Mendelian Genetics Cheat Sheet

Mendelian genetics explains the fundamental principles of heredity, focusing on how traits are passed from parents to offspring through genes and alleles, governed by laws of segregation and independent assortment.

Core Principles

Genes are units of inheritance composed of DNA, coding for proteins.
Alleles are alternative forms of a gene.
Homozygous individuals have two identical alleles for a trait (AA or aa).
Heterozygous individuals have two different alleles for a trait (Aa).
Gametes are haploid cells (egg or sperm) carrying one allele for each gene.
True-breeding organisms are homozygous for the traits studied.
Self-cross involves self-pollination, typically resulting in offspring with similar genetics to the parent.
Law of Segregation: Alleles for a trait separate during gamete formation.
Law of Independent Assortment: Alleles for different traits segregate independently during gamete formation.
Monohybrid cross examines inheritance of a single gene.
Dihybrid cross examines inheritance of two genes.
A test cross determines an unknown genotype by crossing with a homozygous recessive individual.
Incomplete dominance results in three phenotypes (e.g., 1:2:1 ratio).
Codominance shows both alleles expressed equally (e.g., ABO blood type).
Pedigrees map traits across generations in a family.
X-linked traits are carried on the X chromosome, often showing different inheritance patterns in males and females.

Action Steps

Identify the trait(s) being studied.
Determine the genotypes of the parents.
Determine the possible gametes each parent can produce.
Use a Punnett square to predict offspring genotypes and phenotypes.
Calculate probabilities for specific genotypes or phenotypes.
For test crosses, use a homozygous recessive individual (e.g., 'aa' or 'yyrr').
Interpret pedigrees by analyzing trait presence across generations and sexes.
Consider dominance patterns (complete, incomplete, codominance) when predicting outcomes.

Formulas

Monohybrid cross phenotypic ratio: 3:1
Dihybrid cross phenotypic ratio: 9:3:3:1
Test cross (one gene) phenotypic ratio: 1:1
Test cross (two genes) phenotypic ratio: 1:1:1:1
Incomplete dominance phenotypic ratio: 1:2:1

Key Terms

Gene: A unit of inheritance composed of DNA, coding for a protein.
Allele: An alternative form of a gene (e.g., A or a).
Homozygous: Having two identical alleles for a trait (e.g., AA or aa).
Heterozygous: Having two different alleles for a trait (e.g., Aa).
Gamete: A haploid reproductive cell (egg or sperm).
Phenotype: The observable physical traits of an organism.
Genotype: The genetic makeup of an organism (e.g., AA, Aa, aa).
True-breeding: Organisms that are homozygous and produce offspring with the same traits when self-crossed.
Monohybrid Cross: A cross involving one gene or one trait.
Dihybrid Cross: A cross involving two different genes or traits.
Test Cross: A cross between an individual with an unknown dominant genotype and a homozygous recessive individual.
Incomplete Dominance: A type of inheritance where one allele is not completely dominant over another, resulting in a blended phenotype.
Codominance: A type of inheritance where both alleles are fully expressed in the phenotype.
Pedigree: A chart showing the inheritance of a trait through several generations of a family.
X-linked Trait: A trait whose gene is located on the X chromosome.

Real World Examples

Pea plant seed color and shape inheritance.: Illustrates Mendel's laws of segregation and independent assortment, leading to predictable phenotypic ratios (e.g., 9:3:3:1 for dihybrid crosses).
Human ABO blood groups.: Demonstrates codominance (alleles IA and IB) and simple dominance (allele i is recessive to IA and IB).
Snapdragon flower color.: An example of incomplete dominance, where crossing red and white flowers produces pink offspring.
Human genetic disorders like cystic fibrosis or Huntington's disease.: Studied using pedigrees to determine if they are autosomal recessive or dominant.
Color blindness or hemophilia in humans.: Examples of X-linked recessive traits, more common in males due to having only one X chromosome.

Timeline

Mid-1800s: Gregor Mendel conducted experiments with pea plants, laying the foundation for genetics.
Early 1900s: Chromosomal theory of inheritance proposed, linking genes to chromosomes.
1953: Watson and Crick discovered the double helix structure of DNA.
Late 1900s - Present: Advancements in molecular genetics, gene sequencing, and genetic engineering.

People

Gregor Mendel: Father of Genetics; discovered basic principles of heredity through pea plant experiments.
James Watson & Francis Crick: Discovered the double helix structure of DNA.

Quiz

What is the law stating that alleles separate during gamete formation?: Law of Segregation
If a homozygous dominant (AA) plant is crossed with a homozygous recessive (aa) plant, what is the genotype of the F1 generation?: Aa
A trait that is expressed in heterozygotes but shows a blend of parental phenotypes is an example of:: Incomplete Dominance
Which of the following is NOT a possible genotype for the ABO blood group?: ii