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Questions and Answers
What is the phenomenon where the phenotypic effects of some mammalian genes depend on their parent of origin?
What is the assumption for most genes regarding their phenotypic effects?
Which chromosome is associated with Prader-Willi syndrome and Angelman syndrome?
What is the characteristic of individuals with Prader-Willi syndrome?
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What is the main difference between Prader-Willi syndrome and Angelman syndrome?
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What happens to the imprinting status of a gene in gamete-producing cells?
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What determines the imprinting status of a gene?
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What is the result of genomic imprinting on the same allele?
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What is the process where one allele is silenced and the other allele is expressed?
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What happens to genes when methyl groups are added to cytosine nucleotides on one of the alleles?
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What is the result of genomic imprinting?
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How many mammalian genes are subject to imprinting?
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What is the disorder associated with genomic imprinting that leads to mental retardation?
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What is the characteristic of the abnormal X chromosome in Fragile X syndrome?
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Who first observed cytoplasmic genes in plants?
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Where are extranuclear genes located?
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What is the inheritance pattern of extranuclear genes?
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What determines the coloration of offspring in cytoplasmic genes?
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What is the frequency of Fragile X syndrome in males?
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Study Notes
Genetic Mapping
- Geneticists use recombination data to map a chromosome's genetic loci, creating an ordered list of genes along a particular chromosome.
- Alfred Sturtevant developed a method for constructing a genetic map by using crossing over of linked genes.
- The frequency of recombinant offspring reflects the distances between genes on a chromosome.
- The greater the distance between two genes, the higher the probability of a crossover and therefore a higher recombination frequency.
Sturtevant's Mapping Method
- Sturtevant used the testcross design to map the relative position of three fruit fly genes: body color (b), wing size (vg), and eye color (cn).
- The recombination frequencies between cn and b, cn and vg, and b and vg were used to determine the order of the genes.
- The arrangement of the genes places the eye color gene between the other two.
Recombination Frequency
- One map unit (or centimorgan) is equivalent to a 1% recombination frequency.
- The recombination frequency is not always additive due to multiple crossing over events.
Linked Genes
- Linked genes tend to be inherited together because they are located on the same chromosome.
- Each chromosome has hundreds or thousands of genes.
- Genes located on the same chromosome are passed along as a unit, resulting in deviations from independent assortment.
Morgan's Observations
- Morgan observed the inheritance of body color and wing size in fruit flies.
- He crossed F1 heterozygous females with homozygous recessive males.
- The results showed a large number of wild-type and double-mutant flies, indicating that the genes are linked.
Chromosome Mapping
- Genes located far apart on a chromosome are mapped by adding the recombination frequencies between the distant genes and intervening genes.
- Sturtevant and his colleagues were able to map the linear positions of genes in Drosophila into four groups, one for each chromosome.
Limitations of Linkage Maps
- A linkage map provides an imperfect picture of a chromosome.
- Map units indicate relative distance and order, not precise locations of genes.
- The frequency of crossing over is not uniform over the length of a chromosome.
Chromosomal Alterations
- Several serious human disorders are due to alterations of chromosome number and structure.
- Aneuploid zygotes may have an imbalance of gene products, leading to developmental problems.
- Certain aneuploid conditions, such as Down syndrome, can lead to survival to birth and beyond.
Down Syndrome
- Down syndrome is caused by three copies of chromosome 21.
- It affects one in 700 children born in the United States.
- The condition severely alters an individual's phenotype in specific ways.
Nondisjunction
- Most cases of Down syndrome result from nondisjunction during gamete production in one parent.
- The frequency of Down syndrome correlates with the age of the mother.
Genomic Imprinting
- Genomic imprinting is a process where a gene on one homologous chromosome is silenced, while its allele on the homologous chromosome is expressed.
- The imprinting status of a gene depends on whether it resides in a female or a male.
- The same alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.
Imprinting Disorders
- Prader-Willi syndrome and Angelman syndrome are due to the same cause, a deletion of a specific segment of chromosome 15.
- The difference between the disorders is due to genomic imprinting.
- Prader-Willi syndrome is inherited from the father, while Angelman syndrome is inherited from the mother.
Linked Genes
- Linked genes tend to be inherited together because they are located on the same chromosome.
- Each chromosome has hundreds or thousands of genes.
- Genes located on the same chromosome, linked genes, tend to be inherited together because the chromosome is passed along as a unit.
Morgan's Observation
- Morgan observed linkage and its deviations when he followed the inheritance of characters for body color and wing size.
- The wild-type body color is gray (b+) and the mutant black (b).
- The wild-type wing size is normal (vg+) and the mutant has vestigial wings (vg).
Deviation from Independent Assortment
- According to independent assortment, this should produce 4 phenotypes in a 1:1:1:1 ratio.
- Morgan observed a large number of wild-type (gray-normal) and double-mutant (black-vestigial) flies among the offspring.
- These phenotypes correspond to those of the parents.
Linkage and Crossing Over
- Morgan reasoned that body color and wing shape are usually inherited together because their genes are on the same chromosome.
- The other two phenotypes (gray-vestigial and black-normal) were fewer than expected from independent assortment.
- A second crossing over “cancels out” the first and reduces the observed number of recombinant offspring.
- Genes far apart (for example, b-vg) are more likely to experience multiple crossing over events.
Genes on the Same Chromosome
- Some genes on a chromosome are so far apart that a crossover between them is virtually certain.
- In this case, the frequency of recombination reaches its maximum value of 50% and the genes act as if found on separate chromosomes and are inherited independently.
Mapping Genes
- Genes located far apart on a chromosome are mapped by adding the recombination frequencies between the distant genes and intervening genes.
- Sturtevant and his colleagues were able to map the linear positions of genes in Drosophila into four groups, one for each chromosome.
Linkage Maps
- A linkage map provides an imperfect picture of a chromosome.
- Map units indicate relative distance and order, not precise locations of genes.
- The frequency of crossing over is not actually uniform over the length of a chromosome.
Sex-Linked Genes
- Sex-linked genes have unique patterns of inheritance.
- In addition to their role in determining sex, the sex chromosomes, especially the X chromosome, have genes for many characters unrelated to sex.
- Sex-linked genes follow the same pattern of inheritance as the white-eye locus in Drosophila.
Sex-Linked Recessive Disorders
- If a sex-linked trait is due to a recessive allele, a female will have this phenotype only if homozygous.
- Heterozygous females will be carriers.
- Because males have only one X chromosome (hemizygous), any male receiving the recessive allele from his mother will express the trait.
- The chance of a female inheriting a double dose of the mutant allele is much less than the chance of a male inheriting a single dose.
Human Sex-Linked Disorders
- Duchenne muscular dystrophy affects one in 3,500 males born in the United States.
- Affected individuals rarely live past their early 20s.
- This disorder is due to the absence of an X-linked gene for a key muscle protein, called dystrophin.
Alterations of Chromosome Number or Structure
- Nondisjunction occurs when problems with the meiotic spindle cause errors in daughter cells.
- This may occur if tetrad chromosomes do not separate properly during meiosis I.
- Alternatively, sister chromatids may fail to separate during meiosis II.
Aneuploidy
- As a consequence of nondisjunction, some gametes receive two of the same type of chromosome and another gamete receives no copy.
- Offspring resulting from fertilization of a normal gamete with one after nondisjunction will have an abnormal chromosome number or aneuploidy.
- Trisomic cells have three copies of a particular chromosome type and have 2n + 1 total chromosomes.
- Monosomic cells have only one copy of a particular chromosome type and have 2n - 1 chromosomes.
Polyploidy
- Organisms with more than two complete sets of chromosomes have undergone polyploidy.
- This may occur when a normal gamete fertilizes another gamete in which there has been nondisjunction of all its chromosomes.
- The resulting zygote would be triploid (3n).
- Alternatively, if a 2n zygote failed to divide after replicating its chromosomes, a tetraploid (4n) embryo would result from subsequent successful cycles of mitosis.
Genomic Imprinting
- In many cases, genomic imprinting occurs when methyl groups are added to cytosine nucleotides on one of the alleles.
- Heavily methylated genes are usually inactive.
- The animal uses the allele that is not imprinted.
- In other cases, the absence of methylation in the vicinity of a gene plays a role in silencing it.
Fragile X Syndrome
- Fragile X syndrome, which leads to various degrees of mental retardation, also appears to be subject to genomic imprinting.
- This disorder is named for an abnormal X chromosome in which the tip hangs on by a thin thread of DNA.
- Inheritance of fragile X is complex, but the syndrome is more common when the abnormal chromosome is inherited from the mother.
Extranuclear Genes
- Not all of a eukaryote cell's genes are located in the nucleus.
- Extranuclear genes are found on small circles of DNA in mitochondria and chloroplasts.
- These organelles reproduce themselves.
- Their cytoplasmic genes do not display Mendelian inheritance.
- They are not distributed to offspring during meiosis.
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Description
Learn how geneticists use recombination data to map genetic loci on a chromosome and how Alfred Sturtevant developed a method for constructing a genetic map. Understand the concept of linkage analysis and its applications.
