General Genetics

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General Genetics

• Chapter 14
• Mendel and the Gene Idea

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Objectives

• Understand Mendels three Laws governing genetics
• Understand the meaning of the relevant vocabulary
  discussed in class
• Be able to predict the results of a mono
  dihybrid cross using a Punnett square
• Be familiar with patterns of inheritance for
  genes on sex chromosomes
• Understand the concept of Linked Genes
• Understand how recombination of genes affect
  genetic variability
• Be familiar with special situations regarding
  genotype/phenotype predictions

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Descriptors

• Gene (Character) is a feature that is heritable
• Locus specific area on chromosome where the gene
  is found
• Allele (Trait) is a variation of a character
• Genotype the genetic makeup of an organism
  (combination of genes in its nucleus)
• Phenotype the physical appearance of an organism

• The physical appearance of an organism reflects
  its genetic makeup
• Each gene codes for a different polypeptide
• Polypeptide combinations may alter the appearance
  of an organism

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Mendelian Genetics

• Three possible genotypes for each gene in the
  diploid cell
• Homozygous dominant both alleles of a gene are
  of the Dominant variety
• Homozygous recessive both alleles of a gene are
  of the Recessive variety
• Heterozygous the diploid cell has one dominant
  and one recessive allele for each gene

• Gregor Mendel described three Laws of Genetics
• Law of Segregation each parent has two copies of
  a gene but only one is passed to the offspring
  via the gametes (separation of homologous pairs)

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• Law of Independent Assortment genes residing on
  different chromosomes separate without regard for
  one another
• describes the broad range of variation seen in
  organisms
• Law of Dominance some alleles for a gene are
  fully expressed if present (dominant) in the
  phenotype while others have no affect (recessive)
• hierarchy of alleles

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Genetics is Probability

• In a diploid organism (2n), each allele has a
  5050 chance of being found in a particular
  gamete (1/2).
• To calculate the likelyhood of two alleles
  recombining during fertilization, we must
  multiply our probability for each allele together
  (1/2 x 1/2 1/4)

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Punnett Square

• Device used to predict potential genotypes of
  offspring
• Along each axis are placed the gamete
  possibilities for each parent
• Internal boxes represent union of genotypes for
  offspring produced by the union of the
  corresponding axial gametes
• Phenotypes can be determined for each potential
  zygote

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Sex Linked Inheritance

• Males and females differ in their sex chromosome
  combination (females XX males XY)
• Barr bodies
• Because the X contains genes and the Y does
  not, inheritance patterns of sex-linked genes
  vary between the sexes
• recessive traits more prevalent in males

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Concept of Linked Genes

• Linked genes are those that reside on the same
  chromosome and tend to be inherited together
• Genes residing on the autosomal chromosomes
  (pairs 1-22) are said to be autosomal genes
• Sex-linked genes are found on the sex chromosomes
  (pair 23, usually on the X)

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Recombination of Genes

• Production of offspring with a new combination of
  traits is called genetic recombination
• Independent assortment may recombine genes that
  are unlinked
• Linked genes can become unlinked through
  recombination events like crossover (during
  meiosis)

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Gene Mapping

• Maps of genes on chromosomes can be constructed
  from recombination data
• Recombination data for linked genes reflects the
  distance of the 2 loci from one another
• The farther apart 2 loci are from one another the
  more frequent the observed recombination due to
  crossover

Linkage map genetic map based on recombination
frequencies
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Special Situations

• Pleiotropy the ability of a gene to affect an
  organism in many ways
• Epistasis gene at one locus influences the
  expression of a gene at another locus (different
  gene)
• Polygenic Inheritance additive effect of 2 or
  more genes on a phenotypic character

• Incomplete dominance the phenotype of a
  heterozygous genotype is intermediate in
  appearance
• Codominance each allele in the genotype for a
  particular gene will be expressed in the
  phenotype

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Incomplete Dominance (heterozygous genotype is
intermediate in appearance)
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Codominance (each genotypic allele will be
expressed)
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Pleiotropy (single gene affects multiple
phenotypes)

• Sickle Cell Disease
• Production of abnormal hemoglobin
• Prevalent amongst African Americans, but rare in
  people of other races

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Epistasis(one gene influences the expression of
another gene)
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Polygenic Inheritance(additive effect of 2 or
more genes on a phenotypic character)