Lecture10-1_Genetics

Chapter 8: Genetics

Lecture Outline

1. Definitions

2. Genetics and Heredity

   • Genotype: Homozygous, Heterozygous

   • Phenotype and Expression Pattern

3. Laws of Inheritance

   • Law of Segregation

   • Law of Independent Assortment

   • Law of Dominance

4. Predicting Offspring Genotypes

   • Punnett Squares and Probability

   • Test Cross

   • Dihybrid Cross (2 characteristics)

5. Mendel’s Experiments

   • Experimental Design

   • Results and Analysis

6. Non-Mendelian Patterns

   • Incomplete Dominance

   • Codominance

   • Polygenic / Continuous Traits

   • Sex-linked Traits

1. Definitions

What is Genetics?

• Genetics: The study of heredity and the variation of inherited characteristics. It encompasses the molecular structure and function of genes and the methods by which these traits are inherited.

• Heredity: This is the process through which genetic information is transmitted from parents to offspring, influencing their phenotype and potential behaviors.

Milestones in Genetics

• Development of the Rules of Inheritance by Gregor Mendel, which laid the groundwork for modern genetics.

• Analysis of DNA function and structure by James Watson and Francis Crick, establishing the double-helix model of DNA.

• The Human Genome Project: A monumental scientific endeavor that mapped the entire human genome, providing insights into genetic disorders and human ancestry.

Traits, Alleles, and Variation

• Trait: An observable characteristic that can be influenced by genetic and environmental factors, such as flower color, height, and leaf shape.

• Heritable: Traits that can be passed down through generations due to their genetic basis, frequently governed by multiple genes and environmental interactions.

• Allele: Different forms or versions of a gene that can lead to variations in traits, e.g., the color of flowers can be due to alleles coding for purple or white flowers.

• In diploid organisms, two copies of each chromosome exist, meaning there are usually two alleles for each gene, one inherited from each parent, contributing to the genetic diversity in offspring.

Traits - Discrete vs. Continuous

• Discrete Traits: These traits exhibit clear, defined categories (e.g., Mendel's pea plants manifesting only two variations: green or yellow). They are often the result of single-gene influences.

• Continuous Traits: Traits that exhibit a range of variations and do not fit neatly into categories, influenced by multiple genes and environmental factors (e.g., height, skin color, and susceptibility to diseases).

2. Laws of Inheritance

Johann Gregor Mendel (1822 – 1884)

• Mendel, often referred to as the "father of genetics," conducted extensive experiments on pea plants over a decade, identifying patterns in how traits are inherited and establishing the fundamental principles of genetics.

Key Contributions:

• Formulation of three core laws: Law of Segregation, Law of Independent Assortment, and Law of Dominance, derived from his meticulous cross-breeding experiments.

Law of Segregation

• States that allele pairs segregate during gamete formation and recombine at fertilization. Each parent contributes one allele, resulting in a variety of offspring genotypes and phenotypes.

• Mechanism: This separation occurs during Metaphase I of meiosis when homologous chromosomes line up at the equatorial plane.

• Implication: A homozygous parent (e.g., YY) produces gametes with the same allele, while a heterozygous parent (e.g., Yy) produces gametes with both alleles, leading to predictions of genotypic ratios in offspring.

Law of Independent Assortment

• Asserts that alleles of different genes assort independently during gamete formation, hence will not influence one another’s inheritance unless they are linked on the same chromosome.

• Example: This means that if one gene governs flower color and another governs seed shape, the inheritance of these traits is not affected by each other, leading to vast genetic variation.

Law of Dominance

• Dominant traits mask the expression of recessive traits, meaning a heterozygous organism (Yy) will express the dominant phenotype.

• Phenotypic Expression: The trait observed in the organism is the result of the genotype, along with the expression pattern dictated by dominance/recessiveness.

3. Predicting Offspring Genotypes and Phenotypes

Punnett Squares as a Predictive Tools

• Purpose: To visually represent and predict the possible genotype and phenotype combinations of offspring based on parental genotypes.

• Test Cross: A method employed to ascertain the genotype of an organism expressing a dominant phenotype by crossing it with an organism with a known recessive phenotype.

Punnett Square Setup – Notation

• Identify the trait in question (e.g., Flower Color).

• Determine the alleles for the trait (dominant and recessive).

• Assign letters to represent alleles (e.g., "P" for purple, "p" for white).

Punnett Square Offspring Genotypes

• Calculate the potential genotypes of offspring by considering every combination of alleles from the parents through the square.

• For example, in a cross of heterozygous parents (Pp x Pp):

  • 25% chance PP (homozygous dominant)

  • 50% chance Pp (heterozygous)

  • 25% chance pp (homozygous recessive)

  • Resulting genotypic ratio: 1 PP : 2 Pp : 1 pp.

Probability in Genetics

• Probability: The likelihood of a specific event occurring; in genetics, this refers to the chance of inheriting a particular trait.

• Product Rule: To find the probability of two independent events happening, you multiply their individual probabilities.

• Sum Rule: To determine the probability of one of multiple mutually exclusive events, you add the probabilities of each event.

4. Dihybrid Cross: Two Attributes

• Dihybrid crosses simultaneously assess two different traits, such as seed color (Yellow vs Green) and seed shape (Round vs Wrinkly).

• For instance, if both parents are heterozygous for both traits (YyRr), they produce various combinations, which can lead to a phenotypic ratio of 9:3:3:1 among the offspring, showcasing the diversity of genetic inheritance patterns.

5. Non-Mendelian Patterns

• In addition to Mendelian inheritance, several other inheritance patterns exist:

  • Incomplete Dominance: A form of intermediate inheritance where the dominant allele does not completely mask the effects of the recessive allele, leading to a blend of the two traits (e.g., red and white flowers producing pink offspring).

  • Codominance: Both alleles in a gene pair are fully expressed, resulting in offspring with a phenotype that is neither dominant nor recessive (e.g., AB blood type).

  • Polygenic Traits: Traits controlled by multiple genes, leading to a continuous range of possibilities (e.g., height, skin tone).

  • Sex-linked Traits: Traits that are associated with genes located on sex chromosomes, often leading to differences in expression between males and females (e.g., color blindness, hemophilia).