1. Chromosomes and Inheritance

2. Thomas
Hunt
Morgan

3. The common fruit fly – Drosophila melanogaster

4. Gene Linkage Each chromosome has hundreds or thousands of genes.
Genes located on the same chromosome, linked genes, tend to be inherited together because the chromosome is passed along as a unit.
Results of crosses with linked genes deviate from those expected according to independent assortment.

5. P Generation (homozygous) Double mutant (black body, vestigial wings)
EXPERIMENT
P Generation (homozygous)
Double mutant (black body, vestigial wings)
Wild type (gray body, normal wings)
b b vg vg
b b vg vg
Figure 15.9 INQUIRY: How does linkage between two genes affect inheritance of characters?

6. P Generation (homozygous) Double mutant (black body, vestigial wings)
EXPERIMENT
P Generation (homozygous)
Double mutant (black body, vestigial wings)
Wild type (gray body, normal wings)
b b vg vg
b b vg vg
F1 dihybrid (wild type)
Double mutant
TESTCROSS
b b vg vg
b b vg vg
Figure 15.9 INQUIRY: How does linkage between two genes affect inheritance of characters?

7. Wild type (gray-normal)
EXPERIMENT
P Generation (homozygous)
Double mutant (black body, vestigial wings)
Wild type (gray body, normal wings)
b b vg vg
b b vg vg
F1 dihybrid (wild type)
Double mutant
TESTCROSS
b b vg vg
b b vg vg
Testcross offspring
Eggs
b vg
b vg
b vg
b vg
Wild type (gray-normal)
Black- vestigial
Gray- vestigial
Black- normal
b vg
Figure 15.9 INQUIRY: How does linkage between two genes affect inheritance of characters?
Sperm
b b vg vg
b b vg vg
b b vg vg
b b vg vg

8. Wild type (gray-normal)
EXPERIMENT
P Generation (homozygous)
Double mutant (black body, vestigial wings)
Wild type (gray body, normal wings)
b b vg vg
b b vg vg
F1 dihybrid (wild type)
Double mutant
TESTCROSS
b b vg vg
b b vg vg
Testcross offspring
Eggs
b vg
b vg
b vg
b vg
Wild type (gray-normal)
Black- vestigial
Gray- vestigial
Black- normal
b vg
Figure 15.9 INQUIRY: How does linkage between two genes affect inheritance of characters?
Sperm
b b vg vg
b b vg vg
b b vg vg
b b vg vg
PREDICTED RATIOS
If genes are located on different chromosomes: 1 : 1 : 1 : 1
If genes are located on the same chromosome and parental alleles are always inherited together: 1 : 1 : :
RESULTS
965 :
944 :
206 :
185

9. Genetic Recombination
The production of offspring with new combinations of traits inherited from two parents is genetic recombination.
Genetic recombination can result from independent assortment of genes located on nonhomologous chromosomes or from crossing over of genes located on homologous chromosomes.

10. F1 dihybrid female and homozygous recessive male in testcross
b+ vg+
b vg
F1 dihybrid female and homozygous recessive male in testcross
b vg
b vg
b+ vg+
b vg
Figure 15.UN01
Most offspring or
b vg
b vg

11. Mendel and recombination
Mendel’s dihybrid cross experiments produced some offspring that had a combination of traits that did not match either parent in the P generation.
If the P generation consists of a yellow-round parent (YYRR) crossed with a green-wrinkled seed parent (yyrr), all F1 plants have yellow-round seeds (YyRr).
A cross between an F1 plant and a homozygous recessive plant (a test-cross) produces four phenotypes.
Half are parental types, with phenotypes that match the original P parents, either with yellow-round seeds or green-wrinkled seeds.
Half are recombinants, new combination of parental traits, with yellow-wrinkled or green-round seeds.
A 50% frequency of recombination is observed for any two genes located on different (nonhomologous) chromosomes.

12. Gametes from yellow-round dihybrid parent (YyRr)
Gametes from green- wrinkled homozygous recessive parent (yyrr) yr
YyRr
yyrr
Yyrr
yyRr
Figure 15.UN02
Parental- type offspring
Recombinant offspring

13. Figure 15.10 Chromosomal basis for recombination of linked genes.
Testcross parents
Gray body, normal wings (F1 dihybrid)
Black body, vestigial wings (double mutant)
b vg
b vg
b vg
b vg
Replication of chromosomes
Replication of chromosomes
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
bvg
b vg
b vg
b vg
Figure Chromosomal basis for recombination of linked genes.
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 2,300 total offspring 
  17%

14. Gray body, normal wings (F1 dihybrid)
Testcross parents
Gray body, normal wings (F1 dihybrid)
Black body, vestigial wings (double mutant)
b vg
b vg
b vg
b vg
Replication of chromosomes
Replication of chromosomes
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
Figure Chromosomal basis for recombination of linked genes.
b vg
b vg
Meiosis II
Recombinant chromosomes
bvg
b vg
b vg
b vg
b vg
Eggs
Sperm

15. 965 Wild type (gray-normal) 391 recombinants 2,300 total offspring
Recombinant chromosomes
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
Figure Chromosomal basis for recombination of linked genes.
Sperm
Parental-type offspring
Recombinant offspring
Recombination frequency
391 recombinants 2,300 total offspring 
  17%

16. Alfred Sturtevant

17. Linkage Maps
A linkage map is an ordered list of the genetic loci along a particular chromosome.
Sturtevant hypothesized that the frequency of recombinant offspring reflected the distances between genes on a chromosome.
The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore a higher recombination frequency.
The greater the distance between two genes, the more points between them where crossing over can occur.
Sturtevant used recombination frequencies from fruit fly crosses to map the relative position of genes along chromosomes, a linkage map.

18. Recombination frequencies
RESULTS
Recombination frequencies 9%
9.5%
Chromosome
17%
Figure RESEARCH METHOD: Constructing a Linkage Map b cn vg

19. Long aristae (appendages on head) Gray body Red eyes Normal wings
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

20. Linkage
distances on
fruit fly
chromosome 2

21. More on Linkage Maps
A linkage map provides an imperfect picture of a chromosome.
Map units indicate relative distance and order, not precise locations of genes.
The frequency of crossing over is not actually uniform over the length of a chromosome.
Combined with other methods like chromosomal banding, geneticists can develop cytological maps.
These indicated the positions of genes with respect to chromosomal features.
More recent techniques show the absolute distances between gene loci in DNA nucleotides.

22. Errors and Exceptions in Chromosomal Inheritance
1. Alterations of chromosome number or structure cause some genetic disorders
2. The phenotypic effects of some mammalian genes depend on whether they are inherited from the mother or the father (imprinting)
3. Extranuclear genes exhibit a non-Mendelian pattern of inheritance

23. More Errors and Exceptions in Chromosomal Inheritance
Sex-linked traits are not the only notable deviation from the inheritance patterns observed by Mendel.
Gene mutations are not the only kind of changes to the genome that can affect phenotype.
Major chromosomal aberrations and their consequences produce exceptions to standard chromosome theory.

24. Nondisjunction
Nondisjunction occurs when problems with the meiotic spindle cause errors in daughter cells.
This may occur if tetrad chromosomes do not separate properly during meiosis I.
Alternatively, sister chromatids may fail to separate during meiosis II.
As a consequence of nondisjunction, some gametes receive two of the same type of chromosome and another gamete receives no copy.
Offspring resulting from fertilization of a normal gamete with one after nondisjunction will have an abnormal chromosome number or aneuploidy.
Trisomic cells have three copies of a particular chromosome type and have 2n + 1 total chromosomes.
Monosomic cells have only one copy of a particular chromosome type and have 2n - 1 chromosomes.

25. Meiosis I
Nondisjunction
Figure Meiotic nondisjunction.

26. Meiosis I Nondisjunction Meiosis II Non- disjunction
Figure Meiotic nondisjunction.

27. Nondisjunction of homo- logous chromosomes in meiosis I (a)
Meiosis II
Non- disjunction
Gametes
Figure Meiotic nondisjunction.
n  1
n  1
n  1
n  1
n  1
n  1 n n
Number of chromosomes
Nondisjunction of homo- logous chromosomes in meiosis I
(a)
Nondisjunction of sister chromatids in meiosis II
(b)

28. Aneuploidy
If the organism survives, aneuploidy typically leads to a distinct phenotype.
Aneuploidy can also occur during failures of the mitotic spindle.
If aneuploidy happens early in development, this condition will be passed along by mitosis to a large number of cells.
This is likely to have a substantial effect on the organism.
Several serious human disorders are due to alterations of chromosome number and structure.
Although the frequency of aneuploid zygotes may be quite high in humans, most of these alterations are so disastrous that the embryos are spontaneously aborted long before birth.
These developmental problems result from an imbalance among gene products.
Certain aneuploid conditions upset the balance less, leading to survival to birth and beyond.

29. Down’s syndrome or Trisomy 21
Figure Down syndrome.

30. Aneuploidy and Sex Chromosomes
Nondisjunction of sex chromosomes produces a variety of aneuploid conditions in humans.
Unlike autosomes, this aneuploidy upsets the genetic balance less severely.
This may be because the Y chromosome contains relatively few genes.
Also, extra copies of the X chromosome become inactivated as Barr bodies in somatic cells.

31. Aneuploidy and Sex Chromosomes
Klinefelter’s syndrome, an XXY male, occurs once in every 1000 live births.
These individuals have male sex organs, but are sterile.
There may be feminine characteristics
Their intelligence is normal.
Males with an extra Y chromosome (XYY) tend to somewhat taller than average. Occurs in about 1 in every 1000 live births.
Trisomy X (XXX), which occurs once in every 1000 live births, produces healthy females. May have learning disabilities.
Monosomy X or Turner’s syndrome (X0), which occurs once in every 2500 births, produces phenotypic, but immature females. Most have normal intelligence.

32. Polyploidy
Organisms with more than two complete sets of chromosomes, have undergone polypoidy.
This may occur when a normal gamete fertilizes another gamete in which there has been nondisjunction of all its chromosomes.
The resulting zygote would be triploid (3n).
Alternatively, if a 2n zygote failed to divide after replicating its chromosomes, a tetraploid (4n) embryo would result from subsequent successful cycles of mitosis.
Polyploidy is common among plants and much less common among animals.
The spontaneous origin of polyploid individuals plays an important role in the evolution of plants – in fact as many as 60% of all flowering plant species may have arisen via hybridization which is frequently followed by polyploidy

33. Hybridization
followed by
Polyploidy

35. The Red Viscacha Rat from Argentina –
A tetraploid mammal?

36. Mammalian Chromosome Variation
Indian muntjac deer, 2n=6
Red Viscacha Rat, 4n=102
Siberian roe deer, 2n=70 + up to 14 B’s (accessory chromosomes)
Transcaucasian vole, 2n=17, both sexes XO