Principles of Inheritance and Variation | CBSE Class 12 Biology Notes

This note covers NCERT Class 12 Biology Chapter 4, Principles of Inheritance and Variation: Mendel's experiments and laws, incomplete dominance and co-dominance, the dihybrid cross, the chromosomal theory of inheritance, linkage, polygenic inheritance and pleiotropy, sex determination, mutation, pedigree analysis, and the Mendelian and chromosomal disorders.

The crosses are worked out step by step, because ratios and Punnett squares are central to this chapter.

What are inheritance and variation, and why did Mendel succeed?

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. Variation is the degree by which progeny differ from their parents.

Humans knew from as early as 8000 to 1000 B.C. that one of the causes of variation was hidden in sexual reproduction. They used the variation naturally present in wild plants and animals to selectively breed organisms with desirable characters.

Through artificial selection and domestication from ancestral wild cows, for example, we have well-known Indian breeds such as the Sahiwal cows of Punjab. Our ancestors, however, had very little idea of the scientific basis of these phenomena.

Gregor Mendel conducted hybridisation experiments on garden peas for seven years (1856 to 1863) and proposed the laws of inheritance in living organisms. For the first time, statistical analysis and mathematical logic were applied to problems in biology.

His experiments had a large sampling size, which gave greater credibility to his data, and he confirmed his inferences on successive generations of his test plants.

Mendel used true-breeding lines: lines that, after continuous self-pollination, show stable trait inheritance and expression for several generations. He selected 14 true-breeding pea varieties, as pairs that were similar except for one character with contrasting traits.

What did Mendel's monohybrid cross show?

To study the inheritance of one gene, Mendel crossed tall and dwarf pea plants. The steps and results were as follows.

1. He crossed true-breeding tall and dwarf plants and grew the seeds to get the first hybrid generation, the Filial₁ progeny or F₁.
2. All the F₁ plants were tall, like one parent; none were dwarf. For every pair of traits, the F₁ resembled only one of the parents.
3. He self-pollinated the tall F₁ plants. In the F₂ generation some offspring were dwarf: the character not seen in F₁ was expressed again.
4. One-fourth of the F₂ plants were dwarf and three-fourths were tall, a 3:1 ratio. The plants were either tall or dwarf, with no in-between heights, so the traits did not blend.

The parental row shows a tall plant crossed with a dwarf plant. An arrow leads to the F₁ generation, where two tall plants are shown crossed with each other (selfing).

A second arrow leads to the F₂ generation, which shows three tall plants and one dwarf plant.

See Fig. 4.3 in your NCERT textbook

Mendel proposed that something was being passed down unchanged from parent to offspring through the gametes. He called these things factors; we now call them genes, the units of inheritance. Genes coding for a pair of contrasting traits are called alleles: slightly different forms of the same gene.

The capital letter is used for the trait expressed in F₁ and the small letter for the other, so T is tall and t is dwarf. A plant may be TT, Tt or tt. TT and tt are homozygous;

Tt is heterozygous. These letter combinations are the genotype, while the descriptive terms tall and dwarf are the phenotype. Since the Tt plant looks exactly like TT, T is the dominant factor and t is recessive.

A Tt plant, heterozygous for one character, is a monohybrid, and TT × tt is a monohybrid cross.

The Punnett square

During gamete formation by meiosis the alleles of a pair segregate, and only one allele goes into each gamete, with a 50 per cent chance of either. The Punnett square, developed by the British geneticist Reginald C.

Punnett, shows the gametes along two sides and all possible zygotes in the boxes. For Tt × Tt, 1/4 of fertilisations give TT, 1/2 give Tt and 1/4 give tt.

So the F₂ phenotypic ratio is 3 tall : 1 dwarf, but the genotypic ratio is 1 TT : 2 Tt : 1 tt.

The same result follows from the binomial expansion: (1/2 T + 1/2 t)² = 1/4 TT + 1/2 Tt + 1/4 tt.

Mendel also found that dwarf F₂ plants, when self-pollinated, kept producing dwarf plants in F₃ and F₄, so their genotype was homozygous tt.

What is a test cross, and what are the laws of dominance and segregation?

A tall F₂ plant may be TT or Tt, and the phenotype alone cannot tell us which. To find out, Mendel crossed the tall F₂ plant with a dwarf plant.

This is a test cross: an organism showing the dominant phenotype, whose genotype is to be determined, is crossed with the recessive parent instead of being self-crossed.

1. Take the plant with the dominant phenotype and unknown genotype, for example a violet-flowered pea.
2. Cross it with a homozygous recessive plant, here a white-flowered pea.
3. If all the progeny show the dominant trait, the unknown plant is homozygous dominant.
4. If half the progeny show the dominant trait and half the recessive, a 1:1 ratio, the unknown plant is heterozygous.

A violet flower of dominant phenotype (genotype unknown, marked with a question mark) sits in the middle.

On the left it is treated as WW and crossed with a homozygous recessive ww white flower: all four boxes give Ww, and the result reads "all flowers are violet", interpreted as "unknown flower is homozygous dominant".

On the right it is treated as Ww and crossed with ww: two boxes give Ww and two give ww, and the result reads "half of the flowers are violet and half of the flowers are white", interpreted as "unknown flower is heterozygous". The figure labels the flower-colour alleles W and w.

See Fig. 4.5 in your NCERT textbook

Law of Dominance (the First Law)

1. Characters are controlled by discrete units called factors.
2. Factors occur in pairs.
3. 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 F₁, why both are expressed in F₂, and the 3:1 proportion in F₂.

Law of Segregation (the Second Law)

The alleles do not blend, and both characters are recovered as such in F₂ though one is not seen in F₁. During gamete formation the two factors of a pair segregate, so each gamete receives only one of them.

A homozygous parent produces one kind of gamete; a heterozygous parent produces two kinds, each with one allele, in equal proportion.

What are incomplete dominance and co-dominance?

When other traits in other plants were studied, the F₁ sometimes did not resemble either parent and was in between the two. This is incomplete dominance.

In the dog flower (snapdragon, Antirrhinum sp.), a cross between true-breeding red (RR) and white (rr) plants gives pink F₁ plants (Rr). Selfing F₁ gives an F₂ of 1 red (RR) : 2 pink (Rr) : 1 white (rr).

The genotypic ratio is the usual 1:2:1, but the phenotypic ratio has changed from 3:1 to 1:2:1, because R is not completely dominant over r.

P generation: a red (RR) flower and a white (rr) flower give gametes R and r. The F₁ generation is "all pink (Rr)".

The F₁ female and male gametes (R and r from each) feed a diamond-shaped checkerboard giving RR, Rr, Rr and rr.

Below it the phenotypic ratio is red : pink : white = 1 : 2 : 1 and the genotypic ratio is RR : Rr : rr = 1 : 2 : 1. A photograph of red and pink snapdragon flowers is included.

See Fig. 4.6 in your NCERT textbook

Why are some alleles dominant?

Suppose a gene codes for an enzyme that transforms a substrate S. A modified allele could produce (i) the normal or a less efficient enzyme, (ii) a non-functional enzyme, or (iii) no enzyme at all.

In the first case the two alleles are equivalent. In the other two, the phenotype depends only on the unmodified, functioning allele, which is therefore the dominant allele; the modified allele is generally recessive.

Co-dominance and multiple alleles

In co-dominance the F₁ resembles both parents. The ABO blood groups are controlled by gene I, which decides the kind of sugar polymer protruding from the red blood cell membrane. The gene has three alleles, Iᴬ, Iᴮ and i.

Iᴬ and Iᴮ produce slightly different sugars and i produces none. Iᴬ and Iᴮ are completely dominant over i, but when Iᴬ and Iᴮ are together both express, so the cells carry both A and B sugars.

So there are six genotypes and four phenotypes (A, B, AB and O). ABO is also an example of multiple alleles: more than two alleles govern the same character. Since one individual carries only two alleles, multiple alleles can be found only in population studies.

Note: Dominance depends on the phenotype examined. In pea seeds, one gene with alleles B and b controls starch synthesis. BB seeds make large starch grains and are round; bb seeds make smaller grains and are wrinkled.

Bb seeds are round, so B seems dominant, but their starch grains are of intermediate size, so for grain size the alleles show incomplete dominance.

What does a dihybrid cross show?

Mendel crossed a pea plant with yellow, round seeds and one with green, wrinkled seeds. The F₁ seeds were all yellow and round, so yellow is dominant over green and round over wrinkled.

Using Y and y for seed colour and R and r for seed shape, the parents are RRYY and rryy, their gametes are RY and ry, and the F₁ hybrid is RrYy.

On selfing F₁, 3/4 of the F₂ had yellow seeds and 1/4 green, and round and wrinkled also segregated 3:1, exactly as in monohybrid crosses. Taken together, the four phenotypes appeared in the ratio 9:3:3:1:

(3 round : 1 wrinkled) × (3 yellow : 1 green) = 9 round yellow : 3 wrinkled yellow : 3 round green : 1 wrinkled green.

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

Female and male gametes RY, rY, Ry and ry line the two upper sides of a large diamond of 16 boxes, each showing an F₂ genotype with a drawing of the seed.

The phenotypic ratio printed below is round yellow : round green : wrinkled yellow : wrinkled green = 9 : 3 : 3 : 1.

See Fig. 4.7 in your NCERT textbook

Law of Independent Assortment

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

In the RrYy plant, 50 per cent of gametes carry R and 50 per cent r, and each of these independently carries Y or y.

So there are four kinds of gametes, RY, Ry, rY and ry, each with a frequency of 1/4.

The 16 boxes of the Punnett square contain nine different genotypes, which fall into four phenotypes:

The genotypic ratio is therefore 1:2:2:4:1:2:1:2:1, not 9:3:3:1. Only the phenotypes fall into the 9:3:3:1 ratio.

What is the chromosomal theory of inheritance?

Mendel published his work in 1865, but it remained unrecognised till 1900, for four reasons:

1. Communication was not easy in those days, so his work could not be widely publicised.
2. His concept of genes as stable, discrete units that did not blend was not accepted by contemporaries as an explanation for the apparently continuous variation seen in nature.
3. His use of mathematics to explain biological phenomena was totally new and unacceptable to many biologists of his time.
4. He could not provide any physical proof for the existence of factors or say what they were made of.

In 1900 three scientists, de Vries, Correns and von Tschermak, independently rediscovered Mendel's results. Advances in microscopy had revealed structures in the nucleus that doubled and divided before each cell division; these were called chromosomes (coloured bodies, as they were seen by staining). By 1902 chromosome movement during meiosis had been worked out.

Walter Sutton and Theodore Boveri noted that the behaviour of chromosomes was parallel to the behaviour of genes, and used chromosome movement to explain Mendel's laws.

Sutton united chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance. The two alleles of a gene pair are located on homologous sites on homologous chromosomes.

During meiosis I, two chromosome pairs can align at the metaphase plate independently of each other. In one possibility the long orange and short green chromosomes go to the same pole; in the other, the long orange and short red go together. This is the chromosomal basis of independent assortment.

What did Morgan discover about linkage and recombination?

Thomas Hunt Morgan and his colleagues verified the chromosomal theory experimentally. Morgan worked with the fruit fly Drosophila melanogaster, which suited such studies because it can be grown on a simple synthetic medium, completes its life cycle in about two weeks, gives a large number of progeny from a single mating, has clearly distinguishable males and females, and shows many hereditary variations visible under low power microscopes.

Morgan made dihybrid crosses in Drosophila to study sex-linked genes. For example, he crossed yellow-bodied, white-eyed females with brown-bodied, red-eyed males and intercrossed their F₁ progeny. The two genes did not segregate independently, and the F₂ ratio deviated very significantly from 9:3:3:1.

The genes were on the X chromosome, and when two genes were on the same chromosome, parental combinations were much more frequent than non-parental ones. Morgan coined the term linkage for this physical association of genes on a chromosome, and recombination for the generation of non-parental gene combinations.

Some genes were very tightly linked and showed very low recombination; others were loosely linked. The genes white and yellow showed only 1.3 per cent recombination, while white and miniature wing showed 37.2 per cent.

Morgan's student Alfred Sturtevant used the frequency of recombination between gene pairs as a measure of the distance between them and "mapped" their positions on the chromosome.

Genetic maps are now a starting point for sequencing whole genomes, as in the Human Genome Sequencing Project.

Cross A: a yellow, white female (y w on both X chromosomes) is crossed with a wild-type male (y⁺ w⁺).

The F₁ female is wild type and the F₁ male is yellow, white. Among the gametes, parental types are 98.7 per cent and recombinant types 1.3 per cent, and the F₂ boxes are labelled wild type, white, "yellow, white" and yellow.

Cross B does the same with white, miniature (w m) and wild type (w⁺ m⁺): parental types are 62.8 per cent and recombinant types 37.2 per cent. Dominant wild-type alleles carry a superscript plus sign.

The note under the figure says the strength of linkage between y and w is higher than between w and m.

See Fig. 4.11 in your NCERT textbook

What are polygenic inheritance and pleiotropy?

Mendel studied traits with distinct alternate forms. Many traits instead vary along a gradient: humans are not just tall or short but show a whole range of heights.

Such traits are generally controlled by three or more genes and are called polygenic traits. Polygenic inheritance also takes into account the influence of the environment.

Human skin colour is a classic example. In a polygenic trait the effect of each allele is additive.

Suppose three genes A, B and C control skin colour, with the dominant forms A, B and C for dark colour and the recessive forms a, b and c for light colour.

Then AABBCC has the darkest skin, aabbcc the lightest, and a genotype with three dominant and three recessive alleles an intermediate colour. The number of each type of allele determines how dark or light the skin is.

A pleiotropic gene is a single gene that shows multiple phenotypic expressions. In most cases the mechanism is the gene's effect on metabolic pathways that contribute to different phenotypes.

An example is phenylketonuria in humans, caused by a mutation in the gene for the enzyme phenylalanine hydroxylase, a single gene mutation. It shows up as intellectual disability (mental retardation) and a reduction in hair and skin pigmentation.

How is sex determined?

Henking (1891) traced a specific nuclear structure through spermatogenesis in a few insects and saw that 50 per cent of the sperm received it and 50 per cent did not.

He named it the X body but could not explain its significance. Later work showed it was a chromosome, so it was named the X-chromosome.

Because it is involved in sex determination it was called a sex chromosome, and the rest were named autosomes.

In the XO and XY types, males produce two different types of gametes, so these are examples of male heterogamety.

In birds, females produce two types of gametes in terms of sex chromosomes, which is female heterogamety; the female's two different sex chromosomes are named Z and W to distinguish this mechanism.

Sex determination in humans

Humans have 23 pairs of chromosomes, of which 22 pairs are autosomes, the same in both sexes. Females have XX and males XY.

During spermatogenesis 50 per cent of sperm carry X and 50 per cent carry Y, while females produce only one type of ovum, with an X. An ovum fertilised by an X-bearing sperm develops into a female (XX), and by a Y-bearing sperm into a male (XY).

So the genetic makeup of the sperm determines the sex of the child, and each pregnancy has a 50 per cent probability of either. Blaming women for giving birth to daughters rests on a false notion.

Sex determination in the honey bee

In the honey bee, sex depends on the number of chromosome sets. A fertilised egg develops into a female (queen or worker) and an unfertilised egg develops into a male (drone) by parthenogenesis. This is the haplodiploid system.

Males produce sperm by mitosis; they have no father and so cannot have sons, but they have a grandfather and can have grandsons.

The female parent (32) forms gametes of 16 by meiosis, and the male parent (16) forms gametes of 16 by mitosis.

In F₁, an egg that develops without fertilisation gives a male (16), and an egg fertilised by a sperm gives a female (32).

See Fig. 4.13 in your NCERT textbook

What is mutation, and how is pedigree analysis used?

Mutation is a phenomenon that alters DNA sequences and consequently changes the genotype and phenotype of an organism. Besides recombination, it is another source of variation in DNA.

One DNA helix runs continuously through each chromatid, so loss (deletion) or gain (insertion or duplication) of a segment of DNA alters the chromosome. Such chromosomal aberrations are commonly observed in cancer cells.

A change in a single base pair of DNA is a point mutation, and sickle cell anaemia is a classical example. Deletions and insertions of base pairs cause frame-shift mutations. Chemical and physical factors that induce mutations are called mutagens; UV radiation is one.

Pedigree analysis

Controlled crosses like those in the pea cannot be made in humans, so the family history of a trait is studied instead.

An analysis of a trait over several generations of a family is called pedigree analysis, and it is used to trace the inheritance of a specific trait, abnormality or disease.

It also shows whether a trait is dominant or recessive, and whether it is linked to a sex chromosome.

Pedigrees are drawn for autosomal dominant traits (for example myotonic dystrophy) and autosomal recessive traits (for example sickle-cell anaemia).

Which Mendelian disorders should you know?

Genetic disorders fall into two groups. Mendelian disorders are mainly determined by alteration or mutation in a single gene and are transmitted on the lines of Mendel's principles; their pattern can be traced by pedigree analysis.

They may be dominant or recessive, and some, such as haemophilia, are linked to a sex chromosome.

Colour blindness and haemophilia

Males have one X chromosome and females two. So the son of a woman who carries the colour blindness gene has a 50 per cent chance of being colour blind, while the carrier mother is not, because her normal dominant gene suppresses the recessive one.

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

A heterozygous carrier female may pass haemophilia to her sons. A female haemophiliac is extremely rare, because her mother must be at least a carrier and her father haemophilic (unviable in the later stage of life).

The family pedigree of Queen Victoria, who was a carrier, shows a number of haemophilic descendants.

How the sickle-cell mutation acts

1. A single base substitution at the sixth codon of the beta globin gene changes GAG to GUG.
2. As a result, glutamic acid (Glu) is replaced by valine (Val) at the sixth position of the beta globin chain.
3. The mutant haemoglobin polymerises under low oxygen tension.
4. This changes the red blood cell from a biconcave disc to an elongated, sickle-like structure.
5. Only HbSHbS individuals show the disease. HbAHbS carriers appear unaffected but have a 50 per cent chance of passing the mutant gene to each child, and show the sickle-cell trait.

Two micrographs of red blood cells: round biconcave cells on the left and elongated sickle-shaped cells on the right.

Below the normal cells, the Hb (A) gene reads GAG over CTC, its mRNA reads GAG, and the HbA peptide from positions 1 to 7 reads Val, His, Leu, Thr, Pro, Glu, Glu.

Below the sickle cells, the Hb (S) gene reads GTG over CAC, the mRNA reads GUG, and the HbS peptide reads Val, His, Leu, Thr, Pro, Val, Glu, with valine at position 6.

See Fig. 4.15 in your NCERT textbook

Thalassaemia

In α thalassaemia, production of the α globin chain is affected. It is controlled by two closely linked genes, HBA1 and HBA2, on chromosome 16 of each parent, and results from mutation or deletion of one or more of the four genes; the more genes affected, the less α globin is made. β thalassaemia is controlled by a single gene, HBB, on chromosome 11 of each parent, and results from mutation of one or both genes.

Thalassaemia is a quantitative problem of making too few globin molecules, whereas sickle-cell anaemia is a qualitative problem of making an incorrectly functioning globin.

What are chromosomal disorders?

Chromosomal disorders are caused by the absence, excess or abnormal arrangement of one or more chromosomes. A normal human cell has 46 chromosomes (23 pairs): 22 pairs of autosomes and one pair of sex chromosomes.

• Aneuploidy is the gain or loss of a chromosome, caused by failure of segregation of chromatids during cell division. An extra copy of one chromosome is a trisomy; lack of one chromosome of a pair is a monosomy.
• Polyploidy is an increase in a whole set of chromosomes, caused by failure of cytokinesis after telophase. It is often seen in plants.

The figure of an individual with Down's syndrome also labels a flat back of the head, many "loops" on the finger tips, a broad flat face, a big and wrinkled tongue and congenital heart disease.

The Klinefelter figure shows tall stature with feminised characters, and the Turner figure shows short stature with underdeveloped feminine characters. These conditions can be studied by analysing karyotypes.

Glossary

• True-breeding line — A line that, after continuous self-pollination, shows stable inheritance and expression of a trait for several generations.
• Allele — One of the slightly different forms of the same gene that code for a pair of contrasting traits, such as T and t.
• Genotype — The genetic constitution of an organism for a character, written with letters such as TT, Tt or tt.
• Phenotype — The observable form of a character, described in words such as tall or dwarf.
• Test cross — A cross of an organism with the dominant phenotype and unknown genotype with the recessive parent, to find its genotype.
• Incomplete dominance — Inheritance in which the heterozygote is intermediate between the two parents, as in pink snapdragons.
• Co-dominance — Inheritance in which both alleles of a heterozygote are expressed, as in blood group AB.
• Linkage — The physical association of genes on the same chromosome, so that parental combinations appear more often than new ones.
• Recombination — The generation of non-parental gene combinations in the progeny.
• Pleiotropic gene — A single gene that shows multiple phenotypic expressions, as in phenylketonuria.
• Male heterogamety — The condition in which males produce two kinds of gametes with respect to sex chromosomes, as in the XO and XY types.
• Point mutation — A change in a single base pair of DNA, as in sickle-cell anaemia.
• Aneuploidy — Gain or loss of one or more chromosomes due to failure of segregation of chromatids during cell division.
• Karyotype — The chromosome complement of an individual, analysed to study disorders such as 47, XXY.

Common errors and misconceptions

• Misconception: The F₂ genotypic ratio of a monohybrid cross is 3:1. Correct: The phenotypic ratio is 3:1; the genotypic ratio is 1 TT : 2 Tt : 1 tt.
• Misconception: The dihybrid F₂ genotypic ratio is 9:3:3:1. Correct: 9:3:3:1 is the phenotypic ratio. There are nine genotypes in the ratio 1:2:2:4:1:2:1:2:1.
• Misconception: Incomplete dominance means the alleles blend. Correct: The F₂ recovers red and white plants in a 1:2:1 ratio, so the alleles stay distinct; only the heterozygote's phenotype is intermediate.
• Misconception: In blood group AB, one allele is dominant. Correct: Iᴬ and Iᴮ are co-dominant and both sugars are expressed; both are dominant only over i.
• Misconception: The mother decides the sex of a child. Correct: The ovum always carries X; the sperm carries X or Y, so the sperm decides the sex.
• Misconception: In birds, as in humans, the male is heterogametic. Correct: Birds show female heterogamety: females are ZW and males ZZ.
• Misconception: Thalassaemia and sickle-cell anaemia are the same kind of defect. Correct: Thalassaemia is a quantitative defect (too little globin); sickle-cell anaemia is qualitative (faulty globin).
• Misconception: Turner's syndrome has 47 chromosomes. Correct: Turner's syndrome is 45, XO; Down's (trisomy 21) and Klinefelter's (XXY) have 47.

Exam-style questions with model answers

Q1. A diploid organism is heterozygous for 4 loci. How many types of gametes can it produce? [1 mark]

1. Each heterozygous locus gives two kinds of gametes, and the loci assort independently, so the number of gamete types is 2 raised to the power 4, that is 2 × 2 × 2 × 2 = 16.

Q2. Differentiate between homozygous and heterozygous, and between monohybrid and dihybrid. [2 marks]

1. A homozygous individual has two identical alleles for a character, such as TT or tt; a heterozygous individual has two different alleles, such as Tt.
2. A monohybrid is heterozygous for one character and a monohybrid cross follows one pair of traits; a dihybrid cross follows two pairs of traits, such as seed colour and seed shape.

Q3. A child has blood group O. The father has blood group A and the mother blood group B. Work out the genotypes of the parents and the possible genotypes of the other offspring. [3 marks]

1. A child with blood group O is ii, so it received an i allele from each parent. The father must therefore be Iᴬi and the mother Iᴮi.
2. The father's gametes are Iᴬ and i, and the mother's are Iᴮ and i.
3. The possible offspring are IᴬIᴮ (AB), Iᴬi (A), Iᴮi (B) and ii (O), each with a probability of one in four.

Q4. A tall plant with yellow seeds (TtYy) is crossed with a tall plant with green seeds (Ttyy). What proportion of the offspring is (a) tall and green, (b) dwarf and green? [3 marks]

1. For height, Tt × Tt gives 3/4 tall and 1/4 dwarf. For seed colour, Yy × yy gives 1/2 yellow and 1/2 green.
2. Since the two genes assort independently, the probabilities are multiplied.
3. (a) Tall and green = 3/4 × 1/2 = 3/8. (b) Dwarf and green = 1/4 × 1/2 = 1/8.

Q5. How is sex determined in human beings? [3 marks]

1. Humans have 22 pairs of autosomes and one pair of sex chromosomes: XX in females and XY in males.
2. Males produce two types of sperm, 50 per cent carrying X and 50 per cent carrying Y, while females produce only X-bearing ova.
3. Fertilisation of the ovum by an X-bearing sperm gives a female (XX) and by a Y-bearing sperm a male (XY). So the sperm determines the sex, and each pregnancy has an equal chance of a boy or a girl.

Q6. Explain the dihybrid cross and the Law of Independent Assortment, with a Punnett square. [5 marks]

1. Mendel crossed a plant with round, yellow seeds (RRYY) with one with wrinkled, green seeds (rryy). Their gametes, RY and ry, gave F₁ plants that were all round and yellow (RrYy), so round and yellow are dominant.
2. When the F₁ was selfed, each plant formed four kinds of gametes, RY, Ry, rY and ry, each with a frequency of 1/4, because the segregation of R and r is independent of the segregation of Y and y.
3. Writing these four gametes along the two sides of a Punnett square gives 16 combinations in F₂: RRYY 1, RRYy 2, RrYY 2 and RrYy 4 (round yellow, 9); RRyy 1 and Rryy 2 (round green, 3); rrYY 1 and rrYy 2 (wrinkled yellow, 3); rryy 1 (wrinkled green, 1).
4. The F₂ phenotypes appear as 9 round yellow : 3 wrinkled yellow : 3 round green : 1 wrinkled green, which equals (3 round : 1 wrinkled) × (3 yellow : 1 green).
5. The Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair.

Q7. Describe sickle-cell anaemia: its inheritance, molecular basis and effect on red blood cells. [5 marks]

1. Sickle-cell anaemia is an autosome-linked recessive trait controlled by one pair of alleles, HbA and HbS. It is transmitted to children when both parents are heterozygous carriers.
2. Only HbSHbS individuals show the disease. HbAHbS individuals appear unaffected but are carriers with a 50 per cent chance of passing the mutant gene to each child, and show the sickle-cell trait.
3. The cause is a single base substitution at the sixth codon of the beta globin gene, GAG changing to GUG, which is a point mutation.
4. As a result, glutamic acid is replaced by valine at the sixth position of the beta globin chain of haemoglobin.
5. The mutant haemoglobin polymerises under low oxygen tension, changing the red blood cell from a biconcave disc to an elongated, sickle-like shape.

Key takeaways

• Mendel used 7 pairs of contrasting traits in true-breeding pea lines and applied statistics to inheritance for the first time.
• A monohybrid F₂ shows a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio; a test cross with the recessive parent reveals an unknown genotype.
• The Law of Dominance and the Law of Segregation come from monohybrid crosses; the Law of Independent Assortment comes from dihybrid crosses giving 9:3:3:1.
• Snapdragon flower colour shows incomplete dominance (1:2:1), while the ABO blood groups show co-dominance and multiple alleles.
• Sutton and Boveri linked Mendel's laws to chromosome behaviour, and Morgan's Drosophila crosses revealed linkage and recombination.
• Polygenic traits such as skin colour are additive and graded; a pleiotropic gene, as in phenylketonuria, has several effects.
• Sex is determined by XO, XY and ZW systems, and by haplodiploidy in honey bees; in humans the sperm decides the sex.
• Mendelian disorders arise from single-gene mutations, while chromosomal disorders such as Down's, Klinefelter's and Turner's syndromes come from abnormal chromosome numbers.

Test yourself

Why did Mendel's work remain unrecognised till 1900?

Communication was poor, his idea of discrete non-blending factors and his use of mathematics were not accepted, and he could not show what the factors were.

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.

What is the F₂ phenotypic ratio for flower colour in snapdragon?

The F₂ shows 1 red : 2 pink : 1 white, the same as the genotypic ratio RR : Rr : rr.

How many genotypes and phenotypes are possible in the ABO system?

Three alleles give six genotypes and four phenotypes: blood groups A, B, AB and O.

What recombination frequencies did Morgan find for white with yellow, and white with miniature wing?

White and yellow showed 1.3 per cent recombination, a tight linkage, while white and miniature wing showed 37.2 per cent.

Why can a male honey bee not have sons?

A drone develops from an unfertilised egg, so it has no father; its sperm only fertilise eggs that become females, but it can have grandsons.

Why is colour blindness far more common in males than in females?

Its genes are on the X chromosome. Males have one X, so one recessive allele is enough; females need it on both X chromosomes.

What is the karyotype in Klinefelter's syndrome?

Klinefelter's syndrome has an additional X chromosome, giving a karyotype of 47, XXY; affected individuals are sterile.