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Proof for the chromosome theory of inheritance Sex chromosomes Although these were convincing correlations, actual proof of the chromosome theory required the study of sex chromosomes. Remember, Mendel had found that reciprocal crosses produce equal results with respect to the progeny. In general geneticists confirmed his results. However exceptions arose. The most famous exception was that discovered by Thomas Hunt Morgan in the fruit fly Drosophila melanogaster. Drosophila eyes are normally bright red. Morgan discovered a white-eyed male. He performed the following crosses: 1 Morgans crosses CROSS P1 White Red F1 Both males and females were Red Red is dominant to white Selfing 3:1 red:white (1 gene for eye color) All white eyed flies were male!!!! F2 Reciprocal cross CROSS P2 Red White F1 Red F2 White Self Cross ??? All females were red and males were white in the F1!!! 2 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: Sex fourth second third 3 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. X X X Y XX XY XX XY 2 :2 If the gene for eye color resides on a X chromosome There is no counterpart for this gene on the Y chromosome4 Morgans crosses CROSS1 White Red Red Selfing 3:1 red:white All white eyed flies were male!!!! 5 Formal explanation Females have 2 copies of the eye color gene and males have one copy W (red) is dominant over w (white) CROSS1 white XwY Red XWXW F1 Xw XW XW XW Xw y XWY Red Red XW Xw XWY Red Red 6 Formal explanation Females have 2 copies of the eye color gene and males have one copy W (red) is dominant over w (white) Self cross Red XWY Red XWXw F2 XW XW Xw XW XW y XWY Red Red XW Xw XwY Red White 7 Morgans crosses Reciprocal cross CROSS2 White Red Red White All females were red and males were white in the F1!!! 8 Formal explanation The reciprocal cross Red XWY White XwXw F1 XW Xw Xw XW Xw Red y XwY White XW Xw XwY Red White In the F1 all the females are red and all the males are white 9 Formal explanation White XwY Red XWXw F2 Xw XW Xw XW Xw y XWY Red Red Xw Xw XwY White 50% Red and 50% white!!! This is in effect a test cross White 10 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. THIS is Simply a Correlation! Correlation does not equal causation! 11 Sex determination Bridges a student of Morgan set up the cross outlined above in large numbers P cross: white females XwXw x x red males XWY As expected, he obtained red-eyed females (XwXW) and white-eyed males (XwY) BUT About 1 in every 2500 progeny he obtained white-eyed fertile female or a red-eyed sterile male Cherish Your Exceptions 12 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? disjunction Non-disjunction 13 Primary exception About 1 in every 2000 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? autosome X autosome X disjunction Non-disjunction Bridges suggested that occasionally during meiosis the X chromosomes fail to separate. ｷ Normal separation of the X chromosomes produces Xw gametes ｷ Failure of X chromosome separation (non disjunction) Creates XwXw and nullo gametes and these gametes give rise to the sterile red eyed males! 14 Bridges and non-dysjunction white red XWY XwXw F1 XW Xw Xw Xw Xw O y XW Xw XwY Red white XW Xw XwY Red White XW Xw Xw Xw XwY Lethal white female fertile XW Red male Sterile Y Lethal 15 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 -Hypothesis Genes reside on chromosome The exceptional red-eyed males should be X0 and The exceptional white eyed females should be XXY How do you show this? Look at the chromosomes under the microscope THAT IS WHAT BRIDGES SAW under the microscope in the females! 16 Look at the chromosomes under the microscope THAT IS WHAT BRIDGES SAW under the microscope in the females! 17 Non-Dysjunction in Meiosis I XaXA x XaY Replication XaXaXAXA x XaXaYY Non Dysjunction in Non Dysjunction in meiosisI in mother meiosis I in father XaXaXAXA and O XaXaYY and O Normal meiosis II XaXA and O XaY and O 18 Non Dysjunction in meiosis II XaXA x XaY Replication XaXaXAXA x XaXaYY Normal meiosisI in mom XaXa and XAXA Normal meiosis I in dad XaXa and YY Non Dysjunction in meiosis II XaXa or XAXA & nullo XaXa or YY & nullo Aneuploid: Having a chromosome number that is not a19 multiple of the haploid number for the species Quiz What classes of progeny would be expected if you could do the following cross XwXwY x XWY 20 Answer-- Triploids What classes of progeny would be expected if you could do the following cross White XwXwY XW XwXw Y Xw XwY XW XwXw lethal XWY Red male XW Xw red XWY Y Y XwXw white female YY lethal Y Xw Red female White male XW XwY Y XwY Red female Normal females are red eyed Normal males are white eyed Non-disjunct females are white eyed Non-disjunct males are red eyed White male 21 Sex in organisms Sex chromosomes and sex: 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) Species XX XY XXY XO Drosophila Female male female male Human Female male male female Non-sex chromosomes are called autosomes Humans have 22 autosomes, Drosophila has 3 Hemizygous Gene present in one copy in a diploid organism Human males are hemizygous for genes on the X-chromosome 22 Karyotype Bridges confirmed aneuploidy by visualizing abnormal chromosome numbers in Drosophila using the microscope. 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 Telocentric Acrocentric Metacentric 23 Chromosome characteristics Count Number of chromosomes/chromosome pairs Centromere Telomere Centromere Chromosome arms Chromosome arms Telomere Unstained chromosome Stained chromosome 24 Chromosome number/size (haploid) Organism Yeast (S. cerevisiae) Mold (Dictyostelium) Arbidopsis Lily Nematode (C. elegans) Fly (Drosophila) Mouse Human number 16 7 5 12 6 4 20 23 Evolutionary significance of variability in number is not known Human chromosomes Ch # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 X Y Chromosome size 246.1 243.6 199.3 191.7 181.0 170.9 158.5 146.3 136.3 135.0 134.4 132.0 113.0 105.3 100.2 90.0 81.8 76.1 63.8 63.7 46.9 49.3 153.6 22.7 Chromosomes also vary in size 25 Banding Chromosomes can be stained Cells in metaphase can be fixed and stained with dyes. Dyes stain chromosomes and each chromosome has a characteristic banding pattern. In a diploid, homologous chromosomes have the same banding pattern. Stained chromosomes are photographed, cut and arranged in decreasing size 26 Karyotype • The human karyogram. The chromosomes are shown with the Gbanding pattern obtained after Giemsa staining. Chromosome numbers and band numbers • Constitutive heterochromatin is very compact chromatin which has few or no genes 27 Karyotyping Karyotyping provides a rapid means to identify alterations in the number of chromosomes Chromosome 21 In humans a very large number of conceptions are aneuploid Over 70% of spontaneous miscarriages and early embryonic deaths are caused due to Aneuploidy ~5-7% of early childhood deaths are to aneuploidy Humans have a rate of aneuploidy that is 10 times greater than other mammals! Non-dysjunction in meiosisI is the primary cause Monosomy- one chromosome of a pair is missing Trisomy- extra chromosome is present Chromosome 21 trisomies survive to adulthood Downs syndrome occurs in 1 in 200 conceptions and 1 in 900 live births. 28 A Aneuploidy-Trisomy A a A a A A a A A a A Non-dysjunction In MeiosisI a a a A A A A A a a a A A a Non-dysjunct In MeiosisII a a a 29 Triploidy Species that are triploid, reproduce asexually (plant species) What are the consequences of triploidy during mitosis and meiosis? Haploid Diploid Triploid Mitosis in triploid 30 Meiosis and triploids MeiosisI Meiosis I Triploids produce unbalanced gametes This is for one chromosome. If there are n chromosomes in an organism, then balanced gametes (equal copies of all chromosomes) is very rare. 31 What happens when you cross a triploid plant to a triploid plant? 4N 3N 3N 2N 32
