Mendel and the Gene Idea

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Mendel and the Gene Idea

• Chapter 14

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Genetic terms

• P parental generation of a cross
• F1 the first generation after the parental (the
  results of the first cross)
• F2 a cross between F1 individuals yields F2

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Mendels Law of Segregation

• Alleles - alternative forms of genes
• Each individual has 2 alleles for the same gene
• Each parent passes one allele for each gene to
  his/her offspring
• What process does this law represent?
• Meiosis
• What stage of meiosis?
• Anaphase I

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Genetic terms

• Homozygous - 2 identical alleles for a gene
• Heterozygous - 2 different alleles for a gene
• Dominant - The allele that is expressed in the
  heterozygous individual
• Recessive - the allele that is masked in the
  heterozygous individual

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The test cross

• How can you determine the genotype of a dominant
  individual?
• In sheep, white is dominant to black
• It is customary to determine the letter for the
  alleles of a given gene by using the first letter
  of the dominant allele.
• Therefore W white and w black
• How can you tell if white sheep is homozygous or
  heterozygous?

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• Conduct a test cross, where the unknown dominant
  individual is crossed with the known recessive.
• W ___ x ww

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Setting up the Punnett square

• The possible gametes of each individual are
  determined
• Each gamete will have one of the two alleles of
  the parent
• Generate a Punnett square and write the alleles
  for parent 1 on the left side of the Punnett
  square
• Place the alleles for parent 2 above the square
• Fill the squares for each parent

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Interpreting the results of a Punnett square

• Genotypes that resulted from this monohybrid
  cross (Ss x Ss)

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ProbabilityRule of multiplication

• The odds of 2 or more independent events
  occurring together is the product of each
  independent event.
• In a cross between pea plants that are
  heterozygous for flower color (Pp), what is the
  probability that the offspring will be homozygous
  recessive?
• ½ x ½ ¼

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Rule of addition

• Probability that events that can occur in 2 or
  more different ways will occur, is the sum of the
  separate problems.
• In a cross between pea plants that are
  heterozygous for flower color (Pp), what is the
  probability of the offspring being heterozygous?
• ¼ ¼ ½

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Law of Independent Assortment

• The odds of two different traits occurring
  together in the same offspring is the product of
  each independent trait occurring in the
  offspring.
• In which phase of meiosis does this event occur?
• Metaphase I
• What assumption did Mendel make for this law to
  work?
• The traits occur on different chromosomes

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Dihybrid Cross

• R red r white
• T tall t short
• What is the F2 phenotypic ratio of offspring?
• Punnett Square Approach
• Fork Lined Approach

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• What is the probability that a cross between 2
  organisms with genotypes AaBbCc and AaBbCc will
  produce an offspring with aabbcc?
• 1/4 x 1/4 x 1/4 1/64

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Incomplete Dominance

• One allele is not completely dominant over the
  other, so the heterozygote has an intermediate
  phenotype between the 2 homozygotes
• In snapdragons, red snapdragons crossed with
  white and produce all pink flowers. Explain this
  cross

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Dominant Alleles

• Organismal level
• Biochemical level
• Molecular level
• Complete dominance ? Incomplete Dominance ?
  Codominance

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Multiple Alleles
Blood Type Genotypes Antigens in blood Antibodies in serum
A IAIA, or IAi A Anti B
B IBIB, or IAi B Anti A
AB IAIB A, B ---
O ii --- Anti A and Anti B
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Terms

• Pleiotropy - multiple effects from a single gene
• Epistasis - one locus alters the phenotypic
  expression of a second gene
• Polygenic inheritance - additive effect of 2 or
  more genes on a single trait

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Pedigrees

• Family trees used to track human traits
• Male
• Female

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Genetic Disorders

• Recessive disorders
• Cystic Fibrosis
• Chloride transport channels are defective or
  absent in the plasma membrane
• Results in mucus building up in lungs, pancreas,
  digestive tract pleiotropic effects
• Sickle cell disease
• Sickle shaped cells clog blood vessels
• Pleiotropic effects

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Genetic Disorders

• Dominant disorder
• Huntingtons disease
• Degenerative disease of the nervous system
• No effect until about age 35
• DNA can be analyzed to screen for the disease

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Genetic Screening

• Carrier recognition
• Fetal testing
• Amniocentesis
• CVS
• Ultrasound
• Fetoscopy