1. CHAPTER 13 LECTURE SLIDES
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2. Chromosomes, Mapping, and the Meiosis–Inheritance Connection
Chapter 13

3. What Carried Mendel’s “Factors”?
Carl Correns – 1900
First suggests central role for chromosomes
One of papers announcing rediscovery of Mendel’s work
Walter Sutton – 1902
Chromosomal theory of inheritance
Based on observations that similar chromosomes paired with one another during meiosis

4. T.H. Morgan – 1910 Working with fruit fly, Drosophila melanogaster
Discovered a mutant male fly with white eyes instead of red
Crossed the mutant male to a normal red-eyed female
All F1 progeny red eyed = dominant trait

5. Morgan crossed F1 females x F1 males
F2 generation contained red and white- eyed flies
But all white-eyed flies were male
Testcross of a F1 female with a white-eyed male showed the viability of white-eyed females
Morgan concluded that the eye color gene resides on the X chromosome
FIRST DIRECT EVIDENCE FOR CHROMOSOMAL THEORY OF INHERITANCE! – showed that genes lie on chromosomes

8. Sex Chromosomes
Sex determination in Drosophila is based on the number of X chromosomes
2 X chromosomes = female
1 X and 1 Y chromosome = male
Sex determination in humans is based on the presence of a Y chromosome
Having a Y chromosome (XY) = male

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
X chromosome
Y chromosome
2.8 µm
© BioPhoto Associates/Photo Researchers, Inc.
Humans have 46 total chromosomes
22 pairs are autosomes
1 pair of sex chromosomes
Y chromosome highly condensed
Recessive alleles on male’s X have no active counterpart on Y
“Default” for humans is female
Requires SRY gene on Y for “maleness”

10. Hemophilia
Disease that affects a single protein in a cascade of proteins involved in the formation of blood clots
Form of hemophilia is caused by an X-linked recessive allele
Heterozygous females are asymptomatic carriers
Allele for hemophilia was introduced into a number of different European royal families by Queen Victoria of England

12. Dosage compensation
Ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1
In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body
Females heterozygous for genes on the X chromosome are genetic mosaics

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15. Chromosome theory exceptions
Mitochondria and chloroplasts contain genes
Traits controlled by these genes do not follow the chromosomal theory of inheritance
Genes from mitochondria and chloroplasts are often passed to the offspring by only one parent (mother)
Maternal inheritance
In plants, the chloroplasts are often inherited from the mother, although this is species dependent

16. Genetic Mapping
Early geneticists realized that they could obtain information about the distance between genes on a chromosome
Based on genetic recombination (crossing over) between genes
If crossover occurs, parental alleles are recombined producing recombinant gametes

19. Review of Expectations – 2 genes on same chromosome
R - Round, r-oval
W-White, w-black P1
RRWW x rrww F1
RrWw
F2 - testcross RW rw
RrWw
rrww
F2 Expected if
Same 1
Chromosome
Round_White
oval_black

20. Review of Expectations- 2 genes on diff. chromosomes
R - Round, r-oval
W-White, w-black P1
RRWW x rrww F1
RrWw
F2 - testcross RW Rw rW rw
RrWw
Rrww
rrWw
rrww
F2 Expected if 1
Indep. Asst.
Round_White
Round_black
oval_White
oval_black

21. Review of Expectations- 2 genes on same chromosome – but now with crossover events
R - Round, r-oval
W-White, w-black P1
RRWW x rrww F1
RrWw
F2 - testcross RW Rw rW rw
RrWw
Rrww
rrWw
rrww
F2 Expected if
<1
<<1
Crossover between genes
Round_White
Round_black
oval_White
oval_black
RECOMBINANTS

22. Alfred Sturtevant Undergraduate in T.H. Morgan’s lab
Put Morgan’s observation that recombinant progeny reflected relevant location of genes in quantitative terms
As physical distance on a chromosome increases, so does the probability of recombination (crossover) occurring between the gene loci

23. Probability of crossover depends on distance

24. Recombination Frequency
Genotype of gametes formed by double heterozygous parent (Cc Ss)
C S
c s
C s
c S
CcSs
ccss
Ccss
ccSs
Appearance (phenotype)
Colored, Smooth
colorless, wrinkled
Colored, wrinkled
colorless, Smooth
% total recombinants
if Independent Assortment
25%
50%
if closely linked (no crossover) -- 0%
if there's some distance between them
45% 5%
10%
if there's more distance between them
40%
20%
if there's even more distance between them
35%
15%
30%
Now they're so far apart they almost sort independently due to the high probability of crossover events C c S s C c S s C c S s C c S s C c S s C c S s C c S s

25. Multiple crossovers
If homologues undergo two crossovers between loci, then the parental combination is restored
Leads to an underestimate of the true genetic distance
Relationship between true distance on a chromosome and the recombination frequency is not linear

27. Working examples- note the cute way to cross to test for linkage
1. In tomatoes tall growth habit is the result of a dominant gene, D, dwarf growth to its recessive allele, d. Smooth epidermis is due to a dominant gene, P, pubescent epidermis to its recessive allele, p. A homozygous tall smooth variety was crossed with a dwarf pubescent variety. The F1 were test crossed to dwarf pubescent. The results of the test cross were as follows:
tall, smooth - 96
tall, pubescent - 4
dwarf, smooth - 3
dwarf, pubescent - 95
Are these genes linked (on the same chromosome)? If so, what is the percent of crossing over?

28. Worked solution P1 == DDPP x ddpp Parental gametes DP and dp
F1 == DdPp
TestCross DP Dp dP dp
DdPp
Ddpp
ddPp
ddpp
Tall, smooth
Tall, pubes
dwarf, smooth
dwarf, pubes 96 4 3 95
Same Chromosome because mostly parental types
% Crossover = (4+3) / ( )
"=" 7
recombinants
198
total offspring
% crossover

29. 2. In rabbits color is due to a dominant gene, D, albinism to its recessive allele, d. Black is the result of a dominant gene, B, brown to its recessive allele, b. Brown (homozygous) rabbits are crossed with albinos carrying black in the homozygous state. The F1 are crossed to double recessive. From many such crosses the total results are:
black - 68
brown - 132
albino - 200
Are these genes linked?

30. Worked solution P1 == DDbb x ddBB Parental gametes Db and dB
F1 == DdBb
TestCross Db DB db dB
Ddbb
DdBb
ddbb
ddBb
Colored, brn
Colored, blk
Albino, brown
Albino, black
132 68 ?
200 albinos=
68+132
Same Chromosome because mostly parental types
% Crossover = (68+68?) / ( (68+132) )
"="
136
recombinants
400
total offspring 34
% crossover

31. In corn purple plant color is due to a dominant gene, P, green plant color to recessive allele, p. Normal leaves are due to a dominant gene, N, narrow leaves to its recessive allele, n. Homozygous green plants with homozygous normal leaves were crossed with homozygous purple plants having narrow leaves. The F1 were crossed with green plants with narrow leaves. The results of this test cross were as follows:
purple plants with normal leaves - 197
purple plants with narrow leaves - 201
green plants with normal leaves - 199
green plants with narrow leaves - 203
Are these genes linked? If so what is the percent cross over?

32. Worked solution-note that parental gametes are mixed
P1 == ppNN x PPnn
Parental gametes
pN and Pn
F1 == pPNn
TestCross pN pn PN Pn
ppNn
ppnn
PpNn
Ppnn
green, norm
Grn, narrow
Purple, norm
Purp, narrow
199
203
197
201
Different Chromosomes because equal probabilities

33. In corn tallness is due to a dominant gene, D, dwarfness to its recessive allele, d. Normal leaves is dominant, N, crinkly leaves is recessive, n. Homozygous tall, crinkly-leaved corn was crossed to dwarf, homozygous normal-leaved corn. The F1 which was tall normal leaved, was crossed to double-recessive dwarf crinkly-leaved corn. The results were as follows:
Tall crinkly-leaved - 83
Tall, normal-leaved - 19
dwarf crinkly-leaved - 17
dwarf normal-leaved - 81
Are these two pairs of genes linked? If so, what is the percent crossing over?

34. Worked solution-note that parental gametes are mixed
P1 == DDnn x ddNN
Parental gametes
Dn and dN
F1 == DdnN
TestCross Dn DN dn dN
Ddnn
DdNn
ddnn
ddNn
Tall, crinkly
Tall, norm
dwarf, crinkly
dwarf, norm 81 19 17 83
Same chromosome because mostly parental types
(19+17)/(200) = 36/200=18%

35. Constructing maps
The distance between genes is proportional to the frequency of recombination events
recombination recombinant progeny
frequency total progeny
1% recombination = 1 map unit (m.u.)
1 map unit = 1 centimorgan (cM) =

36. Three-point testcross
Uses 3 loci instead of 2 to construct maps
Gene in the middle allows us to see recombination events on either side
In any three-point cross, the class of offspring with two crossovers is the least frequent class
In practice, geneticists use three-point crosses to determine the order of genes, then use data from the closest two-point crosses to determine distances

38. Human genome maps Data derived from historical pedigrees
Difficult analysis
Number of markers was not dense enough for mapping up to 1980s
Disease-causing alleles rare
Situation changed with the development of anonymous markers
Detected using molecular techniques
No detectable phenotype

39. SNPs Single-nucleotide polymorphisms
Affect a single base of a gene locus
Used to increase resolution of mapping
Used in forensic analysis
Help eliminate or confirm crime suspects or for paternity testing

41. Sickle cell anemia
First human disease shown to be the result of a mutation in a protein
Caused by a defect in the oxygen carrier molecule, hemoglobin
Leads to impaired oxygen delivery to tissues

42. Heterozygotes appear normal
Homozygotes for sickle cell allele exhibit intermittent illness and reduced life span
Heterozygotes appear normal
Do have hemoglobin with reduced ability
Sickle cell allele is particularly prevalent in people of African descent
Proportion of heterozygotes higher than expected
Confers resistance to blood-borne parasite that causes malaria

43. Nondisjunction
Failure of homologues or sister chromatids to separate properly during meiosis
Aneuploidy – gain or loss of a chromosome
Monosomy – loss
Trisomy – gain
In all but a few cases, do not survive

44. Smallest autosomes can present as 3 copies and allow individual to survive
13, 15, 18 – severe defects, die within a few months
21 and 22 – can survive to adulthood
Down Syndrome – trisomy 21
May be a full, third 21st chromosome
May be a translocation of a part of chromosome 21
Mother’s age influences risk

46. Nondisjunction of sex chromosomes
Do not generally experience severe developmental abnormalities
Individuals have somewhat abnormal features, but often reach maturity and in some cases may be fertile
XXX – triple-X females
XXY – males (Klinefelter syndrome)
XO – females (Turner syndrome)
OY – nonviable zygotes
XYY – males (Jacob syndrome)

48. Genomic imprinting
Phenotype exhibited by a particular allele depends on which parent contributed the allele to the offspring
Specific partial deletion of chromosome 15 results in
Prader-Willi syndrome if the chromosome is from the father
Angelman syndrome if it’s from the mother

49. Detection
Pedigree analysis used to determine the probability of genetic disorders in the offspring
Amniocentesis collects fetal cells from the amniotic fluid for examination
Chorionic villi sampling collects cells from the placenta for examination