Lecture 9

A tetrad of four chromatids

during meiosis, each bivalent (set of chromosomes) consists of…

Ascus

in some fungi, each meiotic tetrad is contained in a sac-like structure and can be recovered as an intact group

Ascospore

Each product of meiosis is included in a reproductive cell; all of them formed from one meiotic cell remain together in the ascus

Tetrad

the assemblage of four ascospores or four pairs of ascospores in a single ascus; results from the events of meiosis in a single cell

Genotypes of the tetrad

tell us about the events during meiosis in that single cell

Tetrad analysis

ascospores can be separated by dissection and plated individually to examine the genotype of the four products of meiosis

Yeast

makes unordered tetrads

Unordered tetrads

four meiotic products (spores) arranged at random within the ascus

Ordered tetrads

four pairs, eight haploid spores, arranged in a definite order directly related to the planes of the meiotic divisions

Analyzing tetrads

researchers can release the spores of each ascus, induce the haploid cells to germinate under appropriate conditions, and then analyze the genetic makeup of the resulting haploid cultures

Unordered vs ordered ascus

helps to differentiate between two types of meiosis

Types of meiosis

crossover and non-crossover

Spores in yeast ascus

not arranged in any particular order

Classification of a tetrad

based solely on the number of parental and recombinant spores in the ascus

Parental ditype

all four spores in the resulting tetrad will have the parental configuration of alleles

Nonparental ditype

four recombinant spores, two of each type

Tetratype

four different spores, two recombinants (one of each type) and two parentals (one of each type)

[NPD + 1/2(T)]/total tetrads x 100

recombination frequency

Meiotic products in a TT tetrad

only half produced by a single crossing-over are recombinant

First-division segregation patterns

found in the ascus when crossing over between the gene and centromere does not take place; causes separation of A allele from the a allele during meiosis I

Second-division segregation patterns

found in the ascus when crossing over between the gene and the centromere takes place; delays the separation of the A allele from the a allele until meiosis II