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Chapter 15 Chromosomal basis for inheritance.

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Presentation on theme: "Chapter 15 Chromosomal basis for inheritance."— Presentation transcript:

1 Chapter 15 Chromosomal basis for inheritance

2 Mendel Genetics Mendel published his work in 1866
1900 his work was rediscovered. Parallels between Mendel’s factors & chromosome behavior

3 Mendel’s Genetics 1902 Walter Sutton Chromosomal theory of inheritance
Genes are located on chromosomes Located at specific loci (positions) Behavior of chromosomes during meiosis account for inheritance patterns

4 Figure 15.2 The chromosomal basis of Mendel’s laws
P Generation Yellow-round seeds (YYRR) Green-wrinkled seeds ( yyrr) Y y r Y R R r y Meiosis Fertilization R Y y r Gametes All F1 plants produce yellow-round seeds (YyRr) F1 Generation R R y y r r Y Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. Meiosis LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. R r r R Metaphase I Y y Y y 1 1 R r r R Anaphase I Y y Y y Figure 15.2 The chromosomal basis of Mendel’s laws R r Metaphase II r R 2 2 Y y Y y y Y Y y Y Y y y Gametes R R r r r r R R 1/4 YR 1/4 yr 1/4 Yr 1/4 yR F2 Generation An F1  F1 cross-fertilization 3 3 9 : 3 : 3 : 1

5 Fruit fly Thomas Morgan studied fruit flies Drosophila melanogaster
Proved chromosomal theory correct Studied eye color Red is dominant, white is recessive Crossed a homozygous dominant female with a homozygous recessive male

6 Wild type (w+)

7 Mutant (w)

8 Fruit fly F1 offspring were all red eyed
F2 classic 3:1 ratio red:white phenotypes Showed the alleles segregate Supported the Chromosomal theory BUT only males were white eyed All females were red eyed or wild type

9 EXPERIMENT RESULTS CONCLUSION Fig. 15-4 + w w + w w + +
Generation F1 All offspring had red eyes Generation RESULTS F2 Generation CONCLUSION P + X w X w Generation X Y + w w Sperm Eggs F1 + + Figure 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have? + w w Generation w w + w Sperm Eggs + + + w w F2 w Generation + w w w w + w

10 Fruit fly Eye color gene is on the X-chromosomes Sex-linked genes:
Genes found on the sex chromosomes X-chromosome has more genes than Y-chromosome Most sex-linked genes are on the X-chromosome





15 Human Males Y chromosome is very condensed 78 genes
Male characteristics Sperm production & fertility

16 Males SRY is a gene on the Y chromosome Sex determining region of Y
Present gonads develop into testes Determines development of male secondary sex characteristics Not present then individual develops ovaries

17 Females X chromosome has 1000 genes
One of the 2 X chromosomes is inactivated Soon after embryonic development Choice is random from cell to cell Female is heterozygous for a trait Some cells will have one allele Some cell have the other

18 Females Barr body: Condensed inactive X chromosome Stains dark

19 X chromosomes Allele for orange fur Early embryo: Allele for black fur
Fig. 15-8 X chromosomes Allele for orange fur Early embryo: Allele for black fur Cell division and X chromosome inactivation Two cell populations in adult cat: Active X Inactive X Active X Black fur Orange fur Figure 15.8 X inactivation and the tortoiseshell cat

20 Sex-linked Mom passes gene on the X-chromosome to the son
Males have one X-chromosome Recessive gene is expressed Recessive alleles on the X are present No counter alleles on the Y

21 Sex-linked disorders Mom passes sex-linked to sons & daughters
Dad passes only to daughters

22 Sex-linked disorders Sex-linked genetic defects Hemophilia
1/10,000 Caucasian males

23 Sex-linked disorders Colored blindness Red-green blindness
Mostly males Heterozygous females can have some defects


25 Sex-linked disorders Duchenne muscular dystrophy
Almost all cases are male Child born healthy Muscles become weakened Break down of the myelin sheath in nerve stimulating muscles Wheelchair by 12 years old Death by 20

26 Independent assortment

27 Independent assortment
Dihybrid testcross 50% phenotypes similar to parents Parental types 50% phenotypes not similar to parents Recombinant types Indicates unlinked genes Mendel’s independent assortment

28 Test cross

29 Linked genes Do not assort independently Genes are inherited together
Genes located on same chromosome Differs from Mendel’s law of independent assortment

30 Linked genes Test cross fruit flies Wild-type (dihybrid)
Gray bodies and long wings Mutants (homozygous) Black bodies and short wings (vestigial) Results not consistent with genes being on separate chromosomes

31 Black body, vestigial wings
Fig Testcross parents Gray body, normal wings (F1 dihybrid) Black body, vestigial wings (double mutant) b+ vg+ b vg b vg b vg Replication of chromo- somes Replication of chromo- somes b+ vg+ b vg b+ vg+ b vg b vg b vg b vg b vg Meiosis I b+ vg+ Meiosis I and II b+ vg b vg+ b vg Meiosis II Recombinant chromosomes Figure Chromosomal basis for recombination of linked genes b+ vg+ b vg b+ vg b vg+ Eggs Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal b vg b+ vg+ b vg b+ vg b vg+ b vg b vg b vg b vg Sperm Parental-type offspring Recombinant offspring Recombination frequency 391 recombinants =  100 = 17% 2,300 total offspring

32 Linked genes More parental phenotypes Than if on separate chromosomes
Greater than 50% Gray body normal wings or black body vestigial Non-parental phenotype 17% Gray-vestigial or black-normal wings Indicating crossing over

33 Genetic recombination:
New combination of genes 2 genes that are farther apart tend to cross over more 2 genes on the same chromosome can show independent assortment Due to regularly crossing over

34 Genetic map Ordered list of gene loci Linkage map:
Genetic map based on recombination frequencies Distance between genes in terms of frequency of crossing over Higher percentage of crossing over the further apart the genes are Centimorgan (Thomas Hunt Morgan) A map unit

35 Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial
Fig Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes 48.5 57.5 67.0 104.5 Figure A partial genetic (linkage) map of a Drosophila chromosome Long aristae (appendages on head) Gray body Red eyes Normal wings Red eyes Wild-type phenotypes

36 Human genetic map Genetic distance is still proportional to the recombination frequency Use pedigrees Newer technology



39 Alterations in chromosomes
Chromosome number Chromosome structure Serious human disorders

40 Alterations in numbers
Nondisjunction Failure of homologues or sister chromatids to separate properly Aneuploidy: Gain or a loss of chromosomes due to nondisjunction Abnormal number of chromosomes Occurs about 5% of the time with humans

41 Nondisjunction

42 Meiosis I Meiosis II Gametes (a) Nondisjunction of homologous
Fig Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n + 1 n + 1 n – 1 n – 1 n + 1 n – 1 n n Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II

43 Monosomics Lost a copy of a chromosome (not a sex chromosome)
Usually do not survive Trisomes: gained a copy of a chromosome Many do not survive either 35% rate of aneuploidy (spontaneous abortions)

44 Polyploidy More than 2 sets of chromosomes 3n or 4n Plants

45 Fig Figure A tetraploid mammal

46 Alterations in Structure
1. Deletion: Missing a section of chromosome 2. Duplication: Extra section of chromosome Attaches to sister or non-sister chromatids

47 Alterations in Structure
3. Inversion: Reverse orientation of section of chromosome 4. Translocation: Chromosome fragment joins a nonhomologous chromosome

48 Reciprocal translocation
Fig A B C D E F G H A B C E F G H Deletion (a) A B C D E F G H A B C B C D E F G H Duplication (b) A B C D E F G H A D C B E F G H (c) Inversion A B C D E F G H M N O C D E F G H (d) Reciprocal translocation M N O P Q R A B P Q R

49 Human disorders Trisomes Babies with extra chromosomes can survive
Chromosome 13, 15, 18, 21 and 22 These are the smallest chromosomes

50 Trisomy 13

51 Trisomy 18

52 Down syndrome Trisomy 21 1866 J. Langdon Down 1 in 750 births
Similar distribution in all racial groups Similar distribution in chimps and other primates


54 Down Syndrome Mental retardation Heart disease
Intestinal problems/surgery Hearing problems/hearing loss Unstable joints Leukemia Single crease in the palm

55 Down syndrome 20 years or younger 1 in 1700 20-30 years 1 in 1400


57 Nondisjunction Higher incidence in woman’s eggs than in the men’s sperm Woman’s eggs are in prophase I (meiosis) when she is born Her eggs are as old as she is!!! Men produce new sperm daily

58 Down Syndrome Primarily from nondisjunction
Chromosome in woman’s eggs. Therefore age of mom is very important

59 Sex chromosomes X chromosomes fail to separate properly
Some eggs with 2 X chromosomes Some eggs with no X chromosome Produce XXX Appears normal

60 Sex chromosomes XXY Klinefelter syndrome (1 in 500 male births)
Is a male with some female features Sterile Maybe slightly slower than normal OY does not survive, need the X chromosome

61 Sex chromosomes XO, Turner syndrome
Female that has short statue, web neck Sterile 1 in 5000 births

62 Sex Chromosomes XYY 1 in 1000 births Normal fertile males
May be taller than normal

63 Translocation Philadelphia chromosome
Reciprocal exchange of chromosome #22 and #9 exchange portions Shortened translocated #22 CML

64 Translocated chromosome 22 (Philadelphia chromosome)
Fig Reciprocal translocation Normal chromosome 9 Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome) Normal chromosome 22

65 Deletion Cri du chat “Cry of the cat” Deletion of chromosome 5
Mental retardation Small head Die in infancy

66 Genomic imprinting Variation in phenotype
Depends on allele is inherited from male or female Usually autosomes Silencing of one allele in gamete formation

67 Normal Igf2 allele is expressed Paternal chromosome Maternal
Fig Normal Igf2 allele is expressed Paternal chromosome Maternal chromosome Normal Igf2 allele is not expressed Wild-type mouse (normal size) (a) Homozygote Mutant Igf2 allele inherited from mother Mutant Igf2 allele inherited from father Normal size mouse (wild type) Dwarf mouse (mutant) Figure Genomic imprinting of the mouse Igf2 gene Normal Igf2 allele is expressed Mutant Igf2 allele is expressed Mutant Igf2 allele is not expressed Normal Igf2 allele is not expressed (b) Heterozygotes

68 Organelle genes Extracellular genes Cytoplasmic genes

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