Mammalian Artificial Chromosomes

Mammalian artificial chromosomes (MACs) are conceptually similar to YACs, but instead of yeast sequences they contain mammalian or human ones. In this case the telomeric sequences are multimers (multiple copies) of the sequence TTAGGG, and the commonly used centromeric sequence is composed of another repeated DNA sequence found at the natural centromeres of human chromosomes and called alphoid DNA.

Because the alphoid DNA is needed in units of many kilobases, these MAC DNAs are grown as YACs or, more recently, as BACs. When added to suitable cell lines, these MAC DNAs form chromosomes that mimic those in the cell, with accurate segregation and the normal complement of proteins at telomeres and centromeres. Their primary use is not in genome mapping but as vectors for delivery of large fragments of DNA to mammalian cells and to whole animals for expression of large genes or sets of genes. They are still in development, and although gene expression has been demonstrated they have not been used in a practical application. see also Chromosome, Eukaryotic; Human Genome Project; Mapping; Telomere.

Howard Cooke cultivars plant varieties resulting from selective breeding

Bibliography

Grimes, B., and H. Cooke. "Engineering Mammalian Chromosomes." Human Molecular Genetics 7, no. 10 (1998): 1635-1640.

Willard, H. F. "Genomics and Gene Therapy: Artificial Chromosomes Coming to Life." Science 290 (2000): 1308-1309.

Classical Hybrid Genetics

Common garden peas (Pisum sativum) are wonderful when eaten raw. Gregor Mendel (1822-1884) no doubt ate his share. Today he is recognized for using peas to establish the science of genetics.

Mendel investigated hereditary patterns of hybrids. Hybrids are offspring from two organisms that are of different breeds, varieties, or species. Hybrids create new cultivars, from new apple varieties to tangelos to hybrid corn. Some mammals produce hybrids; a mule is the progeny of a horse and a donkey.

Mendel was interested in new flower varieties and absorbed by what hybrids reveal about inheritance. Nineteenth-century scientists wanted to know how organisms created a vast diversity of forms while faithfully maintaining distinct sets of characteristics. Constancy was a known quality of life. Then, as now, people had no trouble recognizing the difference between a housefly and a bee. But what prevented people from suddenly sprouting flowers or losing their human features? Genes, DNA, meiosis, and chromosomes were all unknown. Hybrid genetics was a means to discover answers to fundamental biological questions.

Mendel's aim was to discover the mathematical rules behind the reappearing patterns he saw in hybrids. After testing many varieties of peas, he decided to study seven specific traits: shape of the ripe seeds, seed color, seed coat color, form of the ripe pods, color of the unripe pods, position of the flowers, and length of the stems.

Initially, Mendel determined the inheritance patterns for one trait at a time, ignoring other traits. He crossed two varieties that differed sharply in a specific trait. For instance, for the trait of seed shape, he used one variety whose seeds were wrinkled and another whose seeds were smooth. The seed-shape trait was represented by two specific inherited features, smooth and wrinkled, that were alleles.

Mendel also crossed plant varieties , each with specific combinations of alleles, in order to follow the fates of two or even three traits at the same time. For instance, he crossed a tall plant with wrinkled, green seeds and a short plant with round, yellow seeds. (The underlined words represent alleles for three traits: height, seed shape, and seed color.) To cross two varieties, Mendel first had to make sure the flowers did not pollinate themselves. To do that, he cut off their anthers, their pollen-bearing parts. Then he used the anthers from one variety to pollinate another.

Mendel was the first person to follow specific alleles (which he called "factors") as they were passed from parental varieties through several generations of offspring. He selected plant varieties that were true-breeding: Each generation of plants looked like the antecedent generations, with regard to the studied traits. Mendel used artificial pollination (described above) to perform an initial outcross, a mating between individuals differing in their alleles for at least one trait. The outcross created hybrids, or, more precisely, heterozygotes for the chosen traits. Next, Mendel let the heterozygotes self-pollinate, or intercross (i.e., the mating of inbred animals or self-pollinating plants), that are heterozygous for one or more traits. He then let two subsequent generations of progeny self-pollinate.

Mendel discovered that hybrids (heterozygotes) resembled only one parental variety, despite clearly identified input from both parents. Regarding the stem-length trait for example, one parental variety had tall stems, whereas the other variety had very short (dwarf) stems, yet all their hybrid offspring had only tall stems. Mendel named the displayed feature "dominant," because the hybrid's appearance of a tall stem (phenotype) reflected input from only one parent. He identified the dominant factor or allele for each of the seven traits (seed shape, seed color, length of stem, etc.). The other hidden feature (named "recessive" by Mendel) resurfaced in the next generation, when both hybrid (heterozygous) parents donated their hidden (recessive) allele to a descendent. For example, dwarf stems appeared again in progeny, instead of the conspicuous tall stems seen in the heterozygous parents. A descendent with dwarf stems had a "homozygous recessive" genotype for this trait. An individual's genotype described the two alleles received for a trait, whether hidden or displayed. If a plant had a dominant phenotype for one or several traits, but the genotype was unknown, a method called a test cross was used to reveal the presence of a recessive allele for each of the traits in question. The dominant pheno-type to be tested might carry one dominant and one recessive allele, or two dominant alleles (see Mendel's laws, below), depending on what its parents had donated. The dominant phenotype was crossed with a plant known to be homozygous recessive for a particular trait. If half the offspring showed the dominant phenotype and the other half the recessive

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