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Chromosomal Basis of Heredity Cheat Sheet by

Meiosis I

Prophase I
Chromo­somes condense, homologous pairs align, synapsis (precise alignment, ensires 1 copy of each gene in a daughter cell). Synapsed set called a tetrad (4 chroma­tids). Then, crossi­ng-over occurs (exchange of chromosome segments between pairs of homologous chromo­somes, or nonsister chromatids). Chiasma forms, visible structure from crossover. X and Y chromo­somes pair and synapse thru terminal ends (PARs).
Metaphase I
Nucleoli and envelope broken down, centroiles w/ spinde entern­uclear area, kinete­chore microt­ubules attach to sister kineto­chores, tetrads align at metaphase plate.
Anaphase I
Chromo­somes of tetrad separate (now dyads). Sister chromatids remain attached at centro­meres. DNA content is halved.
Telophase I
Spindles dissas­emble, cytoki­nesis forms two haploid cells.


Meiosis II

Prophase II
Chromo­somes condense and spindles form, kineto­chores attach to tubules..
Metaphase II
Alignment on metaphase plate.
Anaphase II
Centro­meres separate, daughter chromo­somes (still haploid) pulled to opposite sides.
Telophase II
Chromo­somes decond­ense, nuclear envelope forms, cytoki­nesis. Four haploid cells produced, each with one chromosome from each homologous pair.

Meiosis Results

Generates haploid nuclei with half the number of chromo­somes found in diploid cell. (2N -> N) Diploid number restored in fertil­iza­tion.
Indepe­ndent assortment of genes paternal and maternal chromo­somes have an equal chance of aligning on one side of metaphase plate.
Number of chromosome arrang­ements is 2n-1, n= # chromo­somes pairs (haploid number).
Number of chromosome combin­ations resulting from indepe­ndent assortment is 2n (number of different gametes).
Number of kinds of genotypes is 3n.


ONE copy of each chromo­some. (N)
TWO copies (homol­ogues) of each chromo­some. (2N)
Homologous pairs
Same gene loci, structure, and pair during meiosis.


Altern­ative forms of a single gene on the same locus that determine the same trait, but can produce different phenot­ypes.


Informative cheat sheet, well worth reading. It would be easier to understand if you included a short dictionary.

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