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Chapter 14 (((((((((((Useful Genetic Vocabulary, homozygote - an organism…
Chapter 14
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- alternative versions of genes account for variations in inherited characters
the gene for flower color in pea plants, for example, exists in two versions: one for purple flowers and one for white flowersthese alternate versions of a gene are called an allele
- for each character, an organism inherits two copies (that is, two alleles) of a gene, one from each parent
- if the two alleles at a locus differ, then one (the dominant allele) determines the organism's appearance; the other (the recessive allele) has no noticeable effect on the organism's appearance
4. the law of segregation states that two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes
thus 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
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F2 generation - second filial generation (produces from two F1 generations to cross pollinate with other F1 generations or self pollinate
all of his experiments were started with varieties that were true-breeding , over any generations of self-pollination these plants have produced only the same variety as the parent plant
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each pea flower has both pollen-producing organs (stamens) and an egg-bearing organ (carpel)
in nature, peas usually self-fertilize: pollen grains from the stamens land on the carpel of the same flower and the sperm released from the pollen grains fertilize eggs present in the carpel
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he removed the immature stamens of a plant before they produced pollen then dusted pollen from another plant onto the altered flowers
then, each resulting zygote then developed into a plant embryo encased in a seed (pea)
this allowed the knowledge of the exact parents of each new seed
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around 1857, Mendel began to breed garden peas to study inheritance
he chose peas because they have a bunch of variables, including different flower colors (purple and white), their short generation time and the large number of offspring from each mating
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collecting information and history on a particular trait and assembling this information into a family tree describing the trait s of parents and children across the generations
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in humans, for example, pleiotropic alleles are responsible for the multiple symptoms associated with certain hereditary diseases such as cystic fibrosis and sickle-cell disease
IN THE GARDEN PEA this determines the flower color and the color of the coating on the outer surface of the seed, which can be grey or white
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for any character, the observed dominant/recessive relationship of alleles depends on the level at which we examine the phenotype
example, Tay-Sachs disease an inherited disorder in humans is an example. The brain cells of a child with this disease cannot metabolize certain lipids because a crucial enzyme does not work properly
as these lipids accumulate in brain cells, the child suffers from neurological disorders and degeneration of motor and mental performance and dies within a few years
only children who inherit two copies of the Tay-Sachs allele (homozygotes) have the disease
this means that at the organismal level, the Tay-Sachs allele qualifies as recessive
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in Mendel's classic pea crosses, the F1 offspring always looked like one of the two parental varieties because one allele in a pair showed complete dominance over the other
for some genes, however, neither allele is completely dominant, and the F1 hybrids have a phenotype somewhere between those of the two parental varieties
this phenomenon is called incomplete dominance
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in this variation, the two alleles each affect the phenotype in separate, distinguishable ways
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thousands of genetic disorders are known to be inherited as simple recessive traits
these disorders can range in lack of severity from mild, such as albinism, to life-threatening, such as cystic fibrosis
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an allele that codes for either a malfunctioning protein or no protein at all
in the case of all disorders classifying as recessive, heterozygotes typically have the normal phenotype because one copy of the normal allele produces a significant amount of the specific protein
thus, a recessively inherited disorder only shows up in the homozygous individuals who inherit a recessive allele from each parent
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