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How Does Sexual Reproduction Generate Variation Among Offspring?

  • The ultimate source of genetic variation is mutation of genes.

  • There are also three sources of variation that arise during sexual reproduction.

    • Independent assortment of chromosomes occurs due to random orientation of homologous pairs during the first meiotic metaphase.

    • Recombinant chromosomes are formed due to crossing over during the first meiotic prophase.

    • The random nature of fertilization adds to the genetic variation arising from meiosis.

    • Heritable variation is a prerequisite for evolution, since natural selection acts on variation within populations.

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How Are Genes Inherited?

  • Gregor Mendel proposed that traits are inherited as discreet units, thus founding the science of genetics.

    • Mendel used the experimental method and mathematical analysis during his study of inheritance in
      pea plants.

    • He crossed plants with different characteristics and observed three generations of peas.

    • His results gave rise to the concepts of alleles, homologous chromosome pairs, and dominant vs. recessive genes.

    • It is important to know some additional terminology and understand alleles to utilize these concepts effectively.

    • Mendel's laws are: 1) the law of segregation and 2) the law of independent assortment.

  • Punnett squares can be used to illustrate Mendel's laws.

    • Using Punnett squares in a test cross can determine an unknown genotype.

    • Outcomes of hybrid crosses can be predicted by the laws of probability.

    • Using Punnett squares in a dihybrid cross illustrates independent assortment of genes.

  • Mendelian genetics can be used to analyze a human pedigree.

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What Are Some Complexities Of Genetics?

  • Genes control the phenotype of organisms by coding for specific proteins; alleles can show different degrees of dominance.

    • In incomplete dominance, heterozygous individuals have a phenotype intermediate between the two alleles.

    • In co-dominance, both alleles are fully expressed.

    • The degree of dominance can vary depending on the level at which the phenotype is examined.

  • In the majority of cases, multiple alleles exist for a characteristic.

  • In polygenic inheritance, several genes act together to affect a phenotype.

  • The environment may interact with genes to determine phenotype.

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