I. Chromosomal Theory of Inheritance As early cytologists worked out the mechanism of cell division in the late 1800 s, they began to notice similarities in the behavior of BOTH chromosomes & Mendel s factors a) Chromosomes & alleles occur in pairs in diploid cells. b) Homologous chromosomes & alleles segregate during meiosis. c) Fertilization restores the paired condition for both chromosomes & alleles. These similarities formed the basis of Chromosomal Inheritance Theory (CIT), suggesting that alleles may RESIDE on chromosomes. If so, chromosome behavior during division may explain allele segregation & independent assortment. Figure 1: Testing the CIT: T.H. Morgan s Fruit Fly Mating Experiments T.H. Morgan attempted to test the validity of the chromosome theory by using it to predict the outcome of this mating under two possible scenarios: (1) if the alleles for wing shape & body color are unlinked AND (2) if these alleles are linked (reside on the same chromosome) Figure 1.1: Testing the CIT: Gametes Formed Assuming Unlinked Alleles 1
Figure 1.2: Testing the CIT: Expected Results Assuming Unlinked Alleles Figure 1.3: Testing the CIT: Gametes Formed Assuming Linked Alleles Figure 1.4: Testing the CIT: Expected Results Assuming Linked Alleles 2
Figure 1.5: Testing the CIT: Observed Results These results suggest that the alleles for body color & wing shape are linked due to the disproportionate number of parental phenotypes obtained. But why are there ANY recombinant phenotypes? Alfred Sturtevant, an undergraduate student of Morgan s, suggested that the existence of recombinant phenotypes could be due to crossing over between chromosomes during meiosis. In order to test this hypothesis, he studied the inheritance of THREE traits in Drosophila: body color, wing shape, & eye color. Figure 1.6: Testing the CIT: Sturtevant s Crosses Assuming that alleles do indeed exist on chromosomes, Sturtevant suggested that these frequencies represented the relative distance between these alleles on the same chromosome. From this he constructed the first Gene Map... 3
Figure 1.7: Testing the CIT: Gene Linkage Map If alleles are in fact on chromosomes & are affected by crossing over, this model should be able to accurately predict the frequency of a VERY unlikely occurrence: the outcome of TWO cross over events between the b-cn-vg loci Figure 1.7: Testing the CIT: Sturtevant s Results Due to the double cross-over, these phenotypes should exist at a total frequency of. This is exactly what Sturtevant observed. Since the gene/chromosome map was able to predict such an unlikely occurrence, the chromosome theory became accepted. Sample Problem: Constructing Gene Linkage Maps Determine the sequence of genes along a chromosome based on the following recombination frequencies: A-B: 8 Map Units A-D: 25 Map Units B-D: 33 Map Units A-C: 28 Map Units B-C: 20 Map Units 4
Figure 2: Gene Linkage & MAP Units Figure 2.1: Gene Linkage: Maximum Recombination Frequencies/MAP Units The maximum recombination frequency between linked alleles via cross over is 50% (50 MU s). This would only be observed if the alleles are FAR APART (opposite ends) of the same chromosome. Linked Genes: a) MAP UNITS (MU) = recombination frequency between alleles via crossing over (1 MU = 1% recombination frequency). Provides information regarding the relative distance between loci on a chromosome (greater the recombination frequency, the greater the distance between loci) b) **Any recombination frequency LESS THAN 50% suggests that the alleles in question are LINKED & can only be recombined via crossing over. Alleles exhibiting a recombination frequency of 50% are likely unlinked are recombined via independent assortment (50% probability of aligning on either side of metaphase plate). Recombinants: 5
II. Sex Chromosomes & Sex Linkage Figure 3: Chromosomal Basis for Gender Determination During fertilization, it is the sperm cell (X OR Y) that determines the gender of the child upon fusing with an ovum (X). If present in the resulting zygote, it is the Y chromosome that will direct its development into a male. If absent, the zygote will develop into a female. Figure 4: Chromosomal Basis for Gender Determination: Y Chromosome (SRY) 6
Figure 5: X-Linked Alleles: Drosophila Eye Color Figure 5.1: X-Linked Alleles: Hemizygous Males In contrast, females would need to inherit TWO copies of the (w) allele to exhibit white eyes white-eyed females less frequent. 7
Sex-Linked Genes: Figure 5.2: Generalized Sex-Linked (X) Inheritance Pattern Mutated X-linked alleles that cause disease are those associated with hemophilia & color-blindness (both more common among males). Sample Problems: Probability of X-Linked Inheritance If a heterozygous woman has children with an unaffected man, what is the probability of the following outcomes? a) An affected son: b) Four unaffected offspring in a row: c) An unaffected daughter OR son: d) Two out of five offspring are unaffected: 8
III. Epigenetic Inheritance Is observed when a modification occurs to a gene that alters the gene s expression without altering the gene s base sequence. Such modifications may occur during gametogenesis, embryonic development, or as a result of environmental factors. Epigenetic modifications become fixed during an individual s lifetime & are thus passed to all subsequent generations within a cell line. Although fixed across cell generations, epigenetic changes are NOT permanent over the course of generations of individuals (may be lost during gametogenesis or embryonic development). Examples of epigenetic inheritance include Dosage Compensation & Genomic Imprinting. Dosage Compensation For most alleles, two functional copies (1 on each homolog) are needed in order to express vital traits. For X-linked alleles, the ideal copy number is either one OR two, depending on the species... Figure 6: Dosage Compensation: Drosophila melanogaster In humans, a single dosage of X-linked alleles is all that is required to promote normal metabolism. In 1961, Mary Lyon proposed that dosage compensation in mammals occurs by the inactivation of a single X chromosome during female embryonic development... Figure 6.1: Dosage Compensation: Barr Body (Inactive X) 9
Figure 6.2: Dosage Compensation: Barr Bodies vs X-Chromosome Count Figure 6.3: Dosage Compensation: X-Inactivation Mechanism in Mammals If the Xce region is strong, Xist expression is inhibited X chromosome REMAINS ACTIVE (no Barr formation). If the Xce region is weak, Xist expression is promoted X chromosome CONDENSES (Barr formation). 10
Figure 6.4: Dosage Compensation: Epigenetic Inheritance of Barr Body When a cell divides, the Barr body is replicated & both copies remain compacted. Thus, once determined, the X chromosome that assumes the Barr body is maintained across all subsequent cell generations. Dosage Compensation: Barr Body: Figure 6.5: Phenotypic Effects of Random X-Inactivation: Calico Coats in Cats Female mammals that are heterozygous for a sex-linked trait will express BOTH the dominant & recessive form of the trait depending on which X chromosome is inactivated in a particular cell line. Such individuals are said to exhibit a Variegated phenotype for these traits. In the above example, the orange allele is dominant to non-orange & both are sex-linked. In some cell lines, the X chromosome possessing the orange allele forms a Barr body in which case only black or white fur will be expressed. In other cell lines, the X chromosome possessing the black allele forms a Barr body in which case only orange fur is expressed. The end result in such heterozygotes is a coat exhibiting a random distribution of white, black, & orange fur color known as calico. 11
Genomic Imprinting For most alleles, we inherit two working copies, one from mom & one from dad. For other alleles, we inherit only one working copy. The other is tagged or Imprinted w/a chemical group that effects its expression (CH 3 groups SILENCE an allele, COCH 3 groups ACTIVATE an allele). Figure 7: Genomic Imprinting Genomic Imprinting: a) Some alleles are imprinted during oogenesis, whereas others are imprinted during spermatogenesis. As a result, only ONE of the inherited alleles for a trait is expressed (offspring has only one functional copy of an allele). An example of imprinting involves the Igf2 allele in mice that codes for insulin-like growth factor 2, a hormone necessary for normal growth (defects in this gene result in dwarfism). In mice, it is always the PATERNAL copy of this allele that is chemically tagged or imprinted (and thus expressed) Figure 7.1: Genomic Imprinting: Mouse Igf2 Allele In the above example, the normal size mouse has inherited the normal Igf2 allele from dad & the mutant Igf2- allele from mom. Since this allele is only imprinted during the formation of the father s gametes, the offspring will only express the normal Igf2 allele & thus exhibit normal growth. 12
The dwarf mouse has inherited the normal Igf2 allele from mom & the mutant Igf2- allele from dad. Once again, since this allele is only imprinted during the formation of the father s gametes, the offspring will only express the mutant allele & exhibit the dwarf phenotype. The imprinted gene is maintained throughout the life of the offspring (all its cells will possess & maintain the imprint) until such time that the imprint is erased & reestablished during the formation of its own gametes (fig. 7.2). In most cases, imprinting in mice usually involves methylation that acts to silence the imprinted allele Figure 7.1: Genomic Imprinting: Erasing & Reestablishing Imprints During Gametogenesis If inherited imprints are not erased during oogenesis, BOTH the alleles may bear the imprint in the resulting zygote (will result in the silencing of both copies of the allele, a potentially lethal condition). Figure 7.3: Genomic Imprinting: Effects of Dietary Supplements 13
Figure 7.3: Genomic Imprinting: Effects of Parental Nurturing 14