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Chapter 14 Mendel and the Gene Idea:
Gregor Mendel - monk who discovered…
Chapter 14 Mendel and the Gene Idea:
- Gregor Mendel - monk who discovered the basic principles of heredity by breeding garden peas
- Experimental, Quantitative Approach: he tracked traits for at least the P, F1 and F2 generations
Identifying laws of inheritance
- 2 fundamental principles of heredity
- Law of Segregation: the two alleles for a heritable character segregate (in others, separate from each other) during gamete formation and end up in different gametes
- Law of Independent Assortment: states that two or more genes assort independently-that is, each pair of alleles segregates independently of any other pair of alleles - during gamete formation. This law only applies to genes (allele pairs) located on different chromosomes
Mendel's law of segregation
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Probability laws that govern Mendelian inheritance
- reflect the same rules of probability, meaning the outcome of any particular toss is unaffected by what has happened on previous trails
Multiplication rule: states that to determine the probability of one event and the other occurring, we multiply the probability of one event by the probability of the other event.
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Addition Rule: the probability that any one of two or more mutually exclusive (one event or the other) will occur is calculated by adding their individual probabilities.
Complexity of Inheritance patterns
- Complete dominance: F1 offspring that always looks like one of the two parental varieties because one allele in a pair is dominant and the phenotype of the heterozygote and the dominant homozygote is indistinguishable
- Incomplete dominance: when neither allele of a pair of gene is completely dominant, the phenotype is somewhere between the two parental varieties .
- codominance: another variation on dominance relationships between alleles; in this variation, the two alleles each affect the phenotype in separate, distinguishable ways
- Pleiotropy: most genes have multiple phenotypic effects, Greek pleion, meaning more.
- Epistasis: phenotypic expression of a gene at one locus alters that a gene at a second locus, Greek for standing upon.
- Quantitative Characters: Characters that vary in population gradations along a continuum
- Polygenic inheritance: quantitative variation, an additive effect of two or more genes on a single phenotypic character.
- Multifactorial: many factors, both genetic and environmental, collectively influence phenotype
Incomplete Dominance
Tay-Sachs disease:
-an inherited disorder in humans, where the brain cells of a child with the disease cannot metabolize certain lipids because a crucial enzyme does not work properly. As lipids accumulate in brain cells, the child begins to suffer seizures, blindness, and degeneration of motor and mental performance and dies
Blood Type Alleles:
- An individual's blood cells may have carbohydrate 's A, B, AB, or O. (Type A Blood, Type B Blood, Type AB Blood, or neither Type O)
- In addition, blood groups MN system, who have alleles for both M and N show codominance for the M and N red blood cell surface antigens
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Epistasis
Human Traits follow Mendelian patterns of inheritance
- Pedigree: the collection of information about a family's history for a particular trait and assemble this information into a family tree describing the trait across the generations.
- Carriers: a recessively inherited disorder shows up only in the homozygous individuals (aa) who inherit a recessive allele from each parent, and heterozygotes may transmit the recessive allele to their offspring
Recessive Inherited Disorders
EX: PTC (phenylthiocarbamide) a recessive trait, the inability of individuals to taste a specific chemical
Ex: Cystic fibrosis: lethal genetic disease, the normal membrane protein that functions in the transport of chloride ions between certain cells and the extracellular fluid is defective or absent in the plasma membranes of children who inherit two recessive alleles for cystic fibrosis. this causes chronic bronchitis, and recurrent bacterial infections
Ex: Sickle-cell disease, caused by a single amino acid in the hemoglobin protein of red blood cells; in homozygous individuals all hemoglobin is sickle-cell,
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Dominant Inherited Disorders
Ex: Achondroplasia: a form of dwarfism, dominant allele, and homozygous recessive.
Ex: Huntington's Disease: caused by a lethal dominant allele, first appear after reproductive age, that has no obvious phenotypic effect until the 35 to 45 years old. Once it begins it is irreversible and inevitably fatal.
Fetal Testing
- Amniocentesis: one diagnostic test that can determine whether the developing fetus has a serious recessive disease
- Chorionic villus sampling (CVS): a physician inserts a narrow tube through the cervix into the uterus and suctions out a tiny sample of tissue from the placenta, the organ that transmits nutrients and fetal wastes between the fetus and the mother.
Definitions 1
- Character (Gene): A heritable feature/factor that varies among individuals. For each character, an organism inherits two versions (that is, two alleles) of a gene, one from each parent. EX: flower color
- Trait (Allele): a variant for a character. Alternative versions of a gene EX: such as purple or white color of flower
- true-breeding: over many generations of self-pollination will always produce the same trait of a particular character
- Hybridization: mating, or crossing, of two true-breeding varieties.
- P generation (parental generation): true-breeding parents.
- F1 generation (first filial generation): filial from the Latin word for "son", the hybrid offspring.
- F2 generation (second filial generation): production from allowing these F1 hybrids to self-pollinate (or to cross-pollinate with other F1 hybrids)
- Dominant allele/trait: determines the organism's appearance, it is the SEEN phenotype not because it somehow subdues a recessive allele
- Recessive allele/trait: has no noticeable effect on the organism's appearance
- Punnett square: a handy diagrammatic device for predicting the allele composition of offspring from a cross between individuals of known genetic makeup
- homozygote: an organism that has a pair of identical alleles for a gene encoding a character
- homozygous:
- Heterozygote: an organism that has two different alleles for a gene, produce gametes with different alleles, so they are not true-breeding
- heterozygous:
- Phenotype: we distinguish between an organism's appearance or observable traits, also includes physiological traits. EX: PP and Pp
- Genotype: the genetic makeup of an organism
- testcross: breeding an organism of unknown genotype with a recessive homozygote because it can reveal the genotype of that organism
Peas: Stamens- pollen producing organs, Carpel- egg-bearing organ
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Definitions 2
- Monohybrids: the F1 progeny produced in his crosses of true-breeding parents, meaning that they are heterozygous for the one particular character being followed in the cross
- Monohybrid cross: the cross between F1 progeny heterozygotes.
- Dihybrids: individuals heterozygous for the two characters being followed in the cross (YyRr)
- Dihybrid cross: a cross between F1 dihybrids, which helpds determine which of two hypotheses is correct
- Wild type: the phenotype for a character most commonly observed in natural populations
- Mutant Phenotypes: the alternatives to the wild type, due to alleles assumed to have originated as changes, or mutations, in the wild-type allele
Chapter 15
Chromosomal Basis of Inheritance
- Chromosome theory of inheritance: in 1902, Walter Sutton and Theodor Boveri developed this theory, Mendelian genes have specific loci(sites) along chromosomes, and it is the chromosomes that undergo segregation and independent assortment.
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Patterns of Inheritance from Sex-linked genes
- Sex-linked gene: a gene located on either chromosome, a single gene on a sex chromosome
- X-linked genes: Human X chromosome contains approximately 1,100 genes
- Y-linked genes: Human Y chromosome, passed along virtually intact from a father to all his sons, very few disorders are transferred from father to son
- Intersex: chromosomes born with intermediate sexual characteristics
- transgender: anatomical features that do not match an individual's sense of their own gender
X-Linked Disorders
- Duchenne muscular dystrophy: a disease characterized by a progressive weaking of the muscles and loss of coordination
- Hemophilia: X-linked recessive disorder defined by the absence of one or more of the proteins required for blood clotting, causing prolonged bleeding because firm clot is slow to form
- Barr body - Inactive X in each cell of a female condenses into a compact object, that give rise to eggs, resulting in every female gamete (egg) having an active X after meiosis.
Genetic disorders
- caused by alterations of chromosome numbers or structure
- nondisjunction: in which the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II. One gamete receives two of the same type of chromosome and another gamete receives no copy
- aneuploidy: when either of the aberrant gametes unites with a normal one at fertilization, the zygote will also have an abnormal number of a particular chromosome.
- Monosomic: the aneuploid zygote; fertilization involving a gamete that has no copy of a particular chromosome will lead to a missing chromosome in the zygote (so that the cell has 2n-1 chromosomes)
- Trisomic: aneuploid cell; if a chromosome is present in triplicate in the zygote (so that the cell has 2n+1 chromosomes)
- polyploidy: the term for chromosomal alteration; the specific terms triploidy (3n) and tetraploidy (4n) indicate three and four chromosomal sets
Meiotic Nondisjunction
Alterations of Chromosome Structure
- Deletion: occurs when a chromosomal fragment is lost; the affected chromosome is missing certain genes
- Duplication: a broken fragment may become reattached as an extra segment to a sister or nonsister chromatid
- Inversion: a chromosomal fragment may also reattach to the original chromosome but in the reverse orientation
- Translocation: a rearrangement, the fourth possible result of chromosomal breakage, the fragment to join a nonhomologous chromosome
Deletion & Duplication
Inversion &
Translocation
Examples
Down Syndrome (Trisomy 21) - one aneuploid condition, Down syndrome, result of an extra chromosome 21, so that each body cell has a total of 47 chromosomes.
- Klinefelter syndrome: have male sex organs, but their testes are small and produce little or no sperm, symptoms include taller than average height, less muscle mass, and enlarged breast tissue, (XYY)
- Cri du chat (Cry of the cat): results from a specific deletion in chromosome 5, they have a small head with unusual facial features, severe intellectual disabilities, and a cry that sounds like the mewing of a distressed cat
- CML (chronic myelogenous leukemia): chromosomal translocations occuring during mitosis, when a reciprocal translocation happens during mitosis of cells that are precursors of white blood cells.
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Exceptions to Standard Inheritance patterns
- genomic imprinting: variation in phenotype depending on whether an allele is inherited from the male or female parent
Extranuclear genes or cytoplasmic genes: eukaryotic cell's genes that are located outside the nucleus; these cells reproduce themselves and transmit their genes to daughter organelles.
- Ex: mitochondrial mutations inherited from a persons mom may contribute to at least some types of diabetes and heart disease, even Alzheimer's disease.
Mitochondrial myopathy: causes weakness, intolerance of exercise, and muscle deterioration.
Leber's hereditary optic neuropathy: which can produce sudden blindness in people as young as their 20's and 30's; the four mutations cause this disorder affect oxidative phosphorylation during cellular respiration, a crucial function for the cell.
Genomic Imprinting: variations in a phenotype depending on whether an allele us unherited from the male or female parent, and occurs during gamete formation and results in the silencing of a particular allele of certain genes. Affects embryonic development.
Linked genes inherited due to location
- Linked genes: genes located near each other on the same chromosome tend to be inherited together in genetic crosses, and are genetically linked, two or more genes on the same chromosome that tend to be inherited together
- Genetic recombination: the production of offspring with combinations of traits that differ from those found in either P generation parent
- parental types: short for phenotypes, offspring inherit a phenotype that matches either of the phenotypes of the P generation
- recombinant types (recombinants): two nonparental phenotypes are found among offspring, the offspring have new combinations.
- crossing over: accounts for the recombination of linked genes, occurs while replicated homologous chromosomes are paired during prophase of meiosis I, a set of proteins breaks the DNA molecules of one maternal and one paternal chromatid and rejoins each to the other.
Genetic Map: an ordered list of the genetic loci along a particular chromosome
Recombination frequency: the percentage of recombinant offspring, dependent on the distance between genes on a chromosome.
Chromosomal basis for recombination of linked genes
- Linkage Map: a genetic map based on recombination frequencies
- map units: distance between genes
- cytogenetic maps: which locate genes with respect to chromosomal features that can be seen in the microscope
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
The chromosomal basis of Mendel's laws