Lecture 8: Geneticsand Heritable Disease Objectives: Understand the basis of genetic inheritance Understand the basis of genetic variation Relate meiosis, to sex and haploid cells Understand chromosome structure and how it affects general health Explain how small changes in DNA information result metabolic changes Key Terms: Gene, Chromosome, Allele, Locus, loci, Mutation, Diploid and haploid, Phenotype and genotype, Homologous vs. heterozygous, Meiosis vs. Mitosis, Karyotype, X and Y chromosome, Sex determination, Linkage, linkage groups, Full and incomplete linkage, Genetic Markers, Crossover (Recombination), Pedigree, Autosomal and sex-linked, Recessive vs. Dominant, Duplication, Inversion and Translocation, Down Syndrome, Turner Syndrome, Klinefelter Syndrome, Prisoners Syndrome. Chapter 11 for background
Published online February 12, 2004 Evidence of a Pluripotent Human Embryonic Stem Cell Line Derived from a Cloned Blastocyst Woo Suk Hwang et al. 1 College of Veterinary Medicine, Seoul National University, Seoul 151-742, Korea; Somatic cell nuclear transfer (SCNT) technology has recentlybeen used to generate animals with a common genetic composition. In this study, we report the derivation of a pluripotent embryonicstem cell line (SCNT-hES-1) from a cloned human blastocyst. SCNT-hES-1 cells display typical ES cell morphology and cellsurface markers and are capable of differentiating into embryoidbodies in vitro and of forming teratomas in vivo containingcell derivatives from all three embryonic germ layers in SCIDmice. After continuous proliferation for >70 passages, SCNT-hES-1cells maintain normal karyotypes and are genetically identicalto the somatic nuclear donor cells. Although we cannot completelyexclude the possibility of a parthenogenetic origin of the cells,imprinting analyses provide support that the derived human EScells have a somatic cell nuclear transfer origin.
Nucleus of a diploid (2n) Reproductive cell with two pairs of homologous chromosomes OR Possible alignments of the two homologous chromosomes during metaphase I of meiosis A A a a A A a a B B b b b b B B The resulting alignments at metaphase II: A A a a A A a a B B b b b b B B allelic combinations possible in gametes: B B b b b b B B A A a a A A a a 1/4 AB 1/4 ab 1/4 Ab 1/4 aB Fig. 10.8, p. 158
part of the lumen of a seminiferous tubule MITOSIS MEIOSIS I MEIOSIS II Sertoli cell spermatogonium (diploid) primary spermatocyte secondary spermatocyte early spermatids late spermatid immature sperm (haploid) head (DNA in enzyme-rich cap) tail (with core of microtubules) midpiece with mitochondria Fig. 39.14, p. 659
secondary oocyte Ovulation. Mature follicle ruptures and releases the secondary oocyte and the first polar body. first polar body antrum A corpus luteum forms from remnants of the ruptured follicle. A primordial follicle; meiosis I has been arrested in the primary oocyte inside it. When no pregnancy occurs, the corpus luteum degenerates. primordial follicle Fig. 39.17b, p. 662
oviduct FERTILIZATION ovary OVULATION uterus opening of cervix zona pellucida vagina follicle cell granules in cortex of cytoplasm sperm enter vagina nuclei fuse Fig. 39.20, p. 665
inner cell mass (see next slide) oviduct uterus FERTILIZATION ovary IMPLANTATION endometrium Fig. 39.21a, p. 666
start of amniotic cavity start of embryonic disk Trophoblast (surface layer of cells of the blastoyst) endometrium blastocoel inner cell mass start of yolk sac uterine cavity DAYS 6-7 DAYS 10-11 chorionic cavity chorion chorionic villi blood-filled spaces amniotic cavity start of chorionic cavity connecting stalk yolk sac Fig. 39.21b, p. 667 DAY 14 DAY 12
Blastula Cell migrations in early gastrula Fig. 39.7, p. 652
a Dorsal lip is excised from donor embryo, then grafted to an abnormal site in another embryo. b Graft induces a second invagination. Fig. 39.10, p. 654 c Gastrula develops into a double embryo. Most of its tissues originated from the host embryo.
Human Embryos Cloned for Stem Cells • In work that observers call both remarkable and inevitable, scientists in Korea have produced an embryonic stem (ES) cell line from cloned human cells • This advance holds promise for replacing cells damaged by diseases such as Parkinson's and diabetes. • In doing so, the team has apparently overcome some of the obstacles that to date have hampered human cloning, • This work is likely to reignite the smoldering debate over how such research should be regulated.
How did they do it? • The secret to their success may be the gentle way in which they removed the nucleus from a human egg. • Then they added the nucleus from a Cumulus cell, a kind of cell that surrounds the developing eggs in an ovary. • After prompting the reconstructed egg to start dividing, the team allowed it to develop for a week to the blastocyst stage, when the embryo forms a hollow ball of cells. • They then isolated the inner-cell mass, which would develop into the fetus. • When these cells are grown in culture, they can become ES cells.
What’s an ES cell good for? • ES reproduce indefinitely and can form all the cell types in the body. • The ES cell line the team derived seems to form bone, muscle, and immature brain cells, for example. • Scientists have hoped to create ES cells with genes that match those of a patient, an idea called therapeutic cloning or "cloning for stem cells."
Key Terms • Cloned human embryo • Embryonic stem cell • Blastula, Blastocyst • Pluripotent • Karyotype • How does cloning work: • Where does the egg come from • Where does the DNA come from • How many copies of each chromosome
Ethical Questions Destroying Embryos is the Basis of the Ethical Debate Questions: What is the moral status of the developing embryo
Ethical Questions Destroying Embryos is the Basis of the Ethical Debate Questions: Is this simply tissue or is it something more?
Ethical Questions Destroying Embryos is the Basis of the Ethical Debate Questions: Is this a twin? The genetic make up is identical
Ethical Questions Destroying Embryos is the Basis of the Ethical Debate Questions: What is the purpose? Making donor tissue? Making a baby?
Ethical Questions Destroying Embryos is the Basis of the Ethical Debate Questions: Is regenerative medicine ethical?
1- Scientific Imperialism • Science is the Truth Arbiter • Therefore, anything goes if scientists say so. Objectivism is the belief that a scientist can be removed from or independent of his surroundings and experiences while making observations, conclusions and recommendations.
2- Postmodern Relativism • Plurality of Truths • Science is only one form of Subjective Truth • Science has made errors in the past, Therefore, science and scientists should be: • Questioned… • Evaluated… • Regulated… Subjectivism holds that science and scientists are not objective, but antecedents to surroundings, training, personal experience, etc.
3- Godisms • Mankind is created and ultimately Truth is God Revealed. • Science is a product of mankind, therefore science must be carefully evaluated for its potential good and/or bad outcomes. Since Truth is ultimately Revealed and science is error prone, science is subjective and an ethical society must take care to evaluate and judge science’s pursuits and products carefully.
Lecture Outline The structure of our genes Intro to chromosomes Karyotypes Linkage and pedigree Genetic disorders The big problems Recombination Broken chromosomes Extra and missing chromosomes The small problems Mutations
Genes • Units of information about heritable traits • In eukaryotes, distributed among chromosomes • Each has a particular locus • Location on a chromosome
Homologous Chromosomes • Homologous autosomes are identical in length, size, shape, and gene sequence • Sex chromosomes are nonidentical but still homologous • Homologous chromosomes interact, then segregate from one another during meiosis
Alleles • Different molecular forms of a gene • Arise through mutation • Diploid cell has a pair of alleles at each locus • Alleles on homologous chromosomes may be same or different
Sex Chromosomes • Discovered in late 1800s • Mammals, fruit flies • XX is female, XY is male • In other groups XX is male, XY female • Human X and Y chromosomes function as homologues during meiosis
Karyotype Preparation - Stopping the Cycle • Cultured cells are arrested at metaphase by adding colchicine • This is when cells are most condensed and easiest to identify
Karyotype Preparation • Arrested cells are broken open • Metaphase chromosomes are fixed and stained • Chromosomes are photographed through microscope • Photograph of chromosomes is cut up and arranged to form karyotype diagram
Human Karyotype 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 XX (or XY)
X X X Y Y X X X XX XX XY XY Sex Determination eggs sperm Female germ cell Male germ cell sex chromosome combinations possible in new individual
The Y Chromosome • Fewer than two dozen genes identified • One is the master gene for male sex determination • SRY gene (Sex-determining region of Y) • SRY present, testes form • SRY absent, ovaries form
appearance of structures that will give rise to external genitalia appearance of “uncommitted” duct system of embryo at 7 weeks Effect of YChromosome 7 weeks Y present Y absent Y present Y absent testes ovaries 10 weeks ovary testis birth approaching
The X Chromosome • Carries more than 2,300 genes • Most genes deal with nonsexual traits • Genes on X chromosome can be expressed in both males and females
X X X Y Discovering Linkage One cross homozygous dominant female recessive male x Gametes: heterozygous female heterozygous male All F1 offspring have red eyes
Discovering Linkage Reciprocal cross homozygous recessive female dominant male x Gametes: X X X Y heterozygous females recessive males F1 offspring Half are red-eyed females, half are white-eyed males
Discovering Linkage • Morgan’s crosses showed relationship between sex and eye color • Females can have white eyes • Morgan concluded gene must be on the X chromosome
Linkage Groups • Genes on one type of chromosome • Fruit flies • 4 homologous chromosomes • 4 linkage groups • Indian corn • 10 homologous chromosomes • 10 linkage groups
A A a B B b A a B b a b Full Linkage AB ab Parents: x F1 offspring: All AaBb meiosis, gamete formation 50%AB 50%ab With no crossovers, half of the gametes have one parental genotype and half have the other
A a a c c C A C Incomplete Linkage AC ac x Parents: F1 offspring All AaCc meiosis, gamete formation Unequal ratios of four types of gametes: a a A A C c C c Most gametes have parental genotypes A smaller number have recombinant genotypes
Crossover Frequency Proportional to the distance that separates genes A B C D Crossing over will disrupt linkage between A and B more often than C and D
Linkage Mapping in Humans • Linkage maps based on pedigree analysis through generations • Color blindness and hemophilia are very closely linked on X chromosome • Recombination frequency is 0.167%
Pedigree • Chart that shows genetic connections among individuals • Standardized symbols • Knowledge of probability and Mendelian patterns used to suggest basis of a trait • Conclusions most accurate when drawn from large number of pedigrees
I II III IV V *Gene not expressed in this carrier. Pedigree for Polydactly female male 5,5 6,6 * 5,5 6,6 6,6 5,5 6,6 5,5 6 7 5,5 6,6 5,5 6,6 5,5 6,6 5,5 6,6 5,6 6,7 12 6,6 6,6
Genetic Abnormality • A rare, uncommon version of a trait • Polydactyly • Unusual number of toes or fingers • Does not cause any health problems • View of trait as disfiguring is subjective
Genetic Disorder • Inherited conditions that cause mild to severe medical problems • Why don’t they disappear? • Mutation introduces new rare alleles • In heterozygotes, harmful allele is masked, so it can still be passed on to offspring
Autosomal Recessive Inheritance Patterns • If parents are both heterozygous, child will have a 25% chance of being affected
Galactosemia • Caused by autosomal recessive allele • Gene specifies a mutant enzyme in the pathway that breaks down lactose enzyme 1 enzyme 2 enzyme 3 GALACTOSE-1- PHOSOPHATE GALACTOSE-1- PHOSOPHATE LACTOSE GALACTOSE +glucose intermediate in glycolysis
Autosomal Dominant Inheritance Trait typically appears in every generation