Download 15 - GEOCITIES.ws

AP Biology, Chapter 15
The Chromosomal Basis of Inheritance
Summary
Introduction
RELATING MENDELISM TO CHROMOSOMES
Mendelian inheritance has its physical basis in the behavior of chromosomes during
sexual life cycles
1. Explain how the observations of cytologists and geneticists provided the basis for the
chromosomal theory of inheritance.
a. Chromosomal theory of inheritance = genes are stably arranged on
chromosomes
b. Both genes and chromosomes:
i. In pairs in diploid cells
ii. Separate in meiosis
iii. Unite again in fertilization
Morgan traced a gene to a specific chromosome: science as a process
2. Describe the contributions that Walter Sutton, Theodor Boveri, and Thomas Hunt
Morgan made to current understanding of chromosomal inheritance.
a. Sutton
i. Demonstrated Mendel's laws in grasshoppers
ii. Suggested meiotic separation accounted for Mendel's laws
b. Boveri
i. Studied sea urchins
ii. Showed sperm and eggs are 1N
iii. Showed that a complete set of chromosomes is required for
development - each kind of chromosome is unique
iv. Suggested Mendel's factors are carried by chromosomes
b. T.H. Morgan
i. Gathered overwhelming evidence
ii. Identified dozens of new Dropsophila mutations
iii. Demonstrated linkage by extensive crossing
iv. Demonstrated sex-linkage: a particular gene on a particular
chromosome
3. Explain why Drosophila melanogaster is a good experimental organism.
a. Prolific breeders
b. Generation time = 2 weeks
c. 2N = 8; chromosomes easily distinguishable
Linked genes tend to be inherited together because they are located on the same
chromosome
4. Define and compare linked genes and sex-linked genes. Explain why the inheritance of
linked genes is different from independent assortment.
a. Comparison
i. Linked genes are located on the same chromosome
ii. Sex-linked are on the X chromosome
b. Genes on the same chromosome tend to separate together, not randomly
Independent assortment of chromosomes and crossing over produce genetic
recombinants
5. Distinguish between parental and recombinant phenotypes.
a. Dihybrid cross with linked genes
i. P: b+b+ vg+vg+ x bb vgvg
ii. F1: all are b+b vg+vg
iii. Testcross: b+b vg+vg x bb vgvg
b. Testcross results
i. If they assort independently:
1/4 b+b vg+vg
1/4 b+b vgvg
1/4 bb vg+vg
1/4 bb vgvg
ii. They don't = linkage
965 b+b vg+vg
206 b+b vgvg
185 bb vg+vg
944 bb vgvg
c. Explanation
i. The b+ vg+ and b vg from the P generation tended to stay together
and gave b+b vg+vg and bb vgvg, respectively, the parental types
ii. The chromosome can recombine to for b+ vg and b vg+ and those give
b+b vgvg and bb vg+vg, respectively, the recombinant types
d. Generic definitions
i. Parental types result from the original parental combination of alleles
ii. Recombinant types result from rarer, new combinations
6. Explain why linked genes do not assort independently.
a. Linked genes are part of the same molecule
b. Separated as a single object in anaphase I
Geneticists use recombination data to map a chromosome's genetic loci
7. Explain how crossing over can unlink genes.
a. In crossing over chromosomes trade segments
b. Alleles on traded segments become linked in new combinations
8. Explain how Sturtevant created linkage maps.
a. Mated flies until he had pure b+b+ cn+cn+ vg+vg+ and bb cncn vgvg
b. Made the trihybrid: b+b+ cn+cn+ vg+vg+ x bb cncn vgvg  b+b cn+cn vg+vg
c. Set up a three-point testcross: b+b cn+cn vg+vg x bb cncn vgvg
d. Results
i. Parental types: b+b cn+cn vg+vg and bb cncn vgvg
ii. Single cross over (SCO) recombinant types
. bb cn+cn vg+vg and bb cn+cn vg+vg
. bb cncn vg+vg and b+b cn+cn vgvg
iii. Double cross over (DCO) recombinant types
. bb cn+cn vgvg x b+b cncn vg+vg
9. Define map unit.
a. Distance giving 1% recombinants
10. Explain why Mendel did not find linkage between seed color and flower color.
a. 50 map units = 50% recombinants = 1:1 ratio of parental to recombinant
b. Same ratio as independent assortment
c. Widely separated linked genes show independent assortment
11. Explain how genetic maps are constructed for genes located far apart on a
chromosome.
a. Gaps are filled in with newly discovered genes
b. Overlapping mappable segments are connected
12. Explain the impact of multiple crossovers between loci.
a. Multiple crossovers cause underestimation of distance
13. Explain what additional information cytological maps provide over linkage maps.
a. Crossover frequencies are not uniform along the chromosome
b. Cytological maps locate genes with respect to physical features
SEX CHROMOSOMES
Intro
The chromosomal basis for sex varies with the organism
14. Explain how sex is genetically determined in humans and the significance of the SRY
gene.
a. Humans: XX, female; XY, male
i. Grasshoppers: X-, male; XX, female
ii. Birds: ZZ, male; ZW, female
iii. Bees: haploid, male; diploid, female
b. Intact SRY gene is required for the formation of testes
c. SRY is Y-linked
Sex-linked genes have unique patterns of inheritance
15. Explain why sex-linked diseases are more common in human males.
a. Sex-linked genes are on the X chromosome
b. Y chromosome is mainly non-homologous
c. Most genes on the X chromosome are not paired
16. Describe the inheritance patterns and symptoms of color blindness, Duchenne
muscular dystrophy, and hemophilia.
a. Color blindness; sex-linked recessive
i. Some types: red, green, and red-green
ii. Difficulty distinguishing reds and/or greens
b. Duchenne muscular dystrophy; sex-linked recessive
i. Progressive weakening of muscles
ii. Rarely live past early 20s
iii. Lack the protein dystrophin, a link between the cytoskeleton and cell
membrane
c. Hemophilia; sex-linked recessive
i. Lack one of the blood proteins required for clotting
ii. Controlled by injections of recombinant clotting factor
17. Describe the process of X inactivation in female mammals. Explain how this
phenomenon produces the tortoiseshell coloration in cats.
a. Human cells require a single active copy of the X chromosome
b. In females, after embryonic development, one remains condensed, the Barr
body
c. Inactivation involves DNA methylation
d. This inactivation is random
e. Sex-linked heterozygous genes randomly turned off create a mosaic
phenotype
f. In cats, calico is such a phenotypic mosaic
ERRORS AND EXCEPTIONS IN CHROMOSOMAL INHERITANCE
Intro
Alterations of chromosome number or structure cause some genetic disorders
18. Distinguish nondisjunction, aneuploidy, trisomy, triploidy, and polyploidy. Explain how
these major chromosomal changes occur and describe the consequences.
a. Nondisjunction
i. Failure of homologous chromosomes to separate
properly singly or complete spindle or cytokinetic failure
ii. Each chromosome has many genes, pleiotropic effects
iii. Altered ratios of genes disrupt function
b. Types
i. Aneuploidy: abnormal chromosome number
ii. Trisomy: one extra chromosome; three of one type
iii. Monosomy: one missing chromosome; only one of one type
iv. Triploidy: three sets of chromosomes; 3N
v. Polyploidy: many sets of chromosomes
19. Distinguish deletions, duplications, inversions, and translocations.
a. Chromosomal mutations all result from breakage and rejoining
i. Many genes may be involved; ratio and pleiotropic effects
ii. Points of breakage and rejoining may be damaged; mutation
a. Types
i. Deletions: loss of a section of a chromosome
ii. Duplications: doubling of a section
iii. Inversions: flipping over asection
iv. Translocations: a piece of a chromosome becomes attached to a nonhomologous chromosome
20. Describe the type of chromosomal alterations implicated in the following human
disorders: Down syndrome, Klinefelter's syndrome, extra Y, triple-X syndrome, Turner
syndrome, cri du chat syndrome, and chronic myelogenous leukemia.
a. Down syndrome: trisomy 21
b. Klinefelter's syndrome: XXY
c. Extra Y: XYY
d. Triple-X syndrome: XXX
e. Turner syndrome: XO
f. Cri du chat syndrome: deletion from chromosome 5
g. Chronic myelogenous leukemia: 22  9 translocation
The phenotypic effects of some genes depend on whether they are inherited from the
mother or the father (imprinting)
21. Define genomic imprinting and provide evidence to support this model.
a. Example: a specific deletion from chromosome 5
i. Inherited from the mother: Angelman syndrome
ii. Inherited from the father: Prader-Willi syndrome
b. Genes showing imprinting
i. Usually inactivated by methylation
ii. Usually are required in embryonic development
iii. Must be present as one active copy
c. After development unimprinted; germ line cells perform methylation
Extranuclear genes exhibit a non-Mendelian pattern of inheritance
22. Give some exceptions to the chromosome theory of inheritance. Explain why
extranuclear genes are not inherited in a Mendelian fashion and how they can contribute
to disease.
a. DNA isn't just found on large chromosomes in the nucleus
i. Mitochondria and chloroplasts have small circular chromosomes
ii. Prokaryotes and some eukaryotes may carry variable numbers of
plasmids
b. Not included in the mitotic or meiotic spindle
i. Segregated randomly by cytokinesis
ii. In organisms with dimorphic gametes, mitochondria and chloroplasts
come from the mother
c. Mitochondrial defects affect high energy tissues: mitochondrial myopathy
d. Also implicated in diabetes, heart disease, Alzheimer's, aging