Exam practice

See the table in Activity 4.3. In short: mitosis takes place in body cells, has one division, makes 2 identical diploid cells, and keeps the chromosome number the same (equational); meiosis takes place in the cells that make gametes, has two divisions, makes 4 different haploid cells, halves the chromosome number (reductional) and includes crossing over.

Characters are decided by genes on the chromosomes. Through the sperm and the egg, each child receives one set of chromosomes, with its genes, from each parent, so the child has many of the same characters as its parents.

A woman’s eggs all carry an X chromosome (22 + X), but a man makes two kinds of sperm, 22 + X and 22 + Y. If an X sperm fertilizes the egg the child is a girl (XX); if a Y sperm does, a boy (XY). So it is the father’s sperm that decides.

Each fertilization is a separate chance event, like tossing a coin: whichever sperm, X or Y, reaches the egg first. Even though a man makes equal numbers of X and Y sperm, a family has only a few children, so by chance all of them can be of one sex, just as a coin can land heads three times in a row. (In a family of three children, the chance of all sons is ½ × ½ × ½ = 1 in 8.)

In meiosis, a diploid cell (2n) produces four haploid cells (n): the number of chromosomes is reduced to half.

In mitosis, each daughter cell gets exactly the same number of chromosomes (and the same genes) as the mother cell: the number stays equal.

Sexual reproduction needs gametes, and gametes are made only by meiosis. Meiosis also halves the chromosome number, so that when two gametes fuse the zygote has the normal diploid number. Without it, there would be no gametes, or the chromosome number would double in every generation.

During meiosis, crossing over swaps pieces between the chromosomes of each pair, and the pairs separate at random, so every gamete gets a different mixture of the parents’ genes. When these different gametes fuse, every offspring is different: that is variation.

A gene is a segment of DNA in a chromosome that carries the code for a particular character (by coding for a protein). It is the unit of heredity, passing characters from parents to offspring.

A chromosome is a thread-like structure in the nucleus, made of DNA and histone protein, which carries the genes. Chromatin fibers coil up into visible chromosomes when a cell divides.
Roles: they carry the genes that decide the characters of the organism; they pass the genetic information to daughter cells at every cell division, and to the next generation through the gametes; the sex chromosomes decide the sex; and by crossing over and mutation they produce variation, which helps evolution.

The body begins as a single cell, the zygote. Mitosis divides it again and again (1 → 2 → 4 → 8…) to make trillions of cells, and every new cell has the same chromosomes and genes. This is how the body grows. Mitosis also replaces dead and damaged cells (skin, blood cells, the lining of the gut) and heals wounds and broken bones, and in some plants and animals it allows asexual reproduction. Because the daughter cells are identical, it keeps the genetic information of every body cell the same.

Meiosis makes the gametes (sperm and eggs) in the testes and ovaries. It halves the chromosome number, so that when a sperm and an egg fuse, the zygote has the correct diploid number, and it shuffles the genes, so each offspring is new. Mitosis then turns the single-celled zygote into a complete organism. In asexual reproduction (cuttings, tubers, budding), mitosis alone produces the new individual, which is identical to the parent.

No haploid gametes would form. Either the organism couldn’t reproduce sexually at all, or, if its diploid cells fused, the chromosome number would double every generation (46 → 92 → 184 in humans), which the cells couldn’t survive. There would also be no crossing over, so less variation among offspring.

Each character (height, the color of eyes or flowers, blood group) is controlled by genes, segments of DNA on the chromosomes. A body cell has two copies of each gene, one from each parent. When gametes are made by meiosis, each gamete gets one copy of each gene. At fertilization the two copies come together again in the zygote, and every cell of the new body gets them by mitosis. The genes then direct the making of proteins that produce the character. In this way characters pass from parents to offspring, and the combination of genes from both parents decides which characters show.

A man’s cells are 44 + XY and a woman’s 44 + XX. By meiosis, a man makes two kinds of sperm in equal numbers (22 + X and 22 + Y), and a woman makes only one kind of egg (22 + X). If an X sperm fertilizes the egg, the zygote is 44 + XX and grows into a girl; if a Y sperm does, it is 44 + XY and grows into a boy. So the chance of each is 50%. (Draw the chart as in Figure 4.7.)

50%. The egg always carries an X; half the sperm carry X and half carry Y, so an X sperm and a Y sperm are equally likely to fertilize the egg. (In real numbers, slightly more boys are born worldwide, about 105 boys for every 100 girls, so the true chance is nearer 49%; the answer expected is 50%.)

No. Every man makes X and Y sperm in roughly equal numbers. Which one fertilizes each egg is a matter of chance, so a couple can easily have only sons (or only daughters). For three children, the chance of all sons is $\tfrac{1}{2} \times \tfrac{1}{2} \times \tfrac{1}{2} = \tfrac{1}{8}$: about 1 family in 8. Try it in the sperm lottery above.

A: reproductive cell → meiotic (meiosis) cell division → gamete

B: somatic cell → mitotic (mitosis) cell division → somatic cells (two identical daughter cells)

For the differences, see the table in Activity 4.3: meiosis gives 4 haploid, different cells (reductional); mitosis gives 2 diploid, identical cells (equational).

Children get half their genes from the mother and half from the father, so they share many characters with both. But meiosis shuffles the genes (crossing over and random sorting of chromosomes), so every egg and sperm is different and every child gets a different mixture. Some characters are recessive and stay hidden in a parent but show in the child, and the environment also shapes how a child grows. So children resemble their parents but are not copies of them.

Peas are bisexual and self-pollinating, so they stay pure; they can be cross-pollinated by hand; they have a short life cycle; they have many clear pairs of contrasting characters; each plant gives many seeds; and they are easy to grow.

The pure tall parent (TT) gives a T allele and the pure dwarf parent (tt) a t allele, so all the F₁ plants are Tt. Tall (T) is dominant over dwarf (t), so it hides the dwarf character, and all the F₁ plants are tall (the law of dominance).

A hybrid (Tt) makes two kinds of gametes, T and t, in equal numbers (the law of segregation). When two hybrids are bred, these gametes combine at random, giving TT, Tt and tt in the ratio 1 : 2 : 1, so both tall (TT, Tt) and dwarf (tt) offspring appear, in the ratio 3 : 1.

Every person’s DNA is unique (except identical twins), it is the same in every cell of the body, and it doesn’t change during life. Even a tiny trace of blood, hair, skin or saliva left at a crime scene holds the person’s DNA, which can be matched with a suspect’s. When samples are handled properly, the chance of two unrelated people matching is extremely small, so it is very reliable evidence.

Genetic engineering changes the characters of living things by changing their DNA: adding, removing or replacing genes or even single base pairs. To do that, scientists must know the sequence of the DNA, where each gene is, what it codes for and how it is controlled. So detailed study of DNA is the basis of genetic engineering.

When two different species are crossed (horse × donkey, lion × tiger), the hybrid gets one set of chromosomes from each parent, and the two sets don’t match (a horse has 64 chromosomes, a donkey 62, a mule 63). In meiosis, the chromosomes can’t pair up properly, so normal gametes can’t form, and the hybrid is sterile.

DNA tests are extremely sensitive: a few cells from the person collecting the sample, or from another sample, can contaminate it and give a false result, and heat, damp or sunlight can destroy the DNA. A mix-up of labels could convict an innocent person. So samples must be collected with gloves and clean tools, sealed, labeled, stored correctly and transported safely, with a record of everyone who handled them.

Genetics is the branch of biology that studies genes, heredity and variation: how characters are passed from parents to offspring, and why offspring differ.

DNA testing is the study of a person’s (or other living thing’s) DNA from a sample of blood, hair, saliva, skin or other tissue, to identify them or find their relatives. It is used in criminal investigations (to match evidence from a crime scene with a suspect), in paternity tests (to find a child’s biological father), to identify victims of disasters and accidents, to diagnose genetic diseases, and to study ancestry and wildlife.

Genetic engineering of bacteria to make human insulin; genetically modified crops such as Bt cotton and golden rice; gene editing with CRISPR; DNA testing (fingerprinting) in crime and paternity cases; DNA sequencing; molecular markers in plant and animal breeding; testing embryos for genetic disorders in IVF.

DNA carries the genes that decide every character of a living thing. Genetic technology works by reading DNA (sequencing, to find genes and what they do), comparing it (DNA testing, molecular markers), and changing it (genetic engineering: cutting, adding or replacing genes). Because DNA is passed to offspring, a change made in it can be inherited. Without understanding and handling DNA, none of these technologies would be possible.

• Medicine: making human insulin, growth hormone, vaccines and clotting factors in bacteria or yeast; gene therapy to treat inherited diseases.
• Farming: crops that resist pests, diseases and drought, give bigger harvests or contain more nutrients (Bt cotton, golden rice), so less pesticide is used.
• Animals: livestock that grow faster or resist disease.
• Industry and environment: microbes that make enzymes, or break down oil spills and waste.
• Research: finding out what genes do.

A monohybrid cross is a cross between two pure organisms that differ in one pair of contrasting characters.
Let R = red (dominant) and r = white (recessive). (Mendel’s peas actually had purple flowers; the method is the same.)

Parents: pure red RR × pure white rr

Gametes: R and r

F₁: all Rr, red (hybrid)

F₁ self-pollinated: Rr × Rr; gametes R, r from each parent

F₂: phenotypic ratio red : white = 3 : 1; genotypic ratio RR : Rr : rr = 1 : 2 : 1.

Guinea pigs: black coat (B) is dominant over white (b).

P: pure black BB × pure white bb → gametes B and b → F₁: all Bb, black (law of dominance).

F₁ × F₁: Bb × Bb → F₂: 1 BB : 2 Bb : 1 bb, that is 3 black : 1 white (law of segregation).

These are exactly the ratios Mendel found in peas, so his laws apply to animals too (and to people: for example, in the inheritance of albinism, a recessive character).

Law of dominance: when two pure individuals with a pair of contrasting characters are crossed, only one character (the dominant) appears in F₁; the other (recessive) stays hidden. E.g. TT × tt gives all Tt, tall.

Law of purity of gametes (segregation): the two alleles of a gene stay together in a hybrid without mixing, and separate when gametes are formed, so each gamete gets only one allele and is pure. E.g. Tt makes T and t gametes; self-pollinated, Tt × Tt gives 1 TT : 2 Tt : 1 tt (3 tall : 1 dwarf), so the recessive character reappears unchanged in F₂.

• i.Dominant (round) : recessive (wrinkled) = 3 : 1.
• ii.Genotypic ratio RR : Rr : rr = 1 : 2 : 1; phenotypic ratio round : wrinkled = 3 : 1.
• iii.The RR plants are pure round-seeded, because both their alleles are for round seeds; they make only R gametes, so when self-pollinated they give only round seeds. The Rr plants are round too, but hybrid: they carry the hidden wrinkled allele.

The black parent was pure (BB) and the white one was bb. All the gametes of the black parent carried B and all those of the white parent carried b, so every offspring was Bb. Black (B) is dominant, so it hid the white allele (b) and all the F₁ were black. White can show only in a bb animal, which this cross can’t produce. (If the black parent had been Bb, about half the young would have been white.)

By a DNA (paternity) test. Half of a child’s DNA comes from the mother and half from the father. Samples (such as blood or a cheek swab) are taken from the child, the mother and the suspect(s). The parts of the child’s DNA that didn’t come from the mother must have come from the father; if they match the suspect’s DNA at many places, he is the father, with near certainty, and if they don’t match he is excluded. The samples must be collected and handled carefully by trained people, and the result can then be used as evidence in court.

Artificial insemination (AI). Semen is collected from a healthy bull of an improved breed, tested, divided into small straws and frozen in liquid nitrogen. At the camp, a trained technician thaws a straw and, when each cow is in heat (ready to conceive), places the semen into her uterus with a special instrument. One bull’s semen can serve many cows, so many cows can be inseminated at once, and the calves are of an improved breed.

It is largely a boon, if used responsibly.

As a boon: it gives medicines like insulin and vaccines; it can treat inherited diseases; GM crops can resist pests and drought, need less pesticide, and give more and more nutritious food; microbes can clean up pollution; and it helps research.

The worries: possible harm to the environment (genes spreading to wild plants, effects on other insects); a loss of crop variety; farmers depending on companies for seeds; ethical questions about changing humans or animals; and possible misuse.

Conclusion: with careful testing, laws and honest labeling, the benefits are much larger than the risks, so it is a boon for our age.

With artificial insemination, a farmer doesn’t have to buy and feed an expensive breeding bull or buffalo; for a small fee, his cow or buffalo can be made pregnant with the semen of an excellent bull of an improved breed, even one far away or no longer alive. The calves grow faster and give much more milk or meat, so the farmer earns more. AI also prevents diseases spread by mating and injuries to the animals, and keeps good breeding records. Better animals and less cost bring the farmer more income, and happiness.

Many couples can’t have children because of blocked fallopian tubes, few or weak sperm, problems with ovulation, age or unexplained causes. IVF brings the egg and sperm together in a laboratory and places the embryo directly in the uterus, getting round most of these problems. It can use donor eggs or sperm if needed, and embryos can be tested for some genetic disorders. Millions of children worldwide, including in Nepal since 2005, have been born to couples who could not otherwise have had them, so IVF has truly been a boon.