ib bio hl genetics
Study Guide: Chapter 4 - Genetics
Key Concepts & Main Ideas
Introduction to Genetics
Genetics studies genes and their transmission across generations.
Every organism has a unique genome made up of genes and alleles.
Genes determine characteristics; alleles are gene variations causing differences among individuals 1.
Genome & Chromosome Structure
The genome: all genetic information in an organism; present in every cell.
Prokaryotes have a single circular chromosome, 'naked' DNA, plus plasmids.
Eukaryotes have multiple linear chromosomes associated with histones, enclosed in a nucleus [[2], [3], [4]].
Diploid cells contain homologous chromosome pairs; haploid cells contain one set.
Chromosomes have specific loci for genes; alleles are different forms of a gene [[2], [5]].
Karyotypes and karyograms show chromosome number/type and can reveal abnormalities such as trisomy (Down syndrome) 6.
Sex chromosomes determine gender (XX female, XY male); X-inactivation leads to Barr bodies in females [[7], [8]].
Principles of Inheritance
Gregor Mendel’s work laid the foundation for understanding inheritance.
Gametes are haploid; fertilization creates diploid zygotes.
Dominant alleles mask recessive ones; heterozygous genotypes show dominant phenotype [[10], [11]].
Punnett grids predict genotypes and phenotypes from parents' genotypes [[12], [13]].
Codominance: both alleles expressed (e.g., blood groups IA, IB).
Incomplete dominance: heterozygous phenotype is intermediate (e.g., flower color) [[14], [15]].
Sex-linked traits (on X chromosome) show different inheritance patterns, e.g., hemophilia, color blindness [[16], [17]].
Pedigree charts trace inheritance over generations 17.
Genetic Diseases
Many genetic diseases are recessive; some are dominant.
Examples: PKU (recessive, treatable), Huntington's disease (dominant) [[18], [19]].
Consanguinity increases risk of recessive disorders 22.
Variation & Polygenic Inheritance
Continuous variation (e.g., height, skin color) results from polygenes and environment.
Discrete variation (e.g., blood group) results from single genes [[23], [24]].
Phenotypic plasticity: same genotype produces different phenotypes in different environments.
Environmental sex determination (ESD) occurs in some reptiles; temperature affects offspring sex 25.
Dihybrid Crosses and Linked Genes
Dihybrid crosses involve two genes; unlinked genes assort independently (Mendel’s law).
Linked genes are located on the same chromosome and inherited together.
Crossing over during meiosis can separate linked genes, producing recombinants [[26], [27], [28]].
T.H. Morgan’s work with Drosophila demonstrated gene linkage and crossing over [[30], [31]].
Statistical Testing in Genetics
Chi-squared (χ²) test assesses if observed results fit expected Mendelian ratios.
Null hypothesis: no significant difference between observed and expected results.
Degrees of freedom = categories - 1.
Critical values from χ² tables determine acceptance or rejection of null hypothesis [[36], [37], [38]].
Important Details
Alleles represented by letters (dominant uppercase, recessive lowercase).
Homozygous: two identical alleles; heterozygous: two different alleles.
Codominance example: ABO blood group system.
X-inactivation leads to mosaicism (e.g., tortoiseshell cats).
Linked gene notation uses horizontal lines (e.g., A—B / a—b).
Mendelian ratios (monohybrid: 3:1, dihybrid: 9:3:3:1) often altered by linkage.
Critical Arguments & Ethical Considerations
Karyotyping and prenatal diagnosis raise ethical questions about pregnancy termination.
Genetic testing can impact privacy and insurance rights.
Consanguineous marriages increase genetic disease risk; cultural factors vary.
Mendel’s data may have been biased or selectively reported; later work refined genetics understanding 35.
Debate over calling Mendel’s principles “laws” given exceptions.
Study Questions
Define genome, diploid, and homologous chromosomes.
Explain the relationship between a locus, gene, and allele.
List differences between prokaryotic and eukaryotic chromosomes.
How are karyograms useful in prenatal diagnosis?
What is X-inactivation, and why does it occur?
Differentiate between genotype and phenotype.
How do Punnett grids help predict genetic outcomes?
Define codominance and incomplete dominance with examples.
What is a sex-linked trait? Give examples.
Explain the significance of polygenic inheritance.
How do linked genes differ from unlinked genes in inheritance?
Describe the role of crossing over in gene recombination.
What is the purpose of the chi-squared test in genetics?
Discuss ethical issues surrounding genetic testing and prenatal diagnosis.
Key Dates
1860s: Gregor Mendel’s pea plant experiments.
1911: T.H. Morgan’s publication on sex-linkage.
2003: Completion of human genome sequencing.
Memory Aids
Genome = All genetic info (think "Gen" = "General" = whole)
Dominant = uppercase letter; Recessive = lowercase.
Homozygous = same alleles; Heterozygous = different alleles.
Codominance = "Co-" = together, both expressed.
Incomplete dominance = "In-between" phenotype.
X-inactivation = female cells "turn off" one X → Barr body, think "Barr = Barred out."
Linked genes = "Linked like a chain" on the same chromosome.
Chi-squared test: χ² sounds like "check squared" – used to check data fit.
Mendel’s ratios: Monohybrid (3:1), Dihybrid (9:3:3:1).