8.3
A gene is a sequence of DNA that resides at a particular site on a chromosome the locus (plural loci). Genetic linkage of genes on a single chromosome can alter their pattern of inheritance from those described by Mendel s law.
Genetic linkage was discovered by Thomas Hunt Morgan, using the fruit fly Drosophila melanogaster. Much genetic research has been done with Drosophila, which is considered a model organism because of its small size, ease of breeding, and short generation time.
Some crosses performed with Drosophila did not yield expected ratios according to the law of independent assortment. Instead, some alleles for body color and wing shape were inherited together. Morgan hypothesized that the two loci were linked on the same chromosome and could not assort independently.
Figure 8.13 Some Alleles Do Not Assort Independently (Part 1) Morgan showed that genes for body color and wing size in Drosophila are linked, so their alleles don t sort independently
Figure 8.13 Some Alleles Do Not Assort Independently (Part 2) Morgan expected to see four phenotypes in a ratio of 1:1:1:1 but that is not what he observed The color gene and wing size did not sort independently, instead they were frequently inherited together
But if linkage were absolute, the F 1 would only have the two parental phenotypes in a 1:1 ratio. Some of the offspring had recombinant phenotypes, resulting from crossing over during prophase I in meiosis. Two of the four chromatids in the tetrad exchange chromosome segments. Genes at different loci on the same chromosome can recombine and separate by crossing over The result is two recombinant gametes from each event of crossing over
Figure 8.14 Crossing Over Results in Genetic Recombination
Recombinant offspring generally appear in proportions related to the recombination frequency between the two genes: Calculated by dividing the number of recombinant progeny by the total number of progeny. Recombinant frequencies are higher for loci that are farther apart on the chromosome because genes that are further apart are more likely to cross over
Figure 8.15 Recombination Frequencies
Recombinant frequencies can be used to infer the locations of genes along a chromosome and make a genetic map. Fruit flies have one pair of sex chromosomes (involved in sex determination) and 3 pairs of autosomes (all the other chromosomes). Recombination frequency of y and w is low, so they are closer on the map, but y and v is more frequent
Males have X and Y sex chromosomes that are different in size, and many genes on the X chromosome are not present on the Y. Thus, males have only one copy of some genes. This means that the males only have one copy of the X gene, this eye color locus is on the X chromosome A gene that is present as a single copy in a diploid organism is called hemizygous.
Morgan s experiments showed that the gene for eye color was carried on the X chromosome. The wild-type is red (R), the mutant allele is white (r).
Figure 8.16 A Gene for Eye Color Is Carried on the Drosophila X Chromosome
The term sex-linked inheritance refers to inheritance of a gene that is carried on a sex chromosome. In mammals, the X chromosome is larger and carries more genes that the Y; most sex-linked inheritance involves genes that are carried on the X chromosome.
Figure 8.17 Red Green Color Blindness Is Carried on the Human X Chromosome A heterozygous carrier has a normal phenotype This women inherited the mutant X from her mother This man inherited the mutant X from his mother and expresses the mutant phenotype Two siblings inherited the mutant X from their mother. The son expresses the mutation; his sister is a carrier
Patterns in X-linked recessive phenotypes: They appear much more often in males than females. A male with the mutation can pass it only to daughters. Daughters who receive one X-linked mutation are heterozygous carriers. Mutant phenotype can skip a generation if it passes from a male to his daughter (normal phenotype) and then to her son.
Mitochondria and plastids also have a small chromosome. Egg cells have abundant cytoplasm and organelles, but the only part of the sperm that takes part in fertilization is the nucleus. So, mitochondria and plastids are inherited only from the mother. Inheritance of organelles and their genes is thus non-mendelian and is called maternal, or cytoplasmic, inheritance.
Some organelle genes are important for organelle assembly and function, and mutations can have large effects. In plants, a mutation that affects proteins that assemble chlorophyll molecules into photosystems results in all white plants.