Lesson 11 of 11 · 12 min
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Must-know facts
19 facts
- 01Mendel worked on garden pea from 1856 to 1863 using 14 true-breeding varieties and 7 pairs of contrasting characters.
- 02Green pod colour and yellow seed colour are dominant in pea; axial flower position and inflated pod are also dominant.
- 03Monohybrid F₂: phenotype 3:1, genotype 1:2:1. Dihybrid F₂: phenotype 9:3:3:1 out of 16.
- 04Monohybrid test cross of a heterozygote gives 1:1; dihybrid test cross gives 1:1:1:1.
- 05Snapdragon (Antirrhinum sp.) shows incomplete dominance: F₂ is 1 red : 2 pink : 1 white, so phenotypic and genotypic ratios match.
- 06ABO: three alleles, six genotypes, four phenotypes; Iᴬ and Iᴮ co-dominant, both dominant over i.
- 07Pea starch grain size (gene B) shows incomplete dominance even though seed shape shows complete dominance.
- 08Mendel's work was rediscovered in 1900 by de Vries, Correns and von Tschermak.
- 09Sutton and Boveri matched chromosome behaviour to Mendel's factors, Sutton named it the chromosomal theory of inheritance, and Morgan verified it experimentally in Drosophila melanogaster.
- 10Morgan's recombination: yellow body–white eye 1.3%; white eye–miniature wing 37.2%.
- 11Sturtevant used recombination frequency to build genetic maps.
- 12Human skin colour is the NCERT example of polygenic inheritance (three genes); phenylketonuria is the example of pleiotropy.
- 13Henking (1891) named the X body; grasshopper is XO; humans and Drosophila are XY; birds are ZW with heterogametic females.
- 14Honey bee: haplodiploid; females (queen, worker) are diploid with 32 chromosomes, drones are haploid with 16 and arise from unfertilised eggs by parthenogenesis.
- 15Sickle-cell: Glu → Val at position 6 of beta-globin, GAG → GUG; autosomal recessive.
- 16Haemophilia and colour blindness are X-linked recessive; colour blindness affects about 8% of males and about 0.4% of females.
- 17α-thalassemia: HBA1 and HBA2 on chromosome 16; β-thalassemia: HBB on chromosome 11.
- 18Down's syndrome = trisomy 21; Klinefelter's = 47, XXY; Turner's = 45, X0.
- 19Aneuploidy comes from failed chromatid segregation; polyploidy from failed cytokinesis after telophase.
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Key terms
18 terms
- Allele
- One of the alternative forms of a gene, such as T and t for pea height.
- True-breeding line
- A line that shows the same form of a trait over many generations of self-pollination.
- Homozygous
- Having two identical alleles of a gene.
- Heterozygous
- Having two different alleles of a gene.
- Test cross
- Crossing a dominant-looking individual with the recessive parent to reveal its genotype.
- Incomplete dominance
- Heterozygote shows a phenotype in between the two homozygotes.
- Co-dominance
- Both alleles are fully and separately expressed in the heterozygote.
- Multiple alleles
- More than two alleles of one gene existing in a population.
- Linkage
- Physical association of genes on the same chromosome, so they tend to be inherited together.
- Recombination
- Production of gene combinations not present in either parent.
- Polygenic trait
- A trait controlled by three or more genes whose effects add up.
- Pleiotropy
- One gene influencing more than one phenotypic trait.
- Male heterogamety
- The male produces two kinds of gametes with respect to sex chromosomes (XO or XY).
- Female heterogamety
- The female produces two kinds of gametes (ZW females in birds).
- Point mutation
- A change affecting only one base pair of DNA.
- Pedigree analysis
- Tracing a trait through a family tree over generations.
- Aneuploidy
- Gain or loss of individual chromosomes after failed chromatid segregation.
- Polyploidy
- Presence of extra complete chromosome sets after failed cytokinesis.