CONCEPTS IN BIOLOGY

PART III. MOLECULAR BIOLOGY, CELL DIVISION, AND GENETICS

 

10. Patterns of Inheritance

 

10.7. Linkage

Although Mendel’s insight into the nature of inheritance was extremely important, there were many aspects of inheritance that Mendel did not explain. Linkage is a situation in which the genes for different characteristics are inherited together more frequently than would be predicted by probability. Linkage can be explained by examining chromosomes.

Linkage Groups

Each chromosome has many genes located along its length. Mendel’s inheritance patterns don’t really describe the inheritance patterns of individual genes; they describe the inheritance patterns of chromosomes. Homologous chromosomes separate from each other (segregation). Non-homologous chromosomes separate from each other independently (independent assortment.) Because each chromosome has many genes on it, these genes tend to be inherited as a group. A linkage group is a set of genes located on the same chromosome. This means that they tend to be inherited together. The process of crossing-over, which occurs during prophase I of meiosis I, may split up these linkage groups. Crossing-over happens between homologous chromosomes donated by the mother and the father and results in a mixing of the allele combinations in gametes. This means that the child can have gene combinations not found in either parent alone. The closer two genes are to each other on a chromosome, the less likely crossing-over will occur between them and separate them.

Autosomal Linkage

People and many other organisms have two types of chromosomes—sex chromosomes and autosomes. Sex chromosomes control the sex of an organism. Autosomes are chromosomes that are not directly involved in sex determination; they have the same kinds of genes on both members of the homologous pair of chromosomes. Of the 23 pairs of human chromosomes, 22 are autosomes. An example of autosomal linkage is found in figure 10.9. The three genes listed in this figure are on the same chromosome. If the genes sit closely enough to each other, they are likely to be inherited together.

FIGURE 10.9. Chromosome

These are just three genes that are found on human chromosome number 4. Because all these genes are found on one chromosome or strand of DNA, they are considered to be members of one linkage group. Each chromosome represents a group of linked genes.

Sex Determination

Genes determine sexual characteristics in the same manner as other types of characteristics. In many organisms, special sex chromosomes carry sex-determining genes. Sex chromosomes are different between males and females of the same species. Autosomes carry the same genes in both sexes of a species. In humans, all other mammals, and some other organisms (e.g., fruit flies), the sex of an individual is determined by the presence of a certain chromosome combination. In mammals, the genes that determine maleness are located on a small chromosome known as the Y chromosome. The Y chromosome behaves as if it and another larger chromosome, known as the X chromosome, were homologous chromosomes. Males have one X and one Y chromosome. Females have two X chromosomes.

The sex of some animals is determined in a completely different way. In bees, for example, the females are diploid and the males are haploid. Other plants and animals have still other chromosomal mechanisms for determining their sex (Outlooks 10.2).

OUTLOOKS 10.2

The Birds and the Bees ... and the Alligators

The determination of sex depends on the kind of organism it is. For example, in humans, the physical features that result in maleness are triggered by a gene on the Y chromosome. The lack of a Y chromosome results in a female individual. In other organisms, sex is determined by other combinations of chromosomes or environmental factors.

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Organism

Sex Determination

Mammals

Sex is chromosomally determined: XY individuals are male.

Birds

Sex is chromosomally determined: XY individuals are female. Rather than XY the letters WZ are used in birds.

Bees

Males (drones) are haploid and females (workers or queens) are diploid.

Certain species of alligators, turtles, and lizards

Egg incubation temperatures cause hormonal changes in the developing embryo; higher incubation temperatures cause the developing brain to shift sex in favor of the individual becoming a female.

Boat shell snails

Males can become females but will remain male if they mate and remain in one spot.

Shrimp, orchids, and some tropical fish

Males convert to females; on occasion, females convert to males, probably to maximize breeding.

African reed frog

Females convert to males, probably to maximize breeding.

Sex Linkage

Sex linkage occurs when genes are located on the chromosomes that determine the sex of an individual. The Y chromosome is much shorter than the X chromosome and has fewer genes for traits than found on the X chromosome (figure 10.10). Therefore, the X chromosome has many genes for which there is no matching gene on the Y chromosome. Some genes appear on both the X chromosome and Y chromosome. Other genes, however, are found only on the X chromosome or only on the Y chromosome. Females have two copies of the genes that are found only on the X chromosomes. Because males have both a Y chromosome with few genes on it and the X chromosome, many of the recessive characteristics present on the X chromosome appear more frequently in males than in females, who have two X chromosomes. Unusual sex-linked inheritance patterns occur because certain genes are found on only one of the two sex chromosomes. Genes found only on the X chromosome are said to be X-linked genes. Genes found only on the Y chromosome are said to be Y-linked genes.

FIGURE 10.10. Sex Chromosomes

The human X chromosome contains over 1,400 genes and over 150 million base pairs, of which approximately 95% have been determined. The human Y chromosome contains about 200 genes and about 50 million base pairs, of which approximately 50% have been determined. A number of the genes linked on these chromosomes are listed.

Female phenotypes can be affected by the dominant and recessive allele interactions that Mendel identified. Males present a different case. Males only have one copy of the genes that are found on the X chromosome, because they have only one X chromosome. This one allele determines the male’s phenotype. Some X-linked genes can result in abnormal traits, such as color deficiency, hemophilia, brown teeth, and at least two forms of muscular dystrophy (Becker’s and Duchenne’s).

Use the following problem as an example of how to work with X-linked genes. Notice that the same basic format is followed as in previous genetics problems. The major difference is that chromosomes are represented in this problem. Here, an X represents the X chromosome and a Y represents the Y chromosome. Genes that are linked to the X chromosome are shown as superscripts. The X and its superscript should be treated as a single allele. You have used superscripts before in a genetics problem to look at incomplete dominance and codominance.

Problem Type: X-Linked

Cross 6: In humans, the allele for normal color vision is dominant and the allele for color deficiency is recessive. Both alleles are X-linked. People who cannot detect the difference between certain colors, such as between red and green, are described as having “color-defective vision.” A male who has normal vision mates with a female who is heterozygous for normal color vision. What type of children can they have in terms of these traits, and what is the probability for each type?

Gene Key

Gene or Condition: color vision

Allele Symbols

Possible Genotypes

Phenotype

XB = normal color vision

Xb = color-deficient vision

Females (XBXB or XBXb)

Males (XbY)

Female with normal color vision

Male with normal color vision

Y = no gene for color vision

Females (XbXb)

Males (XbY)

Female with color-defective vision

Male with color-defective vision

Note that, in solving sex-linked problems, the general process is the same as that for other genetics problems (table 10.7). The only significant difference is that the alleles are listed as superscripts to the chromosomes, so that the gender and phenotypes can be determined in the last few steps of the problem.

TABLE 10.7. Solution Pathway

10.7. CONCEPT REVIEW

21. What is a linkage group?

22. Provide examples of genes that are linked.