Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles . In the early 1900s, geneticists showed that chromosomal movements in meiosis account for Mendel’s laws.
Morgan’s discovery that the X chromosome in Drosophila carries a gene for eye color supported the chromosome theory of inheritance.
Linked genes tend to be inherited together because they are located on the same chromosome. Each chromosome has hundreds or thousands of genes. Linked genes do not assort independently.
Independent assortment of chromosomes and crossing over produce genetic recombinants. Recombinant offspring, which exhibit new combinations of traits inherited from two parents, result from events of meiosis and random fertilization. These events include crossing over and independent assortment of chromosomes during the first meiotic division. A recombination frequency under 50% indicates that the genes are linked but that crossing over has occurred. During prophase I, paired homologous chromosomes break at corresponding points and switch fragments, creating new combinations of alleles that are then passed on to the gametes.
Geneticists can use recombination data to map a chromosome’s genetic loci. One way to map genes is to deduce their order and a rough indication of the relative distances between them from crossover data. The further apart genes are on a chromosome, the more likely they are to be separated during crossing over. Cytological mapping is a technique that pinpoints the physical locus of a gene by associating a mutant phenotype with a chromosomal defect seen in the microscope.
The chromosomal basis of sex varies with the organism. Sex is an inherited phenotypic character usually determined by the presence or absence of special chromosomes; the exact mechanism varies among different species. Humans and other mammals have an X-Y system, as do fruit flies. An XY male gives either an X chromosome or a Y chromosome to the sperm, which combines with an ovum containing an X chromosome from an XX female. The offspring’s sex is determined at conception by whether the sperm carries X or Y.
Sex-linked genes have unique patterns of inheritance. The sex chromosomes carry certain genes for traits that are unrelated to maleness or femaleness. Hemophilia is a sex-linked recessive disorder whose gene is on the X chromosome. In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during early embryonic development.
Alterations of chromosome number or structure cause some genetic disorders. Errors during meiosis can change the number of chromosomes per cell or the structure of individual chromosomes. Such alterations can affect phenotype. Aneuploidy, an abnormal chromosome number, can arise when a normal gamete unites with one containing two copies or no copies of a particular chromosome as a result of nondisjunction during meiosis. Polyploidy, in which there are more than two complete sets of chromosomes, can result from complete nondisjunction during gamete formation. A variety of rearrangements can result from chromosome breakage. A lost fragment leaves one chromosome with a deletion and may produce a duplication, translocation, or inversion by reattaching to another chromosome. Such alterations cause a variety of human disorders, such as Down syndrome (usually due to trisomy of chromosome 21).
The phenotypic effects of some mammalian genes depend on whether they were inherited from the mother or the father (imprinting). Individuals imprint certain parts of chromosomes in their gamete-producing cells with either a male or a female "stamp," probably in the form of methylation. This affects the way some genes are expressed in offspring. Genomic imprinting helps explain the inheritance pattern of some hereditary disorders, including fragile X syndrome.
Extranuclear genes exhibit a non-Mendelian pattern of inheritance. Mitochondria and chloroplasts contain some of their own genes. Because the zygote’s cytoplasm comes from the ovum, certain features of the offspring’s phenotype depend solely on these maternal cytoplasmic genes. Some diseases affecting the nervous and muscular systems are caused by defects in mitochondrial DNA that prevent cells from making enough ATP.
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