General Biology Exam 1 Review: Key Concepts and Genetics Practice

Scientific Method and Experimental Design

Hypotheses and Variables

Understanding the scientific method is fundamental in biology. Hypotheses and variables are central to experimental design.

• Hypothesis: A testable, falsifiable statement that explains an observation or answers a scientific question.

• Characteristics of a Good Hypothesis:

  • Testable and falsifiable

  • Based on prior knowledge and observations

  • Specific and clearly stated

  • Predicts an outcome

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

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

• Role of the Independent Variable: It is the presumed cause whose effect is being tested on the dependent variable.

• Graphing Data: In a line graph, the independent variable is plotted on the x-axis (horizontal), and the dependent variable is plotted on the y-axis (vertical).

Example: Measuring plant growth (dependent variable) over time (independent variable).

Biodiversity and Ecology

Genetic Diversity and Biodiversity

Biodiversity encompasses the variety of life at all levels, including genetic, species, and ecosystem diversity.

• Genetic Diversity: The variety of genes within a species.

• Contribution to Biodiversity: Genetic diversity increases a population's ability to adapt to environmental changes, resist diseases, and maintain healthy ecosystems.

The Greenhouse Effect and Global Warming

The greenhouse effect is a natural process that warms the Earth's surface, but human activities have intensified this effect, leading to global warming.

• Greenhouse Effect: Greenhouse gases (such as CO2, CH4, and H2O vapor) trap heat in the Earth's atmosphere, maintaining temperatures suitable for life.

• Role in Global Warming: Increased greenhouse gas emissions from human activities enhance this effect, causing average global temperatures to rise.

Example: Burning fossil fuels increases atmospheric CO2, contributing to climate change.

Cell Division and Genetics

Meiosis and Chromosome Structure

Meiosis is a type of cell division that reduces the chromosome number by half, producing gametes for sexual reproduction.

• 2n = 2 Cell During Meiosis I: The cell has two homologous chromosomes (one from each parent), each consisting of two sister chromatids.

• Homologs: Chromosomes with the same genes but possibly different alleles.

• Sister Chromatids: Identical copies of a chromosome, joined at the centromere.

• Non-sister Chromatids: Chromatids from homologous chromosomes (not identical).

Diagram: (Not shown; students should draw two pairs of chromosomes, each with two chromatids, labeling homologs, sister, and non-sister chromatids.)

Sexual Reproduction and Evolution

• Changing Environment Hypothesis: Sexual reproduction increases genetic variation, which can be advantageous in changing environments.

• C. elegans Experiment: Worms grown with pathogens (changing environment) benefit from sexual reproduction due to increased genetic diversity, supporting the hypothesis.

• Meiosis in Sexual Reproduction: Meiosis produces genetically unique gametes, enabling genetic recombination and variation.

• Advantages of Sexual Reproduction: Increases adaptability and survival in variable environments, despite being energetically costly and slower than asexual reproduction.

Mendelian Genetics

Monohybrid Crosses and Dominance

Mendelian genetics explains inheritance patterns of single-gene traits.

• Dominant Allele: Expressed in the phenotype even if only one copy is present (e.g., T for tall in peas).

• Recessive Allele: Expressed only when two copies are present (e.g., t for short in peas).

Example Problems

• Pea Plant Cross (Tt x tt):

  • Genotypes: Tt (heterozygous tall) x tt (homozygous short)

  • Offspring: 50% Tt (tall), 50% tt (short)

  • Phenotypic Ratio: 1 tall : 1 short

• Rabbit Cross (Bb x Bb):

  • Genotypes: Both heterozygous black (Bb)

  • Offspring: BB, Bb, Bb, bb

  • Probability of White Fur (bb): 1/4 or 25%

• Fruit Fly Cross (Rr x rr):

  • Genotypes: Heterozygous red-eyed (Rr) x homozygous white-eyed (rr)

  • Offspring: Rr (red), rr (white)

  • Genotype Ratio: 1 Rr : 1 rr

• Horse Cross (Bb x Bb):

  • Genotypes: Both heterozygous brown (Bb)

  • Offspring: BB, Bb, Bb, bb

  • Phenotypic Ratio: 3 brown : 1 chestnut

Dihybrid and Linked Gene Crosses

• Independent Assortment: Genes on different chromosomes assort independently during gamete formation.

• Linked Genes: Genes located close together on the same chromosome tend to be inherited together.

Example Problems

• Guinea Pig Cross (BbLl x bbll):

  • Genes: B = black, b = brown; L = short fur, l = long fur

  • Possible offspring phenotypes: black short, black long, brown short, brown long

• Fruit Fly Linked Genes (WwEe x wwee):

  • Genes: W = winged, w = wingless; E = red eyes, e = white eyes

  • Linked genes do not assort independently; offspring phenotypes depend on parental combinations.

  • Expected Phenotype Ratio: 1 winged, red-eyed : 1 wingless, white-eyed (if no crossing over)

  • Additional info: If crossing over occurs, recombinant phenotypes may appear in lower frequencies.

Complex Patterns of Inheritance

• Multiple Alleles: More than two alleles exist for a gene in a population (e.g., ABO blood types).

• Polygenic Inheritance: Multiple genes contribute to a single trait, resulting in continuous variation (e.g., height, skin color).

• Incomplete Dominance: Heterozygotes show an intermediate phenotype (e.g., red x white flowers produce pink offspring).

• Complete Dominance: Heterozygotes express the dominant phenotype only.

• Pleiotropy: A single gene affects multiple traits (e.g., Marfan syndrome affects connective tissue, heart, and eyes).

• Gene Interaction (Epistasis): Two or more genes interact to affect a single phenotype (e.g., coat color in Labrador retrievers).

Comparison Table: Inheritance Patterns

Pattern	Description	Example
Complete Dominance	Dominant allele masks recessive	Pea plant height (T/t)
Incomplete Dominance	Heterozygote is intermediate	Snapdragon flower color
Codominance	Both alleles expressed equally	AB blood type
Pleiotropy	One gene, multiple effects	Sickle cell anemia
Polygenic	Many genes, one trait	Human height

Environmental Effects on Phenotype

• Phenotypic Plasticity: The ability of an organism with a given genotype to change its phenotype in response to environmental conditions.

• Example: Identical plants may grow differently in varying soil, light, or water conditions.

Quantitative Traits and Population Variation

• Quantitative Inheritance: Traits controlled by multiple genes, each contributing a small effect.

• Bell Curve Distribution: Most individuals have intermediate phenotypes, with fewer at the extremes, resulting in a normal (bell-shaped) distribution.

Example: Human height is influenced by many genes and environmental factors, producing a continuous range of heights in the population.