (genotype bb).

  • Test Cross Results:

    • If the black lab is BB, all offspring will be black (Bb).

    • If the black lab is Bb, there will be a 1:1 ratio of black (Bb) to chocolate (bb) offspring.

Dihybrid Cross

  • Definition: A genetic cross that examines the inheritance of two traits simultaneously, demonstrating Mendel's law of independent assortment.

  • Gamete combinations for BbDd cross:

    • Possible allele combinations:

    • BD, Bd, bD, bd

  • Resulting offspring phenotypes (9:3:3:1 ratio) include:

    • 9 Black, Hearing

    • 3 Black, Deaf

    • 3 Chocolate, Hearing

    • 1 Chocolate, Deaf

Inheritance Complications

  • Discussion of non-Mendelian inheritance patterns:

    • Incomplete dominance: Heterozygotes exhibit an intermediate phenotype.

    • Codominance: Heterozygotes display both alleles (e.g., ABO blood types).

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

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

    • Polygenic inheritance: Multiple genes contribute to a single phenotype (e.g., skin color, height).

    • Linked genes: Genes located close to each other on the same chromosome tend to be inherited together.

    • Environmental effects: Environmental factors can influence phenotypic expression.

ABO Blood Types

  • Genotypes and Phenotypes:

    • Type A: AA or Ai, surface molecule A.

    • Type B: BB or Bi, surface molecule B.

    • Type AB: AB, surface molecules A and B.

    • Type O: ii, no surface molecules.

    • The ABO blood types are determined by the presence of specific sugars added to the cell surface by corresponding alleles.

Pleiotropy Example - Sickle-cell Disease

  • One gene influences multiple phenotypes, leading to various symptoms such as weakness, pain, and organ damage due to abnormal hemoglobin.

Polygenic Inheritance and Continuous Variation

  • Traits like skin color and height exhibit a range of phenotypes due to the additive effects of multiple genes.

Linked Genes and Crossing Over

  • Genes located on the same chromosome are considered linked and do not assort independently during gamete formation.

  • Crossing Over: A process during meiosis that can separate linked genes, allowing for recombination of alleles.

Sex-Linked Genes

  • Inheritance patterns differ for sex-linked traits (located on X and Y chromosomes).

    • Males inherit one allele from their mother (X) and one Y chromosome.

    • Females need two recessive alleles for expression of X-linked recessive disorders.

  • Example: X-inactivation occurs in females, preventing double expression of genes from both X chromosomes.

Pedigree Analysis

  • Pedigrees are useful tools for tracking inheritance patterns of traits in families. Each symbol in a pedigree diagram represents individuals’ traits (affected or unaffected).

    • Example cases may involve determining genotypes of parents based on offspring traits and analyzing carrier status.

Chapter Summary and Review Materials

  • The chapter reviews Mendelian genetics, including law of segregation, law of independent assortment, and various inheritance complexities.

  • Refer to provided pages for summaries, multiple choice questions, and practical problem-solving strategies in genetics (pages 191-195).