Genetics

Intended Learning Outcomes

• Recall several examples of the applications of genetic study.

• Define and understand genetic vocabulary, including:

  • Phenotype: The observable and measurable traits of an organism resulting from the expression of its genes and the influence of environmental factors.

  • Genotype: The specific combination of alleles an individual has for one or more traits.

  • Locus: The specific physical location of a gene or other significant sequence on a chromosome.

  • Gene: The basic unit of heredity, consisting of a segment of DNA that defines a specific trait.

  • Allele: Different forms of a gene, often resulting from mutations, which can affect the expression of a gene.

  • Dominant: An allele that expresses its trait even when only one copy is present in a heterozygous condition.

  • Recessive: An allele that only expresses its trait when two copies are present, in a homozygous condition.

  • Homozygous: Having two identical alleles for a specific gene (e.g., BB or bb).

  • Heterozygous: Having two different alleles for a specific gene (e.g., Bb).

  • Haploid: A cell containing one complete set of chromosomes (e.g., gametes in humans, n = 23).

  • Diploid: A cell containing two complete sets of chromosomes (e.g., somatic cells in humans, 2n = 46).

• Explain Mendel's experiments and outline his key findings regarding inheritance.

• Use a Punnett square to solve genetic crosses.

Introduction to Genetics & Heredity

Overview

• Genetics seeks to understand the biological basis of inheritance and the variation of life forms.

• Inquiry into why children resemble their parents involves exploring DNA and its role in heredity, challenging the previously accepted theory of blended inheritance.

• Before the 1950s, belief in environmental factors as sole contributors to inheritance was common.

The Paradox of Blended Inheritance

• The idea of blended inheritance suggests that traits from parents mix in offspring, implying that variation would be limited within species. However, this contradicts the observable diversity of life.

• The study of genetics addresses these contradictions by exploring heredity at a molecular level.

Timeline of Genetic Discovery

• 1865: Mendel publishes findings on inheritance patterns in pea plants, laying groundwork for genetics.

• 1902: Sutton and Boveri propose the Chromosome Theory of Heredity.

• 1927: Muller shows X-rays induce mutations.

• 1944: Avery, McLeod, and McCarty show DNA is the hereditary material.

• 1952: Hershey and Chase confirm that DNA is responsible for heredity using radioactive labeling.

• 1953: Watson and Crick propose the double helix structure of DNA.

• 1990-2003: The Human Genome Project maps human DNA.

Application of Genetics

Why Study Genetics?

• Understanding Life: Genetics helps decipher the evolutionary relationships and development processes among organisms.

• Human Origins: Study of genetic sequences provides insights into human ancestry, revealing connections such as the presence of Neanderthal DNA in modern Eurasians.

• Agriculture: Selective breeding and GMOs improve food production to address increasing population needs.

• Aquaculture: Enhances production in fish farming through selective breeding.

• Biofuels: Genetic modifications can improve the efficiency of biofuel production from microalgae.

• Human Disease: Identifying genetic disorders offers pathways for treatment and understanding complex diseases.

Evolutionary Relationships

• Macro and Microevolution: Genetics provides a framework for analyzing both macroevolution (major evolutionary changes) and microevolution (small changes within species).

• Genetic Variation and Adaptation: Studies show how minor genetic mutations can contribute to distinct physical characteristics, illustrated by the beach mouse color pattern study (Hoekstra et al).

Heritability of Traits

• Definition: Heritability is the proportion of variation in a phenotypic trait that can be attributed to genetic variation among individuals in a population, excluding environmental impacts.

Agricultural Applications of Genetics

Green Revolution

• Introduction of high-yielding crop varieties (dwarf wheat, rice) increased food production.

• Selective breeding has produced disease-resistant cultivars, improving food security.

• Downsides: Issues such as monocultures and reduced biodiversity arise from these practices.

Aquaculture Enhancements

• Selective breeding improves key performance metrics like growth rates and disease resistance in aquatic species.

Biofuel Production

• Genetic engineering of microalgae allows for biomass accumulation of lipids, which can be converted into sustainable biofuels while using waste CO2.

Genetic Disorders in Humans

• Muscular Dystrophy: A genetic disorder resulting in muscle deterioration.

• Cystic Fibrosis: A condition affecting mucus production leading to respiratory complications.

• Huntington's Disease: A late-onset genetic disorder characterized by nerve degeneration.

• Diabetes and various others, which significantly affect quality of life and can also have complex inheritance patterns.

Key Genetic Disorders Listed

1. Muscular Dystrophy

2. Hemophilia A

3. Sickle-Cell Anemia

4. Cystic Fibrosis

5. Tay-Sachs Disease

6. Huntington Disease

7. Familial Hypercholesterolemia

8. Alzheimer Disease

9. And others…

Genetics Vocabulary

Key Terms and Definitions

• DNA (Deoxyribonucleic acid): The chemical that contains the genetic instructions for the development and functioning of living organisms. It forms a double helix with a sugar-phosphate backbone and nucleotide base pairs (A-T, C-G).

• Chromatin: The material within a chromosome consisting of DNA and proteins; exists in a less compact form during interphase.

• Chromosome: A thread-like structure made of DNA and proteins, visible during cell division.

• Chromatid: Each of the two identical halves of a replicated chromosome.

• Centromere: The region of a chromosome where the two sister chromatids are joined.

• Haploid (n): A cell with half the number of chromosomes, like gametes (e.g., n = 23 in humans).

• Diploid (2n): A cell with a full set of paired chromosomes (2n = 46 in humans).

• Gene: A unit of heredity; sections of DNA that code for proteins or are involved in regulating traits.

• Allele: Variants of a gene; different versions that can exist at a specific locus.

• Homozygous: Having two identical alleles for a particular trait (e.g., BB or bb).

• Heterozygous: Having two different alleles for a trait (e.g., Bb).

• Locus (loci): The specific site of a gene on a chromosome.

• Phenotype: The physical expression of traits, influenced by genetic makeup and the environment.

• Genotype: The genetic constitution of an organism, representing the alleles present.

• Dominant allele: An allele that expresses its trait in the presence of another allele.

• Recessive allele: An allele that is only expressed in a homozygous state.

Mendel's Experiments

Overview

• Gregor Mendel (1822-1884): Austrian monk who conducted pioneering genetic experiments using pea plants to understand heredity.

• Importance of Methodology: Mendel utilized careful experimental design focusing on measurable traits, ensuring controlled purebred lines, and employing reciprocal crosses.

Key Characters Studied

• Mendel highlighted 7 significant traits with contrasting forms:

  • Stem length: Short vs Tall

  • Seed shape: Round vs Wrinkled

  • Flower color: Purple vs White

  • Pod shape: Inflated vs Constricted

  • Pod color: Green vs Yellow

Findings

F1 Generation

• Cross between pure purple and pure white plants resulted only in purple flowers, contradicting the blended inheritance theory.

• Mendel labeled the purple trait as dominant and white as recessive.

F2 Generation

• When F1 plants self-fertilized, both purple and white plants appeared in a 3:1 ratio (705 purple: 224 white), indicating retention of genetic information.

F3 Generation

• Further self-fertilization indicated a ratio of 1:2:1 among dominant traits: 1 pure breeding dominant, 2 not pure breeding dominant, 1 pure breeding recessive.

Mendel's Conclusions on Inheritance

1. Each trait is determined by two alleles inherited from parents (one from each).

2. Alleles can be dominant or recessive; dominant traits mask recessive traits in heterozygotes.

3. Alleles assort independently during gamete formation, adhering to the law of segregation.

Punnett Squares

Definition

• A Punnett square is a diagram used to predict the genotype and phenotype ratios in offspring from genetic crosses.

• Named after Reginald C. Punnett, it serves as a straightforward method to visualize inheritance patterns.

Application

• Gametes: The alleles from each parent are placed along the edges (rows and columns) of the square, representing possible combinations.

• The inner squares result from crossing these gametes together to provide genotype predictions for the offspring.

Worked Examples

Simple Monohybrid Crosses

• Example: Crossing YY (homozygous dominant) with yy (homozygous recessive).

  • F1 Generation: All Yy (heterozygous dominant).

  • F2 Generation Phenotypic Ratio: 3 Yellow (YY or Yy) : 1 Green (yy).

  • F2 Generation Genotypic Ratio: 1 (YY) : 2 (Yy) : 1 (yy).