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Chapter 14 Mendel and the Gene Idea (Many Human Traits follow Mendelian…

- - - - An alternative to the blending model is a "particulate" hypothesis of inheritance: the gene idea
- - - - The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities
      - Probability in an F1 monohybrid cross can be determined using the multiplication rule
      - Segregation in a heterozygous plant is like flipping a coin: Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele
  - - - We can apply the rules of probability to predict the outcome of crosses involving multiple characters
      - A multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously
      - In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied
- - - - Mendel’s approach allowed him to deduce principles that had remained elusive to others
      - A heritable feature that varies among individuals (such as flower color) is called a character
      - Each variant for a character, such as purple or white color for flowers, is called a trait
      - Peas were available to Mendel in many different varieties
    - - Advantages of using peas
      - 1.Short generation time
      - 2.Large numbers of offspring
      - 3.Mating could be controlled; plants could be allowed to self-pollinate or could be cross-pollinated
    - - -In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization
        
        -The true-breeding parents are the P generation
        
        -The hybrid offspring of the P generation are called the F1 generation
        
        -When F1 individuals self-pollinate or cross-pollinate with other F1 hybrids, the F2 generation is produced
  - - - When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple
      - When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
      - Mendel discovered a ratio of about three purple flowers to one white flower in the F2 generation
      - Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
        
        Mendel called the purple flower color a dominant trait and the white flower color a recessive trait
        
        The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation
  - - - Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring
      - Four related concepts make up this model
      - These concepts can be related to what we now know about genes and chromosomes
    - - First: alternative versions of genes account for variations in inherited characters
      - An example is the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers
      - These alternative versions of a gene are called alleles
      - Each gene resides at a specific locus on a specific chromosome
      - Second: for each character, an organism inherits two alleles, one from each parent
        
        Mendel made this deduction without knowing about chromosomes
        
        The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation
        
        Or the two alleles at a locus may differ, as in the F1 hybrids
        
        Third: if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
        
        In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant
        
        Fourth (the law of segregation): the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
        
        Thus, an egg or a sperm gets only one of the two alleles that are present in the organism
        
        This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis
  - - - An individual with the dominant phenotype could be either homozygous dominant or heterozygous
      - To determine the genotype we can carry out a testcross: breeding the mystery individual with a homozygous recessive individual
      - If any offspring display the recessive phenotype, the mystery parent must be heterozygous
  - - - Mendel derived the law of segregation by following a single character
      - The F1 offspring produced in this cross were monohybrids, heterozygous for one character
      - A cross between such heterozygotes is calleda monohybrid cross
        
        Mendel identified his second law of inheritance by following two characters at the same time
        
        Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
        
        A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently
        
        Using a dihybrid cross, Mendel developed the law of independent assortment
        
        It states that each pair of alleles segregates independently of any other pair of alleles during gamete formation
        
        This law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome
        
        Genes located near each other on the same chromosome tend to be inherited together
- - - - Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:
      - When alleles are not completely dominant or recessive
      - When a gene has more than two alleles
      - When a gene produces multiple phenotypes
        
        The Relationship Between Dominance and Phenotype
        
        In the case of pea shape, the dominant allele codes for an enzyme that converts an unbranched form of starch in the seed to a branched form
        
        The recessive allele codes for a defective form of the enzyme, which leads to an accumulation of unbranched starch
        
        This causes water to enter the seed, which then wrinkles as it dries
  - - - Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
      - In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
      - In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
  - - - Some traits may be determined by two or more genes
      - In epistasis, one gene affects the phenotype of another due to interaction of their gene products
      - In polygenic inheritance, multiple genes independently affect a single trait
    - - In epistasis, expression of a gene at one locus alters the phenotypic expression of a gene at a second locus
        
        For example, in Labrador retrievers and manyother mammals, coat color depends on two genes
        
        One gene determines the pigment color (with alleles B for black and b for brown)
        
        The other gene (with alleles E for color and e forno color) determines whether the pigment will be deposited in the hair
        
        If heterozygous black labs (genotype BbEe) are mated, we might expect the dihybrid F2 ratio of 9:3:3:1
        
        However, a Punnett square shows that the phenotypic ratio will be 9 black to 3 chocolate to 4 yellow labs
        
        Epistatic interactions produce a variety of ratios, all of which are modified versions of 9:3:3:1
  - - - Quantitative characters are those that vary in the population along a continuum
      - Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype
      - Height is a good example of polygenic inheritance: Over 180 genes affect height
      - Skin color in humans is also controlled by many separately inherited genes
    - - Another departure from Mendelian genetics arises when the phenotype for a character 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
        
        A Mendelian View of Heredity and Variation
        
        An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior
        An organism’s phenotype reflects its overall genotype and unique environmental history
- - - - 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
      - Most individuals with recessive disorders are born to carrier parents
      - Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair
        
        If a recessive allele that causes a disease is rare, it is unlikely that two carriers will meet and mate
        
        Consanguineous matings (i.e., between close relatives) increase the chance that both parents of a child carry the same rare allele
        
        Most societies and cultures have laws or taboos against marriages between close relatives
  - - - Cystic fibrosis is the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent
      - The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes, leading to a buildup of chloride ions outside the cell
      - Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
  - - - Sickle-cell disease affects one out of 400 African-Americans
      - It is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
      - In homozygous individuals, all hemoglobin is abnormal (sickle-cell)
      - Symptoms include physical weakness, pain, organ damage, and even paralysis
        
        Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms
        
        About one out of ten African Americans has sickle-cell trait, an unusually high frequency
        
        Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous in regions where malaria is common
  - - - Some human disorders are caused by dominant alleles
      - Dominant alleles that cause a lethal disease are rare and arise by mutation
      - Achondroplasia is a form of dwarfism caused by a rare dominant allele
    - - The timing of onset of a disease significantly affects its inheritance
      - Huntington’s disease is a degenerative disease of the nervous system
      - The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age
      - Once the deterioration of the nervous system begins, the condition is irreversible and fatal
    - - Many diseases, such as heart disease, cancer, alcoholism, and mental illnesses, have both genetic and environmental components
        No matter what our genotype, our lifestyle has a tremendous effect on phenotype