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The Chromosomal Basis of Inheritance (Alterations of chromosome number or…
The Chromosomal Basis of Inheritance
Morgan showed that Mendelian inheritance has its physical basis in the behavior of chromosomes: Scientific Inquiry
Around 1902, Walter Sutton, Theodor Boveri, and others noted these parallels, and a chromosome theory of inheritance began to take form
The most commonly observed character phenotype is called the wild type
Traits that are alternatives to the wild type, such as white eyes in Drosophila, are called mutant phenotypes
Like Mendel, Morgan made an insightful choice in his experimental animal. Morgan worked with Drosophila melanogaster, a fruit fly that eats fungi on fruit
When Morgan crossed his white-eyed male with a red-eyed female, all the F1 offspring had red eyes, suggesting that the red allele was dominant to the white allele
Morgan reasoned that the gene with the white-eyed mutation is on the X chromosome, with no corresponding allele present on the Y chromosome
Morgan’s finding of the correlation between a particular trait and an individual’s sex provided support for the chromosome theory of inheritance
Sex-linked genes exhibit unique patterns of inheritance
In humans and other mammals, there are two varieties of sex chromosomes, X and Y
Short segments at either end of the Y chromosome are the only regions that are homologous with the corresponding regions of the X
In both testes and ovaries, the two sex chromosomes segregate during meiosis and each gamete receives one
The human X chromosome contains approximately 1,100 X-linked genes
Duchenne muscular dystrophy affects one in 3,500 males born in the United States
Hemophilia is an X-linked recessive disorder defined by the absence of one or more proteins required for blood clotting
During female development, one X chromosome per cell condenses into a compact Barr body
Linked genes tend to be inherited together because they are located near each other on the same chromosome
Genes located near each other on the same chromosome tend to be inherited together called linked genes
Independent assortment of chromosomes produces genetic recombination of unlinked genes
Half are the parental types, with phenotypes that match the original P parents, with either yellow-round seeds or green-wrinkled seeds
Half are recombinant types or recombinants, new combinations of parental traits, with yellow-wrinkled or green-round seeds
This process, called crossing over, accounts for the recombination of linked genes
One of Morgan’s students, Alfred Sturtevant, used the crossing over of linked genes to develop a method for constructing a genetic map, an ordered list of the genetic loci along a particular chromosome
A genetic map based on recombination frequencies is called a linkage map. Sturtevant expressed the distance between genes as map units.
By combining linkage maps with other methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes
Alterations of chromosome number or structure cause some genetic disorders
Offspring resulting from the fertilization of a normal gamete with one produced by nondisjunction have an abnormal number of a particular chromosome, a condition known as aneuploidy
Trisomic cells have three copies of a particular chromosome type and have 2n + 1 chromosomes
Monosomic cells have only one copy of a particular chromosome type and have 2n − 1 chromosomes
Organisms with more than two complete sets of chromosomes are polyploid
A deletion occurs when a chromosomal fragment is lost. The chromosome is missing certain genes
A duplication occurs when a fragment becomes attached as an extra segment to a sister chromatid
An inversion occurs when a chromosomal fragment reattaches to the original chromosome, but in the reverse orientation
In translocation, a chromosomal fragment joins a nonhomologous chromosome
One aneuploid condition, Down syndrome, is due to three copies of chromosome 21, or trisomy 21
Some inheritance patterns are exceptions to the standard Mendelian inheritance
Variation in phenotype depending on whether an allele is inherited from the male or female parent is called genomic imprinting
In each generation, old imprints are “erased” in gamete-producing cells, and the chromosomes of the developing gametes are newly imprinted according to the sex of the individual forming the gametes
Karl Correns first observed cytoplasmic genes in plants in 1909, when he studied the inheritance of patches of yellow or white on the leaves of an otherwise green plant
Because a zygote inherits all its mitochondria from the ovum, all mitochondrial genes in most animals and plants demonstrate maternal inheritance
Another mitochondrial disorder is Leber’s hereditary optic neuropathy, which can produce sudden blindness in young adults