chromosomal basis of inheritance
- 4. Genes and Chromosomes Genes ï§ Are located on chromosomes ï§ Can be visualized using certain techniques
- 5. INTRODUCTION ï§ In 1875 process of Mitosis and in 1890 Meiosis was worked out ï§ Independently, Karl Correns, Erich von Tschermak, and Hugo de Vries all found that Mendel had explained the same results 35 years before ï§ But there was still resistance
- 6. ï§ Around 1900, cytologists and geneticists began to see parallels between the behavior of chromosomes and the behavior of Mendelâs factors ï§ Chromosomes and genes are both present in pairs in diploid cells ï§ Homologous chromosomes separate and alleles segregate during meiosis ï§ Fertilization restores the paired condition for both chromosomes and genes.
- 7. Chromosomal Theory of Inheritance In 1902, Walter Sutton, Theodor Boveri, and others noted these parallels and a chromosome theory of inheritance began to take form The chromosome theory of inheritance states that Mendelian genes have specific loci on chromosomes Chromosomes undergo segregation and independent assortment
- 8. Evidence that Genes Associated to Chromosomes ï§ Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome ï§ He provided convincing evidence that chromosomes are the location of Mendelâs heritable factors ï§ Experimental animal, Drosophila melanogaster, a fruit fly species that eats fungi on fruit. âą Fruit flies ï prolific breeders ï generation time of two weeks. âą Fruit flies ï three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males).
- 9. ï§ Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome ï§ He provided convincing evidence that chromosomes are the location of Mendelâs heritable factors Evidence that Genes Associated to Chromosomes
- 10. ï§ In one experiment Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) âą The F1 generation all had red eyes âą The F2 generation showed the 3:1 red:white eye ratio ï§ The white-eyed trait appeared only in males. ï§ All the females and half the males had red eyes Correlating Behavior of a Geneâs Alleles with Behavior of a Chromosome Pair ï§ Morgan concluded that a flyâs eye color was linked to its sex
- 11. Morgan concluded that a flyâs eye color was linked to its sex ï§ Females (XX) may have two red-eyed alleles and have red eyes or may be heterozygous and have red eyes. ï§ Males (XY) have only a single allele and will be red eyed if they have a red-eyed allele or white-eyed if they have a white-eyed allele.
- 12. Morganâs Discovery ï That transmission of the X chromosome in fruit flies correlates with inheritance of the eye-color trait ï Was the first solid evidence indicating that a specific gene is associated with a specific chromosome
- 13. Linkage of Genes Affects Inheritance ï§ Each chromosome ï Has hundreds or thousands of genes ï§ Genes ï located on the same chromosome, linked genes, tend to be inherited together because the chromosome is passed along as a unit ï§ Morgan did other experiments with fruit flies ï To see how linkage affects the inheritance of two different characters
- 14. Linked genes Located on the same chromosome often do NOT assort independently: Parental type B N B b N n B n Parental type b N b n Recombinant types 47% 5% 5% 43%
- 15. Unlinked genes B b Y y Located on different chromosomes assort independently: B Y B y b Y b y 25% 25% 25% 25%
- 16. Genetic Recombination and Linkage ï§ Recombinant offspring are those that show new combinations of the parental traits ï§ Morgan discovered that genes can be linked but due to the appearance of recombinant phenotypes, the linkage appeared incomplete ï§ Morgan proposed that some process must occasionally break the physical connection between genes on the same chromosome ï§ Crossing over of homologous chromosomes was the mechanism
- 17. Recombinant Types Produced when a crossover occurs between the 2 genes being studied AA B a b B A a b b Recombinant types Non recombinant types B a b aA B
- 18. The Chromosomal Basis of Sex ï§ The Chromosomal basis for determining sex is rather simple ï The X-Y System Femaleï homogameticï XX Maleï heterogameticï XY Sex of offspring depends on whether the sperm has an X e.g human
- 19. The Z-W System The X-O System In insectsï Only one type of chromosome Females ï XX, Malesï XO Sex of off springï sperm has X or O e.g In birds, some fishes and some insects Females ï ZW, Malesï ZZ Sex chromosome is in the ovum e.g birds , fishes ,insect etc
- 20. Human Sex linked Disorders ï§ Some recessive alleles found on the X chromosome in humans cause certain types of disorders ï Color blindness
- 21. ï Hemophilia ï Sex linked recessive trait ï Absence of one or more clotting factors in the blood ï Gene that controls formation of clotting factors are recessive and present on the X chromosome ï Individuals bleed excessively when injured ï Approx. 1/10,000 males are affected ï Female Hemophiliacï must have this trait on both the XX ï Maleï only one X chromosome from mother, will be a hemophiliac if mother has this trait
- 22. Alteration of Chromosome Number and Genetic Disorders ï Sex-linked traits are not the only notable deviation from the inheritance patterns observed by Mendel ï Also, gene mutations are not the only kind of changes to the genome that can affect phenotype ï Physical and chemical disturbances, errors in meiosisï damage chromosomes and alter #s ï Large-scale chromosomal alterationsï lead to spontaneous abortions or cause a variety of developmental disorders
- 23. Trisomy 21 ïź Results from an error during meiosis ïź Affects 1/700 children born in the US Downs Syndrome ï Extra chromosome on # 21 ï This affects phenotype ï Facial features Broad round face Flattened nose Small irregular teeth ï Statureï usually short ï Heart defects ï Susceptible to infection, leukemia, Alzheimerâs ï Life span shorter ï Varying degrees of mental retardation
- 24. Aneuploidy Klinefelterâs syndrome, an XXY male, occurs once in every 2000 live births. ï These individuals have male sex organs, but are sterile. ï There may be feminine characteristics, but their intelligence is normal ï Males with an extra Y chromosome (XYY) tend to somewhat taller than average Monosomy X or Turnerâs syndrome (X0), ï occurs once in every 5000 births, produces phenotypic, but immature females.
- 25. Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion A deletion occurs when a chromosome fragment lacking a centromere is lost during cell division. This chromosome will be missing certain genes.
- 26. Duplication A duplication occurs when a fragment becomes attached as an extra segment to a sister chromatid Inversion Occurs when a chromosomal fragment reattaches to the original chromosome but in the reverse orientation.
- 27. Translocation A chromosomal fragment joins a non homologous chromosome. Some translocations are reciprocal, others are not.