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Chromosomes and Human Genetics Chapter 11. Chromosomes & Cancer Some genes on chromosomes control cell growth and division If something affects chromosome.

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Presentation on theme: "Chromosomes and Human Genetics Chapter 11. Chromosomes & Cancer Some genes on chromosomes control cell growth and division If something affects chromosome."— Presentation transcript:

1 Chromosomes and Human Genetics Chapter 11

2 Chromosomes & Cancer Some genes on chromosomes control cell growth and division If something affects chromosome structure at or near these loci, cell division may spiral out of control This can lead to cancer

3 Philadelphia Chromosome First abnormal chromosome to be associated with a cancer Associated with a chronic leukemia –Overproduction of white blood cells

4 A Reciprocal Translocation 12 6 1315 1920 Chromosome 9 and chromosome 22 exchanged pieces

5 An Altered Gene When the reciprocal translocation occurred, a gene at the end of chromosome 9 fused with a gene from chromosome 22 This hybrid gene encodes an abnormal protein that stimulates uncontrolled division of white blood cells

6 Genes Units of information about heritable traits In eukaryotes, distributed among chromosomes Each has a particular locus –Location on a chromosome

7 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

8 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

9 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

10 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)

11 Sex Determination XX XY XX XY X X Y X sex chromosome combinations possible in new individual Y X sperm X X eggs Female germ cellMale germ cell

12 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

13 Effect of Y Chromosome 10 weeks Y present Y absent 7 weeks birth approaching appearance of structures that will give rise to external genitalia appearance of uncommitted duct system of embryo at 7 weeks Y present Y absent testis ovary testesovaries

14 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

15 Discovering Linkage homozygous dominant female recessive male Gametes: XXXY All F 1 offspring have red eyes x heterozygous male heterozygous female One cross

16 Discovering Linkage homozygous recessive female dominant male Gametes: XXXY F 1 offspring x recessive males heterozygous females Half are red-eyed females, half are white-eyed males Reciprocal cross

17 Discovering Linkage Morgans crosses showed relationship between sex and eye color Females can have white eyes Morgan concluded gene must be on the X chromosome

18 Full Linkage x ABab 50% AB 50% ab All AaBb meiosis, gamete formation Parents: F1 offspring: With no crossovers, half of the gametes have one parental genotype and half have the other A B a b A B a b a b A B

19 Incomplete Linkage Parents: F1 offspring Unequal ratios of four types of gametes: All AaCc x meiosis, gamete formation ACac A C A C A C a c a c A c a C a c Most gametes have parental genotypes A smaller number have recombinant genotypes

20 Crossover Frequency Proportional to the distance that separates genes ABCD Crossing over will disrupt linkage between A and B more often than C and D

21 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%

22 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

23 Fig. 11.9, p. 178 male female marriage/mating Individual showing trait being studied sex not specified generationI,II, III, IV... offspring in order of birth, from left to right I II III IV V 67 12 5,5 6,6 6,6 5,5 5,6 6,7 6,6 Gene not expressed in this carrier.

24 Pedigree for Polydactly I II III IV V 67 12 5,5 6,6 6,6 5,5 5,6 6,7 6,6 * Gene not expressed in this carrier. * malefemale

25 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

26 Genetic Disorder Inherited conditions that cause mild to severe medical problems Why dont they disappear? –Mutation introduces new rare alleles –In heterozygotes, harmful allele is masked, so it can still be passed on to offspring

27 Autosomal Recessive Inheritance Patterns If parents are both heterozygous, child will have a 25% chance of being affected

28 Galactosemia Caused by autosomal recessive allele Gene specifies a mutant enzyme in the pathway that breaks down lactose LACTOSEGALACTOSE GALACTOSE-1- PHOSOPHATE GALACTOSE-1- PHOSOPHATE enzyme 1enzyme 2enzyme 3 + glucose intermediate in glycolysis

29 Autosomal Dominant Inheritance Trait typically appears in every generation

30 Huntington Disorder Autosomal dominant allele Causes involuntary movements, nervous system deterioration, death Symptoms dont usually show up until person is past age 30 People often pass allele on before they know they have it

31 Acondroplasia Autosomal dominant allele In homozygous form usually leads to stillbirth Heterozygotes display a type of dwarfism Have short arms and legs relative to other body parts

32 X-Linked Recessive Inheritance Males show disorder more than females Son cannot inherit disorder from his father

33 Examples of X-Linked Traits Color blindness –Inability to distinguish among some of all colors Hemophilia –Blood-clotting disorder –1/7,000 males has allele for hemophilia A –Was common in European royal families

34 Fig. 11.12b, p. 181 I II III IV V VI AlbertVictoria

35 Fragile X Syndrome An X-linked recessive disorder Causes mental retardation Mutant allele for gene that specifies a protein required for brain development Allele has repeated segments of DNA

36 Duplication Gene sequence that is repeated several to hundreds of times Duplications occur in normal chromosomes May have adaptive advantage –Useful mutations may occur in copy

37 Duplication normal chromosome one segment repeated three repeats

38 Inversion A linear stretch of DNA is reversed within the chromosome

39 Translocation A piece of one chromosome becomes attached to another nonhomologous chromosome Most are reciprocal Philadelphia chromosome arose from a reciprocal translocation between chromosomes 9 and 22

40 Translocation chromosome nonhomologous chromosome reciprocal translocation

41 Deletion Loss of some segment of a chromosome Most are lethal or cause serious disorder

42 Polyploidy Individuals have three or more of each type of chromosome (3n, 4n) Common in flowering plants Lethal for humans –99% die before birth –Newborns die soon after birth

43 Nondisjunction n + 1 n - 1 chromosome alignments at metaphase I nondisjunction at anaphase I alignments at metaphase II anaphase II

44 Down Syndrome Trisomy of chromosome 21 Mental impairment and a variety of additional defects Can be detected before birth Risk of Down syndrome increases dramatically in mothers over age 35

45 Fig. 11.18, p. 185 incidence per 1,000 births 20 15 10 5 0 202530354045 mother's age 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

46 Turner Syndrome Inheritance of only one X (XO) 98% spontaneously aborted Survivors are short, infertile females –No functional ovaries –Secondary sexual traits reduced –May be treated with hormones, surgery

47 Klinefelter Syndrome XXY condition Results mainly from nondisjunction in mother (67%) Phenotype is tall males –Sterile or nearly so –Feminized traits (sparse facial hair, somewhat enlarged breasts) –Treated with testosterone injections

48 XYY Condition Taller than average males Most otherwise phenotypically normal Some mentally impaired Once thought to be predisposed to criminal behavior, but studies now discredit

49 Phenotypic Treatments Symptoms of many genetic disorders can be minimized or suppressed by –Dietary controls –Adjustments to environmental conditions –Surgery or hormonal treatments

50 Genetic Screening Large-scale screening programs detect affected persons Newborns in United States routinely tested for PKU –Early detection allows dietary intervention and prevents brain impairment

51 Prenatal Diagnosis Amniocentesis Chorionic villus sampling Fetoscopy All methods have some risks

52 Fig. 11.19, p. 186 Karyotype analysis Fetal cells Centrifugation Growth for weeks in culture medium Removal of about 20 ml of amniotic fluid containing suspended cells that were sloughed off from the fetus A few biochemical analyses with some of the amniotic fluid Quick determination of fetal sex and analysis of purified DNA Biochemical analysis for the presence of alleles that cause many different metabolic disorders

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