lesson 2

• Genetics can explain how these kittens look different
• Analyzing their genes can also help predict the appearance of their offspring
• Most genes encode proteins that have nothing to do with outward appearance. The enzymes essential to these kittens' lives are also the products of genetics
• Studying genetics also allows scientists to breed superior crops and doctors to track genetic illnesses
• Chromosome


  Packets of genetic information
• Haploid


  Cells with only one set of chromosomes, such as sex cells
• Diploid


  Cells with two full sets of chromosomes, formed when two haploid cells fuse during fertilization
• Most cells of a mature individual are diploid
• Homologous chromosomes


  Have the same genes, but might have different versions (alleles) of those genes
• Alleles


  Different versions of the same gene
• Locus


  The location of a gene on a chromosome
• Sexual life cycle


  1. Meiosis

  2. Fertilization

  3. Diploid cells

  4. Haploid cells
• Gregor Mendel used pea plants to study heredity
• Mendel's plant breeding experiments

  1. Hand-pollinating plants

  2. Self-fertilizing

  3. Cross-fertilizing
• True-breeding plants


  Produce offspring identical to themselves
• Dominant allele


  Exerts its effects whenever it is present
• Recessive allele


  Its effect is masked if a dominant allele is also present, usually encodes nonfunctional proteins
• If yellow seed color is dominant, why are some seeds green when a yellow-seed plant is crossed with a green-seed plant? We need more information before we can fully answer this question
• Genotype


  An individual's two alleles for one gene
• Phenotype


  The observable characteristic conferred by a genotype
• Homozygous dominant


  Individuals have two dominant alleles for a gene
• Heterozygous


  Individuals have one dominant and one recessive allele
• Homozygous recessive


  Individuals have two recessive alleles
• It is possible to look at offspring to determine the genotype of the parent
• Punnett square


  Used to reveal which alleles the offspring may inherit based on the genotypes of the parents
• Monohybrid cross


  Both parents are heterozygous for the one gene being evaluated
• When germ cells divide by meiosis, chromosomes (and the alleles on those chromosomes) are randomly distributed among gametes
• If both gametes carry dominant alleles, the offspring will inherit two dominant alleles
• If one gamete carries a dominant allele and the other carries a recessive allele, the offspring will be heterozygous
• If both gametes carry recessive alleles, the offspring will inherit two recessive alleles
• On average, three offspring will have yellow seeds for every one with green seeds
• On average, one offspring will have genotype GG for every two with Gg and for every one with gg
• Punnett square


  A diagram that tracks the inheritance of one or more genes
• Punnett squares


  • Show the relative proportion of offspring phenotypes and genotypes
  • Allow determination of genotypes of yellow-seed pea plants
  • Help explain why some seeds are green when a yellow-seed plant is crossed with a green-seed plant
• Monohybrid cross


  Cross between two individuals that differ in one trait
• In a monohybrid cross, the expected genotypic and phenotypic ratios in the offspring are 1:2:1 and 3:1 respectively
• Dihybrid cross


  Cross between two individuals that differ in two traits
• Mendel's law of independent assortment states that the segregation of alleles for one gene does not influence the segregation of alleles for another gene
• Product rule


  Simplifies tracking inheritance of multiple genes by multiplying the individual probabilities of inheriting each allele
• The product rule cannot be used if genes are linked, because inheriting one allele influences the likelihood of inheriting a linked allele