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