1. 1 Proof for the chromosome theory of inheritance Although these were convincing correlations, actual proof of the chromosome theory required the discovery of sex linkage. Remember, Mendel had found that reciprocal crosses produce equal results with respect to the progeny. In general geneticists confirmed his results. However exceptions did arise. The most famous exception was that discovered by Tomas Hunt Morgan in the fruit fly Drosophila melanogaster. Drosophila eyes are normally bright red. Morgan discovered an exceptional white-eyed male. He performed the following crosses: Sex chromosomes

2. 2 Morgans crosses Reciprocal cross CROSS1 CROSS2

3. 3 X and Y chromosomes Somehow eye color was linked to sex The key to understanding this pattern of inheritance arose from work demonstrating that males and females of a given species often differ in the chromosome constitution. For example, they found that male and female Drosophila both have four chromosome pairs. However in males one of the pairs the members differed in size: Female Drosophila: Male Drosophila:

4. 4 Sex chromosomes Morgan realized that difference in chromosome constitution was the basis of sex determination in Drosophila: Females produce only X-bearing gametes, while males produce X and Y-bearing gametes.

5. 5 Formal explanation Females have 2 copies of the eye color gene and males have one copy W (red) is dominant over w (white) F1 CROSS1

6. 6 Formal explanation Females have 2 copies of the eye color gene and males have one copy W (red) is dominant over w (white) Red X W X w Red X W Y F2

7. 7 Formal explanation White X w Red X W Y F1 The reciprocal cross

8. 8 Formal explanation Red X W X w White X w Y F2

9. 9 Equal numbers of male and female progeny are produced. Morgan realized that he could explain the inheritance patterns of eye color by assuming: 1.The gene determining eye color resides on the X chromosome (red and white eyes represent normal and mutant alleles of this gene) 2. There is no counterpart for this gene on the Y chromosome Thus females carry two copies of the gene, while males carry only a single copy.

10. 10 Sex determination Bridges a student of Morgan set up the cross outlined above in large numbers P cross: As expected, he obtained and About 1 in every 2000 progeny he obtained white-eyed fertile female or a red-eyed sterile male.

11. 11 Primary exception About 1 in every 2500 progeny he obtained a white-eyed fertile female or a red-eyed sterile male. These were called primary exceptional progeny How can these exceptional progeny be explained?

12. 12 Bridges and non-dysjunction white X w red X W Y F1 0ne in 2500 eggs have non-dysjunction

13. 13 Bridges assumed that XXX and Y0 progeny die The only two viable progeny types were XXY and X0 In this model sex is determined by the number of X chromosomes rather than the presence or absence of the Y chromosome This model makes a strong prediction. Genes reside on chromosome The exceptional red-eyed males should be X0 and The exceptional white eyed females should be XXY THAT IS WHAT BRIDGES SAW under the microscope!

14. 14 X a X A xX a Y X a X a X A X A xX a X a YY Dysjunction in meiosisI in motherDysjunction in meiosis I in father X a X a X A X A and OX a X a YY and O Normal meiosis II X a X A and OX a Y and O Dysjunction in Meiosis I

15. 15 X a X A xX a Y X a X a X A X A xX a X a YY Normal meiosisI in motherNormal meiosis I in father X a X a and X A X A X a X a and YY Dysjunction in meiosis II X a X a or X A X A X a X a or YY Dysjunction in meiosis II

16. 16 Chromosome characteristics Centromere Telomere Chromosome arms

17. 17 Karyotype Karyotype gives species specific chromosome organization It is usually a microscopic classification The number of chromosomes The size of each chromosome Position of centromere on each chromosome Chromosomes can be stained Telocentric Acrocentric Metacentric

18. 18 Chromosome number/size (haploid) Organismsizenumber Yeast (S. cerevisiae)1216 Mold (Dictyostelium)707 Arbidopsis1305 Lily50,00012 Nematode (C. elegans)976 Fly (Drosophila)1804 Fugu3652 Mouse300020 Human300023 Evolutionary significance of variability in number and length is not known ChrMb 1246.1 2243.6 3199.3 4191.7 5181.0 6170.9 7158.5 8146.3 9136.3 10135.0 11134.4 12132.0 13113.0 14105.3 15100.2 1690.0 1781.8 1876.1 1963.8 2063.7 2146.9 2249.3 X153.6 Y22.7

19. 19 Banding Cells in metaphase can be fixed and stained with dyes Some dyes that stain chromosomes give a characteristic banding pattern. In a diploid, homologous chromosomes have the same banding pattern Stained chromosomes are photographed, cut and arranged in decreasing size

20. 20 Karyotype The human karyogram. The chromosomes are shown with the G- banding pattern obtained after Giemsa staining. Chromosome numbers and band numbers Constitutive heterochromatin is very compact chromatin which has few or no genes

21. 21 Karyotyping Karyotyping provides a rapid means to identify alterations in the number of chromosomes In humans ~50% of conceptions are aneuploid Over 70% of spontaneous abortions and early embryonic deaths are caused due to Aneuploidy 1 in 170 live births is partially aneuploid ~5-7% of early childhood deaths are to aneuploidy Humans have a rate of aneuploidy that is 10 times greater other mammals Non-dysjunction in meiosis is the primary cause Monosomy- one chromosome of a pair is missing Trisomy- extra chromosome is present Only chromosome 21 trisomies survive to adulthood Downs syndrome occurs in 1 in 200 conceptions and 1 in 900 live births Chromosome 21

22. 22 Aneuploidy Trisomy21 is Non-dysjunction In MeiosisI A A a a

23. 23 Triploidy Species that are triploid, reproduce asexually (plant species) What are the consequences of triploidy during mitosis and meiosis? Haploid Diploid Triploid Mitosis

24. 24 Meiosis and triploids MeiosisI This is for one chromosome. If there are n chromosomes in an organism, then balanced gametes (equal copies of all chromosomes) are very rare.

25. 25 Sex in organisms Sex chromosomes and sex linkage: In Drosophila, it is the number of X's that determine sex while in mammals it is the presence or absence of a Y chromosome that determines sex. Homogametic sex- Producing gametes that contain one type of chromosome (females in mammals and insects, males in birds and reptiles) Heterogametic sex- Producing gametes that contain two types of chromosomes (males in mammals and insects, females in birds and reptiles) SpeciesXXXYXXYXO DrosophilaFemalemalefemalemale HumanFemalemalemalefemale Bridges could tell genotype by where the sex chromosome went and therefore established that chromosomes carried genes

26. 26 Non-sex chromosomes are called autosomes Humans have 22 autosomes, Drosophila has 3 Homogametic sex- XX- females in humans males in birds Heterogametic sex-XY-males in humans Hemizygousgene present in one copy in a diploid organism Human males are hemizygous for genes on the X-chromosome

27. 27 Surname project Y Y Y Y All males in this pedigree will have the SAME Y-chromosome!!!

28. 28 Mendelian genetics in Humans: Autosomal and Sex- linked patterns of inheritance Obviously examining inheritance patterns of specific traits in humans is much more difficult than in Drosophila because defined crosses cannot be constructed. In addition humans produce at most a few offspring rather than the hundreds produced in experimental genetic organisms such as Drosophila It is important to study mendellian inheritance in humans because of the practical relevance and availability of sophisticated phenotypic analyses. Therefore the basic methods of human genetics are observational rather than experimental and require the analysis of matings that have already taken place rather than the design and execution of crosses to directly test a hypothesis To understand inheritance patterns of a disease in human genetics you often follow a trait for several generations to infer its mode of inheritance --- dominant or recessive? Sex-linked or autosomal? For this purpose the geneticist constructs family trees or pedigrees. Pedigrees trace the inheritance pattern of a particular trait through many generations. Pedigrees enable geneticists to determine whether a familial trait is genetically determined and its mode of inheritance (dominant/recessive, autosomal/sex-linked)

29. 29 Male FemaleSex Unknown Affected individual 5 Number of individuals Deceased Spontaneous abortion Termination of pregnancy Pedigree symbols:

30. 30 Pedigree symbols: line of descent individual’s lines relationship line Sibship line consanguinity MonozygoticDizygotic

31. 31 Characteristics of an autosomal recessive trait: There are several features in a pedigree that suggest a recessive pattern of inheritance: nguinity is often involved. In the pedigree shown below, an autosomal recessive inheritance pattern is observed: II:1II:2 III:9 I

32. 32 Characteristics of an autosomal dominant trait: 1. Every affected individual should have at least one affected parent. 2. An affected individual has a 50% chance of transmitting the trait 3. Males and females should be affected with equal frequency 4. Two affected individuals may have unaffected children

33. 33 The following pedigree outlines an inheritance pattern Does this fit an autosomal recessive or autosomal dominant pattern of inheritance?

34. 34 Pedigree of Queen Victoria and the transmission of hemophilia. Victoria Albert Alice carrier Beatrice carrier Irene carrier Alix carrier Alice carrier Victoria carrier

35. 35 Characteristics of a X-linked trait: Hemizygous males and homozygous females are affected Phenotypic expression is much more common in males than in females, and in the case of rare alleles, males are almost exclusively affected Affected males transmit the gene to all daughters but not to any sons Daughters of affected males will usually be heterozygous and therefore unaffected. Sons of heterozygous females have a 50% chance of receiving the recessive gene. gY GG GYgGGY gG GY

36. 36 Mammalian X-chromosome inactivation (epigenetics) Mammalian males and females have one and two X chromosomes respectively. One would expect that X-linked genes should produce twice as much gene product in females compared to males. Yet when one measures gene product from X-linked genes in males and females they are equivalent. This phenomenon, known as dosage compensation, means that the activity of X-linked genes is either down regulated in females or up regulated in males. The former proves to be the case: X chromosome inactivation in females is the mechanism behind dosage compensation. In females, one of the X chromosomes in each cell is inactivated. This is observed cytologically. One of the X-chromosomes in females appears highly condensed. This inactivated chromosome is called a Barr-body. In Drosophila the genes on the single male X chromosome is up- regulated 2-fold

37. 37 X-inactivation The inactivation of one of the two X-chromosomes means that males and females each have one active X chromosome per cell. X-chromosome inactivation is random. For a given cell in the developing organism there is an equal probability of the female or the male derived X chromosome being inactivated. The embryo is a mosaic! Once the decision is made in early development, then it is stably inherited. Patches of cells have the male X ON and patches of cells have the female X ON This is a Developmental rule that overlays on top of Mendellian rules!

38. 38 X-inactivation The inactivation of one of the two X-chromosomes means that males and females each have one active X chromosome per cell. X-chromosome inactivation is random. For a given cell in the developing organism there is an equal probability of the female or the male derived X chromosome being inactivated. zygote Embryo Inactivation The embryo is a mosaic! Once the decision is made in early development, then it is stably inherited. Patches of cells have the male X ON and patches of cells have the female X ON This is a Developmental rule that overlays on top of Mendellian rules!

39. 39 Barr bodies ·XXX individuals have 2 Barr Bodies leaving one active X ·XXXX individuals have 3 Barr Bodies leaving one active X ·XXY individual have one Barr Body leaving one active X (Klinefelter's syndrome) ·X0 individuals have no Barr Bodies leaving one active X (Turner's syndrome) Given X-chromosome inactivation functions normally why are they phenotypically abnormal? Part of the explanation for the abnormal phenotypes is that the entire X is not inactivated during Barr-Body formation (Escape loci) Consequently an X0 individual is not genetically equivalent to an XX individual.