Inheritance and Genetics

INHERITANCE

Learning Intentions

• Main Goal: Understand how traits are passed from parents to offspring.
• Predicting Traits: Use Punnett squares to predict single-trait inheritance.
• Success Criteria:
  • Distinguish between genotype and phenotype.
  • Use Punnett squares to predict single-trait inheritance.

Gregor Mendel: The Father of Genetics

• Who: Austrian monk Gregor Mendel.
• When: 1856: began studying inheritance patterns in pea plants.
• What: Revolutionized genetics.
• Before Mendel: Offspring traits were thought to be a blend of parental traits.
• Mendel's Impact: His work forms the foundation of modern genetics.

Mendel’s Discoveries

• Selective Cross-Breeding: Mendel found traits didn't blend (tall + short plants ≠ medium plants).
• Three Main Conclusions:
  • Units/Factors (Genes): Inheritance is determined by 'units' or 'factors' (now called genes) passed on unchanged.
  • Inheritance from Parents: Individuals inherit one unit from each parent for each trait.
  • Traits Can Skip Generations: Traits can be passed on even if they don't appear in an individual.

Genes and Traits

• Gene Definition: A gene is a section of DNA coding for a protein.
• Mutations: Changes in DNA sequence can cause variations in genes, altering the protein produced.
• Alleles: Variations of a gene are called alleles; one allele for each trait is inherited from each parent.
• Proteins and Physical Traits: Proteins influence the physical traits we observe.

Genotypes vs. Phenotypes

• Phenotype: Observable characteristics or traits in an individual (e.g., eye color).
• Genotype: The genetic information on an allele for a trait; not observable without genetic testing.
• Representing Genotypes: Letters represent alleles and variations (e.g., B for brown eyes, b for blue eyes).

Homozygous vs. Heterozygous

• Homozygous: Two identical alleles of the same gene (e.g., BB or bb).
• Heterozygous: Two different alleles of the same gene (e.g., Bb).
• Genotype Representation: Homozygous or heterozygous alleles are shown through the genotype for the gene.

Eye Colour Example

• Brown eyes allele: $B$
• Blue eyes allele: $b$
• Homozygous Examples: $BB$, $bb$
• Heterozygous Example: $Bb$

Dominant vs. Recessive

• Dominant Phenotypes: Seen even with only one allele for the trait (represented by a capital letter, e.g., B for brown eyes).
• Recessive Phenotypes: Expressed only when homozygous for the recessive allele (represented by a lower-case letter, e.g., b for blue eyes).
• Heterozygous Expression: In heterozygous pairs, the dominant phenotype is usually expressed.

Eye Colour Example (Phenotypes)

• Genotypes and Phenotypes:
  • $Bb$: Brown eyes (heterozygous).
  • $BB$: Brown eyes (homozygous dominant).
  • $bb$: Blue eyes (homozygous recessive).

Dwarfism as a Dominant Phenotype

• Corgi Example: The short legs phenotype in corgis is due to a dominant dwarfism gene.
• Dominant Expression: Only one copy of the gene is needed to express the phenotype.

Punnett Squares

• Purpose: Predict the probability of different phenotypes and genotypes.
• Origin: Named after Reginald Punnett, who revisited Mendel's ideas.
• Function: Shows possible allele combinations from parent gametes (sperm and ova) in a zygote (fertilized ova cell).
• Monohybrid Cross: Punnett squares can show the possible inheritance of a single trait.

Probability Example

• Cross: $Bb \, x \, Bb$
• Genotype probabilities:
  • $BB$: Brown eyes.
  • $Bb$: Brown eyes.
  • $bb$: Blue eyes.
• Phenotype probability:
  • 75% chance of brown eyes.
  • 25% chance of blue eyes.

Chromosomes and Inheritance Patterns

• Autosomal Inheritance: Traits coded by genes on chromosomes 1-22 (autosomes).
  • Equal chance of inheritance in XX and XY individuals.
• Sex-Linked Inheritance: Traits coded by genes on sex chromosomes (X or Y).
  • Affects XX and XY individuals differently.
  • Males (XY) and females (XX) inherit these chromosomes differently.

X-Linked Recessive Conditions

• Example: Colour blindness.
• XB: normal allele, Xb: allele for colorblindness
• Probabilities of inheriting:
  • $XBXB$: unaffected female
  • $XBXb$: carrier female
  • $XBY$: unaffected male
  • $XbY$: Affected male
  • 50% of offspring are with $XY$. $XbY$ will be colour blind.
  • 0% chance of colour blind $XX$ (at least one $XB$ allele present).
  • One $XX$ is a carrier ($XBXb$) and may pass on colour blindness to future children.

Genetics Case Studies

• Purpose: Deep dive into real-life examples to extend understanding.
• Real-World Connections: Connect classroom learning to unusual cases.

HeLa Cells

• Henrietta Lacks: Cancer cells taken without permission in the 1950s.
• Impact: First human cells to grow continuously in vitro; used in vaccines, cancer research, space science.
• Ethical Issues: Raises questions about consent in science.

CCR5 Gene Mutation

• CCR5 Gene: Some individuals have a mutation providing near-immunity to HIV.
• Normal CCR5 Gene: Produces a membrane protein that viruses use to enter cells.
• Impact: Scientists study these individuals to find HIV cures; demonstrates that mutations can have positive effects.

Pedigrees

• Pedigree Chart: A diagram that shows the occurrence of phenotypes through several generations of genetically related individuals.
• Purpose: To map out inheritance over generations.

Pedigree Chart Symbols

• Males: Squares
• Females: Circles
• Unknown Individuals: Diamond shape
• Affected Individuals: Coloured in
• Relationships: Connected with a line between the partners
• Generations: Grouped and numbered in Roman numerals
• Individuals in a Generation: Numbered from left to right

Carriers

• Definition: A person who carries the allele for a trait but does not display it in their phenotype.

Limitations with Pedigree Charts

• The pedigree tradition has a history of misrepresenting people with marginalized identities.
• Today, most of the pedigree charts used in science education have no means of representing diverse family structures or representing individuals who are lesbian, gay, bisexual, transgender, queer, or intersex (LGBTQI+).
• There is debate about what is considered ‘standard’ amongst scientists, with different systems used throughout the years and across disciplines.
• There are a number of alternatives, such as genograms (left), which are an alternative to a pedigree chart, used in a variety of fields outside of genetics. Genograms include symbols for LGBT identity, non-biological children, and even habits and social relationships