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Chapter 15: The Chromosomal Basis of Inheritance. Let’s review Ch 13 - Meiosis makes gametes – sperm & egg Ch 14 – Mendel studied peas gametes pass on traits unknown what was in the gametes Make sure I have all test corrections & scantrons Ch 9, 11 – 12 Quarter test - PowerPoint PPT Presentation
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Chapter 15: The Chromosomal Basis of Inheritance
Let’s review- Ch 13 - Meiosis makes gametes – sperm & egg- Ch 14 – Mendel studied peas
- gametes pass on traits- unknown what was in the gametes
- Make sure I have all test corrections & scantrons- Ch 9, 11 – 12- Quarter test- 1st period –
- get donut – Thank Michael!!- get “pig-in-a-blanket” – Thank Regan!!
Yellow-roundseeds (YYRR)
Green-wrinkledseeds (yyrr)
Meiosis
Fertilization
Gametes
All F1 plants produceyellow-round seeds (YyRr)
P Generation
F1 Generation
Meiosis
Two equallyprobable
arrangementsof chromosomesat metaphase I
LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT
Anaphase I
Metaphase II
Fertilization among the F1 plants
9 : 3 : 3 : 1
14
14
14
14YR yr yr yR
Gametes
Y
RRY
y
r
r
y
R Y y r
Ry
Y
r
Ry
Y
r
R
Y
r
y
r R
Y y
R
Y
r
y
R
Y
Y
R R
Y
r
y
r
y
R
y
r
Y
r
Y
r
Y
r
Y
R
y
R
y
R
y
r
Y
F2 Generation
Starting with two true-breeding pea plants,we follow two genes through the F1 and F2 generations. The two genes specify seed color (allele Y for yellow and allele y forgreen) and seed shape (allele R for round and allele r for wrinkled). These two genes are on different chromosomes. (Peas have seven chromosome pairs, but only two pairs are illustrated here.)
The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus.
1
Each gamete gets one long chromosome with either the R or r allele.
2
Fertilizationrecombines the R and r alleles at random.
3
Alleles at both loci segregatein anaphase I, yielding four types of daughter cells depending on the chromosomearrangement at metaphase I. Compare the arrangement of the R and r alleles in the cellson the left and right
1
Each gamete gets a long and a short chromosome in one of four allele combinations.
2
Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation.
3
Figure 15.2 The chromosomal basis of Mendel’s laws
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?
- Thomas Hunt Morgan - 1st to trace a specific gene to a specific chromosome- Noticed a fly with white eyes (wild-type is red)- Wild-type – phenotype most common in the natural population (+)- Mutants – alternative trait to the wild-type
PGeneration
F1
Generation
X
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?
F2
Generation
Expected 3:1Observed 3:1Problem!!!!!!Only males had white eyes…..hmmm.
PGeneration
F1
Generation
X
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?
F2
Generation
PGeneration
F1
Generation
F2
Generation
Ova(eggs)
Ova(eggs)
Sperm
Sperm
XX X X
Y
WW+
W+
W
W+W+ W+
W+
W+
W+
W+
W+
W
W+
W W
W
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.
EXPERIMENT
Wild type(gray body,
normal wings)
P Generation(homozygous)
b+ b+ vg+ vg+
x Double mutant(black body,vestigial wings)
b b vg vg
F1 dihybrid(wild type)(gray body, normal wings)
b+ b vg+ vg
Double mutant(black body,vestigial wings)
b b vg vg
TESTCROSSx
b+vg+ b vg b+ vg b vg+
b vg
b+ b vg+ vg b b vg vg b+ b vg vgb b vg+ vg
965Wild type
(gray-normal)
944Black-
vestigial
206Gray-
vestigial
185Black-normal
Sperm
Parental-typeoffspring
Recombinant (nonparental-type)offspring
RESULTS
Morgan first mated true-breeding wild-type flies with black, vestigial-winged flies to produce heterozygous F1 dihybrids, all of which are wild-type in appearance. He then mated wild-type F1 dihybrid females with black, vestigial-winged males, producing 2,300 F2 offspring, which he “scored” (classified according to phenotype).
- Noticed a disproportionatelylarge number with same phenotype as parents- Deduced 2 genes must beon the same chromosome- Crossing over accounts forthe recombinant phenotypes
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?
P generation: YyRr x yyrr
Gametes from green-wrinkled homozygousrecessive parent (yyrr)
Gametes from yellow-roundheterozygous parent (YyRr)
Parental-typeoffspring
Recombinantoffspring
YyRr yyrr Yyrr yyRr
YR yr Yr yR
yr
50% 50%
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??
Announcements- Get lab notebooks from table- Get test folder from table- Grades posted- New seats - THURSDAY- Genetics problems – Due MONDAY
- Pre-lab for Meiosis due TOMORROW
Testcrossparents
Gray body,normal wings(F1 dihybrid)
b+ vg+
b vgReplication ofchromosomes
b+ vg
b+ vg+
b
vg
vgMeiosis I: Crossingover between b and vgloci produces new allelecombinations.
Meiosis II: Segregationof chromatids producesrecombinant gameteswith the new allelecombinations.
Recombinantchromosome
b+vg+ b vg b+ vg b vg+
b vg
Sperm
b vg
b vgReplication ofchromosomesvg
vg
b
b
bvg
b vg
Meiosis I and II:Even if crossing overoccurs, no new allelecombinations areproduced.
OvaGametes
Testcrossoffspring
Sperm
b+ vg+ b vg b+ vg b vg+
965Wild type
(gray-normal)b+ vg+
b vg b vg b vg b vg
b vg+b+ vg+b vg+
944Black-
vestigial
206Gray-
vestigial
185Black-normal Recombination
frequency = 391 recombinants2,300 total offspring
100 = 17%
Parental-type offspring Recombinant offspring
Ova
b vg
Black body,vestigial wings(double mutant)
b
Figure 15.6 Chromosomal basis for recombination of linked genes
Sturtevant – developed a genetic linkage map from recombination frequencies
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?
Recombinationfrequencies
9% 9.5%
17%
b cn vgChromosome
bcn 9%cnvg 9.5%bvg 17%
- 1% RF = 1 map unit (m.u.) - Some linked genes are so far apart that crossovers occur very often.- 50% RF is MAX- Recall 50% is seen with unlinked genes
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?
P generation: YyRr x yyrr
Gametes from green-wrinkled homozygousrecessive parent (yyrr)
Gametes from yellow-roundheterozygous parent (YyRr)
Parental-typeoffspring
Recombinantoffspring
YyRr yyrr Yyrr yyRr
YR yr Yr yR
yr
50% 50%
Mutant phenotypesShort aristae
Black body
Cinnabareyes
Vestigialwings
Brown eyes
Long aristae(appendageson head)
Gray body
Redeyes
Normalwings
Redeyes
Wild-type phenotypes
IIY
I
X IVIII
0 48.5 57.5 67.0 104.5
Figure 15.8 A partial genetic (linkage) map of a Drosophila chromosome
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?
- SRY – sex-determining region of Y- w/ SRY – gonads develop into testes- w/o SRY – gonads develop into ovaries- X – has genes not associated w/ sex characteristics- Sex-linked is usually X-linked- Fathers pass X-linked alleles to daughters (XX)- Moms pass X-linked alleles to sons or daughters- If X-linked allele is recessive
- ♀ shows phenotype when homozygous- ♂ shows phenotype when hemizygous – more males affected
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?7. How are sex-linked alleles transmitted?
XAXA XaY
Xa Y
XAXa XAY
XAYXAxa
XA
XA
Ova
Sperm
XAXa XAY
Ova XA
Xa
XAXA XAY
XaYXaxA
XA YSperm
XAXa XaY
Ova
Xa Y
XAXa XAY
XaYXaxa
XA
Xa
A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation.
If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder.
If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, where as males without the disorder will be completely free of the recessive allele.
(a)
(b)
(c)
Sperm
Figure 15.10 The transmission of sex-linked recessive traits
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?7. How are sex-linked alleles transmitted?8. What are some sex-linked alleles in humans?
- Duchenne’s muscular dystrophy- dystrophin – key muscle protein is absent- Progressive weakening of muscles & loss of coordination- 1 in 3500 ♂ - rarely live past early 20s
- Hemophilia- Sex-linked recessive- Protein needed for blood clotting
- Color blindness
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?7. How are sex-linked alleles transmitted?8. What are some sex-linked alleles in humans?9. What are Barr bodies?
- 1 of the 2 Xs becomes almost completely inactive during embryonic development
- Inactive X in each ♀ cell condenses into a Barr body- Most genes on the Barr body are not expressed - Barr body chromosomes are reactivated in ovary cells that
give rise to ova- Calico cats
Two cell populationsin adult cat:
Active X
Orangefur
Inactive X
Early embryo:X chromosomes
Allele forblack fur
Cell divisionand X
chromosomeinactivation
Active X
Blackfur
Inactive X
Allele fororange fur
Figure 15.11 X inactivation and the tortoiseshell (calico) cat
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?7. How are sex-linked alleles transmitted?8. What are some sex-linked alleles in humans?9. What are Barr bodies?10. What are some chromosomal errors & exceptions?
- Nondisjunction- Homologous chromosomes fail to separate during meiosis
- Chromosomal rearrangements
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n + 1n + 1 n 1 n – 1 n + 1 n –1 n nNumber of chromosomes
Nondisjunction of homologouschromosomes in meiosis I
Nondisjunction of sisterchromatids in meiosis II
(a) (b)
Figure 15.12 Meiotic nondisjunction
Aneuploidy – an offspring that has an abnormal # of chromosomes (formed from a nondisjunction gamete)
Trisomic – 2n + 1, Monosomic – 2n – 1 Polyploidy – more than 2 complete chromosome SETS: 3n – triploid, 4n - tetraploid
Crossing over is NOT exact (Figure 19.18)
This leads to deletions & duplications.
A B C D E F G HDeletion
A B C E G HF
A B C D E F G H DuplicationA B C B D EC F G H
A
A
M N O P Q R
B C D E F G H
B C D E F G HInversion
Reciprocaltranslocation
A B P Q R
M N O C D E F G H
A D C B E F HG
(a) A deletion removes a chromosomal segment.
(b) A duplication repeats a segment.
(c) An inversion reverses a segment within a chromosome.
(d) A translocation moves a segment fromone chromosome to another,nonhomologous one. In a reciprocal
translocation, the most common type,nonhomologous chromosomes exchangefragments. Nonreciprocal translocationsalso occur, in which a chromosome transfers a fragment without receiving afragment in return.
Figure 15.14 Alterations of chromosome structure
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?7. How are sex-linked alleles transmitted?8. What are some sex-linked alleles in humans?9. What are Barr bodies?10. What are some chromosomal errors & exceptions?11. What are some human disorders due to chromosomal alterations?
- Down syndrome- Trisomy 21 aka nondisjunction of 21st chromosome- Each cell has 47 chromosomes
Figure 15.15 Down syndrome
Chapter 15: The Chromosomal Basis of Inheritance1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?7. How are sex-linked alleles transmitted?8. What are some sex-linked alleles in humans?9. What are Barr bodies?10. What are some chromosomal errors & exceptions?11. What are some human disorders due to chromosomal alterations?
- Down syndrome- Nondisjunction of sex chromosomes
- Klinefelter syndrome – XXY – 1 in 2000 ♂- XYY syndrome – 1 in 1000 ♂- XXX - 1 in 1000 ♀- Turner syndrome - XO – monosomy X – 1 in 5000 ♀