General Genetics Overview

General Genetics

• Mendel's laws are a core concept in the study of inheritance.
• Zoology and microbiology students are expected to understand these laws.
• The study of genetics, also known as heredity, focuses on the transmission of specific characteristics from parents to offspring.

Introduction

• Genetics is closely related to human cultural history, beginning around ten thousand years ago with the transition from nomadic to settled life.
• The development of agriculture prompted human exploration and understanding of natural processes, leading to early insights into inheritance.

Early Ideas about Inheritance

• The need for food led to the exploration of nature and the adoption of agriculture.
• The agricultural revolution spurred rapid improvements in human culture.
• Humans began identifying and cultivating plants like rice, wheat, and barley.
• Animal domestication (horses, cattle, camels, dogs) also occurred alongside plant cultivation.
• Understanding of reproductive methods in animals and plants gradually developed.
• Hybridization and the generation of new varieties of animals and plants were observed.
• Ideas on inheritance were adopted into everyday practice.

Greek Influence

• Hippocrates proposed the concept of "humors" that influenced disease and health, drawn from various body parts, and passed on to offspring.
• His theory is a forerunner to Darwin's early ideas on inheritance.
• Aristotle proposed that semen produced a "vital heat." This "heat" shaped the menstrual blood, giving rise to offspring with similar forms to their parents.

Later Ideas (1600-1850)

• Pre-formationism suggested that sex cells contain a miniature adult form.
• Epigenesis proposed that body structures are not initially present in sex cells but form later.
• Blending inheritance asserted that parental characteristics blend in offspring, like mixing paints.

Mendelian Genetics

• Gregor Mendel, an Austrian monk, is considered the father of genetics.
• His work on pea plants, using statistical methods, laid the foundation of modern genetics.
• His experiments, conducted in 1866, involved observing inheritance patterns in the garden pea plant (Pisum sativum).
• Mendel found that pea plants were easy to cultivate, had a short generation time, exhibited distinct heritable features and characteristics, and were capable of both self-pollination and cross-pollination.
• Mendel studied traits like seed coat color/flower color, seed color, seed shape, pod color, pod shape, stem height, flower placement, and flower color.

Mendel's Methods

• Mendel created true-breeding plants through self-pollination.
• These plants produced offspring identical to themselves.
• He conducted monohybrid crosses, comparing two different traits in true-breeding plants.
• His experiments used reciprocal crosses (e.g., pollen from a purple-flowered plant to a white-flowered plant and vice versa) across generations.

Monohybrid Crosses

• Mendel's monohybrid crosses illustrated that one trait can be fully dominant or recessive to another when these cross.
• When two contrasting true-breeding pea varieties are crossed (e.g., purple flowers X white flowers), the resulting F₁ offspring express only one of the two trait (e.g., all purple flowers).
• F₁ plants produced F₂ generation by self-fertilization.
• Mendel's record of F₂ plants showed a 3:1 ratio of purple to white flowers.

Production of True-Breeding Plants

• He produced true-breeding plants by self-pollination for several generations.
• True-breeding plants share the same traits via identical alleles for a particular trait and create offspring identical to the parent plants.
• True-breeding organisms have identical genetic make-up.

Test Cross

• Mendel's test cross technique was used to determine if a dominant trait is homozygous or heterozygous.
• A dominant-expressing organism is crossed with a homozygous recessive organism.
• The offspring will show a ratio depending on the genotype of the dominant-expressing parent.

Law of Independent Assortment

• Mendel's law of independent assortment asserts that genes are inherited independently.
• The presence of one trait does not influence the inheritance of another trait.
• This principle is demonstrated through a dihybrid cross that follows two different traits.

Dihybrid Cross

• This cross combines two different traits simultaneously.
• It shows that the inheritance of each trait is independent from the other.

Probabilities in Genetics

• Probabilities are used to quantify the likelihood of genetic events.
• Empirical probability is determined by counting observations.
• Theoretical probability is based on knowledge of rules and circumstances.
• The sum rule and product rule are essential for calculations.

Dominance

• Dominance describes how one allele masks or overrules another.
• Complete dominance, incomplete dominance, and codominance are types of dominance relationships.

Incomplete Dominance

• A heterozygote exhibits a distinctive phenotype between the homozygous phenotypes.
• This intermediate phenotype isn't a blend of the parental traits but rather a new phenotype.

Codominance

• Both alleles are expressed in the heterozygote.
• This distinct expression shows both parental trait combinations.

Genetic Vocabulary

• Summarized list of key genetic terms along with brief definitions.

Genes on Chromosomes

• Each locus, or gene location, can have multiple allele forms.
• Provides descriptions of characteristics associated with particular alleles.

Molecular Explanations of Incomplete Dominance and Codominance

• Different gene expression levels determine phenotype types in terms of which allele products are produced.