Genetics and Scientific Method: Study Notes for General Biology

Genetics and Scientific Method

Scientific Method and Hypothesis Formation

The scientific method is a systematic approach used in biology to investigate phenomena, acquire new knowledge, or correct and integrate previous knowledge. Hypothesis formation is a critical step in this process.

• Characteristics of a Good Hypothesis:

  • Testable and falsifiable

  • Based on prior knowledge and observations

  • Specific and clearly stated

  • Predicts an outcome

• Variables in Experiments:

  • Independent Variable: The factor that is changed or manipulated in an experiment.

  • Dependent Variable: The factor that is measured or observed in response to changes in the independent variable.

• Graphing Experimental Data:

  • Independent variable is plotted on the x-axis.

  • Dependent variable is plotted on the y-axis.

• Example: Testing the effect of sunlight (independent variable) on plant growth (dependent variable).

Genetic Diversity and Biodiversity

Genetic diversity is the variety of genes within a species, contributing to overall biodiversity and the ability of populations to adapt to changing environments.

• Sources of Genetic Diversity:

  • Mutation

  • Gene flow

  • Sexual reproduction

  • Genetic recombination during meiosis

• Contribution to Biodiversity: Genetic diversity allows populations to survive environmental changes and resist diseases.

• Example: Variation in beak shapes among Darwin's finches enables adaptation to different food sources.

Chromosomal Makeup and Meiosis

Meiosis is a type of cell division that reduces the chromosome number by half, producing gametes for sexual reproduction. Chromosomal makeup changes during meiosis, leading to genetic variation.

• Chromosome Number: Diploid (2n) cells have two sets of chromosomes; haploid (n) cells have one set.

• Homologous Chromosomes: Chromosomes that have the same genes at the same loci but may have different alleles.

• Example: Human somatic cells are diploid (2n = 46); gametes are haploid (n = 23).

Sexual vs. Asexual Reproduction

Organisms can reproduce sexually or asexually, each method having advantages and disadvantages.

• Sexual Reproduction:

  • Involves fusion of gametes from two parents

  • Increases genetic diversity

  • Requires more energy and time

• Asexual Reproduction:

  • Involves a single parent

  • Produces genetically identical offspring

  • Efficient and rapid

• Example: Bacteria reproduce asexually by binary fission; most animals reproduce sexually.

Mendelian Genetics: Dominance and Probability

Mendelian genetics explains how traits are inherited through dominant and recessive alleles. Probability calculations help predict offspring genotypes and phenotypes.

• Dominant and Recessive Alleles:

  • Dominant allele (T): Expressed when present

  • Recessive allele (t): Expressed only when two copies are present

• Monohybrid Cross: Cross between two individuals focusing on one trait

• Example: Tall (T) vs. short (t) pea plants

• Probability Calculation:

  • Use Punnett squares to determine possible genotypes and phenotypes

  • Probability of a specific genotype = (number of ways to get genotype) / (total possible outcomes)

• Example: Cross between Tt (heterozygous tall) and tt (homozygous short) plants:

  • Punnett square shows 50% Tt (tall), 50% tt (short)

Dihybrid Crosses and Independent Assortment

Dihybrid crosses involve two traits and demonstrate the principle of independent assortment, where alleles of different genes segregate independently during gamete formation.

• Dihybrid Cross: Cross between individuals heterozygous for two traits (e.g., BbTt x BbTt)

• Phenotypic Ratio: Typical ratio for dihybrid cross is 9:3:3:1

• Example: Black-furred (B) and short-tailed (T) guinea pigs

• Equation: $P(A \, \text{and} \, B) = P(A) \times P(B)$

Incomplete Dominance and Codominance

Not all traits follow simple dominance. Incomplete dominance and codominance are patterns where heterozygotes show intermediate or combined phenotypes.

• Incomplete Dominance: Heterozygote displays a phenotype intermediate between two homozygotes

• Codominance: Both alleles are fully expressed in the heterozygote

• Example: Red (RR) and white (WW) flowers produce pink (RW) flowers in incomplete dominance

Epistasis and Gene Interaction

Epistasis occurs when one gene affects the expression of another gene, leading to complex inheritance patterns.

• Epistatic Interaction: One gene masks or modifies the effect of another gene

• Example: Coat color in mice, where one gene determines pigment and another gene controls pigment deposition

Quantitative Inheritance and Polygenic Traits

Quantitative inheritance involves multiple genes contributing to a single trait, resulting in continuous variation and a bell-shaped distribution in the population.

• Polygenic Traits: Traits controlled by two or more genes, such as height or skin color

• Bell Curve Distribution: Most individuals have intermediate phenotypes; few have extreme phenotypes

• Equation: $\text{Phenotype} = \text{Genotype} + \text{Environment}$

• Example: Human height is influenced by many genes and environmental factors

HTML Table: Comparison of Mendelian and Non-Mendelian Inheritance

The following table summarizes key differences between Mendelian and Non-Mendelian inheritance patterns:

Inheritance Type	Key Features	Example
Mendelian	Simple dominance, segregation, independent assortment	Pea plant height (Tall vs. short)
Incomplete Dominance	Heterozygote shows intermediate phenotype	Pink flowers from red and white parents
Codominance	Both alleles fully expressed	AB blood type
Epistasis	One gene affects expression of another	Coat color in mice
Polygenic Inheritance	Multiple genes contribute to trait	Human height, skin color

Additional info: Academic context and examples have been added to clarify and expand upon the original questions, making the notes self-contained and suitable for exam preparation.