1. Chromosomal Inheritance
Chapter 12
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2. Chromosomal Inheritance
Humans are diploid (2 chromosomes of each type)
Humans have 23 different kinds of chromosomes
Arranged in 23 pairs of homologous chromosomes
Total of 46 chromosomes (23 pairs) per cell
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3. The other 22 pairs of chromosomes are autosomes
One of the chromosome pairs determines the sex of an individual (The sex chromosomes)
The other 22 pairs of chromosomes are autosomes
Autosomal chromosomes are numbered from smallest (#1) to largest (#22)
The sex chromosomes are numbered as the 23rd pair
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4. Sex Determination in Humans
Sex is determined in humans by allocation of chromosomes at fertilization
Both sperm and egg carry one of each of the 22 autosomes
The egg always carries the X chromosome as number 23
The sperm may carry either and X or Y
If the sperm donates an X in fertilization, the zygote will be female
If the sperm donates a Y in fertilization, the zygote will be male
Therefore, the sex of all humans is determined by the sperm donated by their father
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5. X-Linked Alleles
Genes carried on autosomes are said to be autosomally linked
Genes carried on the female sex chromosome (X) are said to be X-linked (or sex-linked)
X-linked genes have a different pattern of inheritance than autosomal genes have
The Y chromosome is blank for these genes
Recessive alleles on X chromosome:
Follow familiar dominant/recessive rules in females (XX)
Are always expressed in males (XY), whether dominant or recessive
Males said to be monozygous for X-linked genes
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6. Chromosomal Basis of Sex
XX female
Xy male
Short segments at either end of they Y chromosome are homologous to regions of the X.
These regions allow the X & Y to pair up during meiosis (in testes)
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7. Chromosomal Basis of Sex
In Humans
Anatomical signs of sex begin when an embryo is 2 months old
Hormone conditions determine the sex
Presence of Y chromosomes and the SRY gene triggers the making of testes (testosterone)
Absence of the SRY gene triggers the making of ovaries
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8. Sex Linked Genes Genes carried on the sex chromosomes
Females must be homozygous to be affected by recessive alleles
Heterozygotes and Homozygotes are do not exist for males…due to the y chromosome
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9. X Inactivation of Females
During embryonic development one X chromosome becomes inactive (Barr Body).
All mitotic divisions create cells with the same inactive X.
Males and females have equal dose of gene representation
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10. Genes in the Barr body are not expressed at this time
Barr Bodies reactivate in the ovaries
Ova are made so that all gametes receive the same number of active chromosomes
Barr Body development is random and independent
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11. Ex: multicolored tortoise shell in cats
Sex linked traits for a heterozygote female will express half of one allele and half of the other allele…depending on which is inactive.
Ex: multicolored tortoise shell in cats
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12. Chromosomal Basis of Inheritance
Thomas Hunt Morgan
Studied fruit flies (Drosophila melanogaster)
Studied specific genes related to specific chromosomes
He determined that the gene for eye color in fruit flies were carried on the X chromosome
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13. Eye Color in Fruit Flies
Fruit flies (Drosophila melanogaster) are common subjects for genetics research
They normally (wild-type) have red eyes
A mutant recessive allele of a gene on the X chromosome can cause white eyes
Possible combinations of genotype and phenotype:
Genotype
Phenotype
XRXR
Homozygous Dominant
Female
Red-eyed
XRXr
Heterozygous
XrXr
Homozygous Recessive
White-eyed
XRY
Monozygous Dominant
Male
XrY
Monozygous Recessive
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14. X-Linked Inheritance
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15. Gene Linkage
When several genes of interest exist on the same chromosome
Such genes form a linkage group
Tend to be inherited as a block
If all genes on same chromosome:
Gametes of parent likely to have exact allele combination as gamete of either grandparent
Independent assortment does not apply
If all genes on separate chromosomes:
Allele combinations of grandparent gametes will be shuffled in parental gametes
Independent assortment working
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16. Gene Linkage
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17. Linked Genes Morgan studied 2 traits on his fruit flies
Body color and wing shape
Wild type (gray body b+) mutant (black body b)
Wild type (normal wings vg+) mutant (short wings vg)
b+b vg+vg v. bb vg vg test cross
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18. Linkage Groups
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19. All Parental offspring Actual results:
Expected results:
All Parental offspring
Actual results:
The proportion of parental offspring vs. nonparental offspring was very high
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20. Genetic Recombination
Explains the nonparental type offspring.
Unlinked genes
50% frequency of recombination of unlinked genes
Due to law of segregation and independent assortment
Fig 15.2 and Fig in text.
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21. Linked Genes Linked genes
Crossing over of linked genes when tetrads form in meiosis I.
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22. Constructing a Chromosome Map
Crossing-over can disrupt a blocked allele pattern on a chromosome
Affected by distance between genetic loci
Consider three genes on one chromosome:
If one at one end, a second at the other and the third in the middle
Crossing over very likely to occur between loci
Allelic patterns of grandparents will likely to be disrupted in parental gametes with all allelic combinations possible
If the three genetic loci occur in close sequence on the chromosome
Crossing over very UNlikely to occur between loci
Allelic patterns of grandparents will likely to be preserved in parental gametes
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23. Rate at which allelic patterns are disrupted by crossing over:
Indicates distance between loci
Can be used to develop linkage map or genetic map of chromosome
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24. Crossing Over
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25. Complete vs. Incomplete Linkage
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26. Linkage Mapping
Genetic map based upon recombination frequencies of genes
1 map unit = 1% recombination frequency
The farther apart they are the higher the chance that crossing over occurs
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27. Recombination frequency can only have a value of up to 50%.
Anything at this point is indistinguishable from genes on separate chromosomes.
If the distance between genes is great enough on one chromosome, then linkage is not observed.
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28. Chromosome Number: Polyploidy
Occurs when eukaryotes have more than 2n chromosomes
Named according to number of complete sets of chromosomes
Major method of speciation in plants
Diploid egg of one species joins with diploid pollen of another species
Result is new tetraploid species that is self-fertile but isolated from both “parent” species
Some estimate 47% of flowering plants are polyploids
Often lethal in higher animals
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29. Chromosome Number: Aneuploidy
Monosomy (2n - 1)
Diploid individual has only one of a particular chromosome
Caused by failure of synapsed chromosomes to separate at Anaphase I (nondisjunction)
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30. Diploid individual has three of a particular chromosome
Trisomy (2n + 1) occurs when an individual has three of a particular type of chromosome
Diploid individual has three of a particular chromosome
Also caused by nondisjunction
This usually produces one monosomic daughter cell and one trisomic daughter cell in meiosis I
Down syndrome is trisomy 21
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31. Nondisjunction
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32. Trisomy 21
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33. Chromosome Number: Abnormal Sex Chromosome Number
Result of inheriting too many or too few X or Y chromosomes
Caused by nondisjunction during oogenesis or spermatogenesis
Turner Syndrome (XO)
Female with single X chromosome
Short, with broad chest and widely spaced nipples
Can be of normal intelligence and function with hormone therapy
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34. Chromosome Number: Abnormal Sex Chromosome Number
Klinefelter Syndrome (XXY)
Male with underdeveloped testes and prostate; some breast overdevelopment
Long arms and legs; large hands
Near normal intelligence unless XXXY, XXXXY, etc.
No matter how many X chromosomes, presence of Y renders individual male
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35. Turner and Klinefelter Syndromes
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36. Chromosome Number: Abnormal Sex Chromosome Number
Ploy-X females
XXX simply taller & thinner than usual
Some learning difficulties
Many menstruate regularly and are fertile
More than 3 Xs renders severe mental retardation
Jacob’s syndrome (XYY)
Tall, persistent acne, speech & reading problems
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37. Abnormal Chromosome Structure
Deletion
Missing segment of chromosome
Lost during breakage
Translocation
A segment from one chromosome moves to a non-homologous chromosome
Follows breakage of two nonhomologous chromosomes and improper re-assembly
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38. Deletion, Translocation, Duplication, and Inversion
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39. Abnormal Chromosome Structure
Duplication
A segment of a chromosome is repeated in the same chromosome
Inversion
Occurs as a result of two breaks in a chromosome
The internal segment is reversed before re-insertion
Genes occur in reverse order in inverted segment
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40. Inversion Leading to Duplication and Deletion
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41. Abnormal Chromosome Structure
Deletion Syndromes
Williams syndrome - Loss of segment of chromosome 7
Cri du chat syndrome (cat’s cry) - Loss of segment of chromosome 5
Translocations
Alagille syndrome
Some cancers
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42. Williams Syndrome
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43. Alagille Syndrome
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