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level: Level 1

Questions and Answers List

level questions: Level 1

QuestionAnswer
Relationship between genes and chromosomesGenes are located on chromosomes [REFERENCE PICTURE]
Sutton and Boveri? Chromosome theory of inheritance?- Mendel’s proposed “hereditary units” were only theoretical in 1860 - Soon, biologists saw parallels between chromosome behavior and the behavior of the proposed factors - Around 1902, Sutton and Boveri and others independently noted these parallels and began to develop the chromosome theory of inheritance
Thomas Hunt Morgan- The first solid evidence associating a specific gene with a specific chromosome came in the early 1900s from the work of Thomas Hunt Morgan - His early experiments provided convincing evidence that the chromosomes are the location of Mendel’s heritable factors - For his work, Morgan chose to study Drosophila melanogaster, a common species of fruit fly
Characteristics that make fruit flies a convenient organism for genetic studies- They produce many offspring ​ - A generation can be bred every two weeks​ - They have only four pairs of chromosomes
Wild typeMorgan noted wild type, or normal, phenotypes that were common in the fly populations
mutant phenotypes​- Traits alternative to the wild type are called mutant phenotypes​ - The first mutant Morgan discovered was a fly with white eyes instead of the wild-type red eyes
Generations within Morgan's experiment- In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)​ - The F1 generation all had red eyes​ - The F2 generation showed a 3:1 red to white eye ratio, but only males had white eyes​
Morgan's reasoning on why the generations ended up the way they were- Morgan reasoned that the white-eyed mutant allele must be located on the X chromosome​ - Female flies have two X chromosomes (XX) while males have one X and one Y (XY)​ - Morgan’s finding supported the chromosome theory of inheritance​
Why was Morgan's discovery important?Morgan’s discovery of a trait that correlated with the sex of flies was key to the development of the chromosome theory of inheritance​
Sex chromosomes- Humans and other mammals have two types of sex chromosomes: a larger X chromosome and a smaller Y chromosome​ - A person with two X chromosomes usually develops anatomy we associate with the “female” sex​ - “Male” properties are associated with the inheritance of one X and one Y​ - The X-Y system is not the only chromosomal system of sex determination​
How certain chromosomes lead to the development of certain things? SRY?- Short segments at the ends of the Y chromosomes are homologous with the X, allowing the two to behave like homologs during meiosis in males​ - In mammals, a gene on the Y chromosome called SRY (sex-determining region on the Y) is responsible for development of the testes in an embryo​
Sex-linked gene? Y-linked genes? X-Linked genes?- A gene that is located on either sex chromosome is called a sex-linked gene​ - Genes on the Y chromosome are called Y-linked genes​ - Many Y-linked genes are related to sex determination​ - Only 78 genes, coding for about 25 proteins, have been identified on the human Y chromosome​ - Genes on the X chromosome are called X-linked genes; the human X chromosome contains about 1,100 genes​ - X chromosomes have genes for many characters unrelated to sex
Pattern of inheritance for X-linked genes- For a recessive X-linked trait to be expressed: - a female needs two copies of the allele (homozygous) - a male needs only one copy of the allele (hemizygous) - X-linked recessive disorders are much more common in males than in females
Some disorders caused by recessive alleles on the X-Chromosomes in Humans- Color blindness (mostly X-linked)​ - Duchenne muscular dystrophy​ - Hemophilia​
Barr body​? Mosaic?- In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development ​ - The inactive X condenses into a Barr body​ - If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character​
Inactivation of an X chromosome- Inactivation of an X chromosome involves modification of the DNA and proteins bound to it called histones​ - A part of the chromosome contains several genes involved in the inactivation process​ - One of the genes there becomes active only on the chromosome that will be inactivated​ - The gene is called XIST (X-inactive specific transcript)​
Linked genes- Each chromosome has hundreds or thousands of genes (except the Y chromosome) - Genes that are located on the same chromosome tend to be inherited together and are called linked genes
Morgan's experiments with linkage- Morgan did experiments with fruit flies to see how linkage affects inheritance of two characters​ - Morgan crossed flies that differed in traits of body color and wing size​ - The first cross was a P generation cross to generate F1 dihybrid flies​ - The second was a testcross​
Results from Morgan's experiments with linkage- The resulting flies had a much higher than expected proportion of the combination of traits seen in the P generation flies (parental phenotypes)​ - He concluded that these genes do not assort independently and reasoned that they were on the same chromosome​
Genetic recombination- Nonparental phenotypes were also produced in the testcross, suggesting that the two traits could be separated sometimes​ - This involves genetic recombination, the production of offspring with combinations of traits differing from either parent​ - A 50% frequency of recombination is observed for any two genes on different chromosomes​ - The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination​
Parental typesOffspring with a phenotype matching one of the parental (P) phenotypes are called parental types​
Recombinant types or RecombinantsOffspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants​
Crossing over- Morgan observed that although some genes are linked, nonparental allele combinations are still produced​ - He proposed that some process must occasionally break the physical connection between genes on the same chromosome​ - That mechanism was the crossing over of homologous chromosomes​
Genetic variation- Recombinant chromosomes bring alleles together in new combinations in gametes​ - Random fertilization increases even further the number of variant combinations that can be produced - This abundance of genetic variation is the raw material upon which natural selection works
Alfred Sturtevant? Genetic map?- Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome​ - Sturtevant predicted that "the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency​"
Linkage mapA linkage map is a genetic map of a chromosome based on recombination frequencies​
Map units- Distances between genes can be expressed as map units; one map unit represents a 1% recombination frequency​ - Map units indicate relative distance and order, not precise locations of genes​
Recombinant frequency and Gene locations- Genes that are far apart on the same chromosome can have a recombination frequency near 50%​ - Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes​
How were recombinant frequencies and linkage maps used?- Sturtevant used recombination frequencies to make linkage maps of fruit fly genes​ - They found that the genes clustered into four groups of linked genes (linkage groups)​ - The linkage maps, combined with the fact that there are four chromosomes in Drosophila, provided additional evidence that genes are located on chromosomes​
What leads to miscarriages- Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders​ - Plants tolerate such genetic changes better than animals do​
Nondisjunction- In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis​ - As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy​
Aneuploidy- Aneuploidy results from the fertilization of gametes in which nondisjunction occurred​ - Offspring with this condition have an abnormal number of a particular chromosome​
Monosomic zygoteA monosomic zygote has only one copy of a particular chromosome​
Trisomic zygoteA trisomic zygote has three copies of a particular chromosome​
Polyploidy- Polyploidy is a condition in which an organism has more than two complete sets of chromosomes​ - Triploidy (3n) is three sets of chromosomes​ - Tetraploidy (4n) is four sets of chromosomes​ - Polyploidy is common in plants, but not animals​ - Polyploids are more normal in appearance than aneuploids​
Breakage of a chromosome leads to four types of changes in chromosome structure:- Deletion removes a chromosomal fragment​ - Duplication repeats a segment​ - Inversion reverses orientation of a segment within a chromosome​ - Translocation moves a segment from one chromosome to another​
Human Disorders Due to Chromosomal Alterations- Alterations of chromosome number and structure are associated with some serious disorders​ - Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond​ - These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy​
Down Syndrome (Trisomy 21)- Down syndrome is an aneuploid condition that results from three copies of chromosome 21​ - It affects about one out of every 830 children born in the United States​ - The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained​
Aneuploidy of Sex Chromosomes​- Nondisjunction of sex chromosomes produces a variety of aneuploid conditions​ - Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals​ - About one in 1,000 males is XYY; these males do not exhibit any syndrome​ - XXX females occur with a frequency of about one in 1,000​ - They are healthy, with no unusual physical features, though they are at risk for learning disabilities​ - Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans ​
Disorders Caused by Structurally Altered Chromosomes​- The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5​ - A child born with this syndrome is severely intellectually disabled and has a catlike cry; individuals usually die in infancy or early childhood​ - Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations​ of chromosomes​
Exceptions to Mendelian genetics- There are two normally occurring exceptions to Mendelian genetics​ - One exception involves genes located in the nucleus, and the other involves genes located outside the nucleus​ - In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance​
Genomic imprinting- For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits​ - Such variation in phenotype is called genomic imprinting​ - Genomic imprinting involves the silencing of certain genes depending on which parent passes them on​ - Most imprinted genes are on autosomes​ - The mouse gene for insulin-like growth factor 2 (Igf2) was one of the first imprinted genes to be identified​ - Only the paternal allele of this gene is expressed​
What causes imprinting?- It seems that imprinting is the result of the methylation (addition of —CH3 groups) of cysteine nucleotides​ - Genomic imprinting may affect only a small fraction of mammalian genes​ - Most imprinted genes are critical for embryonic development​
Extranuclear genes (or cytoplasmic genes)- Extranuclear genes (or cytoplasmic genes) are found in organelles in the cytoplasm​ - Mitochondria, as well as chloroplasts, and other plant plastids carry small circular DNA molecules​ - Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg​ - The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant​
Results of defects in mitochondrial genes- Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems ​ - For example, mitochondrial myopathy and Leber’s hereditary optic neuropathy​
Possibilities in avoiding passing along mitochondrial disorders- It may be possible to avoid passing along mitochondrial disorders​ - The chromosomes from the egg of an affected mother could be transferred to an egg of a healthy donor, generating a “two-mother” egg​ - This egg could then be fertilized by sperm from the prospective father and transplanted to the womb of the prospective mother​