Linkage
Although the independent inheritance of genetic traits has been emphasized up to now, some genes will tend to be inherited together. The august monk Mendel did not envision the relationship that we now know to exist between genes and chromosomes. But to understand genetics more completely, it is necessary to expand his otherwise elegant theories to include gene linkage.
Any given gene has a particular chromosomal assignment and a place on that chromosome that we call a locus (plural: loci). Occasionally, traits of interest are on the same chromosome and tend to be inherited together more often than they are split apart: these are linked genes. Linked genes can be separated from each other as part of the normal process of chromosome recombination that occurs uniquely during meiosis. At present, the entire genome of the horse has been sequenced and we know the location and sequence of many genes of interest for the horse (Chapter 6 on horse genomics). Yet our understanding of the relationships between the bits of DNA and complex traits like behavior, performance, and many other phenotypic characteristics that we value in the horse remains to be developed.
The inheritance of gender
Two of a horse’s 64 chromosomes are called the sex chromosomes. The gender of a horse is the result of action by genes on the sex chromosomes. The sex chromosomes are designated X and Y. Females inherit two copies of the X chromosome. Males inherit an X chromosome and a Y chromosome. In mares, the two X chromosomes pair during meosis, undergo genetic recombination and each egg possesses one of the X chromosomes. During meiosis in the male, the X and Y chromosome pair and each sperm inherits either an X or Y chromosome. The pairing of the X and Y chromo some during meiosis seems exceptional because the X and Y chromosomes are quite different in size and gene content. However, they share DNA sequences for a very short region, called the pseudoautosomal region (PAR) and this is the basis for them to pair during meiosis. The genomics and cytogenetics of X and Y chromosomes are discussed further in Chapters 6and 17.
The consequences of sexual reproduction are that the gender of a foal is determined by the contribution from the sire, not the dam. The dam always contributes an X chromosome to her offspring. The sire contributes sperm with either X or Y. As a consequence, the sex of the offspring is determined by the contribution of the sire—specifically, whether he contributes an X or Y chromosome.
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5 The Language and Patterns of Genetics
Terminology allows people to communicate entire concepts with a single word. We noted at the beginning of Chapter 4 that the term “gene” was initially used as an abstract concept. Later, the molecular basis of a gene was determined, and its meaning became, at once, more complex and less abstract. In this chapter we introduce additional genetic terms, conventions, and formats useful to discuss the patterns of inheritance for genetic traits.
Genes and alleles
When a gene at a particular “locus” (the position of a gene on a chromosome) has two alternative forms (variants) we call these “alleles.” For example, the Extension locus has two alleles, one for black hair (E) and one for red hair (e). We have learned the underlying molecular mechanism is a result of variation at the gene MC1R (Marklund et al., 1996). This gene was used as an example in the preceding chapter and is discussed further in Chapter 7.
Gray is another gene which influences coat color. The Gray locus has two alleles, G for gray and g for non-gray. The Gray locus is different from the Extension locus and is caused by a variation in an entirely different gene (discussed in Chapter 12). The allele responsible for gray is not allelic to the colors red or black. It comes from a different locus with a different gene action. Gray is an entirely different property of hair color. However, Gray (G) is an allele for non-Gray (g) at the Gray locus. Black (E) and red (e) are alleles of each other at the Extension locus.
Genotypes: Homozygous and Heterozygous
As