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Chapter 15. Chromosomal basis for inheritance. Mendel Genetics. Mendel published his work in 1866 1900 his work was rediscovered. Parallels between Mendel’s factors & chromosome behavior. Mendel’s Genetics. 1902 Walter Sutton Chromosomal theory of inheritance - PowerPoint PPT Presentation
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Chapter 15
Chromosomal basis for inheritance
Mendel Genetics Mendel published his work in 1866 1900 his work was rediscovered. Parallels between Mendel’s factors
& chromosome behavior
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
Fig. 15-2P Generation Yellow-round
seeds (YYRR)
Y
F1 Generation
Y
R R
R Y
r
r
r
y
y
y
Meiosis
Fertilization
Gametes
Green-wrinkledseeds ( yyrr)
All F1 plants produceyellow-round seeds (YyRr)
R R
YY
r ry y
Meiosis
R R
Y Y
r r
y y
Metaphase I
Y Y
R Rrr
y y
Anaphase I
r r
y Y
Metaphase IIR
Y
R
y
yyy
RR
YY
rrrr
yYY
R R
yRYryrYR1/41/4
1/41/4
F2 Generation
Gametes
An F1 F1 cross-fertilization
9 : 3 : 3 : 1
LAW OF INDEPENDENTASSORTMENT Alleles of geneson nonhomologouschromosomes assortindependently during gameteformation.
LAW OF SEGREGATIONThe two alleles for each geneseparate during gameteformation.
1
2
33
2
1
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
Fruit fly
Wild type (w+)
Mutant (w)
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
Fig. 15-4
PGeneration
Generation
Generation
Generation
Generation
Generation
F1
F2
All offspring had red eyes
Sperm
EggsF1
F2
P
Sperm
Eggs
XX
XY
CONCLUSION
EXPERIMENT
RESULTS
w
w
w
w
ww
w w
+
+
++ +
w
ww w
w
w
w
ww
+
+
+
+ +
+
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
Human Males Y chromosome is very condensed 78 genes Male characteristics Sperm production & fertility
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
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
Females Barr body: Condensed inactive X chromosome Stains dark
Fig. 15-8X chromosomes
Early embryo:
Allele fororange fur
Allele forblack fur
Cell division andX chromosomeinactivationTwo cell
populationsin adult cat:
Active XActive X
Inactive X
Black fur Orange fur
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
Sex-linked disorders Mom passes sex-linked to sons &
daughters Dad passes only to daughters
Sex-linked disorders Sex-linked genetic defects Hemophilia 1/10,000 Caucasian males
Sex-linked disorders Colored blindness Red-green blindness Mostly males Heterozygous females can have
some defects
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
Independent assortment
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
Test cross
Linked genes Do not assort independently Genes are inherited together Genes located on same
chromosome Differs from Mendel’s law of
independent assortment
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
Fig. 15-10Testcrossparents
Replicationof chromo-somes
Gray body, normal wings(F1 dihybrid)
Black body, vestigial wings(double mutant)
Replicationof chromo-somes
b+ vg+
b+ vg+
b+ vg+
b vg
b vg
b vg
b vg
b vg
b vg
b vgb vg
b vg
b+ vg+
b+ vg
b vg+
b vg
Recombinantchromosomes
Meiosis I and II
Meiosis I
Meiosis II
b vg+b+ vgb vgb+ vg+
Eggs
Testcrossoffspring
965Wild type
(gray-normal)
944Black-
vestigial
206Gray-
vestigial
185Black-normal
b+ vg+
b vg b vg
b vg b+ vg
b vg b vg
b vg+
Sperm
b vg
Parental-type offspring Recombinant offspring
Recombinationfrequency =
391 recombinants2,300 total offspring
100 = 17%
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
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
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
Fig. 15-12
Mutant phenotypes
Shortaristae
Blackbody
Cinnabareyes
Vestigialwings
Browneyes
Redeyes
Normalwings
Redeyes
Graybody
Long aristae(appendageson head)
Wild-type phenotypes
0 48.5 57.5 67.0 104.5
Human genetic map Genetic distance is still
proportional to the recombination frequency
Use pedigrees Newer technology
Alterations in chromosomes Chromosome number Chromosome structure Serious human disorders
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
Nondisjunction
Fig. 15-13-3
Meiosis I
Nondisjunction
(a) Nondisjunction of homologous chromosomes in meiosis I
(b) Nondisjunction of sister chromatids in meiosis II
Meiosis II
Nondisjunction
Gametes
Number of chromosomes
n + 1 n + 1 n + 1n – 1 n – 1 n – 1 n n
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)
Polyploidy More than 2 sets of chromosomes 3n or 4n Plants
Fig. 15-14
Alterations in Structure 1. Deletion: Missing a section of chromosome 2. Duplication: Extra section of chromosome Attaches to sister or non-sister
chromatids
Alterations in Structure 3. Inversion: Reverse orientation of section of
chromosome 4. Translocation: Chromosome fragment joins a
nonhomologous chromosome
Fig. 15-15
DeletionA B C D E F G H A B C E F G H(a)
(b)
(c)
(d)
Duplication
Inversion
Reciprocaltranslocation
A B C D E F G H
A B C D E F G H
A B C D E F G H
A B C B C D E F G H
A D C B E F G H
M N O C D E F G H
M N O P Q R A B P Q R
Human disorders Trisomes Babies with extra chromosomes
can survive Chromosome 13, 15, 18, 21 and 22 These are the smallest
chromosomes
Trisomy 13
Trisomy 18
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
Down Syndrome Mental retardation Heart disease Intestinal problems/surgery Hearing problems/hearing loss Unstable joints Leukemia Single crease in the palm
Down syndrome 20 years or younger 1 in 1700 20-30 years 1 in 1400 30-35 years 1 in 750 45 1 in 16
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
Down Syndrome Primarily from nondisjunction Chromosome in woman’s eggs. Therefore age of mom is very
important
Sex chromosomes X chromosomes fail to separate
properly Some eggs with 2 X chromosomes Some eggs with no X chromosome Produce XXX Appears normal
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
Sex chromosomes XO, Turner syndrome Female that has short statue, web
neck Sterile 1 in 5000 births
Sex Chromosomes XYY 1 in 1000 births Normal fertile males May be taller than normal
Translocation Philadelphia chromosome Reciprocal exchange of
chromosome #22 and #9 exchange portions Shortened translocated #22 CML
Fig. 15-17
Normal chromosome 9
Normal chromosome 22
Reciprocaltranslocation Translocated chromosome 9
Translocated chromosome 22(Philadelphia chromosome)
Deletion Cri du chat “Cry of the cat” Deletion of chromosome 5 Mental retardation Small head Die in infancy
Genomic imprinting Variation in phenotype Depends on allele is inherited from
male or female Usually autosomes Silencing of one allele in gamete
formation
Fig. 15-18Normal Igf2 alleleis expressed
Paternalchromosome
Maternalchromosome
Normal Igf2 alleleis not expressed
Mutant Igf2 alleleinherited from mother
(a) Homozygote
Wild-type mouse(normal size)
Mutant Igf2 alleleinherited from father
Normal size mouse(wild type)
Dwarf mouse(mutant)
Normal Igf2 alleleis expressed
Mutant Igf2 alleleis expressed
Mutant Igf2 alleleis not expressed
Normal Igf2 alleleis not expressed
(b) Heterozygotes
Organelle genes Extracellular genes Cytoplasmic genes