As you know, humans have two types of sex chromosomes, X and Y. Females have two X chromosomes and males have an X and Y chromosome. Because there are two sex chromosomes, there is a 50/50 chance that the offspring will be male or female.
15.2 Linked Genes tend to be inherited together because they are located near each other on the same chromosome
Genes that tend to be inherited together because they are on the same chromosome are referred to as linked genes. sex-linked genes carry many genes that are not related to sex. An example of this is color blindness.
Sex linked chromosomes are also called X-lined chromosomes because they reside on the X chromosome. Therefore, fathers can only pass their sex-linked genes on to their daughters, not their sons. Females will express a sex-linked trait only if they are homozygous for it. Males are more likely to express a sex-linked trait because they only have one X and have no other allele to mask the effects.
Genetic Recombination and Linkage
Genetic Recombination is the production of offspring with combinations of traits differing from those found in either parent.
Mendel learned during his experiments that some offspring have combinations of traits. For example, a cross between a pea plant with yellow-round seeds that is heterozygous for both seed color and shape (YyRr) crossed with a plant with green-wrinkled seeds (homozygous, yyrr) can produce offspring that are genetically different from the parents. The offspring will have genotypes of YyRr, yyrr, (Yyrr, yyRr these two are recombiant offspring). In fact, 50% of all offspring are recombinants as in this example.
Recombination of Linked Genes: Crossing Over
crossing over ( down) causes recombination. one maternal and one paternal chromatid break at corresponding points then are rejoined to each other. (they trade places)
Linkage Maping Using Recombination Data: Scientific InquiryA genetic map is an 
ordered list of the genetic loci along a particular chromosome. Alfred H. Sturtevant discovered that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the highter the recombination frequency. This is true. A genetic map that is based on the recombination frequencies is specifically called a linkage map (left). The distance is measured in map units which is equivalent to a 1% recombination frequency. Today, they are often called centimorgans in honor of Morgan. The frequency of crossing over has a maximum value of 50%. Cytogenetic maps locate genes with respect to chromosomal features.
ordered list of the genetic loci along a particular chromosome. Alfred H. Sturtevant discovered that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the highter the recombination frequency. This is true. A genetic map that is based on the recombination frequencies is specifically called a linkage map (left). The distance is measured in map units which is equivalent to a 1% recombination frequency. Today, they are often called centimorgans in honor of Morgan. The frequency of crossing over has a maximum value of 50%. Cytogenetic maps locate genes with respect to chromosomal features.15.3- Sex-linked genes exhibit unique patterns of inheritance
The Chromosomal Basis of Sex
As stated above, humans have two types of sex chromosomes- X and Y. Females have two X's and males have one X and one Y. In both testis and ovaries, the two sex chromosomes segretate like any other pair of chromosomes pair during meiosis. Females can only give an X chromosome, whereas, males can give either an X or a Y. Half the sperm a male produces will contain a Y chromosome and the other half will contain an X. During the second month of development the gene SRY of a Y chromosome will turn on and is required for the development of testes in males. SRY also codes for other genes on the Y chromosome. If these genes are absent the XY individual is male but cannot produce normal sperm.
Inheritance of Sex-Linked Genes
Sex chromosomes have genes that are unrelated to sex. Any gene that is located on either sex chromosome is reffered to as a sex-linked gene. As stated above, females will only express a recessive sex-linked trait if she is homozygous. Men only need to inherit the recessive allele from their mother to express the trait. There are many sex-linked disorders. Examples inlcude Duchenne muscular distrophy which is a degenerative muscle disorder caused by the absense of a muscle protein called dystraphin and hemophilia. Hemophilia is a disorder that affects the blood's ability to clot. (examples of sex-linked traits below)
X-Inactivation in Female Mammals
During embryonic development, one X chromosome in each cell become "inactivated". Because of this, both females and males have the same amount of genes with loci on the X chromosome. The X chromosome that has been inactivated condenses into a compact object called a barr body. The genes on the inactivated X chromosome are not expressed in the ovaries. The are however, reactivated so that every female gamete has an active X chromosome.
The selection of which chromosome is inactivated is random. Because of this, females have a mosaic of two types of cells- those with the active X from the father and ones with the active X from their mother. Descendents of a cell have the same X chromosome that is active and the same one that is inactive. In humans, this mosaicism can be ovserved in a recessive X-linked mutation that prevents for the development of sweat glands. If a woman is heterozygous for this trait she will have patches of skin that don't have sweat glands mixed with patches of skin that do.
The inactivation of an X chromosome involves modification of DNA and scientists have discovered the gene XIST that is active only on the barr-body chromsome. The many copies of the RNA produced from this gene attach to the X chromosome on which they are made which seems to initiate X inactivation.
QUIZ:
1) All are examples of sex-linked disorders:
a. color-blindness b. Duchenne muscular distrophy c. Fragile X-syndrome d. Huntington's Disease
2) True or false: females must be homozygous for a sex-linked trait in order to express it
3) True or false: crossing over occurs more commonly on genes that are closer together
answers....1) D 2) true 3) false
Don't forget- if you are absent tomorrow (Tuesday) you are still requried to take the chapter 15 quiz on Monday. We have a unit exam next Tuesday and a lab on Monday, so its going to be a busy week. Happy Thanksgiving! :)
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