A gene is a segment of DNA that codes for a protein. Typically, genes are composed of coding sequences (exons) and noncoding intervening sequences (introns). Genes are transcribed into messenger RNA (mRNA) that is processed to remove the introns. The message is then attached to ribosomes and translated into polypeptides.

All somatic (body) cells (gametes [sperm and egg] are not called somatic cells) have 46 chromosomes (40 in the mouse), half of which came from the father and half from the mother. A chromosome is a large group of genes that are attached to each other in a single linear sequence of DNA, and which condenses into a microscopically identifiable unit during mitosis (cell division). The gametes, following meiosis (specialized cell division for gamete production), have only half this number of chromosomes, and are said to be haploid (1n); during fertilization, they combine in the zygote (fertilized egg) to yield the diploid (2n) genome of somatic cells. There are 23 pairs of chromosomes in somatic cells; one member of each pair is paternal (from the father) and one maternal (from the mother). There are 22 matched pairs of autosomal chromosomes, plus one pair of sex chromosomes. The latter is matched in females (XX) and unmatched in males (XY); thus there are 24 different kinds of chromosomes, or 25 if we include the mitochondrial DNA as a chromosome.

Hybridomas start as fusions between two somatic cells, and hence are intitally tetraploid (4n); however, they rapidly lose some of the superfluous chromosomes, becoming hypotetraploid (less than 4n but more than 2n) and aneuploid (abnormal set of chromosomes).

Each gene occupies a specific position on a specific chromosome, its locus (plural = loci). The list of loci on a chromosome, or a region of a chromosome, and their distances apart, is called a genetic map. The locus of a gene is rather like an address. The major histocompatibility complex (MHC) is a group of closely linked genes found on human chromosome 6. The set of linked genes in the MHC region of a single chromosome 6 is called an MHC haplotype.

Consider a specific genetic locus, that for the alpha chain of the HLA-A molecule. Typically, the maternal and paternal copies of the gene for HLA-A will be different (each on a different chromosome), coding for slightly different forms of the HLA-A alpha chain protein. These slightly different gene sequences at the same genetic locus are called alleles. While there can be only two alleles for a given locus in the genome of a single individual, there can be many dozens of alleles in the whole population. The MHC loci have an unusually high number of alleles compared to other gene loci.

When the two alleles at a given locus are identical, the genome is said to be homozygous at that locus; if they differ, it is heterozygous. The 20 or so generations of brother x sister matings used to produce an inbred strain of mice produce homozygosity at nearly all loci (100% of the genome). Consequently, all gametes have identical alleles and chromosomes in an individual inbred mouse, and also among the members of an inbred strain.

During meiosis, the division of each pair of chromosomes between the gametes is random. Thus, the combinations of unlinked alleles making up the haploid sets of chromosomes differs in each gamete. Since there are 23 chromosomes in a haploid set, 2²³ genetically different human gametes are possible in a single individual: over eight million. Occasionally, crossing-over events further increase the number of possible gametes by exchanging a subset of the alleles between a pair of chromosomes.

Consider two inbred strains, C57BL/6 (B6, black fur) and A/J (white fur). Their haplotypes are H-2^b and H-2^a. If a female of strain B6 is mated with a male of strain A, the first generation offspring (F₁) will be heterozygous at every locus for which the alleles differ between the two strains. Since all loci in the H-2 region differ between these two strains, the F₁ mice will be 100% heterozygous at the MHC. There will be heterozygosity at the loci H-2K, H-2:I-A, H-2:I-E, H-2D, and H-2L.

In nearly all cases, both alleles at a heterozygous locus are expressed equally at the protein level in every somatic cell (codominant expression). Thus, every cell in the F₁ mouse mentioned above will have on its surface class I MHC antigens of both H-2^b and H-2^a type. This is important for the MHC since it increases the chance that a cell infected with a virus can effectively present at least one peptide from the virus-encoded foreign proteins to T cells.

The alternative to codominant expression is very rare, but does occur in some important cases. This means that only one allele is expressed, and that the allele at the same locus on the corresponding chromosome is silent at the protein level. This must happen for antigen receptors on B and T cells so that each cell may express receptors of a single specificity, enabling clonal selection to work properly. Hence, immunoglobulin and the T cell antigen receptor show allelic exclusion. The only other genes known to show allelic exclusion are those of the X chromosome. Males have XY and females XX. In females, one of the X chromosomes is silent (it condenses into a microscopically visible spot called a Barr body). The decision as to which female X chromosome or antigen receptor locus is silent is made at the somatic level, randomly for different cells. Once made, however, all progeny of that cell inherit the decision.

Inbred strains have been used to isolate particular alleles, thereby determining the functional effects of their gene products. Consider two strains of mice differing at H-2K, one H-2K^q and the other H-2K^s. An F₁ mouse resulting from a cross between these two strains is backcrossed with an H-2^s mouse. An offspring mouse is selected that still expresses H-2K^q, and is backcrossed again to an H-2K^s mouse. After 10 such backcrosses, selecting for H-2K^q each time, all the Q strain genes except for H-2K will have been diluted out. Indeed, recombinations between chromosomes will have replaced all but the immediate neighbors of the H-2K^q gene with H-2^s alleles. After some tricks to produce a homozygous mouse, the result is two strains of mice that differ in a rather small genetic region on a single chromosome. This relationship is called congenic. Our example generated an H-2K^q mouse on an S strain genetic background, which might be designated S.Q.

After making congenic strains, the next step is to separate the genes within the small isolated region. For example, our H-2K^q congenic to the S strain may also have H-2I^q. By crossing repeatedly and selecting for the appropriate offspring, eventually a random recombination event will occur between these two loci, separating the chromosomes into one that is all H-2^s except for H-2K^q, and another that is all H-2^s except for H-2I^q. These mice (once rendered homozygous) are called congenic recombinants.

To test your understanding, try to answer the following questions.

1. Are human identical twins 100% homozygous?

2. Are all mice in the F₁ generation cross between two inbred strains genetically identical with each other? What about the F₂ offspring resulting from a cross between members of the F₁ generation?

3. What percentage of the cells in an (H-2^k x H-2^d)F₁ mouse express H-2D^k?

4. On the average, what percentage of pairs of human siblings will be HLA-identical?

5. What percentage of her genes does a daughter inherit from her mother?

6. Will a son's skin, if grafted onto his father, be rejected as foreign?

7. Will a father's skin, if grafted onto his son, be rejected as foreign?
 
 

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Thanks to Professor Arthur Mange of the Department of Biology for a critical review.