Patterns of Inheritance

Genetics: The study of inheritance.
Gregor Mendel: Pioneer in the field of genetics, known for his systematic studies.
DNA: Carries genes that encode proteins.
- Gene: A segment of DNA defined by its nucleotide sequence (A, C, G, T) that encodes proteins.
- Proteins: Determine various characteristics of an organism (e.g., eye, hair, and skin color).
Chromosomes:
- Packets of genetic information consisting of long DNA strands wrapped around histone proteins.
- Each chromosome contains numerous genes.

Character: A heritable feature that varies among individuals (e.g., eye color, height).
Trait: A variant of a character (e.g., blue or brown eyes, tall or short).
Mendel observed characters with two distinct traits in his studies.
Purebred Varieties: Mendel created these by selective breeding of plants.
Hybrids: Offspring resulting from crossing two different purebred varieties.
- P Generation: Parental plants.
- F1 Generation: Hybrid offspring of the P generation.
- F2 Generation: Resulting offspring from F1 crosses.

Diploid Cells: Contain two alleles for each gene.
Homologous Pairs:
- Composed of chromosomes with the same genes but potentially different versions (alleles).
- Alleles might be identical (e.g., gene A) or different (e.g., gene B).

Mutations: Changes in the nucleotide sequence of DNA; can result in altered proteins.
- Can be harmful, beneficial, or neutral.
Example: A deletion mutation can produce defective proteins.

Purpose: Represent gamete formation and predict allele combinations in offspring.
- Displays expected proportions of offspring’s phenotypes and genotypes.
Application: Can track inheritance for various traits, including genetic disorders like cystic fibrosis.

Monohybrid Cross: A cross between purebred parent plants differing in only one character.
- Example: Crossing purple and white flowered plants.
Mendel’s Four Hypotheses:
1. Alleles: Different versions of genes (e.g., R = round, r = flat).
2. Homozygous: Organism has identical alleles (e.g., RR, rr).
3. Heterozygous: Organism has different alleles (e.g., Rr, rR).
4. Law of Segregation: During gamete formation, alleles segregate so that each gamete carries only one allele per character.
Phenotype vs. Genotype:
- Phenotype: Physical appearance of an organism.
- Genotype: Genetic makeup of an organism.

Dihybrid Cross: Focuses on inheritance patterns of two different genes simultaneously.
Punnett squares can encompass multiple alleles for each gene to show gamete and offspring allele distributions.

Gene Inheritance:
- Genes on different chromosomes are inherited independently.
- Genes on the same chromosome are often inherited together (linked).
Gene Locus: Specific location of a gene on a chromosome; homologous chromosomes have alleles at the same locus.
Crossing Over:
- Separates genes onto different chromosomes, generating new allele combinations.

Single-Gene Disorders: Many human traits follow simple inheritance patterns and are controlled by single genes on autosomes.
- Recessive Disorders: Most genetic disorders are recessive (e.g., cystic fibrosis).
- Cystic fibrosis is the most common lethal genetic disease in the U.S., caused by a recessive allele affecting exocrine function.
- Inbreeding: Increases the probability of homozygous recessive traits due to close relative mating.
Dominant Disorders:
- Some disorders are dominant (e.g., achondroplasia and Huntington’s disease).
- Achondroplasia: Causes dwarfism; homozygous dominant leads to embryo death.
- Huntington’s Disease: A neurodegenerative disorder manifesting in middle age.

Techniques: A variety of tests can identify disease-causing alleles.
- Tests can be performed during pregnancy via amniocentesis or chorionic villus sampling.
Genetic Counseling: Assists individuals in understanding genetic testing outcomes.

Incomplete Dominance: F1 hybrids display a phenotype that is intermediate between the two parent phenotypes.
Example: ABO blood groups exhibit multiple alleles and codominance; both A (IA) and B (IB) genes can express simultaneously in AB blood type.

Pleiotropy: One gene can influence multiple traits.
- Example: Sickle-Cell Disease results in both defective hemoglobin and other complications (e.g., sickle-shaped red blood cells).

Sex-Linked Traits: Traits affected by genes located on sex chromosomes (most commonly X-linked).
- Diseases such as red-green colorblindness and hemophilia exhibit sex-linked inheritance, affecting primarily males.

Pedigrees: Meant to map family history of traits over generations, aiding geneticists in analysis.

Example: Siamese cats exhibit a gene affecting pigment production; the gene's activity is temperature-sensitive, resulting in different fur coloration depending on body temperature.

Polygenic Traits: Traits that are influenced by multiple genes (e.g., human height, skin color).
- Dominant and recessive alleles influence the height and shade of skin tones in individuals.