What traumatic event does Zitkala-Sa describe at the school?

The forcible cutting of her long hair

Why was the cutting of her hair so humiliating to Zitkala-Sa?

Among her people, shingled hair was worn by cowards and mourners

State Mendel's Law of Independent Assortment.

When two pairs of contrasting traits are considered, the alleles of one pair segregate independently of the alleles of another pair during gamete formation.

What is the phenotypic ratio of a typical dihybrid cross in the F2 generation?

The phenotypic ratio is 9:3:3:1.

Explain co-dominance with an example.

In co-dominance both alleles express themselves equally in the heterozygote; for example, the AB blood group expresses both A and B antigens because IA and IB are co-dominant.

What is a test cross?

A test cross involves crossing an organism showing the dominant phenotype with a homozygous recessive individual to determine its genotype.

Why is haemophilia called a sex-linked recessive disorder?

Its gene is carried on the X chromosome and the disorder appears mostly in males who carry the recessive allele on their single X chromosome.

What is the chromosomal basis of Down's syndrome?

Down's syndrome is caused by trisomy of chromosome 21 (an additional copy of chromosome 21) due to non-disjunction.

Differentiate between Klinefelter's and Turner's syndromes.

Klinefelter's syndrome (47, XXY) occurs in males with an extra X chromosome, while Turner's syndrome (45, X0) occurs in females with a missing X chromosome.

What causes sickle-cell anaemia at the molecular level?

It is caused by a point mutation substituting valine for glutamic acid at the sixth position of the beta-globin chain of haemoglobin.

Who proposed the chromosomal theory of inheritance?

Walter Sutton and Theodore Boveri proposed the chromosomal theory of inheritance, linking Mendelian factors to chromosomes.

What is polygenic inheritance?

It is inheritance where a trait is controlled by three or more genes, with the phenotype reflecting the additive effect of alleles, as in human skin colour.

Principles of Inheritance and Variation

Class 12 Biology · Biology · Chapter 4

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CBSE Class 12 · Biology

Genetics + Evolution: Unit VII, 20 marks

Marks booster + cheatsheet: key concepts in 2 lines each, common traps
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CBSE Class 12 · Biology

Genetics + Evolution: Unit VII, 20 marks

Marks booster + cheatsheet: key concepts in 2 lines each, common traps
16 marks: Reproduction, Unit VI
12 marks each: Units VIII and IX
10 marks: Ecology, Unit X
13 chapters, with page numbers
Common traps, corrected

Page 1 free about 8 min read, Full kit about 55 min read

₹159See the full kit

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Summary

This chapter introduces genetics through Mendel's experiments on the garden pea, establishing the principles of inheritance. Mendel's monohybrid crosses gave the Law of Dominance and the Law of Segregation, while dihybrid crosses gave the Law of Independent Assortment, expressed through phenotypic ratios such as 3:1 and 9:3:3:1. Deviations from Mendelian ratios are explained by incomplete dominance (Snapdragon), co-dominance (ABO blood groups), multiple allelism, pleiotropy and polygenic inheritance (human skin colour). The chromosomal theory of inheritance, proposed by Sutton and Boveri, links genes to chromosomes, supported by Morgan's work on Drosophila showing linkage and recombination, and the construction of genetic maps. Sex determination mechanisms (XX-XY, XX-XO, ZZ-ZW) and the chromosomal basis of sex are explained, along with the role of environment. Mutations, sudden heritable changes in DNA, and the resulting variations are described, including chromosomal aberrations (deletion, duplication) and point mutations like in sickle-cell anaemia. The chapter concludes with the genetic basis of human disorders, classifying them as Mendelian (haemophilia, colour blindness, thalassaemia, sickle-cell anaemia, phenylketonuria) and chromosomal (Down's syndrome, Klinefelter's syndrome, Turner's syndrome).

Mendel's laws of inheritanceDeviations: incomplete dominance and co-dominanceChromosomal theory and linkageSex determinationMutation and variationGenetic disorders in humans

Key terms

Law of Segregation

Mendel's principle that the two alleles of a gene separate during gamete formation so that each gamete carries only one allele.

Incomplete dominance

A pattern where the heterozygote shows a phenotype intermediate between the two homozygotes, as in pink-flowered snapdragon.

Co-dominance

A pattern where both alleles express themselves fully and independently in the heterozygote, as in AB blood group.

Linkage

The tendency of genes located close together on the same chromosome to be inherited together.

Pleiotropy

A condition where a single gene affects multiple, apparently unrelated phenotypic traits.

Aneuploidy

Gain or loss of one or more chromosomes due to failure of segregation (non-disjunction), as in Down's syndrome (trisomy 21).

Extra questions & answers

Try one from this chapter

The phenotypic ratio of a typical dihybrid cross in F2 is:

Flashcards & interactive practice