Chapter 15: The Chromosomal Theory of Inheritance (2)

Because the identity of the __ chromosomes in an individual could be inferred by observing the sex of the fly, the behavior of the two members of the pair of __ chromosomes could be correlated with the behavior of the two __ of the eye color gene.

In humans and other mammals, there are two varieties of sex chromosomes, designed __ and __.
-- The __ is much smaller than the __.
- A person with ___ is a female; a person with __ is a male.

Short segments at either end of the __ chromosome are the only regions that are homologous with corresponding regionsof the __.
-- These ___ allow the __ and __ chromosomes in males to pair and behave like __ during meiosis in the __.

In both teh testes and ovaries, the two sex chromosomes __ during meiosis, and each receives one.
Egg= __
Sperm= __

__ is a matter of chance- 50%

There are three other chromosomal systems besides the human X-Y system.
- name the first one in grasshoppers and cockroaches

Name the chromosomal system in birds and fishes, etc.

Name the chromosomal system in bees and ants.

In humans, the anatomical signs of __ begin to emerge when the embryo is about __ months old.
- Before then, the rudiments of the __ are generic- they can develop into either __ or __, depending on whether a __ is present.

In 1990, a British research team identified a gene on the _ chromosome required for the development of the __.
- __
-- In the absence of __, the gonads develop into ovaries.
-- The biochemical, physiological, and anatomical features that distinguish males and females are complex and many genes are involved in their development. In fact, __ codes for a protein that regulates other genes.

Researchers have sequenced the human __ chromosome and have identified __ genes, which code for about __ proteins (some genes are duplicates). ABout 1/2 of these genes are expressed only in the testis, and some are required for normal __ functioning. In their absence, an __ individual is male, but doesn't produce normal sperm.

In addition to their role as carriers of genes that determine sex, these chromosomes, especially X chromosomes, have genes for many characters unrelated to sex.
-- A gene located on either sex chromosome is called a __, although in humans the term has historically referred specifically to a gene on the __ chromosome. __ in humans follow the same pattern of inheritance as the fruit flies eye-color.

Fathers pass __ to all of their daughters but to none of their sons.
In contrast, mothers can pass __ to both sons and daughters.

If a __ is due to a __ allele, a female will display the phenotype onyl if she is a __.
Males don't use homo and heterozygotes. These use __.
-- Any male receiving the __ from his mother will express the trait. For this reason, far more males than females have sex-linked recessive disorders.
-- However, even though the chance of a female inheriting a double dose of the __ is much less than the probability of a male inheriting a single does, there are females with sex linked disorders.

__- affects about one of every 3500 males born in the US; characterized by a progressive weakening of the muscles and loss of coordination; affected individuals rarely live past their early 20s; researchers have traced the disorder to the absence of a key muscle protein calle ddystrophin and have mapped the gene for this protein to a specific locus on teh X chromosome

__- sex-linked recessive disorderdefined by the absence of one or more of the proteins required for blood clotting. When a person with this is injured, leeding is prolonged because a firm clot is slow to form. Bleeding in the muscles or joints, unlike a small cut, can be painful and can lead to serious damage

One X chromosome in each cell in females becomes almost completely inactivated during __.
- As a result, the cells of females and males have the same effective dose (one copy) of these genes. The __ X in each cell of a female condenses into a compact object called a __, which lies along the inside of the nuclear envelope.
- Most of the genes of the X chromosome that forms the __ are not expressed. In the ovaries, __ chromosomes are reactivated in the cells that give rise to eggs, so every female gamete has an __.

__ demonstrated that selection of which X chromosome will form the __ occurs randomly and independently in each embryonic cell present at the time of X __.
-- As a consequence females consist of a mosaic of two types of cells: those with the __ derived from the father and those from the mother.

After an X chromosome is inactivated in a particular cell, all mitotic descendents of that cell have the same inactive X.
- Thus, if a female is __for a __, about half her cells will express one allele, while the others wll express the alternate.

In humans, __ can be observed in a recessive X-linked mutation that prevents the development of sweat glands. A woman who is eterozygous for this trait has patches of normal skin and patches of skin lacking sweat glands.

Inactivation of an X chromosome involves modification of the DNA, including attachmeent of __ groups to one of the __ of DNA __.

Researchers have also discovered an X chromosome gene called __ for __ that is active only on the __ chromosome.

Multiple copies of the __ product of this gene apparently attach to the X chromosome on which they are made, eventually almost covering it.
-- Interaction of this __ with the chromosome seems to initiate __, and regulation of this process by other genes is still being researched.