Chapter 4: Heredity

Concept illustration showing DNA, a gene on a chromosome, a cell, sex determination in a family tree, and a genetic engineering technique editing a piece of DNA.

Flow diagram showing male and female (2n) producing sperm and egg (n) by meiosis, fertilisation forming a zygote (2n), mitosis forming a mass of cells, and differentiation into an offspring (2n).

• Both mitosis and meiosis are completed in two phases: karyokinesis (division of the nucleus) followed by cytokinesis (division of cytoplasm along with the cell membrane).
• Two daughter cells are formed from a mother cell in mitosis; four daughter cells are formed in meiosis.
• Daughter cells formed at the end of meiosis are gametes, whereas cells formed at the end of mitosis are somatic (vegetative) cells.

Diagram of mitosis: a diploid (2n) cell with DNA replicated shown as an X-shaped chromosome, dividing into two identical diploid (2n) daughter cells.

• Except gametes, each cell in the body is diploid (2n), containing two sets of chromosomes - one paternal and one maternal.
• Before cell division, DNA in the chromosomes is replicated, forming two identical copies.
• At the end of cell division, two identical daughter cells are formed, each carrying a copy of the DNA - this is how the genetic characteristics of the mother cell are established in the daughter cells.
• There is no change in the number of chromosomes in the daughter cells formed by mitosis; hence this division is also called equational division.

• Plays a key role in physical growth, as the number of cells with the same genetic makeup increases.
• Helps regenerate cells in injured areas, returning them to their original state.
• Helps in the asexual reproduction of some plants and invertebrates.
• Maintains genetic stability.

Diagram of meiosis: one mother cell with paired red and blue chromosomes dividing into two cells and then into four haploid daughter cells, each with a mix of red and blue chromosome material.

• In the first phase, an exchange of genetic material occurs between two non-sister chromosomes through crossing over, which changes the genetic makeup of each chromosome; this is followed by karyokinesis and cytokinesis to form two haploid cells.
• In the second phase, mitotic cell division of these haploid cells occurs (karyokinesis followed by cytokinesis), finally forming four cells.
• Variation in the genetic makeup of the daughter cells occurs due to crossing over in the first phase.
• This division is also called reductional cell division because the chromosome number is reduced to half in the daughter cells.
• Meiotic cell division occurs in the testes of adult males and ovaries of adult females to form gametes needed for sexual reproduction; the male gamete fuses with the female gamete to form a diploid zygote.

1. 1Plays a main role in the sexual reproduction of organisms.
2. 2Helps in evolution by bringing variation.
3. 3Helps to repair chromosomal disorders.

Double helix structure of DNA with labels for nucleotide, nucleoside, phosphate molecule, sugar, nitrogen base and bond, showing the antiparallel strands.

• A single unit formed by a nitrogen base combined with deoxyribose sugar is called a nucleoside; a unit formed by a nitrogen base, deoxyribose sugar and a phosphate ion is called a nucleotide - nucleotides are the structural units of DNA.
• DNA consists of two antiparallel strands.
• Four types of nitrogen bases are found in DNA: adenine, guanine, cytosine and thymine.
• Adenine is linked with thymine by a double bond, while guanine is linked with cytosine by a triple bond.
• A small segment of DNA that codes for a particular character of an organism is called a gene; millions of such genes are found in a chromosome.
• During cell division, hereditary information is transmitted by DNA from the mother cell to daughter cells, helping transmit characters from one generation to another.
• Transcription occurs in DNA to form RNA, which helps synthesize protein.

Single-stranded RNA structure with labelled nitrogen bases adenine, guanine, cytosine and uracil.

• Four types of nitrogen bases are found in RNA: adenine, guanine, cytosine and uracil.
• Adenine is linked with uracil by a double bond, while guanine is linked with cytosine by a triple bond.
• There are three types of RNA: Messenger RNA (m-RNA), transfer RNA (t-RNA) and ribosomal RNA (r-RNA).
• The main function of RNA is to synthesize proteins.

Structure of a chromosome showing the centromere and sister chromatids.

• Each chromosome is made up of DNA and histone protein, and numerous genes are found in a chromosome.
• The chromosome mainly has two parts: chromatid and centromere.
• Chromatids are the two arms of the chromosome; sister chromatids are the identical copies found in a replicated chromosome.
• The knot-like structure where two chromatids are connected is called the centromere.

Human karyotype showing 23 pairs of chromosomes arranged and numbered 1 to 22, with the XY or XX sex chromosome pair shown last.

• A somatic cell generally has two sets of chromosomes, called diploid (2n). A gamete has one set of chromosomes, called haploid (n).
• A human somatic cell contains 46 (2 x 23) chromosomes - diploid.
• Human gametes (sperm or ovum) contain 23 (1 x 23) chromosomes - haploid.

• Human body cells have a total of 23 pairs of chromosomes: 22 pairs are autosomes and one pair is the sex chromosome (heterosome).
• A male individual's cells contain one pair of sex chromosomes called XY; a female individual's cells contain XX sex chromosomes.
• The mother cell of the male reproductive organ (testis) consists of 44+XY chromosomes; through meiosis, this divides to form sperm with either 22+X or 22+Y chromosomes.
• The mother cell of the female reproductive organ (ovary) consists of 44+XX chromosomes; through meiosis, this divides to form an ovum with only 22+X chromosomes.
• If a sperm with 22+X chromosomes fuses with the ovum (22+X), the child will be female (44+XX); if a sperm with 22+Y chromosomes fuses with the ovum (22+X), the child will be male (44+XY).
• Since 50% of a male's sperm carry an X chromosome and 50% carry a Y chromosome, the probability of having a son or a daughter is 50% at each fertilization.

Sex determination chart showing father (44+XY) and mother (44+XX) undergoing meiosis to form gametes 22+X, 22+Y (father) and 22+X, 22+X (mother), which combine at fertilization to give two daughters (44+XX) and two sons (44+XY).

1. 1Pea plants are bisexual and their flowers are closed, making them naturally self-pollinating plants.
2. 2Cross-pollination can be done if necessary.
3. 3Their life cycle is short, and offspring can be produced faster.
4. 4They have many pairs of contrasting characters.
5. 5Many seeds can be produced at once, giving many offspring.
6. 6They are easy to cultivate.

Monohybrid cross chart from mother plants (tall TT x dwarf tt) through F1 (all Tt, tall), self-pollination, to F2 generation (tall TT, tall Tt, tall Tt, dwarf tt) and further self-pollination to F3 generation.

• Tall plants were produced from pure tall plants.
• Tall and dwarf plants were produced in a ratio of 3:1 from hybrid tall plants.
• Dwarf plants were produced from pure dwarf plants.

• A cross made between two pure organisms considering only one pair of contrasting characters is called a monohybrid cross - e.g., the cross between a pure tall and a pure dwarf pea plant.
• A cross made between two pure organisms considering two pairs of contrasting characters is called a dihybrid cross.

1. 1Law of Dominance
2. 2Law of Purity of Gametes (Law of Segregation)
3. 3Law of Independent Assortment

Chart showing cross-pollination between pure tall (TT) and pure dwarf (tt) pea plants, with gametes T and t combining to give F1 genotype Tt and phenotype tall in all offspring.

Chart showing the cross between pure black guinea pigs (BB) and pure white guinea pigs (bb), with all F1 offspring genotype Bb.

Chart showing self-pollination of F1 tall hybrids (Tt x Tt), with gametes T and t combining to give F2 genotypes Tt, Tt, Tt, Tt, all phenotypically tall in this particular cross shown, illustrating segregation of alleles.

A fragment of DNA of one organism being added to the DNA of another organism using laboratory tools, illustrating genetic engineering.

• Genetic engineering (also called genetic modification) is the process that alters the DNA structure of an organism using laboratory-based techniques - by changing a single nitrogen base pair (A-T or C-G), or deleting or adding a gene in the DNA.
• Gene variants created through genetic engineering can be passed from one generation to the next.
• Genetic technology has an important place in biotechnology and molecular biology, and has made it possible to change and regulate plant characteristics using DNA sequencing information.
• Modern DNA technologies based on molecular markers, transgenic technology and gene expression are widely used in agriculture to improve yield and quality, reduce losses caused by biotic and abiotic factors, and improve the reproductive capacity of organisms.

1. 1Usually increases the population of plants and animals having similar genetic traits.
2. 2There is a chance of spreading infectious diseases genetically.
3. 3Breeding between very closely related species increases the likelihood of congenital genetic problems in offspring.
4. 4It is also called artificial selection because it involves human interference.
5. 5It inhibits some naturally occurring genetic traits and can affect biodiversity, possibly leading species to extinction due to loss of certain traits.

Siamese cat and Labrador Retriever dog as examples of purebred animals produced through inbreeding.

• Liger: the hybrid obtained by crossing a male lion and female tiger; it is the largest known hybrid cat, larger than either parent, and generally behaves like a lion.
• Tigon: the hybrid obtained by crossing a male tiger and female lion; it is smaller than the Liger and smaller than either of its parents, resembles a tiger, but its behaviour (roaring, socialization) is mostly like a lion.
• Beefalo: a hybrid produced by crossing a buffalo (American Bison) and a bull; unlike most other hybrids, beefalo can reproduce.
• Zebroid: the hybrid obtained by crossing a zebra and a horse; zebroids cannot reproduce.
• Mule: the hybrid produced by cross breeding a donkey and a horse; it can carry loads like a donkey and run like a horse, and is sterile.
• Pomato: a hybrid plant produced by crossing potato and tomato, in which tomato is produced on the stem above the soil and potato is produced underground.

Liger and Tigon shown side by side to compare the two big-cat hybrids.

Beefalo, a hybrid of buffalo (American Bison) and domestic bull.

Zebroid, a hybrid of zebra and horse showing partial striping.

Mule, a hybrid of donkey and horse.

Pomato plant with tomatoes growing above the soil on the stem and potatoes growing underground.

1. 1Combines the desirable qualities of two organisms from different breeds, varieties or species.
2. 2Allows humans to produce organisms with desired characters.
3. 3Provides the opportunity to make full use of a wide range of genetic material.
4. 4Animals with better quality than the parent animals can be developed.
5. 5Immunity, strength, age and vigour of an organism can be improved.
6. 6Crop production can be increased from plants produced through this method.

• Without proper understanding and management, problems may arise in future breeding policy.
• Products from cross breeding often sell for a lower price compared to purebred products, so farmers may not earn as much as expected.
• There is a limitation in the sale of cross-bred animals in the export market.
• As the external and genetic characteristics of the hybrid keep changing, the chance of extinction of purebreds increases.
• Natural traits of the parents are not completely transferred to their progeny, so such traits may gradually disappear.

1. 1There is no need to rear a male for breeding, which saves rearing expenses.
2. 2Helps control infection and the spread of disease during mating.
3. 3Semen collected from the male is tested to check fertility before use.
4. 4Collected semen can be used even after the death of the male.
5. 5Collected semen can be easily transported over long distances for fertilization.
6. 6Helps prevent injury to the female or male at the time of fertilization.
7. 7Increases the rate of fertilization.
8. 8Helps keep a good record of reproduction.

• Needs well-trained manpower and special equipment.
• Requires more time than natural breeding.
• Reproduction may not take place, or there might be a chance of infection if equipment is not properly sanitized.

Diagram of IVF showing egg retrieval from the ovary, fertilization with sperm in a laboratory dish, and embryo transfer back into the uterus.

1. 1The best method of conception for couples unable to conceive due to various problems related to conception.
2. 2Allows conception using a couple's own sperm and ovum, or donor sperm and ovum.
3. 3More successful than other assisted reproductive techniques.
4. 4Helpful in solving problems related to various chromosomal disorders in the child.
5. 5Infertility and genetic problems can be solved by this technique.
6. 6Increases fertility and reduces the risk of miscarriage.
7. 7Increases the chance of having a healthy child.

• No guarantee of a successful cycle - it may not succeed and may take more than one cycle.
• There may be various side effects associated with its use.
• The problem of multiple births at the same time can occur.
• Adopting this technique may cause emotional stress in the couple.
• There is a chance the embryo might implant outside the uterus.
• It is an expensive method.
• There is also a chance of premature birth of a baby with low weight.

• Do not confuse mitosis (2 daughter cells, diploid, growth/repair) with meiosis (4 daughter cells, haploid, gamete formation).
• Remember DNA has thymine and is double-stranded; RNA has uracil instead of thymine and is single-stranded.
• Do not say autosomes determine sex - autosomes determine physical characteristics; only sex chromosomes determine sex.
• Remember the correct sex chromosome combinations: father produces 22+X or 22+Y sperm; mother always produces 22+X ova.
• Do not confuse dominant character (expressed in F1) with recessive character (hidden in F1, may reappear in F2).
• Remember the phenotypic ratio in a monohybrid cross F2 is 3:1, while the genotypic ratio is 1:2:1.
• Do not mix up hybrid examples - Liger (lion father x tiger mother) vs Tigon (tiger father x lion mother); Mule and Zebroid are usually sterile, but Beefalo can reproduce.

• Heredity: transmission of parental characters to offspring through chromosomes and genes.
• Mitosis: 1 cell -> 2 diploid (2n) cells; growth, repair; equational division.
• Meiosis: 1 cell -> 4 haploid (n) cells; gamete formation; crossing over brings variation; reductional division.
• DNA: double-stranded, deoxyribose sugar, bases A-T-G-C, stores hereditary information.
• RNA: single-stranded, ribose sugar, bases A-U-G-C, helps protein synthesis (m-RNA, t-RNA, r-RNA).
• Chromosome = DNA + histone protein; gene = smallest DNA segment coding a character.
• Humans: 46 chromosomes (23 pairs) = 22 pairs autosomes + 1 pair sex chromosomes (XX female, XY male).
• Sex determination: father's sperm (22+X or 22+Y) determines the child's sex; mother's ovum is always 22+X.
• Mendel: father of genetics; used pea plants; 3 laws - dominance, purity of gametes (segregation), independent assortment.
• Monohybrid cross F1: all hybrid, dominant phenotype; F2: phenotypic ratio 3:1, genotypic ratio 1:2:1.
• Genetic technology: genetic engineering modifies DNA; DNA testing used in forensic/paternity investigation.
• Selective breeding methods: inbreeding, line breeding, self-pollination, cross breeding (produces hybrids like liger, tigon, mule, beefalo, zebroid, pomato).
• Artificial insemination: semen introduced mechanically, fertilization inside the body.
• IVF: egg and sperm fused outside the body in a lab, embryo transferred to uterus; first IVF baby - Louise Joy Brown (1978); first Nepali IVF baby - Om Mani Tamang (2005).