Heredity

Life on Earth is thought to have started with simple cells without a nucleus (prokaryotic cells, like bacteria). Over a very long time, cells with a nucleus (eukaryotic cells) evolved, and then living things made of many cells.

In sexual reproduction, cells in the male and female reproductive organs divide by meiosis to form gametes (sperm and eggs). A male and a female gamete fuse at fertilization to form a zygote, a single cell. The zygote divides again and again by mitosis and grows into a complete body made of countless cells.

Both kinds of division happen in two steps:

• karyokinesis: the division of the nucleus;
• cytokinesis: the division of the cytoplasm, with the cell membrane, which follows.

Mitosis makes two daughter cells from a mother cell; they are somatic (body, or vegetative) cells. Meiosis makes four daughter cells; they are gametes.

A. Mitosis

Mitosis happens in all the cells of the body except the cells that make gametes. One cell divides into two daughter cells. It is the division for growth, development and repair.

Every body cell except a gamete is diploid: it has two sets of chromosomes, one set from the father and one from the mother. We write this as 2n. Before a cell divides, the DNA in each chromosome is copied (replicated), making two identical copies. At the end of the division, each daughter cell gets one copy. So the two daughter cells are identical to each other and to the mother cell, with the same number of chromosomes. That is why mitosis is also called equational division.

Why mitosis matters:

1. Growth: it increases the number of cells, all with the same genetic make-up, so the body grows.
2. Repair: it makes new cells to replace injured or worn-out ones (your skin replaces its outer layer about once a month).
3. Asexual reproduction: some plants and simple animals reproduce by mitosis alone (a potato tuber, a sugar-cane cutting, a dividing amoeba, a budding hydra).
4. Genetic stability: every cell gets exactly the same chromosomes, so the body’s genetic information stays the same.

B. Meiosis

Meiosis happens only in the testes (in males) and ovaries (in females), in the cells that make gametes. One diploid mother cell divides into four haploid daughter cells, the gametes. A haploid cell (n) has only half the chromosomes of the mother cell: one from each pair. And the daughter cells are not identical: their genetic make-up varies a little.

Meiosis happens in two divisions:

1. Meiosis I. The two chromosomes of each pair (one from the father, one from the mother) come together side by side. Parts of their chromatids swap places: this is crossing over, and it changes the genetic make-up of each chromosome. Then the two members of each pair separate, and two haploid nuclei form (karyokinesis), followed by cytokinesis: two haploid cells.
2. Meiosis II. Each of the two cells divides again, just like mitosis (karyokinesis, then cytokinesis), but with no copying of DNA first. Result: four haploid cells.

Because of crossing over (and because each pair lines up by chance), the four cells are all genetically different. Because the chromosome number is halved, meiosis is also called reductional division.

Meiosis happens in the testes of adult males and the ovaries of adult females. At fertilization, a haploid sperm (n) fuses with a haploid egg (n), so the zygote is diploid (2n) again. Mitosis of the zygote then builds the whole body.

Why meiosis matters:

1. Sexual reproduction: it makes the gametes.
2. Variation: crossing over and the random sorting of chromosomes make every gamete different, and variation is what evolution works on.
3. A constant chromosome number: by halving the number in the gametes, it keeps the number the same generation after generation. Without it, the number would double every generation (46, 92, 184…).

During meiosis I, while the pairs lie together, damaged DNA can be mended. When meiosis goes wrong, it causes disorders: if a pair fails to separate, a gamete gets an extra chromosome, and a baby with three copies of chromosome 21 has Down syndrome.

DNA (deoxyribonucleic acid) is a very long, thread-like molecule inside cells that carries the genetic information. It is found:

• in the cytoplasm of prokaryotic cells (bacteria have no nucleus);
• in the chromosomes in the nucleus of eukaryotic cells, and also, in small amounts, in their mitochondria (and in plants’ chloroplasts);
• in many viruses, wrapped in a protein coat called a capsid.

DNA is built of small units joined in a chain:

• a nucleoside is a nitrogen base joined to a deoxyribose sugar;
• a nucleotide is a nitrogen base + a deoxyribose sugar + a phosphate group.

Nucleotides are the building blocks of DNA, and DNA is a polynucleotide: millions of nucleotides joined in a row. The order (sequence) of the nucleotides is the genetic code. It is different in every individual, which is why individuals have different qualities.

DNA has two strands, twisted round each other like a twisted rope ladder: the double helix. The sides of the ladder are sugar and phosphate; the rungs are pairs of bases, one from each strand. The two strands run in opposite directions: they are antiparallel.

There are four bases in DNA: adenine (A), guanine (G), cytosine (C) and thymine (T), and they always pair the same way:

• A pairs with T, held by two hydrogen bonds;
• G pairs with C, held by three hydrogen bonds.

DNA carries the hereditary characters of a living thing. A short segment of DNA that carries the code for one particular character (by coding for one protein, or one RNA) is called a gene. When a cell divides, DNA is copied and passed to the daughter cells, which is how characters pass from generation to generation. To use a gene, the cell first copies it into RNA (this copying is called transcription), and the RNA is used to make a protein.

RNA (ribonucleic acid) is also a polynucleotide, but each of its nucleotides is made of a nitrogen base, a ribose sugar and a phosphate. RNA differs from DNA in a few ways:

• it is usually single-stranded;
• its sugar is ribose (deoxyribose is ribose with one oxygen atom less: “de-oxy”);
• in place of thymine it has uracil (U): its four bases are adenine, guanine, cytosine and uracil. When RNA pairs with bases, A pairs with U (two hydrogen bonds) and G with C (three);
• it is found mostly in the cytoplasm, and a little in the nucleus and chromosomes.

In some viruses (such as those that cause flu, measles and COVID-19), RNA, inside its capsid, is the genetic material.

There are three types of RNA, and the main job of all three is to make proteins:

1. Messenger RNA (mRNA) carries a copy of a gene’s code from the DNA to the ribosomes.
2. Transfer RNA (tRNA) brings the right amino acids to the ribosome, one by one.
3. Ribosomal RNA (rRNA) forms the ribosomes, where the amino acids are joined into a protein.

Look at the nucleus of a plant or animal cell through a powerful microscope and you see a network of very fine fibers: chromatin fibers. When the cell gets ready to divide, these fibers coil up and become short, thick and clearly visible. Then we call them chromosomes.

Each chromosome is made of DNA and histone protein, and it carries many genes. Each gene decides a particular character of an individual; a gene is a small segment of the DNA in the chromosome.

A chromosome that has been copied, ready to divide, has two main parts:

• chromatids: its two arms (two identical copies, called sister chromatids);
• the centromere: the narrow, knot-like point where the two chromatids are held together.

A gene is a segment of DNA in a chromosome that carries the code for a particular character of an organism. Genes pass hereditary characters from parents to offspring, and they also play a part in evolution: through mutation (a change in the DNA) and recombination (new mixtures, as in crossing over), new characters appear.

Types of chromosomes

By their function, chromosomes are of two types: somatic chromosomes (autosomes) and sex chromosomes.

a. Somatic chromosomes (autosomes) are the chromosomes that decide the body’s characters. The two members of each pair look alike (same size and shape), so they are also called autosomes. Humans have 22 pairs.

b. Sex chromosomes decide the sex of an individual. The two members of the pair may be different (X and Y in a man), so they are also called heterosomes (“different bodies”). Humans have 1 pair: XX in a female, XY in a male.

Number of chromosomes

The number of chromosomes is constant in all the members of a species, but it differs from species to species. Humans have 46 chromosomes in each body cell; gorillas and chimpanzees have 48.

The number is usually given in pairs: humans have 23 pairs. That is because one set of 23 comes from the father and the other set of 23 from the mother.

• A body (somatic) cell has two sets of chromosomes: it is diploid (2n). A human body cell has 46 = 2 × 23.
• A gamete has one set: it is haploid (n). A human sperm or egg has 23.

Sex determination in humans

Genes in the chromosomes decide the characters of living things. In the same way, the genes on the sex chromosomes decide whether an individual is male or female; this is sex determination. The sex chromosomes decide the sex of a fetus.

A human body cell has 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes. A male has XY and a female has XX. So a man’s cells are 44 + XY and a woman’s are 44 + XX.

In the reproductive years, the diploid germ cells in the testes and ovaries (2n = 2 × 23) divide by meiosis to make haploid gametes (n = 23):

• A man’s germ cell (44 + XY) makes two kinds of sperm in equal numbers: 22 + X and 22 + Y.
• A woman’s germ cell (44 + XX) makes one kind of egg: 22 + X.

At fertilization:

• a 22 + X sperm + a 22 + X egg → 44 + XX, a daughter;
• a 22 + Y sperm + a 22 + X egg → 44 + XY, a son.

Half the sperm carry X and half carry Y, so at each fertilization the chance of a son or a daughter is 50%.

A short exercise on cell division, DNA, RNA and chromosomes.

1. Choose the correct option

2. Write the differences

3. Give reasons

4. Answer these questions

Heredity and Mendelism

Every living thing produces offspring like itself, which is how a species carries on from generation to generation. The offspring inherit characters from the earlier generations, yet no two are exactly alike. Characters pass to the offspring in both sexual and asexual reproduction, carried by the genes in the chromosomes in the nucleus. Each gene carries a particular character, so genes bring the father’s and mother’s qualities to their children.

• Heredity is the passing of characters from parents to their offspring.
• Hereditary characters are the characters passed on in this way (the shape of your nose, your blood group, a pea’s flower color).

Genetics is the branch of biology that studies genes, heredity and variation. Genetic engineering and medical genetics are two of its branches. Gregor Johann Mendel was the first scientist to find laws of inheritance by experiment.

Gregor Mendel and his peas

Mendel was born in July 1822 in Heinzendorf, a village in the Austrian Empire (now Hynčice in the Czech Republic). He is called the father of genetics. He grew pea plants in the garden of his monastery and did many experiments to show how hereditary characters pass from parents to offspring. He studied seven pairs of contrasting characters, each controlled by one gene. We write the gene of the dominant character with a capital letter and the recessive one with the small letter:

Character	Dominant	Recessive
height of the plant	tall (TT)	dwarf (tt)
position of the flower	axial, along the stem (AA)	terminal, at the tip (aa)
color of the pod	green (GG)	yellow (gg)
shape of the pod	inflated (II)	constricted (ii)
shape of the seed	round (RR)	wrinkled (rr)
color of the flower	purple (PP)	white (pp)
color of the seed	yellow (YY)	green (yy)

Why Mendel chose the garden pea:

1. Pea flowers have both male and female parts, and the flower stays closed, so the plants pollinate themselves naturally (and stay pure).
2. They can also be cross-pollinated by hand when needed.
3. Their life cycle is short, so many generations can be studied quickly.
4. They have many clear pairs of contrasting characters (tall or dwarf, nothing in between).
5. Each plant gives many seeds, so there are many offspring to count.
6. They are easy to grow.

Mendel’s experiment

Mendel studied each of the seven pairs of characters separately. For height, he took pure tall plants (whose offspring had always been tall) and pure dwarf plants, and cross-pollinated them. These were the parents (P).

1. He grew the seeds. This generation is the first filial generation (F₁) (filius is Latin for son). All the F₁ plants were tall. He found the same with each of the other six pairs: in F₁, only one of the two characters appeared.
   • The character that shows in F₁ is the dominant character.
   • The character that is hidden in F₁ is the recessive character.
2. He let the F₁ plants self-pollinate. In the second filial generation (F₂), both tall and dwarf plants appeared: about 75% tall and 25% dwarf. The dwarf character hadn’t been lost; it had only been hidden.
3. He let the F₂ plants self-pollinate too (F₃). The F₂ dwarf plants gave only dwarf plants (pure dwarf). Of the F₂ tall plants, one third gave only tall plants (pure tall), and two thirds gave tall and dwarf in the ratio 3 : 1 again (they were hybrids).

Results of Mendel’s experiment:

1. Pure tall plants give only tall plants.
2. Hybrid tall plants give tall and dwarf plants in the ratio 3 : 1.
3. Pure dwarf plants give only dwarf plants.

Genes in pairs: the words you need

Mendel explained his results with an idea we can now see under the microscope: every plant has two copies of each gene (one on each chromosome of a pair), and a gamete gets one.

• The two forms of a gene, such as T (tall) and t (dwarf), are called alleles.
• A plant with two of the same alleles (TT or tt) is pure (homozygous). A plant with two different alleles (Tt) is a hybrid (heterozygous).
• When the alleles are different, the dominant allele (T) decides what the plant looks like; the recessive allele (t) shows only when there are two of it (tt).
• The phenotype is the character you can see: tall or dwarf.
• The genotype is the genetic make-up: TT, Tt or tt.

In F₂:

• the phenotypic ratio is tall : dwarf = 3 : 1;
• the genotypic ratio is pure tall (TT) : hybrid tall (Tt) : pure dwarf (tt) = 1 : 2 : 1.

A quick way to work out a cross is the Punnett square: write one parent’s gametes along the top and the other’s down the side, and fill each box with the two alleles that meet there. Each box is an equally likely offspring.

	T	t
T	TT (tall)	Tt (tall)
t	Tt (tall)	tt (dwarf)

Monohybrid and dihybrid crosses

• A monohybrid cross is a cross between two pure plants (or animals) that differ in one pair of contrasting characters. Example: pure tall × pure dwarf peas gives 100% hybrid (Tt) plants in F₁.
• A dihybrid cross is a cross between two pure plants that differ in two pairs of contrasting characters at once.

In a dihybrid cross, Mendel crossed pure round yellow seeds (RRYY) with pure wrinkled green seeds (rryy):

• All the F₁ seeds were round and yellow (RrYy): both dominant characters.
• In F₂, he got four kinds of seed: 315 round yellow, 108 round green, 101 wrinkled yellow and 32 wrinkled green, very close to a ratio of 9 : 3 : 3 : 1. Two of these, round green and wrinkled yellow, were new combinations that neither grandparent had.

Mendel’s laws

From his experiments Mendel drew three laws:

1. the law of dominance;
2. the law of purity of gametes (the law of segregation);
3. the law of independent assortment.

1. The law of dominance

When two pure individuals with a pair of contrasting characters are crossed, only one of the characters shows in the F₁ generation: the dominant character. The character that stays hidden in F₁ is the recessive character.

Example: pure tall (TT) × pure dwarf (tt). The tall parent’s gametes all carry T; the dwarf parent’s all carry t; so every F₁ plant is Tt, and every one is tall. Tall is dominant over dwarf.

Mendel’s laws work for animals too. Cross a pure black guinea pig (BB) with a pure white guinea pig (bb): all the F₁ young are black (Bb). Black is dominant, white is recessive. (Set the Punnett square above to “guinea pig coat”, with BB × bb.)

2. The law of purity of gametes (segregation)

In a hybrid, the two alleles of a gene (T and t) sit together, but they don’t mix or blend; each stays pure. When the hybrid makes gametes, the two alleles separate (segregate), and each gamete gets only one of them, pure. So a gamete is either T or t, never a mixture: the gametes are “pure”.

That is why the dwarf character can come back in F₂. Self-pollinate the F₁ hybrids (Tt): half the gametes of each parent carry T and half carry t, and they meet at random, giving 1 TT : 2 Tt : 1 tt, which is 3 tall : 1 dwarf.

The guinea pigs behave the same way. Cross two F₁ hybrid black guinea pigs (Bb × Bb): about 75% are black and 25% white. The phenotypic ratio is 3 black : 1 white, and the genotypic ratio 1 pure black (BB) : 2 hybrid black (Bb) : 1 pure white (bb).

3. The law of independent assortment

When two pairs of characters are followed together, the alleles of one gene go into the gametes independently of the alleles of the other. A hybrid RrYy makes four kinds of gametes, RY, Ry, rY and ry, in equal numbers, because whether a gamete gets R or r has nothing to do with whether it gets Y or y. That is why the dihybrid F₂ shows 9 : 3 : 3 : 1, including new combinations. (We now know it holds for genes on different chromosomes, because each pair lines up by chance in meiosis I.)

Genetic technology

Technology has advanced fast, and genetic technology is one of its biggest fields. It changes the genetic material (genes and DNA) of living things, easily and quickly, to give them new qualities. Scientists now know what genes are, what they do, and how they can be changed by adding, deleting or substituting pieces of DNA. Genes are found in all living things and passed from generation to generation; they hold the coded instructions for making proteins and passing on characters.

Genetic technology is the changing of genes by understanding how they work, by making use of the natural variation in genes, and by moving genes into new organisms. Among these technologies:

• Genetic engineering (genetic modification) changes the DNA of a living thing using laboratory methods: changing a single base pair (A–T or C–G), or deleting or adding a gene. The new characters it makes can be passed to the next generation. An organism changed this way is a genetically modified organism (GMO).
• Finding genes and what they do (by reading DNA, called DNA sequencing) is itself an important use of genetic technology, and the basis of biotechnology and molecular biology.
• In farming, technologies based on molecular markers (DNA tags that help breeders pick the best plants early), transgenic crops (with a gene from another species) and gene expression are widely used to increase yield and quality, to reduce losses from pests, diseases, drought and cold (biotic and abiotic factors), and to improve the breeding of plants and animals. These are important ways to make farming sustainable.
• In forensic science, DNA technology is used to investigate crimes: the DNA test.

DNA testing in investigations

DNA testing has made it much easier to investigate crimes and identify the guilty. Except for identical twins, every person’s DNA is different, and every cell of a person carries the same DNA. So a trace of blood, hair (with its root), skin, saliva or semen left at a crime scene can be matched with a suspect’s DNA. DNA tests are mostly used in criminal investigations and paternity tests (to find a child’s biological father), and also to identify bodies after disasters and to reunite lost family members.

As a scientific method, DNA testing is very good at establishing facts, but it is complex and highly sensitive: a tiny mistake can give a badly wrong answer. A few cells from the person who collects the sample, or from another sample, can contaminate it. So samples must be collected, labeled, stored and transported with great care, protected from contamination, and the record of who handled them kept unbroken.

Selective breeding

Since ancient times, people have chosen the plants and animals with the best qualities and bred them, to get the farm products they want. Selective breeding is choosing plants or animals with the desired qualities and breeding them together, so that their offspring have those qualities. It is also called artificial selection, because people, not nature, decide which animals breed.

Its aim is to bring a desired character into a breed and establish it in the generations that follow, by choosing the mother, the father or both. Selective breeding uses natural reproduction and the natural variation in genes. For example, if we always reject the short animals and breed only the tall ones, the offspring inherit the genes for tallness; after many generations, tallness is fixed in the breed and shortness disappears.

Some people criticize it because, while breeding for one quality, others may be lost, and harmful hidden characters (recessive genes) can come to the surface.

Disadvantages of selective breeding:

1. It fills a population with plants or animals that are genetically alike.
2. Because they are alike, one disease can spread through all of them at once; none has the genes to resist it.
3. Breeding close relatives makes inherited defects more likely in the offspring.
4. It removes some naturally occurring genes and so reduces biodiversity; a species with less variety is less able to survive changes in the future, and may die out.

Methods of selective breeding

Inbreeding: closely related animals are mated, to establish a population with predictable characters. If inbreeding goes on for generations, the offspring become genetically alike: they are called purebred (inbred). Examples: the Siamese cat and the Labrador Retriever dog.

Line breeding: a milder kind of inbreeding, between more distant relatives, to get animals with the desired characters. It slows down how fast the animals become purebred, and reduces the risk of the ill health that is sometimes seen in purebred animals.

Self-pollination: most plants have male and female parts on the same plant and can pollinate themselves. Plants grown from such seeds are like the mother plant in many ways, though not in all, because the genes are reshuffled in sexual reproduction. Repeated over generations, self-pollination produces pure lines of genetically almost identical plants (as Mendel’s pure peas were).

Crossbreeding: two unrelated individuals are bred. It is used to combine the desired characters of two different individuals; crossing two purebred organisms gives offspring with the characters of interest. The offspring of a crossbreeding are called hybrids. The aim is to improve the quality of the hybrid, but the hybrid’s qualities are not passed on unchanged to all later generations (remember how F₂ splits up).

Most crossbreeding is between different breeds or varieties of the same species, like a Jersey bull and a local cow, or two varieties of rice. Crosses between different species (a horse and a donkey, a lion and a tiger) are rarer; they are called interspecific hybrids, and are often sterile, like the examples below.

Some hybrid animals and plants

• Liger: father a lion, mother a tiger. The best-known hybrid animal. It grows bigger than both parents (the biggest cats in the world are ligers), and it behaves more like a lion: it is sociable and likes water like a tiger. Male ligers are sterile.
• Tigon: father a tiger, mother a lion. It is about the size of its parents or a little smaller, looks rather like a tiger, but in many ways (its roar, its sociable nature) is like a lion.
• Beefalo: a cross between the American bison (often called buffalo in America) and domestic cattle. Unlike most hybrids, beefalo can reproduce.
• Zebroid: a cross between a zebra and a horse (or donkey). Zebroids can’t reproduce.
• Mule: father a donkey, mother a horse. It carries loads like a donkey and runs like a horse. It is sterile.
• Pomato: a plant that grows tomatoes above the ground and potatoes below (see the note after the photos).

Advantages of crossbreeding. It can be done in plants and animals, it produces new varieties, and it benefits farmers:

1. It combines the desirable qualities of two breeds, varieties or species.
2. People can produce organisms with the characters they want.
3. It makes full use of a wide range of genetic material.
4. It can produce animals that are better than either parent (hybrid vigor).
5. It can improve an organism’s resistance to disease, strength, lifespan and vigor.
6. Crop plants produced this way can give bigger harvests.

Disadvantages of crossbreeding:

1. Without proper understanding and management, it can cause problems for breeding plans in the future.
2. Products of crossbred animals may sell for less than those of purebreds, so farmers may not earn as much as they expected.
3. Crossbred animals can be harder to sell in export markets.
4. As hybrids spread, and their characters keep changing, the purebred (local) breeds may die out.
5. The natural characters of the parents are not fully passed on, so some may slowly be lost.
6. Hybrids of different species are often sterile, so they must be bred again each time (see Exercise 3g).

Artificial insemination

Thanks to new technology, fertilization is now possible without mating. Semen can be collected from an excellent male animal, even one far away, and placed in the female’s body: this is now done on a huge scale to produce better offspring.

Artificial insemination (AI) is the technique of collecting semen from a male of an improved breed and placing it, with instruments, into the reproductive tract of a female at the right time (when she is ready to conceive). Offspring produced this way are as normal as those from natural mating.

The first scientific experiment in artificial insemination of an animal was done on a dog by the Italian scientist Lazzaro Spallanzani in 1779–80 (he published it in 1784, the date often given); she gave birth to three healthy puppies. His experiments also showed that the power to fertilize lies in the microscopic sperm in semen, not in its liquid part.

The main aim of AI is to produce many offspring of an improved breed by using the semen of one excellent male for many females. Farmers no longer need to keep an expensive male with special qualities, which saves them money. Today AI is used instead of natural mating in cows, buffaloes, goats, sheep, pigs and other animals all over the world. It is one of the most useful tools for improving livestock.

Advantages of artificial insemination:

1. There is no need to keep a male for breeding, which saves money.
2. It helps to stop infections and diseases that spread during mating.
3. The semen is tested before use, so the male’s fertility is checked.
4. Stored semen can be used even after the male has died.
5. Semen can easily be carried long distances.
6. It prevents injuries to the female or male during mating.
7. It increases the rate of successful fertilization.
8. It makes it easy to keep good breeding records.

Disadvantages of artificial insemination:

1. It needs well-trained technicians and special equipment.
2. It takes more time and effort than natural mating.
3. If the instruments are not properly sterilized, the female may get an infection, or may not conceive.

In vitro fertilization (IVF)

IVF is the most effective method of assisted reproductive technology: a series of medical procedures to help couples with low fertility or genetic problems have children. Conception in IVF happens outside the body, unlike in natural conception. A child born by IVF is physically and mentally normal.

A child born by IVF may or may not resemble its parents, because IVF can use the couple’s own egg and sperm, or, if one of them can’t produce healthy eggs or sperm, an egg or sperm from a known or unknown donor.

In IVF, a mature egg is taken from the woman’s ovary, kept in a petri dish, and fertilized with the man’s sperm in a well-equipped laboratory. A few days later, the embryo is moved into the woman’s uterus, where it grows as in a normal pregnancy. One cycle of IVF takes about three weeks, sometimes longer, depending on the problem.

Advantages of IVF:

1. It is the best way to conceive for couples who can’t because of problems with conception (blocked tubes, low sperm count, unexplained infertility).
2. It can use the couple’s own eggs and sperm, or a donor’s.
3. It is more successful than other assisted reproductive techniques.
4. Embryos can be tested for some chromosomal and genetic disorders before they are placed in the uterus, so parents who carry a serious inherited disease can have a healthy child.
5. It helps with both infertility and some genetic problems.
6. With the embryo tested, the risk of losing a pregnancy because of a chromosome disorder can be lower.
7. It increases the chance of a healthy child for couples who carry genetic disorders.

Disadvantages of IVF:

1. Success is not guaranteed: a cycle may fail, and several cycles may be needed.
2. The hormone medicines can have side effects.
3. If more than one embryo is placed in the uterus, twins or triplets may result, which is riskier for mother and babies.
4. It can be emotionally stressful for the couple.
5. Rarely, the embryo implants outside the uterus (an ectopic pregnancy).
6. It is expensive.
7. The baby is somewhat more likely to be born early and with a low birth weight.

1. Choose the correct option

2. Differentiate between

3. Give reasons

4. Answer these questions