Genetics: Mendel's Experiments

• Genetics is the study of heredity and variation of inherited characters.
• Inheritance is the process by which characters are passed on from parent to progeny.
• Variation is the degree by which progeny differ from their parents.

Mendel’s Experiments

• Gregor Mendel, known as the "Father of Genetics”, was the first to study inheritance systematically.
• He proposed the laws of inheritance, initially unrecognised but later rediscovered.
• Mendel conducted hybridisation experiments on garden peas (Pisum sativum) for seven years (1856-1863).
• He used contrasting characters of the pea plant:
  • Stem height (tall or dwarf)
  • Flower color (violet or white)
  • Flower position (axial or terminal)
  • Pod shape (inflated or constricted)
  • Pod color (green or yellow)
  • Seed shape (round or wrinkled)
  • Seed color (yellow or green)
• True-breeding lines were used for the experiments.
• A true-breeding line has undergone continuous self-pollination, showing stable trait inheritance.
• Artificial pollination/cross-pollination was used to create hybrids.

Inheritance of One Gene

• A monohybrid cross involves the inheritance of one gene.
• Mendel crossed tall and dwarf pea plants to study the inheritance of one gene.
• The first filial generation or F1 generation showed only one parental trait.
• In the F2 generation, both traits appeared in a ratio of 3:1 (dominant:recessive).
• Mendel proposed that something was being passed down unchanged from parent to offspring.
• These factors were called ‘genes’; genes are units of inheritance.
• Genes exist in pairs in diploid organisms, known as alleles, controlling traits.
• The dominant allele expresses itself, while the recessive allele remains hidden when together.
• The genotype is the genetic makeup, the phenotype is the observable characteristic.
• Homozygous organisms have identical alleles, while heterozygous organisms have different alleles.
• Mendel's Law of Dominance states that in a heterozygote, one allele masks the effect of the other.
• The Law of Segregation states that alleles segregate during gamete formation.
• Each gamete carries only one allele for each character.
• The Punnett square is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross, and was developed by Reginald Punnett.
• A test cross is used to determine the genotype of a dominant phenotype.
• The organism with unknown genotype is crossed with a homozygous recessive individual.
• If the progeny show a 1:1 ratio, the unknown genotype is heterozygous.
• If all progeny show the dominant trait, the unknown genotype is homozygous dominant.

Inheritance of Two Genes

• A dihybrid cross involves the inheritance of two genes.
• Mendel crossed pea plants differing in two traits: seed color and seed shape.
• One parent had round, yellow seeds (RRYY) and the other had wrinkled, green seeds (rryy).
• In the F1 generation, all plants showed round, yellow seeds (RrYy).
• In the F2 generation, four phenotypes appeared in a ratio of 9:3:3:1.
  • Round, yellow (9)
  • Round, green (3)
  • Wrinkled, yellow (3)
  • Wrinkled, green (1)
• The Law of Independent Assortment states that alleles of different genes assort independently of one another during gamete formation.
• The 9:3:3:1 ratio can be derived as a combination series of 3 Yellow : 1 Green, with 3 Round : 1 Wrinkled.

Chromosomal Theory of Inheritance

• The chromosomal theory of inheritance was developed by Sutton and Boveri.
• It states that genes are located on chromosomes.
• Chromosomes segregate and assort independently during meiosis.
• This parallels Mendel's laws.

Linkage and Recombination

• Thomas Hunt Morgan and his colleagues worked on fruit flies (Drosophila melanogaster).
• Fruit flies are suitable for genetic studies because:
  • They can be grown on simple synthetic medium in the laboratory.
  • They complete their life cycle in about two weeks.
  • A single mating produces a large number of progeny flies.
  • There is a clear differentiation of the sexes.
  • They have many types of hereditary variations that can be easily seen with low power microscopes.
• Morgan found that genes on the same chromosome are linked.
• Linked genes do not assort independently.
• The closer genes are on a chromosome, the higher the linkage.
• Recombination is the generation of non-parental gene combinations, occurring due to crossing over during meiosis.
• The frequency of recombination between genes on the same chromosome is proportional to the distance between them.
• Alfred Sturtevant used recombination frequency to map the position of genes on chromosomes, creating the first genetic maps.

Polygenic Inheritance

• Polygenic inheritance is controlled by multiple genes.
• It shows a continuous range of phenotypic variation.
• Human height and skin color are examples of polygenic inheritance.
• The phenotype reflects the contribution of each allele, with the effect of each allele being additive.

Pleiotropy

• Pleiotropy is when a single gene can affect multiple traits.
• Phenylketonuria in humans is an example.
• A mutation in the gene coding for phenylalanine hydroxylase causes mental retardation and reduced hair and skin pigmentation.

Sex Determination

• Sex chromosomes determine the sex of an individual.
• In humans, males are XY, and females are XX.
• Other organisms have different mechanisms, such as XO or ZW.
• In the XO system, females have two X chromosomes (XX), and males have only one X chromosome (XO).
• In birds, females have one Z and one W chromosome (ZW), and males have two Z chromosomes (ZZ).
• Sex determination in humans involves the SRY gene on the Y chromosome.
• The SRY gene triggers the development of testes.

Mutations

• Mutation is a change in the DNA sequence.
• Mutations can occur due to errors in DNA replication or exposure to mutagens.
• Mutagens can be physical, chemical, or biological agents.
• Point mutations involve a change in a single base pair.
• Frameshift mutations involve the insertion or deletion of bases, altering the reading frame.
• Chromosomal aberrations involve changes in chromosome structure or number.
  • Deletion: Loss of a part of the chromosome.
  • Duplication: Presence of a part of the chromosome in more than one copy.
  • Inversion: A part of the chromosome gets detached and reattached in reverse orientation.
  • Translocation: Movement of a part of the chromosome to another chromosome.

Genetic Disorders

• Genetic disorders can be Mendelian or chromosomal disorders.
• Mendelian disorders are caused by mutations in a single gene, following Mendelian patterns of inheritance.
  • Examples: Hemophilia, cystic fibrosis, sickle-cell anemia, phenylketonuria
• Chromosomal disorders are caused by changes in the number or structure of chromosomes.
  • Aneuploidy: Gain or loss of one or more chromosomes e.g. Trisomy 21 (Down's Syndrome).
  • Polyploidy: The presence of more than two sets of chromosomes.
• Examples of chromosomal disorders:
  • Down syndrome (Trisomy 21)
  • Turner's syndrome (XO)
  • Klinefelter's syndrome (XXY)

An introduction to genetics, focusing on Mendel's experiments. Covers heredity, variation, and Mendel's use of pea plants to study inheritance. Key traits studied include stem height, flower color, and seed shape.