Genetics & Evolution

• Genetics is the branch of biology that studies heredity and variation.
• Heredity is the passing of features from parents to offspring through reproduction.
• Variation means the differences that still exist between parents and offspring; no two children are identical.
• A feature that is inherited (height, eye colour, blood group) is called a trait.
• Mendel worked on the garden pea (Pisum sativum) from 1856 to 1863.
• Pea was ideal: clear contrasting traits, easy to grow, selfpollinating (pure lines), and many offspring quickly.
• Gregor Johann Mendel, an Austrian monk, is the Father of Genetics.

Genetics is the branch of biology that studies heredity and variation.

Heredity is the passing of features from parents to offspring through reproduction.

Variation means the differences that still exist between parents and offspring; no two children are identical.

A feature that is inherited (height, eye colour, blood group) is called a trait.

Mendel worked on the garden pea (Pisum sativum) from 1856 to 1863.

Gregor Johann Mendel, an Austrian monk, is the Father of Genetics.

• A gene is a stretch of DNA carrying the code for one trait; it is the physical unit of heredity.
• Different versions of a gene are called alleles (e.g. tall T and dwarf t).
• A dominant allele shows even with one copy and is written in CAPITAL (T).
• A recessive allele is hidden unless both copies are present; written in small letter (t).
• Genotype is the set of alleles carried (TT, Tt, tt); phenotype is the visible feature (tall/dwarf).
• Same alleles (TT or tt) = homozygous (pure); different alleles (Tt) = heterozygous (hybrid).
• Body cells carry two copies of each gene, so they are diploid.
• Gene — DNA unit coding one trait
• Allele — Version of a gene (T or t)
• Dominant — Shows with one copy (T)
• Recessive — Hidden unless both copies (t)
• Genotype — Allele set (TT, Tt, tt)
• Phenotype — Visible feature (tall/dwarf)
• Homozygous — Same alleles (TT/tt)
• Heterozygous — Different alleles (Tt)

A gene is a stretch of DNA carrying the code for one trait; it is the physical unit of heredity.

Different versions of a gene are called alleles (e.g. tall T and dwarf t).

A dominant allele shows even with one copy and is written in CAPITAL (T).

A recessive allele is hidden unless both copies are present; written in small letter (t).

Genotype is the set of alleles carried (TT, Tt, tt); phenotype is the visible feature (tall/dwarf).

Same alleles (TT or tt) = homozygous (pure); different alleles (Tt) = heterozygous (hybrid).

Body cells carry two copies of each gene, so they are diploid.

• Pure tall (TT) crossed with pure dwarf (tt) gave an all-tall F1 generation (Tt).
• Self-crossing F1 made dwarf plants reappear in F2, proving traits stay as separate units, not blends.
• Monohybrid F2 phenotype ratio is 3: 1 (3 tall: 1 dwarf).
• Monohybrid F2 genotype ratio is 1: 2: 1 (1 TT: 2 Tt: 1 tt).
• A Punnett square (by Reginald Punnett) is a grid used to predict offspring ratios.
• Generations are labelled P (parents), F1 (first offspring), F2 (F1 self-cross).
• A cross that follows just ONE trait is a monohybrid cross.

Pure tall (TT) crossed with pure dwarf (tt) gave an all-tall F1 generation (Tt).

Monohybrid F2 phenotype ratio is 3: 1 (3 tall: 1 dwarf).

A Punnett square (by Reginald Punnett) is a grid used to predict offspring ratios.

Generations are labelled P (parents), F1 (first offspring), F2 (F1 self-cross).

A cross that follows just ONE trait is a monohybrid cross.

• Law of Dominance: in a heterozygote (Tt) only the dominant allele shows.
• Law of Segregation: alleles separate during gamete formation, so each gamete gets only ONE allele.
• Segregation is also called the Law of Purity of Gametes.
• Law of Independent Assortment: alleles of different genes are inherited independently.
• A dihybrid cross (two traits) gives the F2 ratio 9: 3: 3: 1.
• Dominance — Only dominant allele shows in hybrid
• Segregation — Alleles separate into gametes
• Independent Different genes inherited Assortment independently
• Mendel published his work in 1866, but it was ignored.
• In 1900, de Vries, Correns and von Tschermak independently rediscovered his laws.

Law of Dominance: in a heterozygote (Tt) only the dominant allele shows.

Law of Segregation: alleles separate during gamete formation, so each gamete gets only ONE allele.

Law of Independent Assortment: alleles of different genes are inherited independently.

A dihybrid cross (two traits) gives the F2 ratio 9: 3: 3: 1.

Mendel published his work in 1866, but it was ignored.

In 1900, de Vries, Correns and von Tschermak independently rediscovered his laws.

• Incomplete dominance: neither allele fully wins, F1 is a blend (red x white flower gives pink).
• Co-dominance: both alleles express fully and separately, F1 resembles BOTH parents.
• The best example of co-dominance is the human ABO blood group (genotype IA IB gives group AB).
• Multiple alleles: a gene has more than two versions in the population (IA, IB, i).
• Polygenic inheritance: one trait controlled by three or more genes whose effects add up.
• Polygenic examples are human height and skin colour, giving a continuous range.
• In skin colour, AABBCC is darkest and aabbcc is lightest; each dominant allele adds darkness.
• Incomplete dominance — Blended F1 — Pink flower
• Co-dominance — Both shown — Blood group AB
• Multiple alleles — 3+ versions — IA, IB, i
• Polygenic — Continuous range — Skin colour, height

Incomplete dominance: neither allele fully wins, F1 is a blend (red x white flower gives pink).

Co-dominance: both alleles express fully and separately, F1 resembles BOTH parents.

The best example of co-dominance is the human ABO blood group (genotype IA IB gives group AB).

Multiple alleles: a gene has more than two versions in the population (IA, IB, i).

Polygenic inheritance: one trait controlled by three or more genes whose effects add up.

Polygenic examples are human height and skin colour, giving a continuous range.

• Blood group is controlled by gene I with three alleles: IA, IB and i.
• IA makes A sugar, IB makes B sugar, and i makes no sugar on red blood cells.
• Both IA and IB are dominant over i, but co-dominant with each other.
• There are 6 genotypes but only 4 phenotypes (A, B, AB, O).
• Group O (genotype ii) is the universal donor; group AB is the universal recipient.
• A — IA IA or IA i
• B — IB IB or IB i
• AB — IA IB
• O — i i

Blood group is controlled by gene I with three alleles: IA, IB and i.

IA makes A sugar, IB makes B sugar, and i makes no sugar on red blood cells.

Both IA and IB are dominant over i, but co-dominant with each other.

Group O (genotype ii) is the universal donor; group AB is the universal recipient.

• Chromosomes are thread-like structures in the nucleus made of DNA plus protein.
• The name means 'coloured body', as they stain darkly in a dividing cell.
• The Chromosomal Theory of Inheritance was given by Sutton and Boveri (1902).
• Humans have 46 chromosomes = 23 pairs.
• Homologous chromosomes carry the same genes, one from each parent.
• The centromere is the constriction holding the two arms and decides chromosome shape.
• 22 pairs are autosomes; the 23rd pair (X and Y) are sex chromosomes.
• Middle — Metacentric
• Slightly off-centre — Sub-metacentric
• Near one end — Acrocentric
• At the tip — Telocentric

Chromosomes are thread-like structures in the nucleus made of DNA plus protein.

Homologous chromosomes carry the same genes, one from each parent.

The centromere is the constriction holding the two arms and decides chromosome shape.

22 pairs are autosomes; the 23rd pair (X and Y) are sex chromosomes.

• Sex is decided by the 23rd pair, the sex chromosomes.
• Females are XX and males are XY.
• The mother always gives an X; eggs carry only X.
• The father decides the sex of the child by giving either an X or a Y sperm.
• X sperm + egg gives a girl (XX); Y sperm + egg gives a boy (XY).

Sex is decided by the 23rd pair, the sex chromosomes.

Females are XX and males are XY.

The father decides the sex of the child by giving either an X or a Y sperm.

• DNA (deoxyribonucleic acid) is the genetic material that stores hereditary information.
• Its bases pair as A–T and G–C; the sequence of bases is the genetic code.
• A mutation is a sudden change in the base sequence within a gene.
• Mutation is the ultimate source of new alleles and a major cause of large-scale diversity among organisms.
• Agents that cause mutations are called mutagens (Xrays, UV rays, certain chemicals).
• DNA is a double helix, modelled by Watson and Crick (1953).
• Hugo de Vries proposed the Mutation Theory of evolution.

DNA (deoxyribonucleic acid) is the genetic material that stores hereditary information.

Its bases pair as A–T and G–C; the sequence of bases is the genetic code.

A mutation is a sudden change in the base sequence within a gene.

Mutation is the ultimate source of new alleles and a major cause of large-scale diversity among organisms.

Agents that cause mutations are called mutagens (Xrays, UV rays, certain chemicals).

DNA is a double helix, modelled by Watson and Crick (1953).

Hugo de Vries proposed the Mutation Theory of evolution.

• Genetic disorders are diseases caused by faulty genes or chromosomes, passed to offspring.
• Haemophilia (failure of blood clotting) is an X-linked recessive disorder.
• Colour blindness (red-green) is also X-linked recessive, commoner in males.
• Sickle-cell anaemia and thalassaemia are gene-based blood disorders.
• Down's syndrome is caused by an extra chromosome 21 (47 chromosomes).
• Infectious diseases like cholera, typhoid, malaria and AIDS are NOT genetic diseases.
• Haemophilia — Yes
• Colour blindness — Yes
• Down's syndrome — Yes
• Sickle-cell anaemia — Yes
• Cholera / Malaria — No

Genetic disorders are diseases caused by faulty genes or chromosomes, passed to offspring.

Haemophilia (failure of blood clotting) is an X-linked recessive disorder.

Colour blindness (red-green) is also X-linked recessive, commoner in males.

Down's syndrome is caused by an extra chromosome 21 (47 chromosomes).

• Genetic engineering is the deliberate alteration of an organism's DNA to give new traits.
• It is also called recombinant DNA (rDNA) technology.
• Restriction enzymes act as molecular scissors that cut DNA at specific sites.
• DNA ligase joins DNA pieces together (molecular glue).
• A vector carries the desired gene into the host cell; bacterial plasmids are common vectors.
• Genetically engineered bacteria now produce human insulin (humulin).
• GM crops like Bt cotton are products of genetic engineering.

Genetic engineering is the deliberate alteration of an organism's DNA to give new traits.

Restriction enzymes act as molecular scissors that cut DNA at specific sites.

DNA ligase joins DNA pieces together (molecular glue).

A vector carries the desired gene into the host cell; bacterial plasmids are common vectors.

Genetically engineered bacteria now produce human insulin (humulin).

GM crops like Bt cotton are products of genetic engineering.

• Evolution is the slow change in living organisms over generations to form new species.
• Lamarck proposed Inheritance of Acquired Characters (use and disuse of organs).
• Darwin wrote the book 'On the Origin of Species' (1859).
• Modern Darwinism (Neo-Darwinism) combines natural selection with genetics and mutation.
• Lamarck — Inheritance of acquired characters
• Darwin — Natural selection (Origin of Species)
• de Vries — Mutation theory
• Charles Darwin proposed Natural Selection, the 'survival of the fittest'.
• Hugo de Vries proposed evolution by sudden mutations.

Evolution is the slow change in living organisms over generations to form new species.

Lamarck proposed Inheritance of Acquired Characters (use and disuse of organs).

Charles Darwin proposed Natural Selection, the 'survival of the fittest'.

Hugo de Vries proposed evolution by sudden mutations.

• Fossils are preserved remains of ancient organisms and the best evidence of evolution.
• Homologous organs have the same structure but different functions (human arm, bat wing) and show common ancestry.
• Analogous organs have different structure but the same function (bird wing, insect wing).
• Vestigial organs (appendix, ear muscles in humans) are reduced, useless remnants from ancestors.
• Oparin and Haldane proposed life arose from chemicals in the early ocean (chemical evolution).
• The Miller–Urey experiment showed organic molecules can form from simple gases.
• Homologous — Same — Different
• Analogous — Different — Same
• Vestigial — Reduced — Lost / useless

Fossils are preserved remains of ancient organisms and the best evidence of evolution.

Homologous organs have the same structure but different functions (human arm, bat wing) and show common ancestry.

Analogous organs have different structure but the same function (bird wing, insect wing).

Vestigial organs (appendix, ear muscles in humans) are reduced, useless remnants from ancestors.

Oparin and Haldane proposed life arose from chemicals in the early ocean (chemical evolution).

The Miller–Urey experiment showed organic molecules can form from simple gases.