A. Sex Chromosomes
1. The chromosomal basis of sex
a. In humans and other mammals, there are two varieties of sex chromosomes, X and Y.
1. An individual who inherits two X chromosomes usually develops as a female.
2. An individual who inherits an X and a Y chromosome usually develops as a male.
b. In the X-Y system, Y and X chromosomes behave as homologous chromosomes during meiosis by forming tetrads.
1. In reality, they are only partially homologous and rarely undergo crossing over.
c. Germ cells, the cells that divide to produce eggs or sperm, in testes are XY and in ovaries are XX. The two sex chromosomes segregate during meiosis and each gamete receives one.
1. Each egg receives an X chromosome.
2. Half the sperm receive an X chromosome and half receive a Y chromosome.
3. Because of this, each fertilization has about a fifty-fifty chance of producing a particular sex.
d. In humans, the anatomical signs of sex first appear when the embryo is about two months old.
1. In individuals with the SRY gene (sex-determining region of the Y chromosome), the generic embryonic gonads are modified into testes.
a. Activity of the SRY gene triggers a cascade of biochemical, physiological, and anatomical features because it regulates many other genes.
b. In addition, other genes on the Y chromosome are necessary for the production of functional sperm.
2. In individuals lacking the SRY gene, the generic embryonic gonads develop into ovaries.
2. Sex-linked genes have unique patterns of inheritance.
a. In addition to their role in determining sex, the sex chromosomes, especially the X chromosome, have genes for many characters unrelated to sex.
b. Most of these sex-linked genes are on the X chromosome only. These genes are missing from the Y chromosome (sorry guys).
1. Note the small size of the Y chromosome in the male karyotype compared to the X. The Y is smaller because it lacks some of the genes that are found on the X (sorry guys).
2. Examples of these sex-linked genes on the X chromosome are:
a. The gene that allows us see the colors red and green.
b. A gene that prevents us from going bald.
c. If a sex-linked trait is due to a recessive allele, a female will have this phenotype only if homozygous.
1. Heterozygous females will be carriers.
d. Because males have only one X chromosome (hemizygous), any male receiving the recessive allele from his mother will express the recessive trait. Examples of this expression of recessive traits are:
1. A man who is colorblind has a mutation on his X chromosome in the gene for seeing red and green. Since that gene is missing from the Y chromosome there is nothing to compensate for the mutated gene on the X.
2. Some men who are bald have a mutation in a gene on their X chromosomes. Since that gene is missing from the Y chromosome there is nothing to compensate for the mutated gene.
3. The chance of a female inheriting a double dose of the mutant allele is much less than the chance of a male inheriting a single dose.
4. Therefore, males are far more likely to inherit sex-linked recessive disorders than are females. This is why more men are colorblind or bald than women.
e. Several serious human disorders are sex-linked.
1. Duchenne muscular dystrophy affects one in 3,500 males born in the United States.
a. Affected individuals rarely live past their early 20s.
b. This disorder is due to the absence of an X-linked gene for a key muscle protein, called dystrophin.
c. The disease is characterized by a progressive weakening of the muscles and a loss of coordination.
2. Hemophilia is a sex-linked recessive trait defined by the absence of one or more clotting factors (proteins that slow and then stop bleeding).
B. Errors and Exceptions in Chromosomal Inheritance
1. Alterations of chromosome number: aneuploidy and polyploidy
a. Nondisjunction occurs when problems with the meiotic spindle cause errors in daughter cells. (Fig. 15.11)
1. This may occur if tetrad chromosomes do not separate properly during meiosis I.
2. Alternatively, sister chromatids may fail to separate during meiosis II.
b. As a consequence of nondisjunction, some gametes receive two of the same type of chromosome and another gamete receives no copy.
c. Offspring resulting from fertilization of a normal gamete with one after nondisjunction will have an abnormal chromosome number (aneuploidy).
1. Trisomic zygotes have three copies of a particular chromosome and have 2n + 1 total chromosomes.
2. Monosomic zygotes have only one copy of a particular chromosome and have 2n - 1 chromosomes.
d. If the organism survives, aneuploidy typically leads to a distinct phenotype depending on which chromosome underwent nondisjunction. (See "Several serious human disorders...." below.)
e. Aneuploidy can also occur during mitosis due to failures of the mitotic spindle.
1. If this happens early in development, this condition will be passed along by mitosis to a large number of cells.
2. This is likely to have a substantial effect on the organism.
f. Organisms with more than two complete sets of chromosomes, have undergone polypoidy.
1. This may occur when a normal gamete fertilizes another gamete in which there has been nondisjunction of ALL its chromosomes.
2. The resulting zygote would be triploid (3n).
3. Alternatively, if a 2n zygote failed to divide after replicating its chromosomes, a tetraploid (4n) embryo would result from subsequent successful cycles of mitosis.
g. Polyploidy is relatively common among plants and much less common among animals.
1. The spontaneous origin of polyploid individuals plays an important role in the evolution of plants.
2. Both fishes and amphibians have some polyploid species.
3. Recently, researchers in Chile have identified a new rodent species that may be the product of polyploidy. (Fig. 15.12)
h. Polyploids are more nearly normal in phenotype than aneuploids.
1. One extra or missing chromosome (aneuploidy) apparently upsets the genetic balance during development more than does an entire extra set of chromosomes (polyploidy).
2. Alterations of chromosome structure
a. Breakage of a chromosome can lead to four types of changes in chromosome structure. (Fig. 15.13)
1. A deletion occurs when a chromosome fragment is lost during cell division (mitosis or meiosis).
a. This chromosome will be missing certain genes.
2. A duplication occurs when a fragment becomes attached as an extra segment to a sister chromatid or homologous chromosome.
3. An inversion occurs when a chromosomal fragment reattaches to the original chromosome but in the reverse orientation.
4. In translocation, a chromosomal fragment joins a nonhomologous chromosome.
a. Some translocations are reciprocal, others are not.
b. The effects of deletions and duplications vary depending on size, which chromosome, which genes and at what point in the lifetime of the organism they occur.
c. Reciprocal translocation or inversion can alter phenotype because a gene�s expression is influenced by its location relative to other genes.
3. Several serious human disorders are due to alterations of chromosome number.
a. Aneuploid zygotes, resulting from nondisjunction in meiosis, may be frequent in humans but most of these alterations are so disastrous that the embryos are spontaneously aborted long before birth.
b. Certain aneuploid conditions upset the balance less, leading to survival to birth and beyond.
1. These individuals have a set of symptoms � a syndrome � characteristic of the type of aneuploidy.
c. One aneuploid condition, Down syndrome (trisomy 21), is due to three copies of chromosome 21. (Fig. 15.14)
1. Although chromosome 21 is the smallest human chromosome, it severely alters an individual�s phenotype in specific ways.
d. Nondisjunction of sex chromosomes produces a variety of aneuploid conditions in humans.
1. Unlike autosomes, this aneuploidy upsets the genetic balance less severely.
a. This may be because the Y chromosome contains relatively few genes.
2. Klinefelter�s syndrome, an XXY male, occurs once in every 2000 live births.
a. These individuals have male sex organs, but are sterile.
b. There may be feminine characteristics but their intelligence is normal.
3. Males with an extra Y chromosome (XYY) tend to somewhat taller than average.
4. Trisomy X (XXX), which occurs once in every 2000 live births, produces healthy females.
5. Monosomy X or Turner�s syndrome (X0), which occurs once in every 5000 births, produces phenotypic, but immature females.