Chapter 15: THe Chromosomal Theory of Inheritance (4)
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The phenotype of an organism can be affected by small-scale changes involving individual genes. __ are the source of all new alleles, which can lead to new __ traits.
- Random mutations
Large-scale chromosomal changes can also affect an organism's __. Physical and chemical disturbances, as well as errors during meiosis, can damage chromosoems in major ways or alter their number in a cell.
Large-scale chromosomal alterations often lead to spontaneous __ of a fetus, and individuals born with these types of genetic defects commonly exhibit various developmental disorders.
TRUE OR FALSE:
In plants, such genetic defects may be tolerated to a greater extent than in animals.
-- Ideally, the meiotic spindle distributes chromosomes to daughter cells without error. But there is an occasional mishap, called a __, in which the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II.
---- In these cases, what happens to the gametes?
- one gamete receives two of the same type of chromosome and another gamete receives no copy. The other chromosomes are usually distributed normally.
If either of the aberrant gametes unites with a normal one at fertilization, the zygote will also have an abnormal number of a chromosome, called __. (It may involve more than one chromosome.)
Fertilization involving a gamete that has no copy of a particular chromosome will lead to a missing chromosome in the zygote (2n-1); the __ zygote is said to be __ for that chromosome.
If a chromosome is present in triplicate in the zygote (2n+1), the __ is __ for that chromosome.
Mitosis will sunsequently transmit the __ to all embryonic cells. If the organism survives, it usually has a set of traits caused by the abnormal dose of the genes associated with the extra or missing chromosome.
-- __ is an ex. of trisomy in humans.
__ can also occur during mitosis. If such an error takes place early in embryonic development, then the __ condition is passed along by mitosis to a large number of cells and is likely to have a substantial effect on the organism.
Some organisms have more than two complete chromosome sets in all somatic cells, called __. (3n= __/ 4n= __)
A __ may arise by the fertilization of an abnormal diploid egg produced by nondisjunciton of all its chromosomes.
__ could result from the failure of a 2n zygote to divide after replicating its chromosomes.
Subsequent normal mitotic divisions would then produce a 4n embryo.
- triploid cell
__ is fairly common in the plant kingdom. (Bananas are __ and wheat is __.)
In the animal kingdom, __ are much less common, although they are known to occur among fishes and amphibians.
In general, __ are more nearly normal in appearance than __.
- hexaploid (6n)
- polyploidy species
One extra or missing chromosome apparently disrupts genetic balance more than does an entire extra set of chromosomes.
Errors in meiosis or damamging agents such as __ can cause breakage of a chromosome, which can lead to four types of changes in chromosome structure: __, __, __, and __.
A __ occurs when a chromosomal fragment is lost. The affected chromosome is then missing certain genes. (If the affected __ is deleted, the entire chromosome will be lost.
The deleted fragment may become attached as an extra segment to a sister chromatid, producing a __. ALternatively, a detached fragment could attach to a nonsister chromatid of a homologous chromosome. In that case, though, the __ segments might both be identical beacuse the homologs could carry different alleles of certain genes.
A chromosomal fragment may also reattach to the original chromosome but in the reverse orientation, producing an __.
A fourth possible result of chromosomal breakage is for the fragment to join a nonhomologous chromosome, a rearrangement called __.
__ and __ are especially likely to occur during meiosis. In crossing over, nonsister chromatids sometimes exchange unequal-sized segemts of DNA, so that one partner gives up more genes than it receives. The products of such a __ crossover are one chromosome with a __and one chromosome with a __.
A diploid embryo that is homozygous for a large __ (or has a single X chromosome with a large deletion, in a male) is usually missing a number of essential genes, a condition that is ordinarily lethal.
__ and __ also tend to be harmful.
In __, in which segments are exchanged between __, and in __, the balance of genes is not abnormal- all genes are present in their normal doses. Nevertheless, __ and __ can alter phenotype because a gene's expression can be influenced by its location among neighboring genes' such events sometimes have devastating effects.
- reciprocal translocations
- nonhomologous chromosomes
Alterations of chromosome number and strucure are associated with a number of serious human disorders. (__ in meiosis results in __ in gametes and any resulting zygoes.) However, some types of __ appear to upset the genetic balance less than others, with the result that individuals with certain __ conditions can survive to birth and beyond.
-- These individuals have a set of traits- a __- characteristic of the type of __. Genetic disorders caused by __ can be diagnosed before birth by fetal testing.
- aneuploidy x2
- aneuploidy x2
One aneuploid condition, __, affects approx. one of every 700 children born in the US; result of an extra chromosome 21 (total of 47 chromosomes. Because the cells are __ for chromosome 21, it is often called __; characteristic facial features, short stature, heart defects, susceptibility to respiratory infection, and mental retardation; prone to develop leukemia nad Alzheimers
- trisomy 21
Frequency of __ increases with mom's age. While the disorder occurs in just __ of children born to women under age 30, the risk clims to __ for mothers at 40 and even higher for older moms.
- Most cases result from __ during meiosis I, and some research points to an age-dependent abnormality in a meiosis checkpoint that normally delays __ until all kinetochores are attached to the spindle. __ of some other chromosomes also increase in incidence with maternal age, although infants with other autosomal trisomies rarely survive for long.
__ of sex chromosomes produces a variety of aneuploid conditions. Most of these conditions appear to upset genetic balance less than __ conditions involving autosomes. This may be because the _ chromosome carries relatively few genes and becasue extra copies of the __ chromosome become inactivated as __ in somatic cells.
- Barr bodies
An extra X chromosome in a male, producing __, occurs approx. once in every 2000 live births. People with this disorder, called __, have male sex organs, but the testes are abnormally small and the man is sterile.
- Even though teh extra X is __, some breast enlargement and other femal ebody characteristics are common.
- Klinefelter Syndrome
Males with an extra Y chromosome (__) do not exhibit any well-defined syndrome, but they tend to be somewhat taller than average.
Females with __ X (__) which occurs once in approx. 1000 live births are healthy and cannot be distinguished from XX females except by karyotype.
__ X, called __, occurs about once in every 5000 births and is the only known viable monosomy in humans.
- Turner Syndrome
Although these __ individuals are phenotypically female, they are sterile because their sex organs do not mature. When provided with estrogen replacement therapy, girls with __ do develop secondary sex characteristics. Most have normal intelligence.
Many __ in human chromosomes, even in a heterozygous state, cause sever problems.
- One such syndrome, such as _-, results from a specific __ in chromosome facial features, and has a cry that sounds like a distressed cat. These people usually die in infancy or early childhood.
- cri du chat
Chromosomal __ have been implicated in certai cancers, including __.
- This disease occurs when a reciprocal transloccation happens during mitosis of cells that will become WBCs. In these cells, the exchange of a large portion of chromosome 22 with a small fragment from a tip of chromosome 9 produces a much shortened, easily recognized chromosome 22, called __.
- chronic myelogenous leukemia
- Philadelphia chromosome
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