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#allelic segregation

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Linkage Mapping and Tetrad analysis

A 1:1:1:1 arrangement of alleles in tetrad indicates

Ordered tetrad arrangement displaying 1:1:1:1 pattern alleles like alternating AAaaAAaa blocks or ABab interleaving indicates alleles remained together through meiosis I due crossover between gene and centromere preventing separation at first division. Heterozygous daughter nuclei after first division segregate alleles only at second division, generating alternating genotype blocks rather than contiguous blocks. This pattern defines second division segregation, contrasting with first division segregation where identical alleles cluster together contiguously. Counting such alternating asci yields SDS frequency used for centromere mapping, revealing recombination events between locus and centromere.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 5: Alternating 1:1:1:1 Indicates SDS Gene-Centromere Exchange

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Linkage Mapping and Tetrad analysis

In ordered tetrads, a 2:2 arrangement of alleles indicates

In linear octad containing four pairs sister spores from postmeiotic mitosis, arrangement with two genotype blocks, such as AAAAaaaa contiguous or AAcc pattern where identical sisters adjacent, shows alleles partitioned at meiosis I reductional division. No crossover between gene and centromere preserved linkage, homologous centromeres with alternative alleles diverged at anaphase I, producing first division segregation pattern 4:4 or 2:2 block. Distal genes exhibiting crossover display alternating 2:2:2:2 patterns indicating second division segregation. Discrimination essential for measuring gene-centromere recombination distances.

Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 5: 2:2 Arrangement Indicates FDS in Ordered Tetrads

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