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Chromosomes and Human Genetics

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1 Chromosomes and Human Genetics
Chapter 12

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

3 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

4 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

5 Studying Human Genetics
Studying Human Genetics is much more complicated than using other model systems (e.g. Pea Plants) Humans reproduce slowly, have few offspring, and it is unethical to breed humans for experiments There are many techniques that are used to study human genetics indirectly Karyotypes Pedigree analysis Linkage maps

6 Karyotype Picture of an individual’s chromosomes Making a Karyotype:
Metaphase chromosomes are fixed and stained Chromosomes are photographed through microscope Photograph of chromosomes is cut up and arranged to form karyotype diagram

7 Karyotype Autosomes Sex Chromosomes

8 Sex Chromosomes Discovered in late 1800s Mammals, fruit flies
XX is female, XY is male In other groups XX is female, XY male Human X and Y chromosomes function as homologues during meiosis

9 Sex Determination female (XX) male (XY) eggs sperm Y X X x XX XY X Y
Figure 12.5 Page 198

10 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

11 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

12 Pedigree A chart showing the genetic connections between individuals
A genetic family tree Often groups of people whose heritage is well documented or who are somewhat isolated from others are used to develop pedigrees. E.g. Icelanders, Mormons, Ashkenazi Jews, Amish, Sardinians

13 Pedigree Symbols male female marriage/mating
offspring in order of birth, from left to right Individual showing trait being studied sex not specified I, II, III, IV... generation

14 Example: Pedigree of Polydactyly
II III IV V 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 *Gene not expressed in this carrier.

15 Hemophilia

16 Polydactyly or Extra Digits

17 Linked Genes Genes found on one type of chromosome
Linked genes can assort separately from on another only through crossing over The closer to genes are to each other on a chromosome, the more tightly linked they are (i.e. the more likely they are to assort together during meiosis

18 Markers A & C are not linked
A B C D a b c d A B C D a b c d A B c d a b C D Markers A & B are linked Markers A & C are not linked

19 Full Linkage A B a b x Parents: A B a b AB ab F1 offspring: All AaBb
meiosis, gamete formation Equal ratios of two types of gametes: A B a b 50% AB 50% ab

20 Incomplete Linkage AC ac A C a c x Parents: A C a c F1 offspring:
All AaCc meiosis, gamete formation A a A a Unequal ratios of four types of gametes: C c c C parental genotypes recombinant genotypes

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

22 Linkage mapping Done by using known “landmarks” or markers on chromosomes These markers are either genes or DNA fragments whose location has already been worked out By watching how these markers are inherited by those with & without the disorder, geneticists can predict the location of the gene responsible for the disorder If a gene and a marker are found together 100% of the time, they are completely linked If a gene and a marker are found together 50% of the time they are completely unlinked

23 Human Genetic Conditions
A genetic abnormality is an uncommon or rare trait E.g. Polydactyly A genetic disorder is an inherited condition that may cause medical problems E.g. Cystic Fibrosis A syndrome is a set of symptoms that characterize a disorder E.g. Down Syndome

24 Human Genetic Abnormalities/Disorders
Different patterns of inheritance are observed depending on the condition: Autosomal Recessive Inheritance Autosomal Dominant Inheritance X-linked inheritance

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

26 Albinism

27 Autosomal Dominant Inheritance
Trait typically appears in every generation

28 Huntington Disorder Autosomal dominant allele
Causes involuntary movements, nervous system deterioration, death Symptoms don’t usually show up until person is past age 30 People often pass allele on before they know they have it

29 Achondroplasia 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

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

31 Color Blindness

32 Hemophilia

33 Chromosomal Mutations
Duplication Deletion Inversion Translocation

34 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

35 Duplication normal chromosome one segment repeated three repeats

36 A linear stretch of DNA is reversed
Inversion A linear stretch of DNA is reversed within the chromosome segments G, H, I become inverted In-text figure Page 206

37 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

38 Translocation one chromosome a nonhomologous chromosome
nonreciprocal translocation In-text figure Page 206

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

40 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

41 Polyploidy Individuals have three or more of each type of chromosome (3n, 4n) Common in flowering plants Found in some fish & amphibians Lethal for humans

42 Aneuploidy Individuals have one extra or less chromosome
(2n + 1 or 2n - 1) Major cause of human reproductive failure Most human miscarriages are aneuploids

43 nondisjunction at anaphase I
chromosome alignments at metaphase I n - 1 nondisjunction at anaphase I alignments at metaphase II anaphase II Figure Page 208

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 Aneuploidy Having one extra or one less chromosome
Downs Syndrome trisomy 21 Turners Syndrome XO Klinefelters XXY Downs Syndrome

46 Trisomy and monosomy in sex chromosomes

47 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

48 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

49 Turners Syndrome Klinefelters Syndrome

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

51 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

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

53 Prenatal Diagnosis Amniocentesis Chorionic villus sampling

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