Lesson 11 of 11 · 12 min

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Must-know facts

19 facts

1Mendel worked on garden pea from 1856 to 1863 using 14 true-breeding varieties and 7 pairs of contrasting characters.
2Green pod colour and yellow seed colour are dominant in pea; axial flower position and inflated pod are also dominant.
3Monohybrid F₂: phenotype 3:1, genotype 1:2:1. Dihybrid F₂: phenotype 9:3:3:1 out of 16.
4Monohybrid test cross of a heterozygote gives 1:1; dihybrid test cross gives 1:1:1:1.
5Snapdragon (Antirrhinum sp.) shows incomplete dominance: F₂ is 1 red : 2 pink : 1 white, so phenotypic and genotypic ratios match.
6ABO: three alleles, six genotypes, four phenotypes; Iᴬ and Iᴮ co-dominant, both dominant over i.
7Pea starch grain size (gene B) shows incomplete dominance even though seed shape shows complete dominance.
8Mendel's work was rediscovered in 1900 by de Vries, Correns and von Tschermak.
9Sutton 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.

Common traps

Where marks are lost

Quoting 3:1 as the monohybrid genotypic ratio.

3:1 is the phenotypic ratio; the genotypic ratio is 1:2:1. Only under incomplete dominance do the two ratios coincide.

Assuming yellow pod colour is dominant because yellow seed colour is.

In pea, green pod is dominant, but yellow seed is dominant. Pod and seed colour are separate characters.

Calling ABO blood groups an example of incomplete dominance.

Iᴬ and Iᴮ are co-dominant (both fully expressed in AB); snapdragon flower colour is the incomplete dominance example.

Thinking linked genes always recombine freely, or never recombine at all.

Linked genes recombine less often than unlinked genes, and the frequency rises with the distance between them: 1.3% for a tightly linked pair versus 37.2% for a loosely linked pair.

Treating the male as the heterogametic sex in every animal.

Birds show female heterogamety: females are ZW, males ZZ. Male heterogamety is seen in XO (grasshopper) and XY (humans, Drosophila) systems.

Swapping the karyotypes of Klinefelter's and Turner's syndromes.

Klinefelter's is a male with an extra X (47, XXY); Turner's is a female missing an X (45, X0). Both are sterile.

Calling sickle-cell anaemia sex-linked, or thalassemia a qualitative defect.

Both sickle-cell anaemia and thalassemia are autosomal recessive. Sickle-cell makes a faulty globin (qualitative); thalassemia makes too little globin (quantitative).

Writing the sickle-cell change as valine replaced by glutamic acid.

It is the reverse: glutamic acid at position 6 is replaced by valine, from GAG changing to GUG.

Mixing up pleiotropy and polygenic inheritance.

Pleiotropy is one gene with many effects (phenylketonuria); polygenic inheritance is many genes adding up to one trait (skin colour, height).

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.

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