Sex determination, mutation, pedigree analysis — line by line

Henking, in 1891, traced a specific nuclear structure through spermatogenesis in a few insects.

He observed that after spermatogenesis, 50 per cent of the sperm received this structure and the other 50 per cent did not.

Asked as — the name and the year are both asked, and 1891 gets swapped with 1900 and 1902 in the options. Note that Henking studied spermatogenesis in insects, not oogenesis and not humans.

Henking named the structure the X body, but could not explain its significance.

That is the exact limit of his contribution — he saw it and named it, nothing more. Later investigators concluded that Henking's X body was in fact a chromosome, and it was therefore named the X-chromosome.

Asked as — "X body was named by ___", and "who discovered / who explained the X body" are asked as separate questions. Henking discovered and named; others explained.

In a large number of insects the mechanism is of the XO type. Here, all eggs carry an additional X-chromosome besides the other chromosomes, which are the autosomes.

The female contributes an X every time. So the eggs are all alike — the female is homogametic.

On the male side, some sperms carry the X-chromosome and some do not.

Eggs fertilised by an X-bearing sperm become females. Eggs fertilised by a sperm without an X become males.

Asked as — in XO type there is no Y chromosome at all. The "O" means nothing is present, not that a chromosome called O exists. That is a real misconception and it gets tested.

The in-text question: are the numbers of chromosomes in the male and female equal? In the XO type, no.

The female has two X chromosomes; the male has only one and nothing in its place. So the male has one chromosome fewer than the female. This is the distinguishing feature of XO type against XY type, where the totals are equal.

Because the X-chromosome is involved in determining sex, it was designated the sex chromosome, and all the remaining chromosomes were named autosomes.

This is where both terms enter the chapter. Autosomes are identical in the two sexes; sex chromosomes are not.

Grasshopper is the book's example of XO type: males have only one X-chromosome besides the autosomes, while females have a pair of X-chromosomes.

One named example, and it is the one that appears in options.

In a number of other insects and in mammals including man, the XY type is seen, and here both male and female have the same number of chromosomes.

This is the contrast with XO. Nothing is missing in the male; the missing partner has been replaced by a different chromosome.

In the male an X-chromosome is present, but its counterpart is distinctly smaller and is called the Y-chromosome. Females have a pair of X-chromosomes.

The word to keep is "distinctly smaller" — the Y is not a second X, and it is not the same size.

In human beings and in Drosophila, males have one X and one Y chromosome, while females have a pair of X chromosomes besides the autosomes (Figure 4.12 a, b).

Both organisms, the same XY pattern. Figure 4.12 (a) is the human pair, (b) the Drosophila pair.

In both XO and XY types, the male produces two different kinds of gametes — either with or without an X, or some with X and some with Y. This is called male heterogamety.

Heterogametic = produces two kinds of gametes with respect to sex chromosomes. Homogametic = produces only one kind. In XO and XY types the male is heterogametic and the female homogametic.

Asked as — the terms homogametic and heterogametic are also labelled in Figure 4.12: the human female XX is homogametic, the male XY heterogametic. Both XO and XY are examples of male heterogamety, and questions often ask which types share that feature.

In some other organisms, for example birds, a different mechanism operates (Figure 4.12 c). The total chromosome number is the same in both sexes, but the two different kinds of gametes are produced by the female — this is female heterogamety.

The mechanism is inverted relative to humans. The variable gamete is the egg, not the sperm.

To distinguish it from the earlier mechanism, the two different sex chromosomes of a female bird are designated Z and W. Females are ZW; males have a pair of ZZ chromosomes besides the autosomes.

Different letters are used deliberately, so that nobody confuses a bird's system with the XY system. Figure 4.12 (c) shows the cock as ZZ (two similar chromosomes) and the hen as ZW (two dissimilar).

Asked as — the commonest trap in this section is reversing it. Say it aloud until it sticks: hen ZW, cock ZZ. Compare with humans, where the female is the one with the identical pair.

The in-text question at the end of 4.6.2: how is the mechanism different in birds, and is the sperm or the egg responsible for the sex of the chicks?

The egg. Since the hen is ZW, her eggs are of two kinds — half carrying Z, half carrying W. Every sperm carries Z. A Z-egg fertilised gives ZZ, a male chick; a W-egg gives ZW, a female chick. So in birds the mother determines the sex of the offspring, the exact opposite of humans.

Humans use the XY type. Of the 23 pairs of chromosomes, 22 pairs are exactly the same in both males and females — these are the autosomes.

46 chromosomes, 23 pairs, 22 autosomal pairs, one sex chromosome pair. All four numbers are asked.

During spermatogenesis, males produce two types of gametes: 50 per cent of sperm carry the X-chromosome and the remaining 50 per cent carry the Y-chromosome, besides the autosomes.

So a sperm is either 22 + X or 22 + Y.

Females produce only one type of ovum, carrying an X-chromosome.

Every ovum is 22 + X. The female contributes an X in every case, which is exactly why she cannot influence the sex of the child.

There is an equal probability of the ovum being fertilised by an X-carrying or a Y-carrying sperm.

Ovum + X-sperm → XX, a female. Ovum + Y-sperm → XY, a male.

It is therefore the genetic makeup of the sperm that determines the sex of the child, and in each pregnancy there is always a 50 per cent probability of either a male or a female child.

Two separate examinable statements. The second one — that each pregnancy is an independent 50:50 — is the answer to questions about a couple who already have three daughters. The probability for the next child is still 1/2; earlier children do not change it.

Asked as — NCERT exercise 11, "how is sex determined in human beings?" A full answer needs: XY type, 22 pairs of autosomes, two sperm types in equal proportion, one ovum type, the two fertilisation outcomes, and the conclusion that the sperm decides.

In the honey bee, sex determination is based on the number of sets of chromosomes an individual receives — not on any sex chromosome.

This is a completely different principle from everything before it. There is no X, Y, Z or W involved.

An offspring formed from the union of a sperm and an egg develops as a female — queen or worker. An unfertilised egg develops as a male (drone) by means of parthenogenesis.

Fertilised → female. Unfertilised → male. Both the queen and the worker are females; the drone is the male.

Asked as — the term parthenogenesis, and the fact that queen and worker are both female, are separately asked. A frequent distractor claims the worker is sterile male.

Therefore males have half the number of chromosomes of a female. Females are diploid with 32 chromosomes; males are haploid with 16.

32 and 16. These two numbers are the single most reliably asked facts in the section.

The system is called the haplodiploid sex-determination system.

Haploid male, diploid female — hence the name. Learn the term itself; it appears as a one-word answer.

The system has special characteristic features. The males produce sperms by mitosis. They have no father and so cannot have sons, but they do have a grandfather and can have grandsons.

Work through why each part is true rather than memorising the sentence:

• Sperm by mitosis — the drone is already haploid, so meiosis would halve an unhalvable set. Mitosis keeps the count at 16.
• No father — he came from an unfertilised egg, so only his mother contributed.
• No sons — his sperm can only ever produce fertilised eggs, and every fertilised egg is female.
• Has a grandfather — his mother the queen was herself from a fertilised egg, so she had a father.
• Can have grandsons — his daughters (queens) can lay unfertilised eggs, which become males.

Asked as — almost always as "which statement is INCORRECT about a drone", with "can have sons" or "produces sperm by meiosis" as the false option. This is a near-guaranteed question in any test on this chapter.

Figure 4.13 (page 71) — the honey bee flow chart, in two arms.

Female arm: Female (32) → meiosis → gametes of 16 → an unfertilised one gives a male (16).
Male arm: Male (16) → mitosis → gamete of 16 → on fertilising an egg, gives a female (32).

Asked as — the division type in each arm is the exam point: meiosis in the female, mitosis in the male. Redraw this figure from memory today with all four numbers.

Mutation is a phenomenon that results in the alteration of DNA sequences, and consequently in changes in the genotype and the phenotype of an organism.

Learn this as the definition. Note the chain of consequence stated in it: DNA sequence → genotype → phenotype.

In addition to recombination, mutation is another phenomenon that leads to variation in DNA.

So the chapter now has two named sources of variation: recombination (section 4.3.3) and mutation. Both are asked as a pair.

Loss (deletion) or gain (insertion / duplication) of a segment of DNA results in an alteration in the chromosome. Since genes are located on chromosomes, altering the chromosome produces abnormalities or aberrations.

Note the vocabulary pairing: deletion = loss, insertion and duplication = gain. Together these produce chromosomal aberrations.

Chromosomal aberrations are commonly observed in cancer cells.

One short sentence, frequently lifted verbatim into a question. Cancer cells, not all diseased cells.

Mutation can also arise from a change in a single base pair of DNA. This is known as point mutation. The classical example given is sickle cell anaemia.

Single base pair = point mutation. Keep the example attached to the term; it returns on Day 4 with the full molecular detail (GAG → GUG, Glu → Val at position 6 of the β-chain).

Asked as — NCERT exercise 14, "what is point mutation? Give one example." Answer: a change in a single base pair of DNA; example, sickle-cell anaemia.

Deletions and insertions of base pairs of DNA cause frame-shift mutations.

Distinguish sharply from the previous item. Substituting one base pair → point mutation. Inserting or deleting base pairs → frame-shift. The reading frame downstream is thrown out of register, so every codon after the site is misread.

Asked as — the point/frame-shift pair is a certainty in any test on this chapter, usually with sickle-cell offered under the wrong heading.

The mechanism of mutation is stated to be beyond the scope of the discussion at this level. But many chemical and physical factors induce mutations, and these are called mutagens. UV radiation is a mutagen.

Two categories — chemical and physical — and one named example, which is UV radiation.

Asked as — UV is the only mutagen named in this section, and it is the answer expected. Options usually include infrared, radio waves and visible light, none of which are named here.

The problem: the controlled crosses possible in a pea plant cannot be performed on human beings. So studying the family history of a particular trait provides the alternative.

This is the reason pedigree analysis exists, and it is the reason questions ask for. You cannot design a human cross, so you read the crosses that already happened.

Definition: the analysis of traits across several generations of a family is called pedigree analysis. The inheritance of a particular trait is represented in a family tree over generations.

Both halves belong in a written answer — several generations, and the family-tree representation.

In human genetics, pedigree study is a strong tool used to trace the inheritance of a specific trait, abnormality or disease.

Three words, and questions do ask what pedigree analysis can be used for. Its uses: to establish whether a trait is dominant or recessive, whether it is autosomal or sex-linked, to trace carriers, and to counsel families on the risk in future offspring.

Asked as — NCERT exercise 10, "what is pedigree analysis? Suggest how such an analysis can be useful." Give the definition, then the four uses above.

The standard symbols (Figure 4.13, page 72). Every one of these is examinable, and diagram-based questions depend on them entirely.

Asked as — the double horizontal line is the single most asked symbol, because consanguinity is the clue that points to a recessive disorder. Also note the birth-order convention; questions about "the third child of the second generation" rely on it.

How to actually read a pedigree — the routine to apply in the exam. The book shows worked examples in Figure 4.14, which belongs to tomorrow's portion, but the technique is needed now.

Ask three questions in this order:

1. Does the trait appear in every generation, with every affected person having an affected parent? → dominant. If it disappears and reappears, skipping generations → recessive.
2. Are both sexes affected in roughly equal numbers? → autosomal. If affected individuals are overwhelmingly male → sex-linked.
3. For a recessive trait, is there a consanguineous mating in the tree? → strong support for autosomal recessive.

Shortcut checks — two unaffected parents with an affected child means the trait must be recessive. An affected mother whose all sons are affected while daughters are not points to X-linked recessive. An affected father whose all daughters are affected and no sons points to X-linked dominant. A trait passing only from father to all sons is Y-linked.

A number of disorders in human beings have been found to be associated with the inheritance of changed or altered genes or chromosomes.

Those two words — genes, chromosomes — are the split that opens tomorrow's section: Mendelian disorders come from altered genes, chromosomal disorders from altered chromosomes.