Genetics and Inheritance

Inheritance

C2: Inheritance

• This unit covers inheritance, focusing on Mendel's experiments and various inheritance patterns.

Specific Outcomes

• C2.1k: Describe evidence for dominance, segregation, and independent assortment of genes on different chromosomes as investigated by Mendel.
• C2.2k: Compare ratios and probabilities of genotypes and phenotypes for dominant and recessive, multiple, incompletely dominant, and codominant alleles.
• C2.3k: Explain the influence of gene linkage and crossing over on variability.
• C2.4k: Explain the relationship between variability and the number of genes controlling a trait.
• C2.5k: Compare the pattern of inheritance produced by genes on the sex chromosomes to that produced by genes on autosomes, as investigated by Morgan and others.
• C2.3s: Interpret patterns and trends of inheritance of traits and predict the probability using pedigrees and Punnett squares. Draw and interpret pedigrees.

Terminology

• Inheritance: The process by which genetic information is passed from parents to offspring.
• Gametes: Haploid cells (sperm and egg) that fuse during fertilization to form a diploid zygote.
  • Meiosis occurs to form haploid gametes.
  • Male and female gametes nuclei fuse to form a zygote.
  • The zygote has two sets of each chromosome (diploid).
• Acquired traits: Traits that are influenced by the environment (also referred to as "environmental").
• Phenotypic plasticity: The capacity of an organism to develop traits suited to its environment by varying patterns of gene expression.
  • Organisms can respond to their environment by varying gene expression.
  • Example: Pale skin becoming darker with increased sunlight exposure, and then paler again when sunlight diminishes.
• Genes: Segments of a chromosome that code for a protein.
• Alleles: Different forms of the same gene. You get one allele from each parent.
• Zygosity:
  • Homozygous: Having two identical alleles for a gene.
    • AA - Homozygous for the dominant allele.
    • bb - Homozygous for the recessive allele.
  • Heterozygous: Having two different alleles for a gene (e.g., Aa).
  • Hemizygous: Having only one allele for a gene (e.g., males with sex-linked genes on the X chromosome).
• Genotype: The alleles an organism possesses (e.g., Rr).
• Phenotype: The observable trait of an organism resulting from its genotype and environmental factors (e.g., tall).
  • RR - Homozygous dominant, resulting in a specific phenotype.
  • Rr - Heterozygous, may result in a different phenotype (e.g., pink).
  • rr - Homozygous recessive, resulting in a specific phenotype (e.g., white).
  • Carrier: Heterozygous individual carrying a recessive allele.

Punnett Squares

• Used to predict genotypic and phenotypic outcomes of offspring.

• Combines alleles of two parents.

• IB expects phenotypes to be written in the box underneath resulting genotypes.

• Example:

  • R is dominant for round seeds.
  • r is recessive for wrinkled seeds.
  • Rr = round (1/4)
  • Rr = round (2/4) or (1/2)
  • rr = wrinkled (1/4)

Mendel's Experiments

• Mendel used the pea plant and conducted experiments over 8 years (1853-1861).

• Studied the inheritance of traits in pea plants.

• He selected pure bred (homozygous dominant or homozygous recessive) plants as the parent generation to complete his experiments with.

• Mendel's 7 Observable Traits:

  • Flower Color: Purple, White
  • Plant Height: Tall, Short
  • Seed Color: Yellow, Green
  • Seed Shape: Round, Wrinkled
  • Pod Color: Green, Yellow
  • Pod Shape: Inflated (full), Constricted (flat)
  • Flower Position: Axial, Terminal

• Mendel self-pollinated the pea plant traits that only had two forms.

  • Observation: offspring had same trait as parent plant.
  • What we know now: all homozygous.

• Mendel then cross-pollinated a tall plant with a short (dwarf) plant

  • Expectation: offspring would have intermediate height.
  • Observation: offspring as tall as the tall parent.

• This was the pattern for all of the traits. Each one was not an intermediate, but rather they exhibited the characteristic of one parent plant but not the other.

• Next, Mendel decided to allow the F1 plant to self-pollinate.

  • Observation: the trait that wasn't observed at first, re-appeared in the F2 generation.
  • A 3:1 ratio of tall:dwarf was observed.

• Mendel used algebra to determine what was happening. These symbols now correspond to alleles.

  1. Mendel crossed two purebred plants ($TT = all F1 offspring Tt$)
  2. Mendel self-pollinated the F1 offspring ($Tt = 3 tall, 1 dwarf (tt)$)The allele "T" determines height (it is dominant) The recessive allele (t) has no effect if the dominant allele is present

The Principle of Dominance

• Dominant alleles are always expressed or always appear in an individual.
• Recessive alleles are present but inactive and are not usually expressed.
• When individuals with different traits are crossed, the offspring will express the dominant trait as their phenotype

Mendelian Ratio (3:1)

• Observed in F2 generations.
• Examples:
  • Flower color: dominant vs. recessive: 705:224 (3.15:1)
  • Seed color: 6022:2001 (3.01:1)
  • Seed shape: 5474:1850 (2.96:1)
  • Pod color: 428:152 (2.82:1)
  • Pod shape: 882:299 (2.95:1)
  • Flower position: 651:207 (3.14:1)
  • Plant height: 787:227 (2.84:1)

The Law of Segregation

• Two alleles coding for the same trait separate during gamete formation.
• This is why we separate them in a punnett square because we do not know which gamete the parent will pass on to their child.

Genotypic and Phenotypic Ratios

• The phenotypic ratio is the ratio of offspring with one trait compared to another. Ex: 3 round: 1 wrinkled.
• The genotypic ratio is the ratio of offspring with one genotype compared to other genotypes Ex: 1 RR: 2 Rr: 1 rr.
• In simple mendelian genetics: only 2 phenotypes exist
• When 2 heterozygotes are bred, the phenotypic ratio is 3:1

Test Cross

• A test cross is used to determine whether a dominant phenotype is homozygous or heterozygous.
• You cross the dominant phenotype with a recessive phenotype.

Test Cross Examples

• If unknown is homozygous (BB):
  • Phenotypic Ratio: 100% Black
• If unknown is heterozygous (Bb):
  • Phenotypic Ratio: 50% Black; 50% White
• Example:
  • In Pea plants, the allele for purple flowers is dominant over the allele for white flowers.
  • A plant with purple flowers is crossed with a white-flowered plant and produces 40 offspring in total.
  • 18 offspring are white flowered. What is the genotype of the purple-flowered parent?
  • B. Pp
  • recessive 18 pp (white)
  • Therefore 40-18 = 22 purple.

Non-Mendelian Genetics

• Most traits are complex and do not follow the simple mendelian genetics.
• The following are examples of alternate patterns of inheritance:
  • Incomplete Dominance
  • Codominance
  • Multiple Alleles
  • Lethal Alleles
  • Regulatory Genes

Incomplete Dominance

• Both alleles are equally dominant.
• Heterozygote phenotype is an intermediate blend.
• 3 different phenotypes exist.
• When you breed two heterozygotes together, the resulting phenotypic ratio is 1:2:1
• Both Alleles are represented by Capital letters and a subscript or superscript.
• Example:
  • smooth fur (FSS), rough fur (FRFR) or an intermediate fur texture (FSFR).
  • A rabbit with smooth fur is bred with a rabbit with an intermediate fur texture. What is the probability of producing a rabbit with rough fur?
  • A. 0
  • 3 phenotypes; All capital letters with superscript
  • Fur (F)with superscript R and S
  • Blended FRFR FSFS X FSFR

Codominance

• Both alleles are independently and fully expressed.
• Heterozygote phenotype results in