Principles of Inheritance and Variation

Principles of Inheritance and Variation

This chapter focuses on the principles of inheritance and variation, exploring Mendel’s
laws, gene interactions, polygenic inheritance, pleiotropy, sex determination,
mutations, and genetic disorders.

• Inheritance and Variation
  • Inheritance: Passing of traits from parents to offspring.
  • Variation: Differences between parents and offspring.
  • Genetics: The science of inheritance and variation.
    • Example: Elephants always give birth to baby elephants; mango seeds grow into mango plants.
• Early Understanding
  • Humans knew about variation through sexual reproduction long ago (8000-1000 B.C.).
  • Selective breeding: The practice of breeding plants and animals for desirable traits, utilizing natural variations.
    • Example: Indian breeds like Sahiwal cows originated from selective breeding of ancestral wild cows.

Mendel’s Laws of Inheritance

• Gregor Mendel (Father of Genetics)
  • Mid-19th century scientist.
  • Conducted experiments on garden peas (Pisum sativum) (1856-1863).
  • Proposed laws of inheritance using statistical analysis and mathematical logic.
  • The large sample size of his experiments made his data statistically reliable.
  • Mendel’s results were validated through repeated experiments and observations on successive generations of pea plants.
• Mendel’s Experiments
  • Studied pea plants with contrasting traits, such as tall or dwarf plants, yellow or green seeds.
  • Established a foundational framework for the rules of inheritance.
  • Later other scientists expanded and refined these rules based on further research.
• True-breeding Pea Lines
  • True-breeding: Plants that, through continuous self-pollination, produce offspring with consistent traits across generations.
  • Mendel selected 14 true-breeding pea varieties for his experiments.
  • Traits studied included:

7 Character	Dominant Trait	Recessive Trait
Stem Height	Tall	Dwarf
Flower Color	Violet	White
Flower Position	Axial	Terminal
Pod Shape	Inflated	Constricted
Pod Color	Green	Yellow
Seed Shape	Round	Wrinkled
Seed Color	Yellow	Green

Inheritance of One Gene

• Mendel’s Experiment
  • Initial Cross: Crossed tall (TT) and dwarf (tt) pea plants.
  • First Hybrid Generation (F1): All F1 plants were tall.
  • Second Generation (F2): Self-pollinated F1 tall plants to produce F2. In F2, 3/4 were tall, and 1/4 were dwarf.
  • Findings
    • F1 Generation: Showed traits of only one parent (tall).
    • F2 Generation: Showed traits of both parents in a 3:1 ratio (tall to dwarf).
    • No Blending of Traits: Plants were either tall or dwarf.
• Genes and Alleles
  • Genes: Mendel referred to the units of inheritance as “factors,” now known as genes.
  • Traits: Observable characteristics or features of an organism determined by genetic and environmental factors. Genes contain information for traits.
  • Alleles: Different forms of the same gene (e.g., T for tall, t for dwarf).
  • Genotypes: Combination of alleles (TT, Tt, tt).
  • Phenotypes: Observable traits (tall or dwarf).
• Dominance and Recessiveness
  • Dominant Allele (T): Masks the recessive allele (t) in F1 plants.
  • Heterozygous (Tt): Tt plants are tall because T is dominant.
  • Homozygous (TT or tt): TT plants are tall, and tt plants are dwarf.
• Monohybrid Cross
  • Definition: Cross between two plants with one different trait.
  • Example: Cross between TT (tall) and tt (dwarf)
• Segregation of Alleles
  • Gamete Formation: Alleles separate randomly during gamete formation
  • Each gamete gets one allele (T or t).
  • Fertilization: Combines alleles from both parents (e.g., Tt).
• Punnett Square
  • Purpose: Diagram to predict genotype and phenotype ratios.
  • Representation: Shows possible combinations of alleles from parental gametes.
  • F2 Generation: Shows a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.
• Backcross: Mating (cross) a hybrid with one of its parents.
  • Test Cross
    • Purpose: Determine the genotype of a plant with a dominant trait.
    • Method: Cross the plant with a recessive plant (tt).
    • Analysis: Offspring analysis determines if the dominant plant is TT or Tt.
• Mendel’s Laws of Inheritance
  • Law of Dominance: In heterozygous condition, dominant allele masks the recessive allele.
  • Law of Segregation: Alleles separate randomly during gamete formation, ensuring each gamete receives only one allele.

Mendel’s Laws of Inheritance

1. Law of Dominance

“In a pair of alleles, one allele can mask the expression of the other”.

• Key Points
  • Characters are controlled by discrete units called factors (now known as genes).
  • Factors occur in pairs.
  • In a dissimilar pair, one factor dominates (dominant) while the other is hidden (recessive).
• Explanation
  • In a monohybrid cross (e.g., tall vs. dwarf pea plants):
    • F1 generation shows only the dominant trait.
    • F2 generation shows a 3:1 ratio of dominant to recessive traits.
  • 

2. Law of Segregation

“Alleles separate during gamete formation, with each gamete receiving only one allele from each pair”.

• Key Points
  • Alleles do not blend; they separate during gamete formation.
  • Each gamete receives only one allele from the pair.
  • Homozygous parents produce identical gametes.
  • Heterozygous parents produce two types of gametes in equal proportions.
  • 

a. Incomplete Dominance (exception/anomalous behavior of law of Dominance).

“A genetic scenario where the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes”.

Example

• Flower color in snapdragons (dog flowers).
  • Red (RR) x White (rr) results in Pink (Rr) F1 generation.
  • F2 generation: 1 Red (RR): 2 Pink (Rr): 1 White (rr).
  • 

Explanation

• The dominant allele (R) does not completely mask the recessive allele (r).
• Rr results in an intermediate phenotype (pink).

b. Co-dominance (exception/anomalous behavior of law of Dominance).

“A genetic scenario where both alleles in a heterozygote are fully expressed without blending”.

Example

• ABO blood groups in humans.
  • Gene I has three alleles: 𝐼𝐴, 𝐼𝐵, and i.
  • 𝐼𝐴IA and 𝐼𝐵IB are co-dominant, both express their traits together.
  • 

Explanation

• 𝐼𝐴𝐼𝐵 genotype results in blood type AB (both sugars present).
• Multiple alleles: More than two alleles can exist for a gene, but only two are present in an individual.

Additional Concepts

• Multiple Alleles (Multiple Allelism)
• “The existence of more than two alleles for a genetic trait within a population”.
  • Example: ABO blood grouping with three alleles.
  • Population studies show multiple alleles, though individuals have only two alleles.
  • 
• Single Gene, Multiple Effects (Pleiotropy)
• “A single gene influencing multiple phenotypic traits”.
  • Example: Starch synthesis in pea seeds.
    • BB (large starch grains), bb (small starch grains), Bb (intermediate starch grains).
    • Dominance can depend on which phenotype is examined.
    • 

Inheritance of Two Genes

Mendel’s Experiments with Two Traits

• Example: Cross between pea plants with:
  • Round, yellow seeds (RRYY) (dominant traits)
  • Wrinkled, green seeds (rryy) (recessive traits)
• Results:
  • F1 generation had all round, yellow seeds i.e. RrYy.
  • F2 generation showed a 3:1 ratio for each trait (yellow:green and round:wrinkled).

3. Law of Independent Assortment

“Genes for different traits are inherited independently of each other during gamete formation.”.

• Key Points:
  • Traits are inherited independently of each other.
  • In a dihybrid cross (two traits), the F2 generation shows a 9:3:3:1 ratio.
• Explanation:
  • Example: RrYy plants produce gametes with combinations RY, Ry, rY, and ry.
  • Punnett square helps visualize the independent assortment.
  • 

Law of Segregation vs. Law of Independent Assortment

Feature	Law of Segregation	Law of Independent Assortment
Definition	Alleles for a trait separate during gamete formation, each gamete receives one allele from each pair.	Alleles of different genes assort independently of one another during gamete formation.
Focus & Cross type	Single gene inheritance For a monohybrid cross	Inheritance of two or more genes For a dihybrid cross
Key Principle	Each gamete gets one allele of each gene	Alleles of different genes assort independently

– Chromosomal Theory of Inheritance

• History:
  • Mendel’s work was rediscovered in 1900 by Hugo de Vries, Carl Correns, and Erich von Tschermak.
  • Chromosomes observed during cell division supported Mendel’s ideas.
• Key Contributors:
  • Walter Sutton and Theodore Boveri linked chromosome behavior (properties & segregation) to Mendel’s laws.
  • Thomas Hunt Morgan verified this theory using fruit flies (Drosophila melanogaster).
• key features:
  • Genes on Chromosomes: Genes are found on chromosomes.
  • Chromosome Pairs: Chromosomes come in pairs (homologous pairs) , one from each parent.
  • Meiosis and Inheritance: During meiosis, chromosomes pairs split so each gamete gets one chromosome from each pair (Mendel’s laws of segregation and independent assortment).
  • Linkage and Crossing Over: Genes close together on a chromosome are often inherited together (linkage), but they can mix during meiosis through crossing over, leading to new combinations of alleles. (crossing over).
  • Matches Mendel’s Laws: Chromosome behavior during meiosis matches Mendel’s inheritance patterns.

– Linkage and Recombination

• Linkage: (exception/anomalous behavior of law of Independent Assortment).
  • Genes located on the same chromosome tend to be inherited together.
  • Example: Morgan’s experiments with fruit flies (Drosophila melanogaster) showed that certain traits did not follow the expected 9:3:3:1 ratio.
• Recombination:
  • New combinations of traits can occur due to crossing over during meiosis. Thus breaking the linkage between genes.
  • Some genes show tight linkage (low recombination), others show loose linkage (high recombination).
• Genetic Mapping:
  • Alfred Sturtevant created genetic maps showing the relative position of genes on chromosomes based on recombination frequency.
  • Recombination Frequency: The rate at which crossing over occurs between two genes, used to estimate their distance on a chromosome.
  • Map Units: The measurement unit (centimorgans, cM) for the distance between genes, where 1 cM represents a 1% recombination frequency.

Thomas Hunt Morgan used fruit flies (Drosophila melanogaster) to prove the Chromosomal Theory of Inheritance because: (#slider)

• They thrive on simple synthetic media.
• They have a short generation time (12-14 days).
• Breeding is possible year-round.
• Each mating produces hundreds of offspring.
• Male and female flies are easily distinguishable (males are smaller than females).
• They exhibit many visible hereditary variations observable with a low power microscope.

Key Takeaways

• Mendel’s experiments with two traits showed that traits are inherited independently (Law of Independent Assortment).
• Chromosomes play a crucial role in inheritance, supporting Mendel’s laws.
• Linkage and recombination explain how genes close together on the same chromosome can be inherited together, but can also occasionally be separated.

Polygenic Inheritance

Understanding Polygenic Traits

• Definition: Traits controlled by three or more genes.
• Examples:
  • Human height: People are not just tall or short, but show a range of heights.
  • Human skin color: Shows a variety of shades rather than just light or dark.

How Polygenic Traits Work

• Additive Effect: Each gene contributes to the trait.
• Example of Skin Color:
  • Three genes (A, B, C) control skin color.
  • Dominant alleles (A, B, C) make the skin darker.
  • Recessive alleles (a, b, c) make the skin lighter.
  • Darkest skin: AABBCC (all dominant alleles).
  • Lightest skin: aabbcc (all recessive alleles).
  • Intermediate skin color: Combination of dominant and recessive alleles.

Influence of Environment

• Environment Matters: Traits like skin color and height are also influenced by environmental factors, not just genetics.

Key Points

• Polygenic Traits: Controlled by multiple genes.
• Range of Phenotypes: Unlike Mendel’s traits, polygenic traits show a spectrum of possibilities.
• Additive Effects: The overall phenotype is the sum of the effects of each gene.
• Examples: Height and skin color in humans.

Mutation

What is Mutation?

• Definition: Sudden changes in DNA sequences, leading to variations.
• Impact: Alters genotype (genetic makeup) and phenotype (physical appearance) of an organism or simply cause variations.

Causes of Variation

• Recombination: One way DNA varies.
• Mutation: Another way DNA varies, by altering the sequence.

How DNA is Structured

• DNA Helix: Runs continuously from one end to the other in each chromatid.
• Supercoiled Form: DNA is tightly packed in a supercoiled structure.

Types of Chromosomal Changes

• Deletions: Loss of a segment of DNA.
• Insertions/Duplications: Gain of a segment of DNA.
• Result: Changes in chromosomes, leading to abnormalities or aberrations.
  • Common in cancer cells.

Point Mutation

• Definition: Change in a single base pair of DNA.
• Example: Sickle cell anemia.

Frame-Shift Mutation

• Cause: Deletions or insertions of base pairs.
• Result: Shifts the DNA reading frame, causing major changes.

Mutagens

• Definition: Factors that induce mutations.
• Types:
  • Chemical Mutagens: Various chemicals like Mustard gas, phenol, formalin etc.
  • Physical Mutagens: UV radiation.

Summary

• Mutation: Sudden changes DNA, leading to variations.
  • Causes: can result from errors in DNA replication or external factors like radiation.
• Inheritable: Mutations are inheritable if they occur in germ cells (sperm or eggs).
• Non-Inheritable: are non-inheritable if they occur in somatic (body) cells.
• Types:
  • Point mutations: Single base pair changes.
  • Frame-shift mutations: Deletions or insertions.
• Mutagens: Induce mutations (e.g., UV radiation).
• Effect: Can cause abnormalities, like in cancer cells and sickle cell anemia.

Genetic Disorders

Pedigree Analysis

What is Pedigree Analysis?

• Definition: Studying the inheritance of traits in a family over several generations.
• Purpose: To understand how traits, abnormalities, or diseases are passed down.
• Method: Uses standard symbols to represent traits and relationships.

Why Pedigree Analysis is Important

• Human Inheritance: Since we can’t control human crosses like in plants, we use family history.
• Family Tree: Represents traits across generations.
• Tool: Helps trace specific traits, abnormalities, or diseases in human genetics.

Inheritance and Genes

• Genes on DNA: Every feature in an organism is controlled by genes located on DNA.
• Transmission: DNA carries genetic information from one generation to the next.
• Mutations: Occasionally, changes occur in genetic material, leading to mutations.

Disorders and Inheritance

• Altered Genes: Some disorders are linked to changes in genes or chromosomes.
• Example: Inherited diseases due to mutated genes.

Key Points

• Pedigree analysis helps track how traits are inherited in families.
• It uses a family tree to show inheritance patterns.
• Genes on DNA control traits and are passed down generations.
• Mutations can lead to inherited disorders.

Mendelian Disorders

Genetic Disorders: Two types – Mendelian and Chromosomal.

• (a) Mendelian Disorders: Caused by mutation in a single gene. e.g. Haemophilia.
  • Inheritance: Follow Mendel’s principles; can be traced using pedigree analysis.

• (b) Chromosomal Disorders: due to alterations in chromosome number or structure. e.g. Down’s syndrome.
  • Inheritance: Results from abnormalities during meiosis.

Types of Mendelian Disorders

• Dominant or Recessive: Traits can be either.
• Sex-Linked or Autosomal: Traits can be either linked to sex chromosomes or to autosomes.

Common Mendelian Disorders

1. Colour Blindness
   • Type: Sex-linked recessive.
   • Cause: Mutation in genes on the X chromosome.
   • Effect: Difficulty distinguishing red and green (due to defect in red or green cone cells in the eye).
   • Prevalence: More common in males, 8% of males, 0.4% of females.
2. Haemophilia
   • Type: Sex-linked recessive.
   • Cause: Affected (Lack of) blood clotting protein.
   • Effect: Excessive bleeding from minor cuts.
   • Transmission: Carrier females (mother) to male offspring.
   • Example: Queen Victoria’s descendants.
3. Sickle-Cell Anaemia
   • Type: Autosomal recessive.
   • Cause: Mutation in the beta globin gene (HbS gene).
   • Effect: Red blood cells become sickle-shaped.
   • Genotypes:
     • HbA HbA: Normal
     • HbA HbS: Carrier
     • HbS HbS: Affected
4. Phenylketonuria (PKU)
   • Type: Autosomal recessive.
   • Cause: Lack of enzyme to convert phenylalanine to tyrosine.
   • Effect: Mental retardation due to phenylalanine buildup in brain.
   • Detection: Presence of phenylpyruvic acid in urine.
5. Thalassemia
   • Type: Autosomal recessive.
   • Cause: Mutation or deletion affecting globin chain production in haemoglobin.
   • Types:
     • α Thalassemia: Affects α (alpha) globin chain.
     • β Thalassemia: Affects β (beta) globin chain.
   • Effect: Anemia due to abnormal hemoglobin.
   • Difference from Sickle-Cell: Quantitative vs. qualitative problem i.e. Thalassemia affects quantity of globin, while sickle-cell affects quality in globin synthesis.

Remember: Mendelian disorders are inherited through single gene mutations and can be traced using family trees. They can be dominant or recessive and affect various bodily functions.

Chromosomal Disorders

Definition: Disorders caused by absence, excess, or abnormal arrangement of chromosomes.

Causes

• Aneuploidy: Gain or loss of a chromosome due to segregation failure during cell division.
  • Example: Down’s syndrome (extra copy of chromosome 21).
  • Example: Turner’s syndrome (loss of an X chromosome in females).
• Polyploidy: Increase in a whole set of chromosomes due to cytokinesis failure after telophase.
  • Common in plants.

Human Chromosomes

• Total: 46 chromosomes (23 pairs).
  • Autosomes: 22 pairs.
  • Sex Chromosomes: 1 pair (XX or XY).

Types of Chromosomal Disorders

• Trisomy: Extra copy of a chromosome.
• Monosomy: Missing one chromosome from a pair.
• Both condition can lead to serious health issues.

Examples of Chromosomal Disorders

1. Down’s Syndrome:
   • Cause: Extra copy of chromosome 21 (trisomy 21).
   • First described by Langdon Down (1866).
   • Features:
     • Short stature.
     • Small round head.
     • Furrowed tongue.
     • Partially open mouth.
     • Broad palm with characteristic crease.
     • Retarded physical, psychomotor, and mental development.

2. Klinefelter’s Syndrome:

• Cause: Extra X chromosome (47, XXY).
• Features:
  • Overall masculine development (Male appearance.).
  • Some feminine traits (breast development – Gynaecomastia).
  • Sterility.

3. Turner’s Syndrome:

• Cause: Missing one X chromosome (45, XO).
• Features:
  • Sterility due to rudimentary ovaries.
  • Lack of secondary sexual characteristics.

Chapter Summary:

• Genetics is a part of biology that studies inheritance principles.
• Progeny resembling parents in features attracts biologists’ attention.
• Mendel systematically studied inheritance patterns in pea plants.
• Mendel’s principles are known as ‘Mendel’s Laws of Inheritance’.
• Genes (factors) regulating traits are found in pairs called alleles.
• Expression of traits in offspring follows a definite pattern.
• Dominant traits are expressed in heterozygous conditions (Law of Dominance).
• Recessive traits are expressed only in homozygous conditions.
• Traits don’t blend in heterozygous conditions.
• Segregation of traits happens during gamete formation (Law of Segregation).
• Not all traits show true dominance; some show incomplete dominance or co-dominance.
• Genes independently assort and combine in various combinations (Law of Independent Assortment).
• Different combinations of gametes are represented in Punnett Squares.
• Genes on chromosomes regulate traits (genotype vs. phenotype).
• Mendel’s laws correlate with chromosome segregation during meiosis.
• ‘Chromosomal Theory of Inheritance’ extends Mendel’s laws.
• Linked genes assort together, while distant genes assort independently due to recombination.
• Sex-linked genes are linked to sexes, identified by different sex chromosomes.
• Humans have 22 pairs of autosomes and sex chromosomes (XX for females, XY for males).
• Chickens have ZZ for males and ZW for females in sex chromosomes.
• Mutation is a change in genetic material; point mutation is a single base pair change.
• Sickle-cell anemia results from a single base change in the hemoglobin gene.
• Pedigree analysis studies inheritable mutations in families.
• Some mutations involve changes in whole chromosome sets (polyploidy) or subsets (aneuploidy).
• Down’s syndrome is due to trisomy of chromosome 21 (47 chromosomes).
• Turner’s syndrome has one missing X chromosome (XO).
• Klinefelter’s syndrome has an extra X chromosome (XXY).
• Karyotypes help in studying these conditions.