BIO 340 General Genetics - Transmission Genetics I Notes

Overview of Transmission Genetics I

• BIO 340 General Genetics, Transmission Genetics I (Mendelian genetics)

• Focus: how genes pass from one generation to the next, using Mendelian principles

• Sources in slides: Lecture content, genetic analysis perspectives, and Mendel’s experiments with peas

• Reading assignment reference: Genetic Analysis, Sanders and Bowman (3rd ed), Chapter 2 (2.3, 2.5) and quizzes

Defining a gene: three perspectives

• Molecular genetics perspective

  • Genes are the chemistry and molecular nature of DNA

  • Example sequence and translation: coding strand, template strand, mRNA, codons, and amino acids

  • Coding strand example: 5' ext{ATG ACA CTG GGT ACG CTT TAA} 3'

  • Template strand example (complement): 3' ext{TAC TGT GAC CCA TGC GAA ATT} 5'

  • mRNA transcribed: 5' ext{AUG ACA CUG GGU ACG CUU UAA} 3'

  • Codons and amino acids: MET, THR, LEU, GLY, THR, LEU; stop codon marks end of translation

• Population genetics perspective

  • Allelic frequencies in populations and how they change over generations (population dynamics of genes)

  • Visual example: frequency changes across generations in populations

• Transmission (Mendelian) genetics perspective

  • The fundamental unit of heredity (gene) that passes information from parents to offspring

  • Emphasis on patterns of inheritance across generations

Genotype vs. Phenotype

• Phenotype: observable characteristics of an organism (e.g., tall vs. dwarf, purple vs. white flowers)

• Genotype: the specific allelic makeup at a gene locus (e.g., AA, Aa, aa)

• Relationship nuances

  • Not strictly one-to-one: the same phenotype can arise from different genotypes under varying environments (e.g., identical twins in different environments)

  • Genotype informs phenotype, but environment can modulate expression

Key genetic terms: gene, locus, allele, genotype

• Gene: fundamental unit of heredity

• Locus: physical location of a gene on a chromosome

• Allele: variant form of a gene at a given locus

• Genotype: allelic state of an individual at a locus

• For diploids (2n):

  • Genotypes can be homozygous (two identical alleles) or heterozygous (two different alleles)

• Example: single locus with two alleles A and a

  • Alleles: A, a

  • Genotype examples: AA, Aa, aa

• When multiple alleles exist at a locus (e.g., A1, A2, A3), genotypes include combinations like A1A2, A2A3, etc.

Quick Quiz: alleles vs. genotype (concept check)

• Question: How do alleles relate to genotype?

• Answer: A. Genotype consists of the alleles an organism has for a particular gene

• Rationale: A genotype is the set of alleles present at a locus (e.g., AA, Aa, aa). Alleles are the variants that compose the genotype.

Mendel and his innovations

• Historical context: 19th-century belief in blending inheritance was inadequate to explain results

• Gregor Mendel: mid-1800s, founder of genetic science; studied natural sciences and entered the priesthood to pursue education

• Model organism: Mendel used pea plants (Pisum sativum) to deduce general principles of heredity

• Mendel’s achievements with peas: studied phenotypes, counted offspring, and inferred underlying units of inheritance

• Five innovations that drove Mendel’s success:
  1) Rigorous control of crosses

  • Chose a suitable species; many offspring per cross; short generation time; easy to mate;
    self-pollination can produce true-breeding lines
    2) Use of discontinuous (dichotomous) traits

  • Traits with clear-cut categories (purple vs white, yellow vs green, etc.)

  • Avoided intermediates for clean counting
    3) Use of pure-breeding strains

  • True-breeding lines produce offspring resembling the parent when selfed

  • Cultivated closed flowers to prevent cross-pollination; established stable lines
    4) Quantification of results

  • Large data sets from many crosses; precise counts of phenotypes

  • Identified consistent phenotypic ratios that revealed underlying patterns
    5) Use of replicate, reciprocal, and test crosses

  • Replicates to increase data reliability

  • Reciprocal crosses swapped parental sexes to test inheritance independent of parent sex

  • Test crosses to reveal unknown genotypes by crossing with homozygous recessive

• What Mendel learned from each innovation

  • Controlled crosses enabled reliable data and generation tracking

  • Discontinuous traits allowed straightforward counting (no intermediates)

  • Pure-breeding strains established stable parental phenotypes across generations

  • Quantification revealed consistent ratio patterns across traits

  • Replicate, reciprocal, and test crosses helped validate patterns and reveal genotype information

Using monohydrids crosses to understand inheritance

• Monohybrid cross: focusing on one trait at a time (e.g., flower color)

• Terminology:

  • P (Parental generation): pure-breeding strains*

  • F1 (First filial generation): offspring of P cross

  • F2 (Second filial generation): offspring from F1 crosses

  • Monohybrid cross: cross between two heterozygotes (e.g., Aa x Aa) or between a heterozygote and a homozygous recessive (as a test cross for genotype discovery)

• Classic monohybrid cross result (Aa x Aa): phenotypes segregate in a 3:1 ratio; genotypes segregate in a 1:2:1 ratio

• Example data for several traits (selected):

  • Seed shape: round vs wrinkled → F1 all round; F2 ~ 3:1 round:wrinkled

  • Seed color: yellow vs green → F1 all yellow; F2 ~ 3:1 yellow:green

  • Pod shape: full vs constricted → F1 all full; F2 ~ 3:1 full:constricted

  • Pod color: green vs yellow → F1 all green; F2 ~ 3:1 green:yellow

  • Flower color: violet vs white → F1 all violet; F2 ~ 3:1 violet:white

  • Flower position: axial vs terminal → F1 all axial; F2 ~ 3:1 axial:terminal

  • Stem height: tall vs dwarf → F1 all tall; F2 ~ 3:1 tall:dwarf

• Mendel’s data supported the idea that traits are discrete units (genes) with two versions (alleles) per gene per individual

Mendel’s Law of Segregation

• Core idea: during gamete formation, each copy of a gene separates (segregates) equally so that each gamete carries one allele

• During fertilization, gametes unite at random, so offspring have two alleles for each gene, one from each parent

• Formal statement:

  • Part I: In gamete formation, copies of a gene segregate so that each gamete carries one member of the pair

  • Part II: In fertilization, randomly formed gametes unite without regard to which copy they carry

• Classic 2x2 Punnett cross for Aa x Aa

  • Gametes from each parent: A or a

  • Offspring genotypes: AA, Aa, Aa, aa

  • Genotype ratio: 1:2:1

  • Phenotype ratio (dominant vs recessive): 3:1 when one allele is completely dominant over the other

• Visualization with gametes and zygotes

  • Gametes: A or a from each parent

  • Zygotes: AA, Aa, Aa, aa

  • Resulting phenotype distribution reflects dominance and recessivity

How to visualize gene transmission: Punnett squares

• Example: Aa x Aa cross

  • Parental genotypes: Aa and Aa

  • Possible offspring genotypes: AA, Aa, Aa, aa

  • Phenotypic outputs depend on dominance; typically 3:1 for dominant phenotype to recessive phenotype

• More general approach

  • Each parent contributes one allele per gene per gamete (haploid gametes)

  • Fertilization pairs alleles to form diploid offspring

Test crosses: predicting genotypes

• Test cross purpose: determine unknown genotype of an individual with known phenotype

• Method: cross an individual with an unknown genotype but dominant phenotype with a homozygous recessive (aa)

• Interpretations:

  • If offspring are all heterozygous (Aa) with dominant phenotype, the unknown parent is AA

  • If offspring are 1:1 Aa:aa, the unknown parent is Aa

• Notation example: Aa x aa

  • If unknown parent is AA: all offspring will be Aa

  • If unknown parent is Aa: offspring will be 1:1 Aa:aa

Practice questions (selected from lecture)

• Question 1: alleles vs genotype

  • Answer: A. Genotype consists of the alleles an organism has for a particular gene

• Question 2: Mendel’s helpful strategies

  • Answer: C. Only II and IV (many offspring per generation and discontinuous traits)

• Question 3: Mendel’s Law of Segregation illustration

  • Answer: B. A pea plant with Aa genotype produces half A gametes and half a gametes

• Question 4: Guinea pig coat color cross (true-breeding black x true-breeding white; F1 all black; F1 x F1)

  • White progeny proportion in F2: frac{1}{4} (1/4 white)

• Question 5: In the black coat color cross (Bb x Bb) among black F2 progeny, proportion that are homozygous

  • Answer: frac{1}{3} (BB among black phenotypes is 1 of 3 equally likely black genotypes: BB, Bb, Bb)

• Question 6: Cross of black (B_) with white (bb) in F1 phenotype

  • Answer: A. All black (F1 are Bb)

Genotypes and alleles: deeper definitions

• Heterozygous for a gene A

  • Genotype: Aa

• Homologous chromosomes carry copies of the same gene

• In a diploid organism, each individual has two alleles per gene locus (one from each parent)

• Visual: Gene A on homologous chromosome pair; maternal and paternal chromosomes provide alleles A and a in a heterozygote

Summary of Mendel’s inheritance framework

• Genes exist in discrete units (not blending) and are inherited in predictable patterns

• Each individual carries two alleles per gene; offspring receive one allele from each parent

• Dominant and recessive relationships determine phenotype in heterozygotes

• Pure-breeding lines and controlled crosses enable clear interpretation of inheritance patterns

• The Law of Segregation underpins all Mendelian inheritance and is illustrated by 3:1 phenotypic ratios and 1:2:1 genotypic ratios in monohybrid crosses

Real-world relevance and connections

• Mendelian genetics provides foundational models for inheritance, but many traits are polygenic or influenced by the environment

• The use of model organisms (e.g., peas) demonstrates how simple systems can reveal general genetic principles applicable to more complex organisms

• Transmission genetics informs breeding programs, medicine (genetic testing, risk assessment), and evolutionary biology

• Ethical, philosophical, and practical implications (not deeply covered in these slides) include: implications of genetic testing, privacy, eugenics concerns, gene editing, and how genetic information shapes societal decisions

• Practical note: The slides emphasize fundamental concepts needed to understand transmission genetics, with reading assignments for deeper exploration of 2.3 and 2.5 in the Sanders and Bowman text

Quick reference: key equations and ratios (LaTeX)

• Mendel’s Law of Segregation concepts:

  • Gamete allele probability: P( ext{A}) = frac{1}{2}, \, P( ext{a}) = frac{1}{2}

  • Offspring genotype ratio for Aa x Aa: ext{AA}: ext{Aa}: ext{aa} = 1:2:1

  • Corresponding phenotype ratio for a dominance relationship: 3:1 (dominant:recessive)

• Monohybrid cross phenotype/genotype summary:

  • Phenotype with dominance: 3:1 (dominant:recessive)

  • Genotype: 1:2:1 (AA:Aa:aa)

• Test cross interpretation:

  • If unknown genotype is AA: offspring with aa will be all Aa (phenotype dominant)

  • If unknown genotype is Aa: offspring will be in a 1:1 ratio Aa:aa

Reading to be done before next lecture

• Genetic Analysis, Sanders and Bowman (3rd ed)

• Chapter 2: Transmission genetics 2.3, 2.5

• Complete Reading Quizzes 2 before Thursday’s class