C THE NUCLEUS, CHROMOSOMES, AND EXPRESSION OF THEIR GENES Genomic DNA in most of the animals of interest to the reader consists of approximately 3-4 billion base-pairs of nucleotides arranged as linear polymers, in a variable number of chromosomes. Somatic cells (i.e. non-gametes) have a pair of sex chromosomes along with characteristic numbers of paired autosomes where each chromosome within the pair is derived from each of the two parents. In mammals where the sire's gamete determines sex of the offspring, males possess an Xand a Ychromosome, the Ybeing paternally-derived. Female offspring result from the sire contributing an Xchromosome to the Xpresent in the oocyte, resulting in an XXkaryotype. Avian sex determination is the responsibility of the female parent and the non-autosomal chromosomes are called WZin females and ZZin the homogametic males. 2007 version – page 99
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