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Gen kebutaan warna merah-hijau adalah c ditunjuk satu resesif. Visi warna normal, Ruangan Khusus C, mendominasi. C/c gen terletak hanya pada kromosom X, sehingga kemampuan untuk terangkai warna tergantung sepenuhnya pada kromosom X.Warna kulit adalah contoh yang baik dari sifat kompleks. Itu tergantung pada faktor-faktor lingkungan seperti paparan sinar matahari dan gizi, serta beberapa gen. Anggaplah bahwa warna kulit dikendalikan oleh tiga gen yang terpisah, masing-masing memiliki dua alel: A,; B, b; dan C, c. Seseorang dengan genotipe AABBCC sangat gelap berkulit, individu dengan aabbcc genotipe sangat ringan berkulit, dan seseorang dengan genotipe AaBbCc memiliki warna kulit menengah. Paling mewarisi sifat tidak dikontrol oleh satu gen, tetapi sebaliknya oleh efek gabungan dari dua atau lebih gen, situasi yang dirujuk sebagai polygenic warisan, atau gabungan efek dari banyak gen dan faktor lingkungan, situasi yang disebut sebagai pusaka kompleks. Contoh dari ciri-ciri yang kompleks termasuk warna kulit, warna rambut, warna mata, ketinggian, laju metabolisme dan tubuh membangun. Dalam kompleks pusaka, satu genotipe dapat memiliki banyak fenotipe mungkin, tergantung pada lingkungan, atau satu fenotipe dapat mencakup banyak mungkin genotipe. Genotipe mungkin enam memproduksi empat jenis darah, sebagai berikut:Empat jenis darah (fenotipe) dari kelompok ABO — A, B, AB, dan O-hasil dari warisan enam kombinasi tiga alel berbeda dari gen tunggal disebut I gen: (1) allele IA produces the A antigen, (2) allele IB produces the B antigen, and (3) allele i produces neither A nor B antigen. Each person inherits two I-gene alleles, one from each parent, that give rise to the various phenotypes.Although a single individual inherits only two alleles for each gene, some genes may have more than two alternative forms; this is the basis for multiple-allele inheritance. One example of multiple-allele inheritance is the inheritance of the ABO blood group. People with the homozygous dominant genotype HbAHbA form normal hemoglobin; those with the homozygous recessive genotype HbSHbS have sickle-cell disease and severe anemia. Although they are usually healthy, those with the heterozygous genotype HbAHbS have minor problems with anemia because half their hemoglobin is normal and half is not. Heterozygotes are carriers, and they are said to have sickle-cell trait.In incomplete dominance, neither member of a pair of alleles is dominant over the other, and the heterozygote has a phenotype intermediate between the homozygous dominant and the homozygous recessive phenotypes. An example of incomplete dominance in humans is the inheritance of sickle-cell disease (SCD). Most inherited traits are influenced by more than one gene, and, to complicate matters, most genes can influence more than one trait. Variations on dominant–recessive inheritance include incomplete dominance, multiple-allele inheritance, and complex inheritance.Most inherited traits are influenced by more than one gene, and, to complicate matters, most genes can influence more than one trait. Variations on dominant–recessive inheritance include incomplete dominance, multiple-allele inheritance, and complex inheritance.Most inherited traits are influenced by more than one gene, and, to complicate matters, most genes can influence more than one trait. Variations on dominant–recessive inheritance include incomplete dominance, multiple-allele inheritance, and complex inheritance.Most inherited traits are influenced by more than one gene, and, to complicate matters, most genes can influence more than one trait. Variations on dominant–recessive inheritance include incomplete dominance, multiple-allele inheritance, and complex inheritance.Most inherited traits are influenced by more than one gene, and, to complicate matters, most genes can influence more than one trait. Variations on dominant–recessive inheritance include incomplete dominance, multiple-allele inheritance, and complex inheritance.Most inherited traits are influenced by more than one gene, and, to complicate matters, most genes can influence more than one trait. Variations on dominant–recessive inheritance include incomplete dominance, multiple-allele inheritance, and complex inheritance.Most inherited traits are influenced by more than one gene, and, to complicate matters, most genes can influence more than one trait. Variations on dominant–recessive inheritance include incomplete dominance, multiple-allele inheritance, and complex inheritance.Most inherited traits are influenced by more than one gene, and, to complicate matters, most genes can influence more than one trait. Variations on dominant–recessive inheritance include incomplete dominance, multiple-allele inheritance, and complex inheritance.Most inherited traits are influenced by more than one gene, and, to complicate matters, most genes can influence more than one trait. Variations on dominant–recessive inheritance include incomplete dominance, multiple-allele inheritance, and complex inheritance.Most inherited traits are influenced by more than one gene, and, to complicate matters, most genes can influence more than one trait. Variations on dominant–recessive inheritance include incomplete dominance, multiple-allele inheritance, and complex inheritance.Phenotype refers to how the genetic makeup is expressed in the body; it is the physical or outward expression of a gene. Such individuals are called carriers of the recessive gene.Occasionally an error in cell division, called nondisjunction, results in an abnormal number of chromosomes. In this situation, homologous chromosomes (during meiosis I) or sister chromatids (during anaphase of mitosis or meiosis II) fail to separate properly. A cell from which one or more chromosomes has been added or deleted is called an aneuploid. A monosomic cell (2n -1) is missing a chromosome; a trisomic cell (2n +1) has an extra chromosome. Most cases of Down syndrome are aneuploid disorders in which there is trisomy of chromosome 21. Nondisjunction usually occurs during gametogenesis (meiosis), but about 2% of Down syndrome cases result from nondisjunction during mitotic divisions in early embryonic development.Another error in meiosis is a translocation. In this case, two chromosomes that are not homologous break and interchange portions of their chromosomes. The individual who has a translocation may be perfectly normal if no loss of genetic material took place when the rearrangement occurred. However, some of the person’s gametes may not contain the correct amount and type of genetic material.About 3% of Down syndrome cases result from a translocation of part of chromosome 21 to another chromosome, usually chromosome 14 or 15. The individual who has this translocation is normal and does not even know that he or she is a “carrier.” When such a carrier produces gametes, however, some gametes end up with a whole chromosome 21 plus another chromosome with the translocated fragment of chromosome 21. Upon fertilization, the zygote then has three, rather than two, copies of that part of chromosome 21.Most patterns of inheritance do not conform to the simple dominant–recessive inheritance we have just described, in which only dominant and recessive alleles interact. The phenotypic expression of a particular gene may be influenced not only by which alleles are present, but also by other genes and by the environment.
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