Module 4 Notestothesixth - Mendel discovered the basic...

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Mendel discovered the basic principles of heredity by breeding garden peas in
carefully planned experiments
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Advantages of pea plants for genetic study:
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Varieties with distinct heritable variable characters (such as flower color);
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Mating can be controlled
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Each pea plant has sperm-producing organs (stamens)
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P was the parental generation and F1 is the first filial generation (first generation of
offspring from the parental generation)
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Use "true breeders" which were plants that would always produce the same
color flower when they were self-pollinated
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The "true breeders" were homozygous for the genes that coded for
flower color
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Ex: PP=purple pp=white
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In a typical experiment Mendel mated two contrasting true-breeding parents to
produce hybrids, or heterozygote's (Pp)
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When each hybrid was self pollinated they could produce purple or white
flowers
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There was usually a 3:1 ratio for purple to white flowers
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Mendel called the purple flower a dominant trait and the white flower color was a
recessive trait
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Developed a hypothesis to explain the 3:1 inheritance patter he observed in F2
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The first concept is that alternative versions of genes account for variations in
inherited characters-alleles
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We know now that each gene resides at a specific locus on a specific chromosome
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Figured out that there were two alleles that determined the genotype and
thus determined the phenotype
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The allele is the variation of it, the gene is the location on the chromosome, and the
chromosome is where the gene is found.
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The second concept is that for each character an organism inherits two alleles, one
from each parent
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Homozygous the same heterozygote different
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The third concept is that if two alleles at a locus differ, then one (the dominant
allele) determines the organisms appearance, and the other (the recessive allele) has
no noticeable effect on appearance
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The fourth concept the law of segregation, states that the two alleles for a heritable
character separate (segregate) during gamete formation and end up in different
gametes
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Thus, an egg or sperm gets only one of the two alleles that are present in the
somatic cells of an organism
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The possible combinations of sperm and egg can be shown using a Punnett square

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Mendel's segregation model accounts for the 3:1 ratio he observed in the F2
generation of his numerous experiments
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How can we tell the genotype of an individual with the dominant phenotype?
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We would determine it doing a test cross: breeding the mysterious individual
with a white homozygous individual. If it was a heterozygote there will be a 1:1
ratio of white and purple flowers and if it was a homozygote it will only breed
purple flowers
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Mendel derived the law of segregation by following a single character (monohybrid
cross)
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The F1 offspring produced in this cross were monohybrids, individuals that are
heterozygous for one character
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The genotype for one trait is not determined by another trait
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i.e color does not determine whether or not the seed will be round or wrinkled