Principles of Inheritance and Variation: A Summary
Genetics is the study of heredity and variation in organisms. It explores how traits are passed from one generation to the next through genes, which are segments of DNA located on chromosomes.
Mendel’s Laws of Inheritance
Gregor Mendel, a 19th-century Austrian monk, laid the foundation of genetics through his experiments with pea plants. He discovered the following laws:
- Law of Segregation: Each inherited trait is controlled by a pair of alleles (alternative forms of a gene). During gamete formation, these alleles separate, so each gamete receives only one allele for a trait.
- Law of Independent Assortment: Alleles for different traits are inherited independently of each other, unless they are located on the same chromosome.
DNA and Genetic Code
- DNA (Deoxyribonucleic Acid): The genetic material that carries the instructions for building and maintaining organisms. It is a double-stranded molecule with a structure resembling a twisted ladder.
- Genetic Code: The set of rules that determines how the sequence of nucleotides in DNA is translated into the sequence of amino acids in a protein.
Chromosomal Theory of Inheritance
This theory states that genes are located on chromosomes and that the behavior of chromosomes during meiosis explains Mendel’s laws of inheritance.
Variation in Populations
- Genetic Variation: Differences in the genetic makeup of individuals within a population. This variation is essential for evolution.
- Gene Pool: The total genetic variation present in a population.
Evolution
- Natural Selection: The process by which organisms with traits that increase their chances of survival and reproduction are more likely to pass on their genes to the next generation.
- Genetic Drift: Random changes in allele frequencies in a population, often due to chance.
- Gene Flow: The movement of genes between populations.
- Mutation: Changes in the DNA sequence that can lead to new alleles and genetic variation.
Speciation
The formation of new species through evolutionary processes, often due to geographic isolation or reproductive barriers.
Human Evolution
The process by which humans evolved from their primate ancestors. This involves the study of fossils, genetics, and comparative anatomy.
In conclusion, genetics and evolution are interconnected fields that help us understand the diversity of life on Earth and the mechanisms that drive biological change.
Exercise
1. Mention the advantages of selecting pea plant for experiment by Mendel
Ans :
Mendel selected pea plants for his experiments due to several advantages:
- Distinct Characteristics: Pea plants have many easily observable characteristics, such as flower color, seed shape, and plant height, that can be clearly differentiated.
- Short Life Cycle: Pea plants have a relatively short life cycle, allowing Mendel to conduct multiple generations of experiments within a short period.
- Self-Pollination: Pea plants can self-pollinate, allowing Mendel to control the mating of plants and obtain pure-breeding lines.
- Cross-Pollination: Mendel could also cross-pollinate pea plants by manually transferring pollen between different plants, enabling him to study the inheritance of traits from different parents.
- Large Number of Offspring: Pea plants produce a large number of offspring, providing Mendel with sufficient data to analyze inheritance patterns.
2. Differentiate between the following –
(a) Dominance and Recessive
(b) Homozygous and Heterozygous
(c) Monohybrid and Dihybrid
Ans :
(a) Dominance and Recessive
| Feature | Dominant | Recessive |
| Expression | Masks the expression of the recessive allele | Only expressed when both alleles are recessive |
| Genotype | Can be homozygous dominant (TT) or heterozygous (Tt) | Must be homozygous recessive (tt) |
| Phenotype | Always expressed in the heterozygous condition | Only expressed in the homozygous recessive condition |
| Mendel’s Experiment | Tall pea plants | Short pea plants |
| Symbol | Capital letter (e.g., T) | Lowercase letter (e.g., t) |
(b) Homozygous and Heterozygous
| Feature | Homozygous | Heterozygous |
| Allele Combination | Both alleles are the same | Both alleles are different |
| Genotype | TT or tt | Tt |
| Phenotype | Consistent expression of the trait | Expression depends on the dominant allele |
| Mendel’s Experiment | Pure-breeding lines | Hybrids |
| Genetic Variation | Less genetic variation | More genetic variation |
(c) Monohybrid and Dihybrid
| Feature | Monohybrid Cross | Dihybrid Cross |
| Traits Involved | One trait | Two traits |
| Genotype of Parents | Differ in one gene (e.g., TT x tt) | Differ in two genes (e.g., RRYY x rryy) |
| Phenotypic Ratio in F2 Generation | 3:1 (dominant: recessive) | 9:3:3:1 (dominant-dominant: dominant-recessive: recessive-dominant: recessive-recessive) |
| Mendel’s Experiments | Used to demonstrate Mendel’s laws of inheritance | Used to demonstrate independent assortment |
| Genetic Analysis | Easier to analyze | More complex to analyze |
3. A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
Ans :
To calculate the number of different gametes that can be produced by a diploid organism heterozygous for 4 loci, we can use the following formula:
Number of gametes = 2^n
Where:
- n = the number of heterozygous loci
In this case, n = 4. So, the number of gametes is:
Number of gametes = 2^4 = 16
4 loci can produce 16 different types of gametes.
4. Explain the Law of Dominance using a monohybrid cross.
Ans :
Mendel’s Law of Dominance: A Monohybrid Cross
Mendel’s Law of Dominance states that when two individuals with contrasting traits are crossed, the trait that appears in the offspring is called the dominant trait, while the trait that remains hidden is called the recessive trait.
A Monohybrid Cross Example: Pea Plant Height
Let’s consider a monohybrid cross between two pure-breeding pea plants: one tall (TT) and one short (tt).
- Parental Generation (P):
- TT (tall) x tt (short)
- Gametes:
- TT: T, T
- tt: t, t
- F1 Generation:
- Tt (all tall)
5. Define and design a test-cross
Ans :
Test cross is a genetic technique used to determine the genotype of an individual with a dominant phenotype. The phenotypic ratios in the offspring can reveal whether the individualhomozygous dominant or heterozygous.
6. Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
Ans :
Punnett Square for a Monohybrid Cross
Let’s assume:
- Trait: A dominant trait (e.g., tall height) represented by the allele “T” and a recessive trait (e.g., short height) represented by the allele “t”.
- Parent Genotypes:
- Female: TT (homozygous dominant)
| T | t | |
| T | TT | Tt |
| T | TT | Tt |
Analysis:
- Genotypic ratio: 2 TT: 2 Tt (all individuals are tall)
- Phenotypic ratio: 4 tall: 0 short
7. When a cross in made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy), what proportions of phenotype in the offspring could be expected to be
(a) tall and green.
(b) dwarf and green
Ans :
8. Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dibybrid cross?
Ans :
9. Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dibybrid cross?
Ans :
T.H. Morgan made significant contributions to the field of genetics through his research on the fruit fly, Drosophila melanogaster. His work led to the chromosomal theory of inheritance, which established that genes are located on chromosomes and that they are arranged in a linear order.
Here are some key contributions of T.H. Morgan:
- Chromosomal Theory of Inheritance: Morgan’s experiments showed that genes are located on chromosomes and that they are inherited together in linked groups.
- Sex-Linked Inheritance: Morgan discovered sex-linked inheritance, which explained how certain traits are passed down differently in males and females.
- Mutation: Morgan studied mutations in fruit flies, providing evidence that genes are the units of heredity and that mutations can lead to new traits.
10. What is pedigree analysis? Suggest how such an analysis, can be useful.
Ans :
Pedigree analysis is a genetic technique used to trace the inheritance of traits through families. It involves constructing a family tree-like diagram that shows the relationships between individuals and their affected or unaffected status for a particular trait.
Pedigree analysis is a valuable tool for:
- Identifying Inheritance Patterns: By examining the distribution of a trait within a family, pedigree analysis can help determine whether it is inherited in a dominant, recessive, or sex-linked pattern.
- Genetic Counseling: Pedigrees can be used to assess the risk of inheriting genetic disorders and to provide genetic counseling to individuals and families.
- Gene Mapping: Pedigrees can help identify genes associated with specific traits or disorders, aiding in genetic research and the development of targeted therapies.
- Tracing Family History: Pedigrees can be used to trace family history and identify common ancestors.
- Investigating Disease Outbreaks: Pedigrees can be used to investigate the spread of diseases within a family or community.
11. How is sex determined in human beings?
Ans :
Sex determination in humans is primarily determined by the sex chromosomes present in the fertilized egg.
- Male: XY chromosomes
- Female: XX chromosomes
The sperm cell carries either an X or a Y chromosome. If the sperm is carrying an X chromosome, the resulting embryo will be female (XX). If the sperm is carrying a Y chromosome, the resulting embryo will be male (XY).
It’s important to note that while the sex chromosomes play a crucial role in sex determination, other factors may also influence the development of male or female characteristics. These factors include hormones and environmental influences.
12. A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.
Ans :
13. Explain the following terms with example
(a) Co-dominance
(b) Incomplete dominance
Ans :
(a) Co-dominance
- Definition: A genetic phenomenon where the alleles of a gene are equally expressed in a heterozygous individual, resulting in a phenotype that is a blend of both parental traits.
- Example: ABO blood group system. In individuals with blood type AB, both the A and B alleles are expressed equally, resulting in a distinct blood type.
(b) Incomplete Dominance
- Definition: A genetic phenomenon where the heterozygous phenotype is intermediate between the two homozygous phenotypes.
- Example: Flower color in snapdragons. When a red-flowered snapdragon is crossed with a white-flowered snapdragon, the offspring have pink flowers, indicating an intermediate phenotype.
14. What is point mutation? Give one example.
Ans :
Point mutation is a type of genetic mutation that involves a single nucleotide change in the DNA sequence. This change can result in the substitution, deletion, or insertion of a single nucleotide.
Example:
- Sickle cell anemia: This genetic disorder is caused by a single nucleotide substitution in the gene that codes for the beta-globin protein. The substitution changes one amino acid in the protein, leading to the abnormal shape of red blood cells characteristic of sickle cell anemia.
15. Who had proposed the chromosomal theory of the inheritance?
Ans :
Thomas Hunt Morgan proposed the chromosomal theory of inheritance. His research on Drosophila melanogaster (fruit flies) provided strong evidence that genes are located on chromosomes and that they are inherited in a linear order.
16. Mention any two autosomal genetic disorders with their symptoms.
Ans :
1. Sickle Cell Anemia
- Symptoms:
- Anemia (low red blood cell count)
- Fatigue
- Weakness
- Painful crises (episodes of severe pain)
- Swollen joints
- Jaundice
- Frequent infections
2. Cystic Fibrosis
- Symptoms:
- Persistent coughing and wheezing
- Frequent respiratory infections
- Difficulty breathing
- Salty-tasting sweat
- Poor growth and weight gain
- Digestive problems
