Genetics: Some Basic Fundamentals | ICSE Class 10 Biology Notes

This note covers the ICSE Class 10 Biology topic Genetics: heredity and variation, chromosomes, genes and alleles, the basic terms of genetics, Mendel's experiments on the garden pea, the monohybrid and dihybrid crosses, Mendel's three laws, the chromosomal basis of inheritance, sex determination in human beings, X-linked inheritance of colour blindness and haemophilia, and mutation.

What are genetics, heredity and variation?

Genetics is the branch of biology that deals with the inheritance, as well as the variation, of characters from parents to offspring.

Inheritance is the process by which characters are passed on from parent to progeny; it is the basis of heredity. The rules of heredity determine the process by which traits and characteristics are reliably inherited.

Variation is the degree by which progeny differ from their parents. A child bears all the basic features of a human being, yet it does not look exactly like its parents, and human populations show a great deal of variation.

Earlobes are a simple example. In some people the earlobe is closely attached to the side of the head, and in others it is not. Free and attached earlobes are two variants found in human populations.

How variation builds up over generations

Inheritance from the previous generation provides both a common basic body design and subtle changes in it. The next generation then has differences inherited from its parents as well as newly created differences.

In asexual reproduction, if one bacterium divides and the two bacteria formed divide again, the four bacteria produced are very similar. The only very minor differences between them are caused by small inaccuracies in DNA copying.

Sexual reproduction generates even greater diversity. A field of sugarcane shows very little variation among the plants, but in a number of animals that reproduce sexually, including human beings, quite distinct variations are visible among individuals.

Why variation matters

Variations in a species do not all have equal chances of surviving. Depending on the nature of the variation, different individuals have different kinds of advantages. Bacteria that can withstand heat, for example, will survive better in a heat wave.

Selection of variants by environmental factors forms the basis for evolutionary processes. Humans have also used natural variation to breed useful organisms: the Sahiwal cows of Punjab were developed by artificial selection and domestication from ancestral wild cows.

What are chromosomes, genes and alleles?

Both the father and the mother contribute practically equal amounts of genetic material to a child. Each trait can therefore be influenced by both paternal and maternal DNA, so for each trait there are two versions in each child.

Chromosomes

The genetic material, DNA, is not present in the nucleus as a single long thread. Each set of genes is present as separate, independent pieces, each called a chromosome. The name means "coloured bodies", because chromosomes were seen after staining.

Chromosomes come in pairs

Each body cell has two copies of each chromosome, one from the male parent and one from the female parent. The two members of such a pair are called homologous chromosomes.

Every germ cell takes one chromosome from each pair, of either maternal or paternal origin. When two germ cells combine at fertilisation, they restore the normal number of chromosomes in the progeny. This ensures the stability of the DNA of the species.

A normal human cell has 46 chromosomes (23 pairs). Of these, 22 pairs are autosomes, which are exactly the same in males and females, and one pair is the sex chromosomes, also called allosomes.

Genes and alleles

A gene is the functional unit of inheritance. It is a section of DNA that provides information for one protein, and it contains the information required to express a particular trait. Mendel called these units "factors".

Alleles are slightly different forms of the same gene, which code for a pair of contrasting traits. For example, T (tall) and t (dwarf) are alleles of the gene for height in the pea plant. The two alleles of a gene pair lie at matching sites on the two homologous chromosomes.

How do genes control traits?

Plants have hormones that can trigger growth, so plant height can depend on the amount of a particular plant hormone.

1. A gene carries the information for an enzyme that is important in making the hormone.
2. If the enzyme works efficiently, a lot of hormone is made, and the plant is tall.
3. If the gene has an alteration that makes the enzyme less efficient, less hormone is made.
4. With less hormone, the plant is short.

In this way, genes control characteristics, or traits.

What do the basic terms of genetics mean?

Symbols for alleles

The capital letter of a symbol is used for the trait expressed at the F₁ stage, and the small letter for the other trait. So T stands for tall and t for dwarf, and a plant's pair of alleles for height can be TT, Tt or tt.

Using T for tall and d for dwarf is avoided, because it then becomes difficult to remember whether T and d are alleles of the same gene.

Generations and crosses

The plants crossed at the start are the parental generation (P). Their offspring are the Filial₁ progeny or F₁, and the offspring of the F₁ are the F₂.

A monohybrid cross, such as TT × tt, follows one pair of traits; a dihybrid cross follows two. A true-breeding line is one that, after continuous self-pollination, shows stable inheritance and expression of a trait for several generations.

Who was Mendel, and why did his experiments succeed?

Gregor Johann Mendel, who lived from 1822 to 1884 and is often called the father of genetics, was educated in a monastery and studied science and mathematics at the University of Vienna. After failing the examinations for a teaching certificate, he went back to his monastery and started growing peas.

He carried out hybridisation experiments on garden peas for seven years (1856 to 1863) and proposed the laws of inheritance in living organisms. Many others had studied inheritance in peas before, but Mendel was the first to keep count of the individuals showing a particular trait in each generation.

Why Mendel succeeded

• Mathematics: statistical analysis and mathematical logic were applied to a problem in biology for the first time.
• Large samples: his large sampling size gave greater credibility to his data.
• Repeated checks: he confirmed his inferences on successive generations of his plants.
• Clear-cut characters: he chose characters seen as two opposing traits, such as tall or dwarf plants.
• Pure lines: he used true-breeding pea lines.

The seven pairs of contrasting traits

Mendel selected 14 true-breeding pea varieties, as pairs that were similar except for one character with contrasting traits.

How a cross is made in pea

1. The anthers of a flower of the female parent are removed with scissors. This is emasculation.
2. Pollen from a flower of the male parent is transferred to the emasculated flower. This is pollination.
3. The pollinated flower forms a pod containing seeds.
4. The seeds are collected and grown to give the plants of the first hybrid generation.

What does a monohybrid cross show?

A monohybrid cross follows one character with two contrasting traits. Mendel crossed true-breeding tall pea plants with true-breeding dwarf plants.

1. Parents: a tall plant (TT) is crossed with a dwarf plant (tt).
2. Gametes: the tall parent forms gametes with T, and the dwarf parent forms gametes with t.
3. F₁ generation: all the plants are Tt and tall. None is dwarf, and none is of medium height.
4. Selfing: the F₁ plants are self-pollinated. Each forms T and t gametes in equal proportion.
5. F₂ generation: three-quarters of the plants are tall and one-quarter are dwarf.

The dwarf character, not seen in the F₁, reappeared in the F₂. The F₂ plants were either tall or dwarf, identical to the parental types, and none was of in-between height. The traits showed no blending.

Mendel obtained similar results for the other pairs of traits: only one parental trait appeared in the F₁, and both appeared in the F₂ in the proportion 3:1.

Top row: a tall plant (TT) is crossed with a short plant (tt), and an arrow leads to the F₁, labelled "All Tall offspring (Tt)".

Bottom row: two tall F₁ plants (Tt × Tt) are crossed, and an arrow leads to four F₂ plants labelled Tall (TT), Tall (Tt), Tall (Tt) and Short (tt).

Working the cross with a Punnett square

The Punnett square, developed by the British geneticist Reginald C. Punnett, is a graphical way to calculate the probability of all possible genotypes of offspring in a cross.

1. Write the genotypes of the two parents.
2. Work out the gametes each parent can form; a Tt plant forms T and t gametes.
3. Write the gametes along two sides of the square, usually the top row and the left column.
4. Fill each box with the combination of the gametes in its row and column, and count the boxes.

A tall (TT) plant is crossed with a dwarf (tt) plant; their gametes lead into a diamond-shaped grid of four boxes, all Tt, labelled F₁ generation.

Two tall Tt plants are then selfed, giving a second grid with the boxes TT, Tt, Tt and tt, labelled F₂ generation. Beneath: phenotypic ratio tall : dwarf = 3 : 1; genotypic ratio TT : Tt : tt = 1 : 2 : 1.

Phenotypic and genotypic ratios

Random fertilisation gives F₂ zygotes in the proportion 1/4 TT, 1/2 Tt and 1/4 tt. So the F₂ has a phenotypic ratio of 3 tall : 1 dwarf, but a genotypic ratio of 1 TT : 2 Tt : 1 tt.

Externally, TT and Tt plants cannot be told apart. When Mendel self-pollinated the dwarf F₂ plants, they went on producing dwarf plants in the F₃ and F₄ generations, so the dwarfs were homozygous, tt.

The test cross

To find whether a tall F₂ plant was TT or Tt, Mendel crossed it with a dwarf plant. In such a test cross, an organism showing the dominant phenotype is crossed with the recessive parent instead of being self-crossed.

A violet flower of dominant phenotype, genotype unknown and marked "?", is crossed with a white, homozygous recessive (ww) flower.

Left: if it is WW, all four offspring are Ww; result, all flowers violet, so the unknown flower is homozygous dominant. Right: if it is Ww, the offspring are Ww, Ww, ww and ww; result, half violet and half white, so the unknown flower is heterozygous.

What are Mendel's three laws?

From his monohybrid crosses Mendel proposed two general rules, and from his dihybrid crosses a third. They are now called the Principles or Laws of Inheritance.

First law: the Law of Dominance

• Characters are controlled by discrete units called factors.
• Factors occur in pairs.
• In a dissimilar pair of factors, one member of the pair dominates (dominant) the other (recessive).

This law explains why only one parental character is expressed in the F₁ of a monohybrid cross while both appear in the F₂. It also explains the 3:1 proportion obtained at the F₂.

Second law: the Law of Segregation

The alleles do not blend, and both characters are recovered as such in the F₂, though one of them is not seen in the F₁. The Law of Segregation, also called the law of purity of gametes, states that the two factors of a pair separate during gamete formation, so that a gamete receives only one of the two.

A homozygous parent therefore produces gametes that are all similar, while a heterozygous parent produces two kinds of gametes, each with one allele, in equal proportion.

Third law: the Law of Independent Assortment

The Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair of characters.

What does a dihybrid cross show?

A dihybrid cross follows two characters at once. Mendel crossed a pea plant with round, yellow seeds and one with wrinkled, green seeds, using R for round, r for wrinkled, Y for yellow and y for green.

1. Parents: round, yellow (RRYY) × wrinkled, green (rryy).
2. Gametes: RY from the first parent and ry from the second.
3. F₁ generation: all RrYy, with round, yellow seeds. So round is dominant over wrinkled, and yellow over green.
4. Selfing: each F₁ plant forms four kinds of gametes, RY, Ry, rY and ry, each 1/4 of the total.
5. F₂ generation: a 16-box Punnett square gives 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.

P generation: round yellow (RR YY) and wrinkled green (rr yy) seeds give gametes RY and ry. The F₁ is round yellow (Rr Yy) and is selfed.

The four female and four male gametes (RY, rY, Ry, ry) run along the two upper sides of a diamond-shaped grid of 16 boxes, from RRYY at the top to rryy at the bottom. Beneath: round yellow : round green : wrinkled yellow : wrinkled green = 9 : 3 : 3 : 1.

The F₂ phenotypes

In a similar cross between round, green (RRyy) and wrinkled, yellow (rrYY) parents, the F₁ was again round and yellow (RrYy), and the F₂ seeds were counted.

Where the 9:3:3:1 ratio comes from

Taken one at a time, each character still segregates 3:1, just as in a monohybrid cross. Combining them gives (3 round : 1 wrinkled) × (3 yellow : 1 green) = 9 round yellow : 3 wrinkled yellow : 3 round green : 1 wrinkled green.

The F₂ also shows new combinations of traits. When a tall plant with round seeds is crossed with a short plant with wrinkled seeds, the F₁ is all tall with round seeds, but the F₂ includes tall plants with wrinkled seeds and short plants with round seeds. The two traits are independently inherited.

How do chromosomes explain Mendel's laws?

Mendel published his work in 1865, but it remained unrecognised till 1900. Communication was not easy, his idea of discrete factors that did not blend was not accepted, his use of mathematics was new to biologists, and he could give no physical proof of what factors were.

In 1900, three scientists, de Vries, Correns and von Tschermak, independently rediscovered Mendel's results. Advances in microscopy by then allowed scientists to watch cell division, and by 1902 the movement of chromosomes during meiosis had been worked out.

The chromosomal theory of inheritance

Walter Sutton and Theodore Boveri noted that chromosomes behave in parallel with genes: the pairing and separation of a pair of chromosomes would lead to the segregation of the pair of factors they carried. Sutton called this synthesis the chromosomal theory of inheritance.

A cell has two chromosome pairs: long orange and long yellow, and short green and short red.

In Possibility I, the long orange and short green chromosomes go to the same pole at anaphase of meiosis I; in Possibility II, the long orange goes with the short red. Each column then shows anaphase of meiosis II and four germ cells, with different combinations in the two columns.

Thomas Hunt Morgan and his colleagues verified the theory experimentally with the fruit fly Drosophila melanogaster. Morgan's work also showed that genes on the same chromosome, called linked genes, do not assort independently; closely located genes tend to be inherited together.

How is sex determined in human beings?

Different species use very different strategies to decide sex. In a few reptiles, the temperature at which the fertilised eggs are kept decides whether the young will be male or female, and snails can change sex. In human beings, sex is largely genetically determined.

The human sex chromosomes

Of the 23 pairs of human chromosomes, 22 pairs are exactly the same in males and females; these are the autosomes. The remaining pair, the sex chromosomes, is not always a perfect pair.

Women have a perfect pair of sex chromosomes, both called X. Men have a mismatched pair: a normal-sized X and a distinctly smaller Y chromosome. So women are XX and men are XY; the mechanism is of the XY type.

1. Males produce two types of sperm: 50 per cent carry an X chromosome and 50 per cent carry a Y chromosome, besides the autosomes.
2. Females produce only one type of ovum, carrying an X chromosome.
3. The ovum has an equal chance of being fertilised by an X-bearing or a Y-bearing sperm.
4. Fertilisation by an X-bearing sperm gives an XX zygote, which develops into a girl.
5. Fertilisation by a Y-bearing sperm gives an XY zygote, which develops into a boy.

At the top, the outline of a male body is marked XY and a female body XX. In the Gametes row, the male forms two sperm, X and Y, and the female one egg, X.

Crossing lines join the gametes to two zygotes, XX and XY, and arrows lead to the Offsprings row: the XX zygote becomes a female and the XY zygote a male.

All children inherit an X chromosome from their mother, so the sex of the child depends on what it inherits from the father. In each pregnancy there is a 50 per cent probability of either a boy or a girl.

Note: It is the genetic make-up of the sperm that determines the sex of the child. Blaming or ill-treating women for giving birth to daughters rests on a false notion.

Sex determination in other organisms

Humans and Drosophila, whose males form two kinds of gametes, show male heterogamety. In many birds the female forms two kinds of gametes instead, which is female heterogamety.

What is sex-linked inheritance?

Genes located on the sex chromosomes are called sex-linked genes. Colour blindness and haemophilia are caused by recessive alleles of genes on the X chromosome, so they are X-linked recessive disorders.

Males have only one X chromosome, while females have two. A male with the recessive allele on his single X shows the disorder. In a female with one recessive allele, the matching normal (dominant) allele on her other X suppresses its effect. Such a heterozygous female is a carrier.

Colour blindness

Colour blindness is a sex-linked recessive disorder caused by a defect in either the red or the green cone of the eye. It results in failure to discriminate between red and green colour, and is due to mutation in certain genes on the X chromosome.

It occurs in about 8 per cent of males and only about 0.4 per cent of females. The son of a woman who carries the gene has a 50 per cent chance of being colour blind, while the mother herself is not colour blind, because the gene is recessive.

A daughter will not normally be colour blind unless her mother is a carrier and her father is colour blind.

Haemophilia

In haemophilia, a single protein that is part of the cascade of proteins involved in blood clotting is affected. In an affected person, a simple cut results in non-stop bleeding.

The disease passes from an unaffected carrier female to some of her sons. A haemophilic female is extremely rare, because her mother must be at least a carrier and her father must be haemophilic. The family pedigree of Queen Victoria shows a number of haemophilic descendants, as she was a carrier.

Working an X-linked cross

Write X for the X chromosome with the normal allele and Xʰ for the X with the haemophilia allele. Consider a carrier woman (XXʰ) married to a normal man (XY).

1. The mother forms two kinds of eggs, X and Xʰ, in equal numbers.
2. The father forms two kinds of sperm, X and Y, in equal numbers.
3. The possible children are XX (normal daughter), XXʰ (carrier daughter), XY (normal son) and XʰY (haemophilic son).
4. So half the sons are expected to be haemophilic and half the daughters carriers; no daughter is haemophilic.

Draw the mother (XXʰ) and father (XY) side by side, with their gametes (X, Xʰ and X, Y) in circles below.

Join them in a four-box Punnett square and label the offspring XX normal female, XXʰ carrier female, XY normal male and XʰY haemophilic male.

A son receives his X chromosome from his mother and his Y chromosome from his father. So an affected father does not pass an X-linked disorder to his sons, but each of his daughters receives his X and is at least a carrier.

What is mutation?

DNA carries genetic information from one generation to the next, usually without change, but changes do occasionally take place. Mutation is a phenomenon that results in alteration of DNA sequences and consequently in changes in the genotype and the phenotype of an organism.

Besides recombination, mutation is another phenomenon that leads to variation in DNA. Chemical and physical factors that induce mutations are called mutagens; ultraviolet (UV) radiation is one.

Gene (point) mutations

A mutation caused by a change in a single base pair of DNA is a point mutation. A classical example is sickle-cell anaemia.

• A base change at the sixth codon of the beta globin gene, from GAG to GUG, replaces glutamic acid by valine at the sixth position of the beta globin chain of haemoglobin.
• Under low oxygen tension the mutant haemoglobin polymerises, changing the red blood cell from a biconcave disc to an elongated, sickle-like shape.
• The gene has two alleles, HbA and HbS. Only people homozygous for HbS (HbS HbS) show the disease; heterozygotes (HbA HbS) appear unaffected but are carriers.

Chromosomal mutations

Loss (deletion) or gain (insertion or duplication) of a segment of DNA alters a chromosome, causing chromosomal aberrations. Failure of chromatids to segregate during cell division causes the gain or loss of a chromosome, called aneuploidy. An increase in a whole set of chromosomes, often seen in plants, is polyploidy.

Where does variation come from?

Variations arising during reproduction can be inherited, and they may lead to increased survival of individuals. In sexually reproducing organisms, they arise in several ways.

1. Segregation: the alleles of each pair separate into different gametes, so a hidden recessive allele can reappear in the next generation.
2. Independent assortment: chromosome pairs sort into germ cells independently of each other, giving new combinations of traits.
3. Crossing over: during meiosis, homologous chromosomes pair and exchange genetic material, which leads to recombination.
4. Random fertilisation: any male gamete can fuse with any female gamete, giving zygotes of different genotypes.
5. Mutation: changes in DNA create new alleles and chromosomal changes.

Meiosis keeps the chromosome number of a species constant across generations. It also increases the genetic variability in a population from one generation to the next, and variations are very important for evolution.

Some traits, unlike Mendel's tall or dwarf, spread across a gradient: humans show a whole range of heights. Such polygenic traits are generally controlled by three or more genes and are also influenced by the environment.

Glossary

• Genetics — the branch of biology that deals with the inheritance, as well as the variation, of characters from parents to offspring.
• Heredity — the passing of characters from parents to progeny through inheritance, following rules by which traits are reliably inherited.
• Chromosome — one of the separate, independent pieces into which the genetic material of a cell is organised; each body cell has two copies of each.
• Homologous chromosomes — the two chromosomes of a pair, one from each parent, carrying the alleles of the same genes at matching sites.
• True-breeding line — a line that, after continuous self-pollination, shows stable inheritance and expression of a trait for several generations.
• F₁ generation — the first filial generation, the hybrid offspring produced by crossing the two parents of a genetic cross.
• Monohybrid cross — a cross between parents that differ in one pair of contrasting traits, such as tall and dwarf stem height.
• Dihybrid cross — a cross between parents that differ in two pairs of contrasting traits, such as seed shape and seed colour.
• Punnett square — a grid, developed by Reginald C. Punnett, used to work out all the possible genotypes of offspring in a genetic cross.
• Test cross — a cross of an organism showing the dominant phenotype with the recessive parent, to find out whether it is homozygous or heterozygous.
• Emasculation — the removal of the anthers from a flower chosen as the female parent before it is cross-pollinated.
• Carrier — a heterozygous individual who has one recessive allele for a disorder but does not show it; for X-linked disorders such as haemophilia, carriers are females.
• Point mutation — a mutation caused by a change in a single base pair of DNA, as in sickle-cell anaemia.
• Mutagen — a chemical or physical factor that induces mutations; ultraviolet radiation is an example.

Common errors and misconceptions

• Misconception: The mother decides whether a baby will be a boy or a girl. Correct: Every ovum carries an X chromosome. The sperm, carrying either X or Y, decides the sex, so blaming women for the birth of daughters rests on a false notion.
• Misconception: The tall F₁ plants are genetically the same as the tall parent. Correct: The F₁ plants are Tt. They look like the TT parent, but they carry the t allele, which shows up again in one-quarter of the F₂.
• Misconception: The recessive trait is lost or blended in the F₁. Correct: It is only hidden. The alleles do not blend, and the recessive trait reappears unchanged in the F₂.
• Misconception: The 3:1 ratio of a monohybrid cross is a genotypic ratio. Correct: 3 tall : 1 dwarf is the phenotypic ratio. The genotypic ratio is 1 TT : 2 Tt : 1 tt.
• Misconception: A dihybrid F₂ has exactly 16 plants. Correct: 9:3:3:1 is a ratio of proportions out of 16. One F₂ of 556 seeds, for example, had 315, 108, 101 and 32 seeds of the four types.
• Misconception: A carrier mother shows haemophilia or colour blindness herself. Correct: Her normal dominant allele suppresses the recessive one, so she is unaffected. Males, with only one X, show these disorders far more often.
• Misconception: A colour-blind father passes colour blindness to his son. Correct: A son gets his X chromosome from his mother and his Y from his father, so the father's X-linked allele goes to his daughters instead.

Exam-style questions with model answers

Q1. What are alleles? Give one example. [2 marks]

1. Alleles are slightly different forms of the same gene that code for a pair of contrasting traits; they lie at matching sites on homologous chromosomes.
2. Example: in the pea plant, T (tall) and t (dwarf) are alleles of the gene for stem height.

Q2. A pure tall pea plant is crossed with a pure dwarf pea plant. Show the F₁ and F₂ generations and give the F₂ phenotypic and genotypic ratios. [4 marks]

1. Parents: tall TT × dwarf tt. The tall parent forms T gametes and the dwarf parent forms t gametes.
2. F₁: all plants are Tt and tall, because T is dominant over t.
3. Selfing the F₁ (Tt × Tt): each plant forms T and t gametes in equal proportion.
4. F₂ Punnett square: TT, Tt, Tt and tt.
5. Phenotypic ratio: 3 tall : 1 dwarf. Genotypic ratio: 1 TT : 2 Tt : 1 tt.

Q3. State Mendel's three laws of inheritance. [3 marks]

1. Law of Dominance: characters are controlled by discrete units called factors, which occur in pairs; in a dissimilar pair, one factor (dominant) dominates the other (recessive).
2. Law of Segregation: the two factors of a pair separate from each other during gamete formation, so that each gamete receives only one of the two.
3. Law of Independent Assortment: when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair.

Q4. Explain, with a chart, how sex is determined in human beings. Why is it wrong to blame the mother for the birth of a daughter? [5 marks]

1. Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. Females are XX and males are XY, the Y being smaller.
2. Males produce two types of sperm, 50 per cent with X and 50 per cent with Y. Females produce only one type of ovum, with X.
3. Chart: parents XX × XY; gametes X (egg) and X or Y (sperm); zygotes XX (girl) and XY (boy).
4. An X-bearing sperm fertilising the ovum gives a girl; a Y-bearing sperm gives a boy. Each pregnancy has a 50 per cent chance of either.
5. Since every ovum carries X, it is the sperm from the father that determines the sex, so blaming the mother for a daughter rests on a false notion.

Q5. A pea plant with round, yellow seeds (RRYY) is crossed with one with wrinkled, green seeds (rryy). Work out the F₁, the gametes of the F₁ and the phenotypic ratio of the F₂. Which law does this cross illustrate? [5 marks]

1. Parents RRYY and rryy form gametes RY and ry respectively.
2. F₁: all RrYy, with round, yellow seeds, since round is dominant over wrinkled and yellow over green.
3. On selfing, each F₁ plant forms four kinds of gametes, RY, Ry, rY and ry, each 1/4 of the total, because the two pairs segregate independently.
4. These combine in a 16-box Punnett square to give four phenotypes: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
5. Round green and wrinkled yellow are new combinations not seen in the parents.
6. The cross illustrates the Law of Independent Assortment: the segregation of one pair of characters is independent of the other pair.

Q6. A woman who is a carrier for haemophilia marries a normal man. Using a Punnett square, show the kinds of children they may have. Why is haemophilia more common in males? [5 marks]

1. Let X be the normal X chromosome and Xʰ the X with the haemophilia allele. Mother: XXʰ (carrier). Father: XY (normal).
2. Eggs: X and Xʰ. Sperm: X and Y.
3. Punnett square: XX (normal daughter), XXʰ (carrier daughter), XY (normal son), XʰY (haemophilic son).
4. So half the sons are expected to be haemophilic and half the daughters carriers.
5. Haemophilia is caused by a recessive allele on the X chromosome. A male has only one X, so a single recessive allele produces the disease.
6. A female needs the allele on both X chromosomes, so her mother must be at least a carrier and her father haemophilic, which is extremely rare.

Q7. A colour-blind man marries a woman who has normal vision and is not a carrier. What will their children be like? [3 marks]

1. Let Xᶜ be the X with the colour-blindness allele. Father: XᶜY. Mother: XX.
2. Sperm: Xᶜ and Y. Eggs: X only.
3. Daughters are XXᶜ: they have normal vision, because the normal allele suppresses the recessive one, but all are carriers.
4. Sons are XY: they receive their X from their mother and their Y from their father, so all have normal vision.

Q8. What is mutation? Name the two main kinds of mutation with one example of each. [3 marks]

1. Mutation is a phenomenon that results in alteration of DNA sequences and consequently in changes in the genotype and phenotype of an organism.
2. Gene or point mutation: a change in a single base pair of DNA, for example sickle-cell anaemia, where GAG changes to GUG in the beta globin gene.
3. Chromosomal mutation: a change in chromosome structure or number, for example Down's syndrome, caused by an extra copy of chromosome 21.

Key takeaways

• Genetics deals with inheritance and variation; variation arises during reproduction, is much greater in sexual reproduction, and may help individuals survive.
• Genes are the functional units of inheritance, found on chromosomes; alleles are slightly different forms of a gene that code for contrasting traits.
• Humans have 46 chromosomes in 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes (allosomes), XX in females and XY in males.
• In a monohybrid cross of tall and dwarf pea plants, the F₁ is all tall, and the F₂ shows a 3:1 phenotypic and a 1:2:1 genotypic ratio.
• In a dihybrid cross of round yellow and wrinkled green seeds, the F₁ is round yellow and the F₂ shows a phenotypic ratio of 9:3:3:1.
• Mendel's three laws are the Law of Dominance, the Law of Segregation and the Law of Independent Assortment; the behaviour of chromosomes in meiosis explains segregation and independent assortment.
• In humans the sperm, carrying X or Y, determines the sex of the child, and each pregnancy has a 50 per cent chance of a boy or a girl.
• Colour blindness and haemophilia are X-linked recessive disorders passed on mainly by carrier mothers to their sons, so males are affected far more often.
• Mutation alters the DNA sequence; it may be a point mutation, as in sickle-cell anaemia, or a chromosomal change, as in Down's syndrome.

Test yourself

What did Mendel call the units that we now call genes?

He called them "factors", which he proposed were passed down unchanged from parent to offspring through the gametes.

Name the three scientists who rediscovered Mendel's work in 1900.

De Vries, Correns and von Tschermak independently rediscovered Mendel's results on the inheritance of characters in 1900.

How many kinds of gametes does a dihybrid RrYy plant form, and what are they?

It forms four kinds of gametes in equal proportion: RY, Ry, rY and ry, each one-quarter of the total.

Who proposed the chromosomal theory of inheritance?

Walter Sutton and Theodore Boveri noted the parallel between chromosomes and genes; Sutton united chromosomal segregation with Mendelian principles as the chromosomal theory of inheritance.

What is the chromosome make-up of human sperm and ova?

Half the sperm carry an X chromosome and half carry a Y chromosome, besides the autosomes; all ova carry an X chromosome.

About what percentage of males and of females are colour blind?

Colour blindness occurs in about 8 per cent of males and only about 0.4 per cent of females.

Why is Queen Victoria important in the study of haemophilia?

She was a carrier of haemophilia, and her family pedigree shows a number of haemophilic descendants.