chapter 12

• Character


  A heritable feature that varies among individuals in a population (e.g., flower color is a character for the pea plant species studied by Gregor Mendel)
• Trait


  Each variant for a character (e.g., purple or white for Mendel's pea plants)
• True-breeding


  Organisms that produce offspring with the same phenotype over many generations (e.g., purple flowering pea plants that ALWAYS produce offspring with purple flowers and those offspring ALSO produce offspring ONLY with purple flowers)
• Allele


  ONE specific DNA sequence for a given functional DNA element such as a particular protein coding gene (e.g., The number of alleles present within an organisms cells depends upon the ploidy! Diploid cells (2n) have 2 alleles, Haploid cells only have 1 allele, triploid (3n) cells have 3 alleles, tetraploid (4n) cells have 4 alleles, etc.)
• Dominant allele


  Determines organism's appearance (1 dominant allele is sufficient to DOMINATE or mask the effects of any other allele(s) present)
• Recessive allele


  No noticeable effect on organism's appearance UNLESS BOTH alleles are recessive (BECAUSE no dominant alleles are present!)
• Genotype


  Genetic makeup of an organism (i.e. RR; Rr; rr)
• Phenotype


  Organism's appearance or observable traits (i.e. purple)
• Punnett square


  Diagrammatic device for predicting allele composition of offspring from a genetic cross
• Homozygous


  Organism with pair of identical alleles for a character
• Heterozygous


  Organism that has two different alleles for a gene
• P (Parental), F1 (1st filial) or F2 (2nd filial)


  Define generational relationships
• Mendel's Experiment


  1. P generation: true breeding parents (purple and white)

  2. F1 generation: all purple

  3. F2 generation: 3:1 ratio of purple:white
• Mendel's Findings


  • Led to the Law of Segregation and Law of Independent Assortment
• Law of Segregation


  • Alleles account for variations in inherited characters
  • For each character, an organism inherits two copies of a gene, one from each parent
  • If the two alleles at a locus differ, then the dominant allele determines the organism's appearance
• How to use Law of Segregation

  All possible allele combinations can be predicted using a Punnett square
• Testcross


  Breeding an organism of unknown genotype with a recessive homozygote
• Testcross in action


  1. Cross the "mystery" plant with a white plant

  2. The allele from the mystery plant will determine the appearance of the offspring

  3. All off spring purple = PP

  4. Both purple and white = Pp
• Law of Independent Assortment

  • Two or more genes assort independently
  • Each pair of alleles segregates independently of each other pair of alleles during gamete formation
  • F1 YyRr plant will produce four classes of gametes in equal quantities
• Law of Independent Assortment results in formation of F2 offspring with a phenotypic ratio of 9:3:3:1
• Law of Independent Assortment applies to genes located on different chromosomes
• Degree of dominance


  Alleles can show different degrees of dominance or recessiveness in relation to each other
• Complete dominance


  Phenotype of Rr same as RR
• Incomplete Dominance


  • F1 hybrids have phenotype between those of the parents (i.e. RR = red; Rr = pink; rr = white)
  • 1:2:1 genotypic AND phenotypic ratio
• Co-dominance


  • The two alleles each affect the phenotype in different ways (e.g. ABO blood grouping)
  • An allele is said to be dominant because it is seen in the phenotype NOT because it subdues a recessive allele
• Pedigree


  A family tree that describes the interrelationships of parents and children across generations
• Pedigree Analysis


  Inheritance patterns of particular traits can be traced and described using pedigrees
• Dominant vs Recessive traits in pedigrees

  • Dominant never skips a generation!
  • Recessive PHENOTYPE can skip a generation as a heterozygous recessive trait! Individual called a carrier.
• Determining trait dominance and genotype from pedigree


  1. Is the trait dominant or recessive?

  2. What is the genotype of the indicated individual?
• Recessive alleles


  • Recessively inherited disorders show up only in individuals homozygous for the allele
  • Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal
• Dominant alleles are not necessarily more common in populations than recessive alleles
• For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype</b>
• Tay-Sachs disease


  • At the organismal level, the allele is recessive
  • At the biochemical level, the phenotype (E activity level) is incompletely dominant
  • At the molecular level, the alleles are codominant
• Multiple Alleles


  • Most genes within populations exist as more than 2 allelic forms
  • Ex: four phenotypes of the ABO blood groups are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.
• Pleiotropy


  • Most genes have multiple phenotypic effects, a property called pleiotropy
  • EXAMPLE: pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
• Epistasis


  • One gene affects the phenotype of another because the two gene products interact
  • The phenotypic expression of a gene at one locus alters that of a gene at a second locus
  • Ex: Fur coat color of Labrador retrievers: B= Black; b = brown with complete dominance
  • E = deposits pigment; e = no pigment deposited; ee results in yellow coat regardless of Black/brown locus
  • Gene for pigment deposition is epistatic to gene that codes for black or brown pigment
• Epistasis interactions


  Produce different rations, all of which are modified 9:3:3:1
• Polygenic Inheritance


  • Multiple genes affect a single trait
  • Quantitative characters: heritable feature that varies continuously over a range rather than an "either-or" fashion
  • Ex: Height – at least 180 genes affect height
  • EX: Skin color – three genes (A,B,C) contribute a "unit" of darkness to the phenotype and being incompletely dominant to the other allele (a, b, or c)
  • AABBCC = very dark; aabbcc = very light; AaBbCc = intermediate shade
• Environmental Impact on Phenotype

  • The phenotype for a character can depends on environment as well as genotype
  • The phenotypic range is broadest for polygenic characters
  • Traits that depend on multiple genes combined with environmental influences are called multifactorial
• Law of Independent Segregation

  Shown in Figure 12.2