Chapter 15

Genes exist as part of chromosomes. If a gene is on the X chromosome, but there is no corresponding locus on the Y chromosome we call the male hemizygous for the respective gene. The allele is phenotypically expressed in hemizygous individuals no matter whether it is dominant or recessive. Examine the patterns of sex linked inheritance of such genes depicted in Fig. 15.4, 15.7.

There are many more genes than there are chromosomes, so many genes must be on the same chromosome. This leads to complicated phenotypic ratios in offspring. To appreciate these patterns we must first consider two situations: when two genes are on different chromosomes and where two genes are on the same chromosome without crossing over occurring between them. For the first, assume that an individual that is doubly heterozygous AaBb is test crossed (i.e., mated with aabb). Since the tester individual makes no contribution to the phenotype of the offspring, the offspring phenotypic ratio is established by the ratio of the gamete types produced by the double heterozygous. The ratio is 1AaBb: 1Aabb: 1aaBb: 1aabb. Note that two of these phenotypes (Aabb and aaBb) are different from either parent and are called recombinant types. The two phenotypes (AaBb and aabb) that are like the parents are called parental types. From this we can calculate the recombination frequency as (# of recombinants/total # of offspring) and we would expect this to be 0.5. This is our expectation if the genes are assorting independently as proposed by Mendel. In contrast, in the second case the double heterozygous parent will only produce two types of gametes (either AB, ab or Ab, aB depending on which alleles are linked on the same chromosome). Since offspring from a test cross of this individual would belong to either of the phenotypes dictated by the gametes of this individual, the recombinant frequency would be 0. (Fig. 15.9, 15.10)

When genes are linked, recombinants arise if crossing over occurs between the loci. Geneticists use the recombination frequency X 100 as a genetic map distance between the two loci. This allows geneticists to establish the order of genes on a chromosome and the relative distance between loci. The more crossing over, the farther apart the loci are on the chromosome. (Fig. 15.10, 15,11, 15.12)

Gender is determined in mammals by the presence or absence of a Y chromosome. During development the Y chromosome produces a substance that causes certain embryonic tissue to become testes. Without testes determining factor the tissue develops into ovaries. Not all species use the presence or absence of the Y chromosome to establish gender. (Fig. 15.6)

Mammalian females have two X chromosomes while males only have one. This could lead to an imbalance in the quantity of enzymes that are produced by the two sexes. To avoid such problems mammalian females randomly inactivate one of their X chromosomes in each cell at an early stage of embryonic development. This is referred to as the Lyon hypothesis. All future cells that descend from any such cell have the same X chromosome inactivated. Because of this mature, heterozygous mammalian females often have clumps of cells with one X chromosome active and nearby clumps with the other X chromosome active. A commonly used example of this is the calico cat. Inactivated X chromosomes become condensed and positioned near the inside of the nuclear envelope forming a Barr body. Since only one active X chromosome exists in mature cells, any other X chromosome will form a Barr body. (Fig. 15.8)

Occasionally individuals inherit an extra copy of a chromosome (they are called trisomics) or are missing a copy of a chromosome (monosomics). The general term for such individuals is aneuploids. In some species (particularly plants) individuals may inherit one or more extra complete haploid sets of chromosomes. These are called polyploids. Ninety five percent of Down syndrome cases in humans are caused by trisomy of chromosome 21. (Fig. 15.16)

There may also be structural abnormalities associated with chromosomes. Some have duplications (extra copies of regions). Others have deletions (missing regions). In some individuals a region is rotated 180⁰ and reinserted forming an inversion. If a portion of a chromosome breaks off and joins with a nonhomologous chromosome this constitutes a translocation (Fig. 15.15).