Chapter 2 - Mendelian Patterns of Inheritance

BIOL 1P91 Part 3

• The course is BIOL 1P91, Part 3
• The instructor is Dr. Lori MacNeil
• Email: [email protected]
• Office: MC F229
• Office hours by appointment

Interactive Tool

• A tool called Slido will be used for interactive polls and practice questions during lectures.
• Students can access Slido from any smartphone, tablet, or laptop.
• Slido information is available at slido.com and #Meeting.

Mendelian Patterns of Inheritance

• This is Chapter 17 of a Biology textbook, likely Brooker | Widmaier | Graham | Stiling, Sixth Edition.
• The chapter outline includes Mendel's Laws of Inheritance, the Chromosome Theory of Inheritance, Pedigree Analysis of Human Traits, Sex Chromosomes and X-Linked Inheritance, Variations in Inheritance, Gene Interaction, and Genetics and Probability.

Explaining Inheritance

• Inheritance is the acquisition of traits passed from parent to offspring.
• Historically, pangenesis was a theory postulating inheritance through "seeds" produced by body parts.
• Late 19th-century theories also included that if traits changed over a lifetime, hereditary material would also change, leading to inherited modified traits.
• The Lamarck's theory of acquired characteristics is described in terms of giraffe's neck—traits acquired during an organism's lifetime can be passed on to future generations.
• Blending inheritance, another theory which is also refuted, suggests hereditary traits are blended together in offspring, with these traits passed to subsequent generations.

Gregor Mendel

• Gregor Mendel is considered the "Father of modern genetics".
• He was a priest who studied physics and mathematics.
• His seminal work on pea plants, started in 1856, was published in 1866.
• His work was largely ignored until rediscovered around 1900.
• Mendel's studies have revolutionized the understanding of inheritance.

Garden Pea (Pisum sativum)

• Garden peas have several advantageous properties for genetic studies:
  • Genetic variation: Many plant varieties with visible differences in characteristics.
  • Easy self-fertilizing: Pollination is facilitated by the tightly enclosed flowers.
  • Easy to make crosses: Manual cross-pollination or hybridization is easily performed.
• Differences can be seen in shape, color, etc. for traits

Single-Factor Crosses

• Follows the inheritance of traits for a single character.
• P generation: True-breeding parents (e.g., homozygous tall x homozygous dwarf).
• F₁ generation: Offspring of the P cross.
• F₂ generation: Offspring obtained by allowing F₁ individuals to self-fertilize.

The Data (Mendel's Results)

• Mendel's experiments with peas demonstrated a consistent 3:1 ratio in the F₂ generation (i.e. 3/4 individuals with the dominant trait and 1/4 individuals with the recessive trait).
• His data showed that traits were not blended but rather retained their individual identities through generations

Three Important Ideas from Mendel's Work

1. Traits: Existing in two forms—dominant and recessive
2. Genes: An individual carries two genes for a character, and each gene has variant forms (alleles).
3. Segregation: Two alleles of a gene separate during the formation of gametes, passing one allele to each gamete.

Genotype and Phenotype

• Genotype: The genetic makeup of an individual for a particular trait or set of traits.
• Homozygous dominant: Having two identical dominant alleles (e.g., TT)
• Homozygous recessive: Having two identical recessive alleles (e.g., tt)
• Heterozygous: Having two different alleles (e.g., Tt)
• Phenotype: The observable characteristics of an individual.

Punnett Squares

• A tool to predict the genotypes and phenotypes of offspring in genetic crosses.
• Five steps:
  • Write down the genotypes of the parents.
  • Write down the possible gametes for each parent.
  • Create an empty Punnett square.
  • Fill in the possible genotypes of the offspring by combining the alleles of the gametes.
  • Determine the relative proportions of genotypes and phenotypes in the offspring.

Testcross

• A genetic cross to determine the genotype of an individual that has a dominant phenotype.
• An individual with a dominant phenotype is crossed to a homozygous recessive individual—if any recessive offspring result, the parent was heterozygous.

Two-Factor Crosses

• Follows the inheritance patterns of two different traits in an organism.
• Two hypothetical postulates:
  • Linked assortment (traits inherited together): This predicts a 3:1 ratio in the progeny.
  • Independent assortment (alleles independently distributed to gametes): This predicts a 9:3:3:1 ratio in the progeny.
• Mendel's results validated independent assertion.

Mendel's Results (Data)

• The F₁ generation plants are dihybrids meaning that the plants are hybrid with respect to both traits.
• In F₂ generation results using independent assortment, there is a 9:3:3:1 inheritance pattern.

Mendel's Law of Independent Assortment

• The alleles of different genes are independently assorted during gamete formation.

Pedigree Analysis

• Pedigree analysis examines the presence of a particular trait across generations in a family.
• Used to understand the inheritance of genetic diseases and determine whether the mutant allele is dominant or recessive
• Allows for prediction of the likelihood of and individual being affected with certain traits.

Example: Cystic Fibrosis

• ≈3% of Europeans are heterozygous carriers of the recessive CFTR (disease-causing allele).
• Homozygous individuals exhibit CF symptoms resulting from a mutated CFTR gene (encoding a transmembrane chloride channels).
• This interferes with chloride ion flow, altering water flow out of cells, producing thick and sticky mucus.

Human Disease

• Many human genetic diseases are often recessive, as disease alleles persist in heterozygous carriers, who are unafected.
• Huntington Disease is an example of a human disease that shows dominant traits:
  • Symptoms occur later in life, often after reproduction.
  • The normal allele encodes a functional nerve protein.
  • The mutant allele forms an abnormal, aggregating protein, disrupting nerve function.

Pedigree of a Dominant Trait

• Visual depiction illustrating inheritance of dominant traits through generations.
• Individuals showing a particular trait are identified in a family pedigree diagram.

Autosomes vs. Sex Chromosomes

• Autosomes are chromosome pairs found in both male and female organisms.
• Sex chromosomes are distinctive, differing between the sexes.
• Genes on the sex chromosomes present diverse inheritance patterns

Human Chromosomes

• A human typically has 46 chromosomes.
• 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males).
• The presence of a Y chromosome determines maleness in humans.

X-Linked Traits

• X-linked traits are present on the X sex chromosome but not the Y.
• In humans, the X chromosome is larger and carries more genes than the Y.
• Females can be homozygous or heterozygous for X-linked genes.
• Males are hemizygous for X-linked genes (i.e. exhibiting only one copy of each X-linked gene).

Example: Hemophilia A

• X-linked recessive disease (more frequent in males).
• Results from defective clotting proteins—characterized by excessive bleeding.
• If a heterozygous female and an unaffected male are parents, sons have a 50% chance of having hemophilia; daughters have a 25% chance.

Morgan's Drosophila Experiments

• Drosophila melanogaster experiments demonstrated that specific traits (e.g., eye color) are linked to particular chromosomes.
• Eye color genes were linked to the X chromosome.

Morgan's Drosophila Crosses

• These experiments studied the inheritance of traits (e.g., eye color) in fruit flies.
• Crossing flies with different eye color genes demonstrated some traits are linked across generations.

Protein Function Explains Dominance

• Wild-type alleles are prevalent alleles, encoding proteins that function correctly.
• Mutant alleles are rare, usually arising from a mutation, causing decreased or absent of functional protein products.
• In many cases, having 50% of the normal protein amount leads to a normal trait expressed while a homozygous recessive trait occurs when there is no or almost no functional protein expressed, resulting in a recessive phenotype.

Genotype-Phenotype Connection in Simple Mendelian Inheritance

• The relationship between genotype and phenotype is depicted in a simplified model for a single-gene trait.
• The amount of functional protein produced directly correlates with the phenotype expressed.

Some Single-Gene Traits: Do not exhibit a simple dominant/recessive relationship

• Some traits do not have a simple dominant/recessive relationship between alleles.

Incomplete Dominance

• Heterozygous individuals exhibits a phenotype between the two homozygous phenotypes.
• This is apparent when crossing 2 plants—red and white flowers—producing offspring with pink flowers.

Codominance

• A heterozygous trait in which both alleles are expressed simultaneously and resultantly, leading to both phenotypes in the trait.
• ABO blood types are a well-known example in humans.

ABO Blood Group

• Blood type depends on which two alleles are inherited (multiple alleles).
• Each blood group is characterized by the antigens present on red blood cells and antibodies in the plasma.

Role of the Environment

• Phenotype (observable characteristics) can be influenced by the environment.
• The norm of reaction describes the range of phenotypes seen in individuals with the same genotype under differing environmental conditions.
• For example, genetically identical plants may grow to different heights in different temperatures/environments.

Complex Traits

• Most traits are influenced by multiple genes and environmental factors.
• Gene interactions (e.g., epistasis) occur when a single trait is controlled by the products of two or more genes.

Example: Epistasis in Sweet Peas

• The relationship between genes involved in flower color determination was studied using a breeding experiment.
• The results suggested that more than one gene is involved in determining flower color.
• If the genotype doesn't have the dominant allele for one gene, then flowers will be white regardless of the other gene's genotype.

Enzymatic pathway

• An example demonstrating how genes work together to produce a color (e.g., purple flower pigment) through a series of enzymatic reactions.

Epistasis in Sweet Pea

• The relationship between genes/alleles involved in flower color determination.
• C allele is dominant to c allele for flower color.
• P allele is dominant to p allele for flower color.
• cc genotype masks P, and pp masks C genotypes, leading to white flowers even when dominant alleles exist.

Polygenic Inheritance

• Traits exhibiting continuous variation across a phenotypic range.
• Examples include height, weight, skin color, metabolic rate, and heart size in humans.
• Polygenic inheritance is determined by multiple genes and influenced by environmental factors.

Example: Grain Pigmentation in Wheat

• This demonstrates polygenic inheritance, illustrating how multiple genes influence the continuous variation in grain color in wheat.

Different Types of Mendelian Inheritance Patterns