Prophase Homologous Chromosomes

Prophase Homologous Chromosomes pair and crossing over, or recombination, occurs--a step unique to meiosis. Chromosomes form structures called synapses. The paired chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. At this stage, non-sister chromatids may cross-over at points called chiasmata
A chromatid is one of two identical copies of DNA making up a chromosome, which are joined at their centromeres, for the process of cell division (mitosis or meiosis). The term is used so long as the centromeres remain in contact. When they separate (during anaphase of mitosis and anaphase 2 of meiosis), the strands are called daughter-chromosomes. In other words, a chromatid is "one-half of a replicated chromosome". It should not be confused with the ploidy of an organism, which is the number of homologous versions of a chromosome.
In evolutionary biology, homology has come to mean any similarity between characters that is due to their shared ancestry. There are examples in different branches of biology. Anatomical structures that perform the same function in different biological species and evolved from the same structure in some ancestor species are homologous. In genetics, homology can be observed in DNA sequences that code for proteins (genes) and in noncoding DNA. For protein coding genes, one can compare translated amino-acid sequences of different genes. Sequence homology may also indicate common function. Homologous chromosomes are non-identical chromosomes that can pair (synapse) during meiosis, and are believed to share common ancestry.
Genetic.linkage occurs when particular genetic loci or alleles for genes are inherited jointly. Genetic loci on the same chromosome are physically connected and tend to segregate together during meiosis, and are thus genetically linked. Alleles for genes on different chromosomes are usually not linked, due to independent assortment of chromosomes during meiosis.

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