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Ch. 14 (Mendel's Experiment (Law of Segregation (Mendel's Model…
Ch. 14
Mendel's Experiment
Mendel chose to experiment with peas is because there are a variety. One variety has purple flowers, while another has white flowers.
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Each variant for a character, such as purple or white flowers, is called a trait.
Another reason for using peas is their short generation time and large number of offspring from each mating.
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To achieve cross-pollination of the plants, Mendel removed the immature stamens of a plant before they produce pollen and then dusted pollen from another plant too the altered flowers.
Mendel chose to track only those characters that occurred in alternate forms, such as flower color.
He made sure that he started his experiments that were true-breeding, referring to organisms that produce offspring of the same variety over many generations of self-pollination.
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The true-breeding parents are referred to as the P generation and their hybrid offspring are the F1 generation.
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Law of Segregation
If the blending model of inheritance was correct, F1 hybrids form a cross between purple and white flower pea plants would have pale purple flowers, a trait intermediate between those of P generation.
In the figure above, the F1 offspring had flowers of the same color as the purple flowered parents.
Mendel allowed the F1 plants to self or cross-pollinate and planted their seeds, the white flower reappeared in the F2 generation.
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Mendel reasoned that the heritable factor for white flowers didn't disappear in the F1 plants but hid when purple flowers factor was present.
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Mendel's Model
Mendel developed a model to explain the 3:1 inheritance pattern that he observed among the F2 offspring in his pea experiments.
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First, alternative versions of genes account for variations in inherited characters.
The gene flower color in pea plants exists in two versions, purple and white flowers. The alternative versions of a gene are called alleles.
Second, for each character, an organism inherits two copies of a gene, one from each parent.
Each somatic cell has two sets of chromosomes, one set from each parent.
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Third, if the two ales at a locus differ, the one, the dominant allele, determines the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance.
According to Mendel, F1 plants had purple flowers because the allele for the trait was dominant and for white flowers it was recessive.
Fourth and final part, the law of segregation, says that two alleles for a heritable character segregate during gamete formation and end up in different games.
The segregation corresponds to the distribution of copies of the two members of a pair of homologous chromosomes to different gametes in meiosis.
An egg or a sperm gets only one of the two alleles that are present in the somatic cells of the organism making the gamete.
If an organism has identical alleles for a particular character then the allele is present in all gametes.
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A pungent square is a handy diagrammatic device for predicting the allele composition of offspring from a cross between individuals of known genetic makeup.
Genetic Vocabulary
An organism that has a pair of identical alleles for a gene encoding a character is called a homozygote and is said to be homozygous for that gene.
An organism that has two different alleles for a gene is called a heterozygote and is said to be heterozygous for that gene.
Heterozygotes produce gametes with different alleles, so they are not true-breeding.
We distinguish between an organisms appearance of observable traits, called phenotype and its genetic makeup, its genotype.
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Breeding an organism of unknown genotype with a recessive homozygote is called a testcross because it can reveal the genotype of that organism.
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Polygenic Inheritance
Mendel studied characters that could be classified on an either or basis, but human skin color and height are not one of two discrete characters, but instead vary in the population in gradations along a continuum.
Quantitative characters indicates polygenic inheritance, an additive effect of two or more genes on a single phenotypic character.
Height is a good example of polygenic inheritance: in a study of 25,000 people found that genetic variations associated with genes that affect height.
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For example, if an AABBCC person would have dark skin and aabbcc would indicate light skin.
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Because the alleles have cumulative effect, the genotypes AaBbCc and AABbcc would make the same genetic contribution to skin darkness.
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Epistasis
In epistasis, the phenotypic expression of a gene at one locus alters that of a gene at a second locus.
For a Labrador retriever to have brown fur, its genotype must be bb; these dogs are called chocolate labs.
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The dominant allele, E, results in the deposition of either black or brown pigment, depending on the genotype at the first locus.
If the lab is homozygous recessive for the second locus (ee), then the coat is yellow, regardless of the genotype at the black/brown locus.
The gene for pigment deposition (E/e) is said to be epistatic to the gene that codes for black and brown pigment (B/b).
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Pleiotropy
Most genes have multiple phenotype effects, a property called pleiotropy.
In humans, pleiotropic alleles are responsible for the multiple symptoms associated with certain hereditary diseases, such as cystic fibrosis and sickle-cell disease.
Pedigree Analysis
Unable to manipulate the mating patterns of people, geneticists analyze the results of matings that have already occurred by collecting information about a family's history for a particular trait and assembling this information into a family tree describing the traits of parents and children across generations. This is known as a family pedigree.
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The figure above shows a three generation pedigree that traces the occurrence of a widow's peak on the hairline.
The widow's peak is due to a dominant allele, W. Because the widow's peak allele is dominant, al individuals who lack a widow's peak yet be homozygous recessive (ww).
The grandparents with widow's peak must have the Ww genotype, since some of their offspring are homozygous recessive.
The offspring in the second generation who do have the peaks must be heterozygous because they are the products of Ww x ww matings.
The third generation in this pedigree consists of two sisters. The one with the peak could be homozygous (WW) or heterozygous (Ww).
Recessive Alleles
An allele that causes a genetic disorder codes for either a malfunctioning proton or no protein at all.
Disorders classified as recessive, heterozygotes typically have the normal phenotype because one copy of the normal allele produces a sufficient amount of the specific protein.
Recessively inherited disorders shows up only in the homozygous individuals who inherit a recessive allele from each parent.
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As mentioned before, Tay-Sachs is high among the Jewish people whose ancestors lived in Central Europe.
When a dies-causing recessive allele is rare, it is relatively unlikely that two carriers of the same harmful allele will meet and mate.
Same blood matings are more likely to produce offspring homozygous for recessive traits, including the harmful ones.
Cystic Fibrosis
The most common lethal genetic disease in the US is cystic fibrosis, which occurs to people of European descendants.
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The normal allele for this gene codes for a membrane protein that functions in the transport of chloride ions between certain cells and the extracellular fluid.
The chloride transport channels are defective or absent in the plasma membranes of children who inherit two recessive alleles for cystic fibrosis.
The result in an abnormally high concentration of intracellular chloride, which causes an uptake of water due to osmosis.
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The mucus builds up in the pancreas, lungs, digestive tract and other organs.
Sickle-Cell Disease
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Sickle-cell is caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells; in homozygous individuals, hemoglobin is of the sickle-cell variety.
When the oxygen content of an affected individual's blood is low, the sickle-cell hemoglobin aggregate into long fibers that defer the red cells into a sickle shape.
Sickled cells may clump and clog small blood vessels, often leading to other symptoms throughout the body, including physical weakness, pain, organisms damage and even stroke.
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Although two sickle cell alleles are necessary for an individual to manifest full-blown sickle-cell disease and the condition is considered a recessive one, the presence of one sickle-cell allele can affect the phenotype.
At the organism level, the normal allele is incompletely dominant to the sickle-cell allele.
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