Genetics: Quick Revision Notes

Genes: Traits like tall/short, eye color; made of DNA in the nucleus; Genetics deals with heredity – characteristics passed from parents to offspring.
Each organism receives one set of chromosomes from each parent (maternal and paternal).
Genes are located on chromosomes (mother: ova; father: sperm); each parent contributes half the instructions for a new organism.
Genotype = all the genes present in an organism; Phenotype = observable traits; alleles are alternate versions of a gene.

Gregor Mendel (Austrian monk, 1822) conducted pea plant experiments.
He concluded heredity is controlled by 2 factors (later called genes) for each trait; one from each parent.
He showed that if two alleles differ, one may be dominant over the other (Law of Dominance) and that alleles segregate randomly during gamete formation (Law of Segregation).

Alleles account for variations in inherited traits.
Each characteristic inherited as 2 alleles (one from each parent).
Dominant allele determines appearance when present; recessive allele is not seen in the phenotype if a dominant allele is present.
Alleles segregate into gametes during meiosis (random separation).

Gene: unit of hereditary information, DNA nucleotide sequence controlling a trait.
Locus: position of a gene on a chromosome.
Alleles: different forms of a gene controlling a trait.
Homozygous: same alleles (e.g., $TT$ or $tt$ ).
Heterozygous: different alleles (e.g., $Tt$ ).
Genotype: the pair of alleles (e.g., $TT, Tt, tt$ ).
Phenotype: observable trait (e.g., tall, blue eyes).
Dominant: allele expressed in phenotype when present (capital letter, e.g., $T$ ).
Recessive: allele expressed only when homozygous (lowercase, e.g., $t$ ).
Polygenic inheritance: trait controlled by several genes.
Locus, Alleles, Phenotype, Genotype (recap).

Chromosomes come in homologous pairs; each characteristic is influenced by genes on these chromosomes.
Sex chromosomes: XX = female, XY = male. Sex is determined by the father’s sperm (X or Y).
Normal human karyotype shows 22 autosome pairs + 1 sex chromosome pair.

P1 cross: homozygous tall (TT) × homozygous short (tt).
- Gametes: $T, t$
- F1 genotype: $Tt$ (all tall).
- F2 generation (from crossing F1 with itself): genotype ratio $1:2:1$ ( $TT: Tt: tt$ ).
- Phenotype ratio: $3:1$ tall:short.
If both parents are heterozygous ( $Tt imes Tt$ ): genotype ratio $1:2:1$ ; phenotype ratio $3:1$ tall:short.
Punnett squares are used to visualize these crosses.

Each characteristic is controlled by 2 alleles (one maternal, one paternal).
Alleles can be for brown, blue, green, etc. eye color.
Differing alleles can assort independently for different traits (Law of Independent Assortment).

Co-dominance: both alleles are equally expressed (e.g., human blood groups A and B both expressed when AB).
Incomplete dominance: heterozygote shows intermediate phenotype (e.g., red × white → pink).
Sex-linked disorders: genes on X chromosome (e.g., colour blindness, haemophilia). More common in males because they have only one X.

Alleles: A, B, O. A and B are codominant over O and are not dominant over each other.
Genotypes: $AA, AO, AB, BB, BO, OO$
Phenotypes: A, B, AB, O.
Note: when listing alleles for blood types, follow the correct notation (A, a, B, b, etc.) as used in tests.

Genes on the X chromosome; males (XY) are usually affected if the X carries the allele (no second X to compensate).
Examples: Red-green color blindness, Haemophilia.
Punnett diagrams must reflect X and Y gametes for accurate cross outcomes.

Pedigrees show family relationships and inheritance across generations.
Symbols: squares = males, circles = females; shaded = affected or trait; lines show relationships.
Used to trace inherited conditions (e.g., albinism, polydactyly).

Mutation: sudden, random, inheritable change to DNA.
Gene mutation: change in a gene’s structure.
Chromosomal mutation: change in chromosome number or structure.
Sickle cell anemia: recessive mutation in the beta-globin gene; provides malaria resistance in carriers.
Some disorders are sex-linked (e.g., haemophilia, color blindness).

Cloning = a form of genetic engineering via somatic cell nuclear transfer.
Reproductive cloning = produce a living organism genetically identical to the donor.
Therapeutic cloning = generate tissues/organs for research or transplant.
Dolly the sheep is a famous example.
Pros/Cons: rapid production, disease resistance, conservation vs ethical concerns and reduced genetic diversity.

Recombinant DNA technology combines DNA from different sources using restriction enzymes and DNA ligase.
Applications include vaccines and production of therapeutic proteins (e.g., insulin).
Process: isolate plasmids, cut with restriction enzymes, insert gene of interest, transform bacteria to produce product.

Genotype vs Phenotype: genotype is the genetic makeup; phenotype is the observable trait.
Dominant vs Recessive: dominance determines phenotype in presence of allele; recessive only shows when homozygous.
Punnett diagrams: used to predict offspring genotypes/phenotypes; expressed as ratios like $3:1$ or 1:2:1
.
Alleles for X-linked traits: must use X and Y in diagrams; males are XY, females XX.
Heterozygous/homozygous terminology summarized as:
- Homozygous dominant: $TT$
- Homozygous recessive: $tt$
- Heterozygous: $Tt$