Mendel, Genes, and Inheritance
Chapter 9: Mendel, Genes, and Inheritance
Genes: Described as the blueprint for biological inheritance.
Importance of Understanding Genes
Red Blood Cells in Sickle-Cell Disease:
Normal Condition: Red blood cells are donut-shaped and produce normal hemoglobin, effectively delivering oxygen necessary for cellular respiration.
Sickle Cell Condition:
Caused by a mutation in a gene, altering the nucleotide sequence.
Results in a loss of function for transporting oxygen to cells, leading to misshapen cells (half donut-shaped) that do not adequately contribute to ATP production.
Historical Context of Inheritance Theories
Blending Theory of Inheritance (Pre-1900s):
Suggested hereditary traits mix evenly in offspring via the mixing of parents’ blood.
Does not explain:
Why extremes do not gradually disappear.
Instances where offspring display traits that differ from both parents.
Proven incorrect by Mendel’s findings.
Gregor Mendel: Pioneer of Genetics
Gregor Mendel: Known as the Father of Genetics.
First to apply the scientific method to study inheritance, developing hypotheses and experimental methods.
Mendel’s Experimental Foundations
Mendel’s Hypotheses:
Conducted mitotic experiments that supported two main principles:
Principle of Segregation.
Principle of Independent Assortment.
Mendel's Experiments with Pea Plants (1860s)
Reasons for Choosing Garden Peas:
Easy to grow.
Clear definitions of traits with noticeable variation.
True-Breeding Varieties:
Self-fertilized Plants: Produce offspring with the same trait across generations.
Cross-Pollination: Involves two different parent plants leading to new trait combinations.
Terminology in Genetics
P Generation (Parental Generation): Initial plants used in crosses, producing embryos.
F1 Generation (First Filial Generation): The first generation of offspring.
F2 Generation (Second Filial Generation): The offspring produced from the F1 generation.
Mendel's Flower Color Cross Experiments
P Generation: Crossing purple flowers with white flowers.
F1 Generation: Resulted in all purple-flowered offspring.
F2 Generation:
Results revealed purple and white flowers, indicating traits do not blend but segregate:
Ratio of purple to white flowers is approximately 3:1.
Pea Plant Traits and Ratios in Mendelian Genetics
Key Traits Generated from Crosses:
Seed Shape: Round vs. wrinkled with a ratio of 2.96:1 in the F2 generation.
Seed Color: Yellow vs. green with approx. 3.01:1 ratio.
Pod Shape: Inflated vs. constricted with a ratio of 2.95:1.
Pod Color: Green vs. yellow showing a 2.82:1 ratio.
Flower Position: Axial vs. terminal showing 3.14:1 ratio.
Stem Length: Tall vs. dwarf with a ratio of 2.84:1.
Mendel’s Fundamental Hypotheses About Genetics
First Hypothesis:
Genes for genetic characters occur in pairs, one inherited from each parent, with alleles being different versions of a gene.
Genotype: The two copies of each gene.
Second Hypothesis:
If two alleles are different, one is dominant over the other.
Dominant allele is expressed while recessive allele is masked, with recessiveness present only when two copies are expressed.
Third Hypothesis:
Alleles of genes segregate during gamete formation, leading to half carrying one allele and half another.
This principle is defined as the Principle of Segregation, leading to the formation of a diploid zygote that holds two alleles.
Monohybrid Crosses
Monohybrid Cross: Involves only one character.
Start with dominant purple flower (PP) cross with a white flower (pp).
All F1 offspring will be heterozygous (Pp).
Self-crossing F1 generation (Pp x Pp) leads to a 3:1 phenotype ratio in the F2 generation.
Key Genetic Terminology
Homozygous: Both alleles are identical (PP or pp).
Heterozygous: Two differing alleles (Pp).
Genotype: Genetic constitution (e.g., PP, Pp, pp).
Phenotype: External expression (e.g., color of flowers).
Probability in Genetics
Product Rule:
Used to calculate the probabilities of independent events.
Example: Individual probabilities multiplied for successive events, such as determining outcomes of coin flips.
Sum Rule:
Applied for different events yielding the same result.
Example: Calculating the total probability for obtaining heads or tails in two tosses.
Examples of Probability Calculation:
Heterozygous cross probabilities result in a 3:1 phenotype ratio based on independent assortment of alleles.
Testing Mendel’s Hypothesis
Testcross: Crossing an unknown genotype with a homozygous recessive individual.
Reveals whether the unknown trait is homozygous or heterozygous based on offspring ratios.
Mendel’s Principle of Independent Assortment
States that alleles of genes governing different characters segregate independently during gamete formation.
This principle allows for multiple combinations of traits, forming the basis for dihybrid crosses.
Dihybrid Crosses: Two Characteristics
Example Characters: Pea Shape (R = round, r = wrinkled) and Pea Color (Y = yellow, y = green).
Crossing: Example of RRYY with rryy to produce F1 generation (RrYy).
F2 Generation Ratio: Results in a phenotypic ratio of 9:3:3:1 based on combinations of traits.
Chromosome Theory of Inheritance
Walter Sutton: Established parallels between genes and chromosomes during meiosis.
Chromosomes exist in pairs.
Each chromosome is inherited singly to gametes.
Independent assortment of chromosomes occurs during meiosis.
Each chromosome of a pair is derived from each parent.
Mendelian Traits in Humans
Human traits such as albinism show clear patterns of inheritance related to Mendelian principles.
Modifications to Mendelian Genetics
Incomplete Dominance: Dominant alleles do not completely mask recessive alleles, leading to intermediate phenotypes.
Codominance: Both alleles in heterozygotes manifest equally.
Multiple Alleles: More than two alleles exist for a trait within a population.
Epistasis: One gene may mask or modify the effect of another gene.
Polygenic Inheritance: Traits controlled by multiple genes, leading to a continuous spectrum of phenotypes.
Pleiotropy: When a single gene influences multiple traits.
Incomplete Dominance and Examples
Results in phenotypes that differ from those of either homozygote. Examples include flower colors in plants and genetic conditions like sickle-cell disease exhibiting intermediate expressions.
Codominance in Human Genetics
Example: Human blood types display codominance with multiple alleles producing distinct phenotypes based on combinations.
Multiple Alleles' Impact on Phenotypes
Understanding alleles’ relationships determines phenotypic expressions, while still following Mendel's principles.
Human ABO Blood Groups
Antigens: Glycoproteins that determine blood type.
Identifying alleles IA, IB, and i that correspond to types A, B, AB, and O.
Epistasis and Real World Examples
Explanation of how one gene can mask another in phenotypic outcome, such as in Labrador retrievers which may exhibit black, chocolate, or yellow fur depending on their alleles.
Polygenic Inheritance
Several genes influencing a single trait leading to continuous variation (e.g., human height's bell curve distribution).
Pleiotropy
One gene typically affects multiple traits, as seen in conditions like sickle-cell disease contributing to various complications affecting multiple organ systems.
Summary Questions for Reflection
How do species maintain continuity through generations?
What are Mendel's Hypotheses for inheritance?
How are genes connected to chromosomes compared to each other?
What are modifications proposed relative to Mendel’s hypotheses?