PCAT

Gregor Mendel's experiments with pea plants established the foundation of classical genetics. The Law of Segregation states that each organism carries two alleles for each trait and that these separate during gamete formation, so each gamete carries only one. The Law of Independent Assortment states that alleles for different traits segregate independently, provided the genes are on different chromosomes. Dominant alleles are expressed with at least one copy; recessive alleles are expressed only in the homozygous recessive genotype.

Monohybridcross:Aa×Aa

The monohybrid cross gives a 1 AA : 2 Aa : 1 aa genotypic ratio and a 3 : 1 phenotypic ratio. A dihybrid cross (AaBb × AaBb) gives a 9 : 3 : 3 : 1 phenotypic ratio.

Pattern	DescriptionandExample
Incompletedominance	Theheterozygoteshowsanintermediatephenotype:red(RR)×white(WW)givespink(RW)
Codominance	Bothallelesarefullyexpressed:bloodtypeAB
Multiplealleles	Morethantwoallelesexistinthepopulation:ABObloodtypehasIA IB andi , ,
Polygenicinheritance	Atraitiscontrolledbytwoormoregenes:skincolor height ,
Pleiotropy	Onegeneaffectsmultipletraits:sicklecelldisease
Epistasis	Onegenemaskstheexpressionofanothergene

Chromosomal Genetics

Humans have 46 chromosomes in 23 pairs: pairs 1–22 are autosomes, and pair 23 is the sex chromosomes (XX in females, XY in males). X-linked recessive traits such as color blindness and hemophilia appear more often in males, who have only one X chromosome. Nondisjunction during meiosis produces gametes with an extra or missing chromosome, leading to conditions such as trisomy 21 (Down syndrome), Turner syndrome (45,X), and Klinefelter syndrome (47,XXY).

Molecular Genetics

Mutation	Effect
Silent	Abasesubstitutionthatdoesnotchangetheaminoacid
Missense	Asubstitutionthatchangesoneaminoacid
Nonsense	Asubstitutionthatcreatesaprematurestopcodon
Frameshift	Aninsertionordeletionthatshiftsthereadingframe oftenalteringtheproteindramatically ,

Gene expression is regulated at the levels of transcription (promoters, enhancers, transcription factors), post- transcription (mRNA processing, RNA interference), translation, and post-translation (protein modification and degradation). Epigenetics refers to heritable changes in gene expression without changes in DNA sequence; DNA methylation and histone modification are the key mechanisms, and methylation of promoter regions typically silences a gene.

Biotechnology

Pcr

DNA is cut with restriction enzymes

The polymerase chain reaction

A guide RNA directs the Cas9 nuclease

and inserted into a vector such as a

amplifies specific DNA sequences

to a target sequence, where its cut can

plasmid for replication in a host. Used

through repeated cycles of

be repaired to insert, delete, or modify

to produce insulin, vaccines, and other

denaturation, annealing, and extension.

DNA.

therapeutic proteins.

Classification and Taxonomy

Taxonomy classifies organisms in the hierarchy Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. The three domains are Bacteria, Archaea, and Eukarya. Binomial nomenclature gives each species a two-part italicized name, genus (capitalized) plus species epithet, as in Homo sapiens.

Group	CellTypeandStructure	KeyFeatures
Bacteria	Prokaryotic unicellular; ,	Diversemetabolism;includemanypathogensandbeneficialorganisms
	peptidoglycancellwalls
Archaea	Prokaryotic unicellular;no ,	Oftenextremophiles(hotsprings saltlakes deepsea);moreclosely , ,
	peptidoglycan	relatedtoeukaryotesthantobacteria
Protista	Eukaryotic mostlyunicellular ,	Diverse paraphyleticgroupofalgae amoebas andprotozoans; , , ,
		photosyntheticorheterotrophic
Fungi	Eukaryotic mostlymulticellular; ,	Absorptiveheterotrophs(molds yeasts mushrooms);reproducebyspores , ,
	chitincellwalls
Plantae	Eukaryotic multicellular;cellulose ,	Photosyntheticautotrophs:mosses ferns gymnosperms angiosperms , , ,
	cellwalls
Animalia	Eukaryotic multicellular;nocell ,	Heterotrophs:invertebratesandvertebrates
	walls

Phylogeny and Evolution

Phylogenetic trees (cladograms) show evolutionary relationships. A clade includes an ancestor and all its descendants, grouped by shared derived characters (synapomorphies). Homologous structures share a common ancestor (vertebrate forelimbs); analogous structures perform similar functions but evolved independently (insect and bird wings); vestigial structures are reduced remnants of structures that were functional in ancestors.