12.1 Relatives and recessive homozygotes #
12.1.1 Consanguinity #
From Chapter 9, f(aa) = q²(1 − F) + qF. The second term is identity by descent. For a child of first cousins, F = 1/16 if the parents are not otherwise inbred. At a constructed q = 0.01, not a disease claim, random mating gives q² = 0.0001, while
f(aa) = (0.0001)(15/16) + (0.01)/16 = 0.000719,
about seven times higher, and most of the increase is qF. Rarer alleles are affected more, because q² shrinks faster than qF. A very rare recessive phenotype in a child of relatives suggests a shared ancestor. A recessive allele already common in the population often does not need related parents. The sum over all loci is not calculated here, and no universal multiplier from cousin marriage to birth defects is stated. Allele frequency need not change: this is inbreeding, not selection. On a pedigree, a cousin is not given the unrelated prior 2q. The shared path raises the chance they carry the familial allele.
12.2 Sampling error in human history #
12.2.1 Founder effects as illustrations #
A founder effect is a small group starting a population with a sample of alleles. Expansion does not put back what the founders lacked. The illustrations below are demographic sampling. They are not judgements of character, and they are not a reason to treat any group as uniform or as defined by a disease.
Finland’s history of a limited founding population, expansion, and local isolation left a frequency pattern called the Finnish disease heritage: some otherwise rare recessive alleles are unusually common, and some alleles common elsewhere in Europe are comparatively rare. The name is that pattern, not a description of people beyond ancestry and drift. The same kind of trace exists wherever few founders preceded a large population.
Certain recessive founder alleles, including alleles of HEXA associated with Tay–Sachs disease, are elevated among people of Ashkenazi Jewish ancestry. The same sampling reading fits other founder events, including those in French Canadian and Afrikaner history. Naming one case is a teaching choice, not a claim that the process belongs to one community, and not a stereotype. Disease alleles reveal the sample because they are noticed. Commonness after a founder event is not, by itself, evidence of advantage.
12.3 Where these notes stop #
12.3.1 The boundary with Evolutionary Biology #
Human origins, archaic relationships, speciation, long-term substitution, and ancient admixture are Evolutionary Biology. Sequence comparison is Bioinformatics.
These notes stop at the processes those histories use: Hardy–Weinberg as a null, selection, mutation, migration, non-random mating, and drift, including founder events. How F changes genotype frequencies is in scope for a human population. Where ancestors lived is not stated here. Sickle cell was handled the same way: q̂ = s/(s+t) is genetics; the history of the parasite and of migration is not required and is not given.
12.4 Key terms, summary, and self-check #
Key terms. Consanguinity; identity by descent; founder effect; Finnish disease heritage; founder allele.
Summary. Consanguinity raises f(aa) by the term qF; for first cousins F = 1/16, and rare alleles feel it most. Founder effects are samples, illustrated by the Finnish disease heritage and by alleles such as those at HEXA, not by stereotypes. Human allele-frequency processes belong here; human phylogeny does not.
Self-check.
1. At q = 0.01 and F = 1/16, which term dominates f(aa)?
2. Why is 2q the wrong prior for a first-cousin partner?
3. Why is a rare allele’s high frequency after a founder event not proof of advantage?
4. What is the Finnish disease heritage a statement about?
5. Which task is Evolutionary Biology: computing Δp under migration, or reconstructing archaic admixture?
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# Closing: how the chapters fit together
Chapters 1 and 2 are segregation, first with simple dominance and then with modified ratios. Chapter 3 puts genes on sex chromosomes. Chapter 4 replaces independent assortment with a linkage map that counts double crossovers, and Chapter 5 follows what extra, missing, inverted, or fused chromosomes do, including the BCR–ABL1 junction. Chapter 6 is the same logic in haploids. Chapter 7 supplies new alleles. Chapter 8 leaves the nucleus for organelles and separates them from maternal effect. Chapters 9 and 10 move from single-locus frequencies to many loci, and Chapters 11 and 12 read the same rules in human pedigrees and human populations. Origins and phylogeny stay in Evolutionary Biology.
Mutation chemistry, splicing, recombination proteins, XIST, and the mitochondrial code stay in Molecular Biology. Meiosis and organelle biogenesis stay in Cell Biology. Phylogeny and the long-term fate of these frequencies stay in Evolutionary Biology.
Revision checklist #
1. State monohybrid genotypic and phenotypic ratios, and say what a chi-square p value is.
2. Turn 9:3:3:1 into 9:3:4, 9:7, and 15:1, each with its pathway.
3. Give both white-eye reciprocal F1 results, and male versus female frequency at allele frequency q.
4. From a three-point cross, name the middle gene and compute I = 1 − S.
5. Contrast paracentric and pericentric recombinant chromatids, and say why a translocation heterozygote is semisterile.
6. Distinguish Hfr time-of-entry from lambda specialised transduction, and say what a cis–trans test defines.
7. Separate silent, missense, nonsense, and frameshift mutations, and contrast HTT with FMR1.
8. Separate Mirabilis variegation from snail coiling by the grandchildren.
9. Estimate carrier frequency from q², say why rare recessives hide from selection, and state q̂ under heterozygote advantage.
10. Define h² so it is not “percent genetic”, write R = h²S, and give the worked posterior carrier probability after two unaffected children.
Selected sources #
These notes are an original teaching summary, not a transcript of any textbook.
- Griffiths AJF, Wessler SR, Carroll SB, Doebley J, et al. Introduction to Genetic Analysis. W. H. Freeman.
- Klug WS, Cummings MR, Spencer CA, Palladino MA, et al. Concepts of Genetics. Pearson.
- Hartl DL, Jones EW. Genetics: Analysis of Genes and Genomes. Jones and Bartlett.
- Falconer DS, Mackay TFC. Introduction to Quantitative Genetics. Longman.
- Hartl DL, Clark AG. Principles of Population Genetics. Sinauer.
- Mendel G. Versuche über Pflanzen-Hybriden. Verhandlungen des naturforschenden Vereines in Brünn (1866).
- Hardy GH. Mendelian proportions in a mixed population. Science (1908).
- Morgan TH. Sex limited inheritance in Drosophila. Science (1910).
- Sturtevant AH. The linear arrangement of six sex-linked factors in Drosophila, as shown by their mode of association. Journal of Experimental Zoology (1913).
- Lyon MF. Gene action in the X-chromosome of the mouse (Mus musculus L.). Nature (1961).
- Benzer S. Fine structure of a genetic region in bacteriophage. Proceedings of the National Academy of Sciences of the USA (1955).
- Fisher RA. The correlation between relatives on the supposition of Mendelian inheritance. Transactions of the Royal Society of Edinburgh (1918).
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# Part C. Visual Figures & Scientific Diagrams Gallery
Comprehensive high-resolution visual reference repository for Genetics from the B10logy Visual Atlas.
Autosomal Dominant Pedigree Chart
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# Part C. Visual Figures & Scientific Diagrams Gallery
Essential high-resolution reference diagrams for Genetics from the B10logy Visual Atlas.