Chapter 11 � Introduction to Genetics

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Chapter 11 � Introduction to Genetics Powered By Docstoc
					Chapter 11 – Introduction to

   Vocab: genetics, fertilization, true-breeding, trait,
 hybrid, gene, allele, segregation, gamete, probability,
     Punnett square, homozygous, heterozygous,
    phenotype, genotype, independent assortment,
incomplete dominance, codominance, multiple alleles,
polygenic traits, homologous, diploid, haploid, meiosis,
            tetrad, crossing-over, gene map
 11-1 The Work of Gregor Mendel

Genetics is the scientific study of heredity.
Gregor Mendel
• Austrian monk in what is now the Czech
  Republic in 1822
• Studied garden peas
Pea Plants
• Fertilization is when male and female
  reproductive cells join.
• Male cell = pollen; Female cell = egg
• Pea plants are normally self-pollinating –
  egg and pollen from same plant – one
• True-breeding plants – after self-
  pollinating they have identical offspring
• Mendel began manipulating the pollination
  – cross-pollination
• Mendel studied seven different traits that
  had two contrasting forms
• Trait = a specific characteristic that varies
  from one individual to another
The offspring of crosses between parents
  with different traits are called hybrids.
Mendel crossed plants with the two forms
• P: Parental Generation: Tall x Short
• F1: First filial Generation: all Tall
• F2: Second filial Generation: 75% tall &
                                    25% short
Made two conclusions:
• Inheritance is determined by factors
  passed from one generation to the next
  called genes. Genes have different forms
  called alleles.
• The principle of dominance states that
  some alleles are dominant and others are
  recessive. Recessive alleles are masked
  by dominant alleles.
• Segregation – When each F1 plant flowers
  and produces gametes, the two alleles
  segregate from each other so that each
  gamete carries only a single copy of each
  gene. Therefore, each F1 plant produces
  two types of gametes – those with the
  allele for tallness and those with the allele
  for shortness
    11-2 Probability and Punnett
The likelihood that a particular event might
 occur is called probability.
Coin landing on heads = 50%
Coin landing on heads three times in a row =
 ½ x ½ x ½ = 1/8
The principles of probability can be used to
 predict the outcomes of genetic crosses.
Punnett squares can be used to predict and
  compare the genetic variations that will
  result from a cross.
• Homozygous = organism with two identical
  traits; TT or tt
• Heterozygous = organisms with two
  different traits; Tt
• Phenotype = physical characteristics; tall
  or short
• Genotype = genetic makeup; TT, Tt, or tt
Punnett Square
Parent 1 genotype=TT

Parent 2 genotype=tt
Punnett Square
Parent 1 genotype=TT

Parent 2 genotype=tt       T    T

                       t   Tt

11-3 Exploring Mendelian Genetics

Independent Assortment = looks at the
  inheritance of two traits
The principle of independent assortment
  states that genes for different traits can
  segregate independently during the
  formation of gametes. Independent
  assortment helps account for many
  genetic variations observed in plants,
  animals, and other organisms.
 Beyond Dominant and Recessive
• Some alleles are neither dominant nor
  recessive, and many traits are controlled
  by multiple alleles or multiple genes.
  – Incomplete dominance
  – Codominance
  – Multiple alleles
  – Polygenic traits
1. Incomplete dominance – flower color
  (primrose, 4 o’clocks, snapdragon), horse
  coat color, sickle-cell anemia
• 4 o’clocks:
  – RR= red
  – RW= pink
  – WW= white
• Each dominant allele produces red
  pigment: RW makes ½ as much and
  appears pink
Incomplete Dominance
2. Codominance – both alleles contribute to
  the phenotype
• Chickens – allele for black feathers is
  codominant with white feathers…
  heterozygote is an “erminette” speckled
• Human AB blood type
3. Multiple Alleles – more than two possible
  alleles exist in a population
• Human ABO blood typing system
• Coat color in rabbits
4. Polygenic Traits – traits controlled by two
  or more genes
• At least three genes control pigment in the
  eyes of fruit flies
• The range of skin colors is from more than
  four genes
    Applying Mendel’s Principles

• 1900’s American geneticist Thomas Hunt
  Morgan started studying fruit flies,
  Drosophila melanogaster because it was
  small, reproduce quickly, and have as
  many as 100 offspring
Characteristics of any organism are
 determined by interaction between genes
 and the environment including climate,
 soil, food availability, pollution.
              11-4 Meiosis

• Chromosome number
• Genes are located on chromosomes in the
  cell nucleus.
• Chromosomes are found in pairs called
  homologous chromosomes.
  – Fruit flies have 8 chromosomes and 4
    homologous pairs; 4 from female and 4 from
• A cell with both sets of homologous
  chromosomes is diploid
  – Symbol = 2N; body cells are diploid
• A cell with one set of chromosomes is
  – Symbol = N; gametes are haploid
Phases of Meiosis
• Meiosis is a process of reduction division
  in which the number of chromosomes per
  cell is cut in half through the separations of
  homologous chromosomes in a diploid
• Tetrad = structure formed during meiosis
  in which homologous chromosomes pair
• Crossing over = the 4 chromatids of a
  tetrad may exchange portions resulting in
  new combinations of alleles.
Mitosis results in the production of two
 genetically identical diploid cells, whereas
 meiosis produces four genetically different
 haploid cells.

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