Mendelian Genetics

Common Course Objectives

  • Discuss patterns of inheritance
  • Explain genetic terms
  • Explain genotypic and phenotypic variation
  • Explain dominant and recessive traits
  • Discuss Mendelian model of inheritance
  • Explain chromosomal aberrations

In the Beginning

  • By the 19th century, most people understood that both parents contributed hereditary material to their children, but the nature of that material was unknown.
  • Some thought the hereditary material was a fluid that blended during fertilization, but this didn't explain observed traits like freckles or unexpected hair color.
  • Darwin rejected blending inheritance but couldn't explain heredity, which was central to his theory of natural selection.
  • Darwin noticed trait variation among individuals; advantageous traits would become more frequent over generations.
  • Hereditary information (DNA) is divided into discrete units called genes.
  • An allele is an alternative form of a gene found at the same place on a chromosome, which is critical to understanding heredity.

Mendel

  • Gregor Johann Mendel (1822-1884) was an Austrian scientist and Augustinian friar.
  • He carefully bred pea plants and documented the outcomes of his experiments.
  • Mendel observed that certain traits were passed from plant to plant, generation after generation.
  • He collected evidence of how inheritance works.

Mendel’s Experimental, Quantitative Approach

  • Mendel studied the common garden pea, Pisum sativum, which naturally self-fertilizes.
  • Mendel bred particular individuals together and observed and documented the traits of the offspring.
  • He controlled reproduction by preventing self-fertilization (removing anthers) and cross-fertilizing by brushing carpels with pollen from another plant.
  • Mendel collected the seed from these crosses, planted them, and recorded the traits of the new plants.
  • Many experiments involved plants that "bred true" for a particular trait, meaning all offspring had the same form of the trait as the parent.
  • Breeders cross-fertilize plants by transferring pollen among individuals with different traits.
  • Mendel discovered that the traits of offspring often appear in predictable patterns, indicating that hereditary information is passed in discrete units.

Inheritance in Modern Times

  • Organisms that breed true for a particular trait probably have identical alleles governing that trait.
  • An individual with identical alleles of a gene is said to be homozygous for that allele.
  • The particular set of alleles that an individual carries is called its genotype.
    • Example: Alleles for eye color: B = brown eyes, b = blue eyes; BB, Bb, bb = genotypes
  • Alleles are a major source of variation in a trait, with new alleles produced by mutation.
  • A mutation may cause a trait to change, such as a gene for purple flower color mutating to produce white flowers.
  • Phenotype is an individual’s observable traits; flower color is an example.
  • Any mutated gene is an allele, whether or not it affects phenotype.
  • Offspring produced by crossing two individuals that breed true for different forms of a trait are called hybrids.
  • Hybrids carry different alleles of a gene and are said to be heterozygous.
  • In many cases, one allele influences the effect of the other, visible in the hybrid phenotype.
    • Example: Male ligers are sterile, while females are fertile.
  • Example: Alleles for flower color: B = blue flowers, b = white flowers.
    • BB = blue flowers (homozygous)
    • Bb = blue flowers (heterozygous) hybrid
    • bb = white flowers (homozygous)
  • An allele is dominant when its effects mask that of the recessive allele paired with it.
    • Example: Alleles for flower color: B = blue flowers, b = white flowers.
    • B is the dominant allele.
    • b is the recessive allele.

Mendel’s Law of Segregation

  • Mendel crossed pea plants that bred true for purple flowers with pea plants that bred true for white flowers; all offspring had purple flowers.
  • One gene controls purple flower color in pea plants.
    • The allele that codes for purple flower color (P) is dominant over the allele that codes for white flower color (p).
    • Plant with 2 P alleles (PP) has purple flowers.
    • Plant with 2 p alleles (pp) has white flowers
  • When homologous chromosomes separate during meiosis, the gene pairs on those chromosomes separate, too.
    • Each gamete carries only one of the two genes of each pair.
    • Homozygous dominant (PP) plants can only make gametes carrying the dominant allele.
    • Homozygous recessive (pp) plants can only make gametes carrying the recessive allele.
  • If homozygous plants are crossed: PP x pp
    • Only one outcome is possible: a gamete carrying a P allele will meet up with a gamete carrying a p allele.
    • All offspring will have one P allele and one p allele.
    • Genotype = Pp
    • All offspring will have purple flowers (= phenotype).
  • A Punnett square is a diagram used to predict the genetic and phenotypic outcome of a cross.
  • This pattern is predictable and can be used as evidence of a dominant relationship between alleles.
  • Breeding experiments use these patterns to reveal genotype.
  • In a testcross, an individual with a dominant trait but unknown genotype is crossed with an individual known to be homozygous recessive for the trait.
  • The pattern of traits among the offspring can show whether the tested individual is heterozygous or homozygous.
    • Testcross: Purple flower pea plant x white flower pea plant
      • Purple flower = PP or Pp (unknown genotype)
      • White flower = pp (known genotype)
    • If all offspring had purple flowers, the purple-flowered parent's genotype was PP.
    • The genotype of the offspring would be Pp.
  • A monohybrid cross checks for the dominance relationship between the alleles of a single gene.
  • Individuals identically heterozygous for one gene (Pp) are crossed.
  • The frequency at which the two traits appear among the offspring may show that one of the alleles is dominant over the other.
  • Genotypes:
    • 1 PP
    • 2 Pp
    • 1 pp
  • 3 out of the 4 possible outcomes will include at least one copy of the dominant allele, P, and have purple flowers.
  • Each time fertilization occurs, there is a 1 in 4 chance that the offspring inherits 2 recessive p alleles and has white flowers.
  • The probability that a particular offspring of this cross will have purple or white flowers is 3 purple to 1 white, represented as the ratio 3:1.
  • The 3:1 pattern indicates that purple and white flower color are specified by alleles with a clear dominant-recessive relationship.
    • Purple is dominant.
    • White is recessive.
  • Phenotypic ratios in the F2 offspring of Mendel’s monohybrid crosses were close to 3:1.
  • This formed the basis for his Law of Segregation, which states: “each individual that is a diploid has a pair of alleles (copy) for a particular trait and each parent passes an allele at random to their offspring resulting in a diploid organism”

Monohybrid Cross Example

  • P = purple flower color
  • p = white flower color
  • Two true-breeding flowers crossed: PP x pp
  • Determine gametes: P, P, p, p
  • Set up Punnett Square
  • Do the cross
  • What is the genotypic ratio of the F1 offspring? 4:0 Pp
  • What is the phenotypic ratio of the F1 offspring? 4:0 purple flowers.

Monohybrid Cross with F2 Generation

  • P = purple flower color
  • p = white flower color
  • Two F1 offspring crossed: Pp x Pp
  • Determine gametes: P, p, P, p
  • Set up Punnett Square
  • What is the genotypic ratio of the F2 offspring? 1:2:1
    • 1 PP
    • 2 Pp
    • 1 pp
  • What is the phenotypic ratio of the F2 offspring? 3:1
    • 3 purple flowers
    • 1 white flower

Mendel’s Law of Independent Assortment

  • During meiosis, gene pairs on homologous chromosomes tend to sort into gametes independently of other gene pairs.
  • 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 together as a unit 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.

Dihybrid Cross Example

  • In rabbits, gray fur (G) is dominant to white (g) fur, and black eyes (B) are dominant to red eyes (b).
  • A male rabbit with the genotype GGbb is crossed with a female with the genotype ggBb.
  • What is the male’s phenotype? Gray hair, red eyes
  • What is the female’s phenotype? White hair, black eyes
  • How many offspring have gray fur and black eyes? 8
  • How many offspring have gray fur and red eyes? 8
  • How many offspring have white fur and black eyes? 0
  • How many offspring have white fur and red eyes? 0

Dihybrid Cross Another Example

  • A male rabbit with the genotype GgBb is crossed with a female rabbit with a genotype GgBb.
  • Genotypes:
    • Male: GB, Gb, gB, gb
    • Female: GB, Gb, gB, gb
  • How many offspring have gray fur and black eyes? 9
  • How many offspring have gray fur and red eyes? 3
  • How many offspring have white fur and black eyes? 3
  • How many offspring have white fur and red eyes? 1

Contribution of Crossovers

  • Some genes are located on the same chromosome.
  • Genes are far enough apart that crossing over happens frequently.
  • They often assort into gametes independently, as if they were on separate chromosomes.
  • Other genes are very close together on a chromosome and do not assort independently.
  • Crossing over does not happen very often between these genes.
    • Such genes are said to be linked.
  • Alleles of linked genes stay together during meiosis more frequently than others.
  • The effect of the relative distance between the genes.
    • Genes that are closer together get separated less often by crossovers.
  • The closer genes are, the more likely gametes will get parental combinations of genes of those alleles.
  • Genes that are REALLY close to each other are said to be linked.
    • A linkage group includes all of the genes on a chromosome.

Dominant vs. Recessive Traits

  • The terms dominant and recessive describe the inheritance patterns of certain traits.
    • A dominant trait is expressed.
  • For a recessive allele to produce a recessive phenotype, the individual must have two copies, one from each parent.
  • An individual with one dominant and one recessive allele for a gene will have the dominant phenotype.

Beyond Simple Dominance

  • Not all alleles are clearly dominant or recessive.
  • An allele can be:
    • Completely dominant
    • Incompletely dominant
    • Codominant with its partner on a homologous chromosome

Incomplete Dominance

  • In incomplete dominance, an allele is not fully dominant over its partner on a homologous chromosome.
    • Both are expressed.
  • The combination of alleles produces an intermediate phenotype.
  • Snapdragon flower color is an example of incomplete dominance.
    • R = red pigment
    • r = no color (mutated allele)
    • RR plants make lots of red pigment = red flowers
    • rr plants do not make pigment = white flowers
    • Rr plants make a little bit of pigment = pink flowers
  • A cross between 2 pink-flowered plants (Rr x Rr) produces red, pink, and white flowered offspring in a 1:2:1 ratio.

Codominance

  • Codominant alleles are both expressed at the same time in heterozygotes.
  • Multiple allele systems, such as ABO blood typing, are examples.
  • An enzyme coded for by the ABO gene modifies a carbohydrate on the surface of human red blood cells.
    • A and B alleles code slightly different versions of the enzyme, which modify the carbohydrate a little differently.
    • O allele has a mutation that prevents its enzyme product from becoming active at all.
  • The two alleles you carry for the ABO gene determine which form of the carbohydrate you will have on your blood cells.
    • This determines your blood type.
  • A and B alleles are codominant when paired.
    • AB genotype = AB blood type
      • Can receive blood from any of the other types = universal recipients.
    • O allele is recessive when paired with either A or B.
      • AA or AO genotype = A blood type
      • BB or BO genotype = B blood type
      • OO genotype = O blood type
        • Can donate blood to any of the other types = universal donors.

Epistasis

  • Some traits are affected by multiple gene products.
  • Epistasis occurs when the expression of one gene is modified (e.g., masked, inhibited, or suppressed) by the expression of one or more other genes.
  • Also called polygenic inheritance.
    • Human skin color
    • Coat color in Labs
  • Coat color in Labrador retrievers is a good example of epistasis.
    • Coat color genes only come in black or chocolate.
    • Yellow Labrador retrievers result when recessive epistatic genes called “extension genes” don't allow color pigment to reach the fur.

Pleiotropy

  • Pleiotropy is a type of genetic expression in which only one gene affects multiple traits.
  • Mutations of pleiotropic genes often result in complex genetic disorders.
    • Sickle-cell anemia
    • Cystic fibrosis: a disorder that damages your lungs, digestive tract, and other organs.