General Biology Exam 1 Review: Genetics, Experimental Design, and Biodiversity

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Scientific Method and Experimental Design

Characteristics of a Good Hypothesis

A hypothesis is a testable statement that explains an observation or answers a scientific question. Good hypotheses are:

Testable: Can be evaluated through experimentation or observation.
Falsifiable: Can be proven wrong by evidence.
Specific: Clearly defines variables and expected outcomes.
Based on prior knowledge: Informed by existing research or observations.
Predictive: Suggests a possible outcome that can be measured.

Role of the Independent Variable in an Experiment

The independent variable is the factor that is deliberately changed or manipulated by the experimenter to observe its effect on the dependent variable. It is plotted on the x-axis of a graph.

Purpose: To determine how changes in the independent variable affect the outcome (dependent variable).
Example: In a plant growth experiment, the amount of sunlight is the independent variable.

Graphing Variables

Graphs are used to visually represent the relationship between variables in an experiment.

Line Graph: Time (independent variable) is plotted on the x-axis; the dependent variable (e.g., plant height) is plotted on the y-axis.
Labeling Axes: Always label axes with variable names and units.

Biodiversity and Genetic Diversity

Genetic Diversity and Biodiversity

Genetic diversity refers to the variety of genes within a species. It contributes to biodiversity by enabling populations to adapt to changing environments, resist diseases, and maintain ecosystem stability.

Higher genetic diversity increases the chances of survival under environmental stress.
Example: A population with diverse alleles can better withstand a new pathogen.

The Greenhouse Effect and Global Warming

The greenhouse effect is the process by which certain gases in Earth's atmosphere trap heat, keeping the planet warm enough to support life. Excess greenhouse gases (e.g., CO2, CH4) from human activities enhance this effect, leading to global warming.

Key greenhouse gases: Carbon dioxide, methane, water vapor.
Role in global warming: Increased concentrations lead to higher average global temperatures.

Cell Division and Meiosis

Chromosomal Makeup During Meiosis I

In a cell with 2n = 2 (diploid number of 2), during Meiosis I:

Each chromosome consists of two sister chromatids.
Homologous chromosomes pair up (one from each parent).
Non-sister chromatids are chromatids from homologous chromosomes.

Diagram: Two pairs of chromosomes, each with two chromatids, aligned as homologous pairs.

Meiosis and Sexual Reproduction

Meiosis is essential for sexual reproduction because it reduces chromosome number by half, producing haploid gametes. This allows for genetic recombination and diversity.

Benefits: Increases genetic variation, which is advantageous in changing environments.
Disadvantages: Requires more energy and time than asexual reproduction.

Changing Environment Hypothesis (C. elegans Experiment)

The experiment with C. elegans (nematodes) grown with or without pathogens supports the changing environment hypothesis:

Sexual reproduction is favored in environments with changing threats (e.g., pathogens).
Sexually reproducing populations adapt more quickly to new challenges.

Mendelian Genetics

Monohybrid Crosses and Phenotypic Ratios

Monohybrid crosses involve one gene with two alleles, one dominant and one recessive.

Pea Plant Example: Tall (T) is dominant, short (t) is recessive.
Cross: Tt (heterozygous tall) x tt (homozygous short)

Genotype	Phenotype	Ratio
Tt	Tall	1
tt	Short	1

Phenotypic ratio: 1 Tall : 1 Short

Probability in Genetic Crosses

Rabbit Example: Black (B) is dominant, white (b) is recessive.
Cross: Bb x Bb

Genotype	Phenotype	Probability
BB	Black	1/4
Bb	Black	1/2
bb	White	1/4

Probability of white-furred offspring: 1/4

Genotype Ratios in Crosses

Fruit Fly Example: Red eyes (R) dominant, white eyes (r) recessive.
Cross: Rr x rr

Genotype	Ratio
Rr	1
rr	1

Genotype ratio: 1 Rr : 1 rr

Phenotypic Ratios in Dihybrid Crosses

Horse Example: Brown (B) dominant, chestnut (b) recessive.
Cross: Bb x Bb

Genotype	Phenotype	Ratio
BB	Brown	1
Bb	Brown	2
bb	Chestnut	1

Phenotypic ratio: 3 Brown : 1 Chestnut

Independent Assortment and Linked Genes

Guinea Pig Example: Fur color (B/b) and fur length (L/l) segregate independently.
Cross: BbLl x bbll

Fur Color	Fur Length
Black	Short
Black	Long
Brown	Short
Brown	Long

Possible phenotypic combinations: Black short, black long, brown short, brown long

Fruit Fly Example: Wing shape (W/w) and eye color (E/e) are linked.
Cross: WwEe x wwee

Wing Shape	Eye Color	Expected Ratio
Winged	Red-eyed	1
Wingless	White-eyed	1

Expected phenotype ratio: 1 winged, red-eyed : 1 wingless, white-eyed (assuming no recombination)

Complex Patterns of Inheritance

Multiple Alleles and Range of Phenotypes

Some traits are controlled by more than two alleles, leading to a range of phenotypes. This is called multiple allele inheritance.

Example: Human blood type (A, B, O alleles).
Difference from Mendelian inheritance: More than two possible genotypes and phenotypes.

Incomplete Dominance vs. Complete Dominance

Incomplete dominance occurs when the heterozygote has an intermediate phenotype between the two homozygotes.

Phenotypic ratio in incomplete dominance: 1:2:1 (e.g., red, pink, white flowers).
Complete dominance: Heterozygote shows the dominant phenotype; ratio is typically 3:1.

Environmental Effects on Phenotype

Genetically identical organisms can exhibit different phenotypes due to environmental influences.

Example: Plants grown in different light conditions may vary in height or leaf color.
Reason: Environment affects gene expression and development.

Pleiotropy vs. Gene Interaction

Pleiotropy occurs when a single gene affects multiple traits. Gene interaction involves multiple genes influencing a single trait.

Pleiotropy example: The gene for sickle cell hemoglobin affects red blood cell shape, resistance to malaria, and other symptoms.
Gene interaction example: Skin color is influenced by several genes, each contributing to the overall phenotype.

Quantitative Inheritance and Bell Curve Distribution

Traits governed by quantitative inheritance (polygenic traits) are controlled by multiple genes, resulting in continuous variation and a bell-shaped curve in the population.

Examples: Human height, skin color.
Reason: Many genes contribute small effects, producing a range of phenotypes.