Inheritance and Genetics Lecture Note
Evolution and DNA Review
Evolution is driven by natural selection: the process by which organisms with features that help them adapt to the environment preferentially survive and reproduce, increasing the frequency of those traits in the population.
Fitness in evolution refers to reproductive success: the ability to survive and produce viable offspring, thereby increasing allele representation in the next generation.
Historical ideas about inheritance:
Preformation Hypothesis: early idea that a miniature version of an organism exists in the gametes.
Blending Inheritance: the phenotype of offspring is a uniform blend of parental phenotypes (now considered outdated).
Pangenesis (Aristotle): particles from all body parts contribute to eggs and sperm; each parent contributes a mixture to offspring.
Issues with early theories include lack of discrete units and inability to explain patterns of inheritance.
DNA basics:
DNA has two key properties: stability (faithfully preserves genetic information) and replicability (enables inheritance).
Watson & Crick (1953) elucidated the double-helix structure, building on X-ray data from Rosalind Franklin (who did not receive full credit at that time).
DNA is located in:
Nucleus (eukaryotic cells)
Mitochondria (in animals) and chloroplasts (in plants) as additional DNA-containing organelles
DNA structure:
Nucleotide: a linked set consisting of a phosphate, a sugar (deoxyribose), and a nitrogenous base.
\text{Nucleotide} = {\text{phosphate}, \ \text{deoxyribose}, \ \text{nitrogen base}}
Backbone: alternating phosphates and sugars; rungs (steps) formed by base pairs.
Base pairs (complementarity):
A \text{ pairs with } T
G \text{ pairs with } C
Gene: a section of DNA that codes for a particular trait.
Allele: an alternate version of a gene.
Location and content of DNA:
Found in nucleus; also present in mitochondria and (in plants) chloroplasts.
In eukaryotes, the nucleus contains chromosomes.
Mechanism of inheritance (Mendelian foundations):
Gregor Mendel (1822–1884) proposed laws of inheritance.
Key ideas:
No blending inheritance; inheritance occurs via discrete units (genes).
Genes come in different versions (alleles).
Some alleles are dominant while others are recessive.
Mendelian traits and patterns:
Simple Mendelian traits illustrate how alleles segregate and assort.
Examples of simple traits include:
Hitchhiker's thumb (recessive)
Cheek dimples (dominant)
Widow's peak (dominant)
Polygenic traits exist when one phenotype is influenced by two or more genes.
Polygenic inheritance and quantitative variation:
Phenotypes influenced by multiple genes show continuous variation (e.g., height).
Data typically yield a bell-shaped curve for such traits (normal distribution).
Example graphic concept: multiple additive alleles (e.g., R1, R2, r1, r2) across several loci can produce many phenotypes.
Definitions you need to know:
Dominant: an allele that masks the effect of other alleles for a trait; can refer to the dominant phenotype or trait.
Recessive: an allele masked by a dominant allele; can refer to recessive phenotype or trait.
Genotype: the specific alleles an organism has for a trait.
Phenotype: the physical expression of the genotype for a trait.
Genotype terminology:
Homozygous dominant: two dominant alleles (e.g., PP).
Homozygous recessive: two recessive alleles (e.g., pp).
Heterozygous: one dominant and one recessive allele (e.g., Pp).
Phenotypic patterns and genotype-phenotype mapping:
Complete dominance: the heterozygote phenotype matches the homozygous dominant phenotype; e.g., BB\, Bb\, bb with Bb showing the dominant trait when complete dominance is in effect.
Describing genotypes and phenotypes in crosses:
Punnett-square reasoning leads to classic ratios (e.g., monohybrid cross results).
Law of Segregation (Mendel):
The alleles for a trait segregate during gamete formation and reunite at fertilization.
Each gamete carries one allele for each gene; offspring receive one allele from each parent.
DNA replication and cell division: overview
Somatic cells (body cells) divide by mitosis to produce two genetically identical diploid daughter cells.
Germ cells (gametes) divide by meiosis to halve the chromosome number, producing haploid gametes.
Chromosome basics (human):
Humans have 23 homologous pairs of chromosomes (46 total).
22 autosomes and 1 pair of sex chromosomes (XX for female, XY for male).
Somatic cells are diploid (2n = 46); gametes are haploid (n = 23).
Mitosis vs. Meiosis: key differences
Mitosis: diploid cells, replication and division yield two identical diploid daughter cells; purpose for growth, tissue repair.
Meiosis: reduction division, halving chromosome number, producing four haploid gametes; introduces genetic diversity via crossing over.
Meiosis details and key terms:
Meiosis reduces genetic content by half so offspring have the correct chromosome number when gametes fuse.
Haploid: a cell with a single set of unpaired chromosomes (n).
Crossing-over: exchange of genetic material between homologous chromosomes during meiosis, increasing genetic diversity.
Chromosome behavior in cell division (high level):
During replication, chromosomes are duplicated and then separated into daughter cells.
In mitosis, sister chromatids separate; in meiosis, homologous chromosomes separate in meiosis I, followed by separation of sister chromatids in meiosis II.
Law of Independent Assortment and Linkage:
Law of Independent Assortment: genes for different traits are sorted independently of one another in gamete formation.
Linkage: genes located close together on a chromosome tend to be inherited together; this is an exception to independent assortment.
Sex-linked traits:
Genes located on the sex chromosomes (commonly X-linked).
In humans, females have two X chromosomes (XX) and males have one X and one Y (XY).
Sex-linked inheritance can lead to different phenotypic ratios in males and females (e.g., color vision gene examples with X-linked inheritance).
Mutation and sources of variation:
Mutation: a random change in a gene or chromosome that can create a new trait; effects can be advantageous, deleterious, or neutral.
Thomas Hunt Morgan (1866–1945) contributed to the study of mutation and sex-linked traits.
Variation in populations and gene pools:
Variation arises through mutation, recombination during meiosis (crossing over), and segregation of alleles.
Codominance and ABO blood groups:
Codominance: both alleles in a heterozygote are fully expressed; neither masks the other.
Examples: Roan coat color in cattle and horses; ABO blood type system.
Blood type genetics:
Alleles: I^A, I^B, i
Phenotypes:
I^A I^A\text{ or } I^A i \rightarrow \text{Type A}
I^B I^B\text{ or } I^B i \rightarrow \text{Type B}
I^A I^B \rightarrow \text{Type AB}
ii \rightarrow \text{Type O}
Incomplete dominance:
The heterozygous phenotype is distinct and often intermediate between the homozygous phenotypes.
Classic examples include snapdragons and carnations.
Genes you don’t get from your parents (TEDEd reference):
A referenced concept/animation discussing aspects of inheritance beyond straightforward parental gene transmission (not detailed here).
Key questions and takeaways:
At what level does inheritance operate? Gene-level inheritance underpins much of classical genetics.
The material covers a broad view from molecular to organismal inheritance, including how Mendelian patterns interact with polygenic, codominant, and incomplete-dominant inheritance.
Real-world and ethical relevance:
Understanding inheritance informs agriculture (polygenic traits, selective breeding), medicine (genetic disorders, blood types, sex-linked conditions), and evolutionary biology (variation and adaptation).
Notes on terminology and equations to remember:
Genotype notation examples: PP, Pp, pp
Punnett-square expected ratios (monohybrid): genotype 1:2:1; phenotype 3:1 under complete dominance.
Dihybrid cross phenotype ratio under complete dominance: 9:3:3:1.
Chromosome numbers: humans have 2n = 46; gametes have n = 23.
Base-pairing rules: A\leftrightarrow T, \ G\leftrightarrow C.
Blood type genotypes and phenotypes follow codominance for I^A and I^B alleles and the recessive i allele.