Let's look at the inheritance of short legs and normal legs in MendAliens, a mythical alien species that has the same system of sex determination as humans. If we cross a short-legged female with a normal-legged male, all sons have the same phenotype as the mother and all daughters have the same phenotype as the father. The simplest hypothesis is that the short-legs trait is caused by a recessive allele on the X chromosome, an X-linked gene. Let's find out what happens when you cross two F1 individuals. We get 38 normal females to 33 short-legged females to 41 normal males to 35 short-legged males. Now let's add genotypes to the parental cross. We will use small s for the recessive short-legs allele and capital S for the dominant normal-legs allele. There is a recessive small s allele on each of the parental female's X chromosomes, while the parental male has the normal capital S allele on his X chromosome and no allele on his Y chromosome. The F1 female has the normal phenotype because she inherited the recessive allele small s on the X chromosome from her mother, but she inherited the normal allele capital S on the X chromosome from her father. The F1 male has the recessive phenotype because he inherited the recessive allele on the X chromosome from his mother, and there is no allele on the Y chromosome that he inherited from his father. A trait is inherited as an X-linked recessive trait if a cross between a female with the recessive trait and a true-breeding normal male results in all male offspring having the recessive trait and all female offspring having the normal phenotype, as shown here. Now let's add genotypes to the F1 cross. Assuming that our F1 individuals have the genotypes worked out in the previous step, we would predict that the F2 generation would have a ratio of one normal female to one short female to one normal male to one short male. These expected ratios are approximately matched by our observed F2 results of 38 normal females to 33 short females to 41 normal males to 35 short males. In conclusion, leg length can be explained by a single gene on the X chromosome with the short-legs allele recessive to the normal-legs allele. This cross illustrates the inheritance of an X-linked recessive allele. Now let us analyze a reciprocal cross, in which we switch the phenotypes of the parents. This is quite different from the result we got from the original cross. If we had looked at this first, we might have thought that short legs was caused by a simple recessive allele. This shows the importance of making the F1 cross, as shown in the next step. The F1 cross shows that short legs is not caused by a simple recessive allele. Looking at the F2 offspring of this cross, we see that there is a three to one ratio of normal to short offspring. But when we look at males and females separately, we see that this ratio is not the same for the two sexes. This indicates that the leg-length gene is linked to the X chromosome, with the short-legs allele recessive to the normal-legs allele. Had we not looked at the sexes separately, we might not have seen that X linkage was involved. Notice that for traits caused by an X-linked recessive allele, the F1 and F2 results are different for the parental cross between a female with the recessive trait and a normal male, as shown previously, and the parental cross between a normal female and a male with the recessive trait, as shown here. Thus, a reciprocal cross must always be made when the gene could be X-linked. Let's review how X-linked genes work by examining other scenarios. The pointed-ear trait in MendAliens is caused by an X-linked recessive allele. We will use small e for the pointed-ears, that is, the recessive allele, and capital E for the normal no-ear allele. A female MendAlien with no ears, which is the normal condition, and whose male parent had pointed ears, pairs with a male MendAlien with no ears. Let's make the cross. The proportions in the phenotypes are two females with no ears, one male with no ears, and one with ears. Overall, there will be three with no ears and one with ears. Here's another scenario. Let us now assume that the female MendAlien from the previous scene pairs with a male MendAlien with pointed ears. Let's make the cross. The proportions in the phenotypes are one female with no ears, one female with ears, one male with no ears, and one with ears. Overall, there will be two offspring with no ears and two with ears.
Table of contents
- 1. Introduction to Biology2h 40m
- 2. Chemistry3h 40m
- 3. Water1h 26m
- 4. Biomolecules2h 23m
- 5. Cell Components2h 26m
- 6. The Membrane2h 31m
- 7. Energy and Metabolism2h 0m
- 8. Respiration2h 40m
- 9. Photosynthesis2h 49m
- 10. Cell Signaling59m
- 11. Cell Division2h 47m
- 12. Meiosis2h 0m
- 13. Mendelian Genetics4h 41m
- Introduction to Mendel's Experiments7m
- Genotype vs. Phenotype17m
- Punnett Squares13m
- Mendel's Experiments26m
- Mendel's Laws18m
- Monohybrid Crosses16m
- Test Crosses14m
- Dihybrid Crosses20m
- Punnett Square Probability26m
- Incomplete Dominance vs. Codominance20m
- Epistasis7m
- Non-Mendelian Genetics12m
- Pedigrees6m
- Autosomal Inheritance21m
- Sex-Linked Inheritance43m
- X-Inactivation9m
- 14. DNA Synthesis2h 27m
- 15. Gene Expression3h 20m
- 16. Regulation of Expression3h 31m
- Introduction to Regulation of Gene Expression13m
- Prokaryotic Gene Regulation via Operons27m
- The Lac Operon21m
- Glucose's Impact on Lac Operon25m
- The Trp Operon20m
- Review of the Lac Operon & Trp Operon11m
- Introduction to Eukaryotic Gene Regulation9m
- Eukaryotic Chromatin Modifications16m
- Eukaryotic Transcriptional Control22m
- Eukaryotic Post-Transcriptional Regulation28m
- Eukaryotic Post-Translational Regulation13m
- 17. Viruses37m
- 18. Biotechnology2h 58m
- 19. Genomics17m
- 20. Development1h 5m
- 21. Evolution3h 1m
- 22. Evolution of Populations3h 52m
- 23. Speciation1h 37m
- 24. History of Life on Earth2h 6m
- 25. Phylogeny2h 31m
- 26. Prokaryotes4h 59m
- 27. Protists1h 12m
- 28. Plants1h 22m
- 29. Fungi36m
- 30. Overview of Animals34m
- 31. Invertebrates1h 2m
- 32. Vertebrates50m
- 33. Plant Anatomy1h 3m
- 34. Vascular Plant Transport2m
- 35. Soil37m
- 36. Plant Reproduction47m
- 37. Plant Sensation and Response1h 9m
- 38. Animal Form and Function1h 19m
- 39. Digestive System10m
- 40. Circulatory System1h 57m
- 41. Immune System1h 12m
- 42. Osmoregulation and Excretion50m
- 43. Endocrine System4m
- 44. Animal Reproduction2m
- 45. Nervous System55m
- 46. Sensory Systems46m
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- 48. Ecology3h 11m
- Introduction to Ecology20m
- Biogeography14m
- Earth's Climate Patterns50m
- Introduction to Terrestrial Biomes10m
- Terrestrial Biomes: Near Equator13m
- Terrestrial Biomes: Temperate Regions10m
- Terrestrial Biomes: Northern Regions15m
- Introduction to Aquatic Biomes27m
- Freshwater Aquatic Biomes14m
- Marine Aquatic Biomes13m
- 49. Animal Behavior28m
- 50. Population Ecology3h 41m
- Introduction to Population Ecology28m
- Population Sampling Methods23m
- Life History12m
- Population Demography17m
- Factors Limiting Population Growth14m
- Introduction to Population Growth Models22m
- Linear Population Growth6m
- Exponential Population Growth29m
- Logistic Population Growth32m
- r/K Selection10m
- The Human Population22m
- 51. Community Ecology2h 46m
- Introduction to Community Ecology2m
- Introduction to Community Interactions9m
- Community Interactions: Competition (-/-)38m
- Community Interactions: Exploitation (+/-)23m
- Community Interactions: Mutualism (+/+) & Commensalism (+/0)9m
- Community Structure35m
- Community Dynamics26m
- Geographic Impact on Communities21m
- 52. Ecosystems2h 36m
- 53. Conservation Biology24m
13. Mendelian Genetics
Incomplete Dominance vs. Codominance
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