Unit 1: Introduction to Genetics, Mendelian Genetics and Chromosomal Basis of Inheritance - Subjective Questions

Dominance and recessiveness describe the relationship between alleles of a gene:

• Dominance: An allele is said to be dominant if it expresses its phenotype even in the heterozygous condition (when only one copy is present). It masks the effect of the other allele.
• Recessiveness: An allele is recessive if its phenotype is expressed only in the homozygous condition (when two copies are present). It is masked by the dominant allele in heterozygotes.

Example (Mendel's Pea Plants):

• When Mendel crossed a tall (TT) plant with a dwarf (tt) plant, all F offspring were tall (Tt).
• The tall allele (T) is dominant, and the dwarf allele (t) is recessive.
• The dwarf trait reappeared only in the F generation in a 3:1 ratio (3 tall : 1 dwarf), confirming that recessive alleles are expressed only when homozygous.

Key point: Dominance does not mean an allele is more common or superior — it simply refers to which allele's phenotype is visible in the heterozygote.

Principle of Segregation (Mendel's First Law):

During the formation of gametes, the two alleles of a gene separate (segregate) from each other so that each gamete carries only one allele of each gene.

Explanation:

• Every diploid organism carries two alleles for each trait, one inherited from each parent.
• These alleles remain distinct and do not blend.
• During meiosis, homologous chromosomes separate, causing the paired alleles to segregate into different gametes.
• At fertilization, alleles combine randomly, restoring the diploid number.

Illustration (Monohybrid Cross):

• Parents: TT (tall) × tt (dwarf)
• F: All Tt (tall)
• Self-cross of F (Tt × Tt):

• Genotypic ratio: 1 TT : 2 Tt : 1 tt
• Phenotypic ratio: 3 tall : 1 dwarf

Significance: This principle explains the reappearance of recessive traits in the F generation and is the cytological basis for allele separation during meiosis.

Principle of Independent Assortment (Mendel's Second Law):

During gamete formation, the alleles of one gene segregate independently of the alleles of another gene, provided the genes are located on different chromosomes.

Dihybrid Cross Example:

Mendel crossed pea plants differing in two traits:

• Seed shape: Round (R) dominant over wrinkled (r)
• Seed color: Yellow (Y) dominant over green (y)

Parental Cross: RRYY (round, yellow) × rryy (wrinkled, green)

• F: All RrYy (round, yellow)

F Self-Cross (RrYy × RrYy):

Each parent produces 4 gamete types: RY, Ry, rY, ry

Combining in a 4×4 Punnett square gives 16 combinations.

Phenotypic Ratio (F):

• 9 Round Yellow
• 3 Round Green
• 3 Wrinkled Yellow
• 1 Wrinkled Green

Thus the ratio is 9 : 3 : 3 : 1.

Conclusion: Since each trait separately shows a 3:1 ratio, the combined probability of two independent traits is , , etc., producing 9:3:3:1. This proves genes assort independently.

Thomas Hunt Morgan used the fruit fly Drosophila melanogaster as his model organism because of its short life cycle, easy maintenance, and clearly distinguishable mutant traits.

Discovery of Sex-Linked Inheritance (White Eye):

• Morgan found a white-eyed male among normal red-eyed flies.
• Cross 1: White-eyed male × Red-eyed female → All F red-eyed.
• F: Red and white eyes appeared in 3:1, but all white-eyed flies were males.
• This showed the eye-color gene is located on the X chromosome — the first proof of sex linkage.

Discovery of Linkage:

• Morgan studied two genes on the same chromosome (e.g., body color and wing size).
• Instead of the expected 9:3:3:1 dihybrid ratio, he observed that certain trait combinations stayed together more often than expected.
• He concluded these genes were linked (present on the same chromosome).

Recombination:

• Some offspring showed new combinations due to crossing over.
• The frequency of recombination was used to estimate the distance between genes, forming the basis of genetic mapping.

Significance: Morgan's work confirmed the Chromosomal Theory of Inheritance by physically linking specific genes to specific chromosomes.

Chromosomal Theory of Inheritance was proposed by Walter Sutton and Theodor Boveri (1902–1903).

Statement: Genes are located on chromosomes, and the behaviour of chromosomes during meiosis and fertilization accounts for Mendel's laws of inheritance.

Parallels between Genes and Chromosomes:

• Both genes and chromosomes occur in pairs in diploid cells.
• Both segregate during gamete formation so that each gamete receives one member of each pair (supports the Law of Segregation).
• Both members of a pair separate independently of other pairs (supports the Law of Independent Assortment).
• The diploid condition is restored at fertilization.
• Homologous chromosomes carry corresponding alleles at the same loci.

Sutton's Contribution:

• He observed that the segregation of chromosomes during meiosis exactly parallels the segregation of Mendel's factors (alleles).
• This provided the cytological basis for Mendelian genetics.

Later confirmation: Morgan's Drosophila experiments experimentally confirmed the theory by demonstrating sex-linked inheritance.

Autosomal Inheritance vs Sex-Linked Inheritance:

Key Explanation:

• In autosomal traits, since both sexes have two copies of each autosome, inheritance patterns are symmetrical.
• In sex-linked (X-linked recessive) traits, males are hemizygous (only one X), so a single recessive allele expresses the trait, making males more frequently affected.

Reason for higher expression in males:

• Males are XY — they have only one X chromosome, making them hemizygous for X-linked genes.
• A single recessive allele on the X chromosome will be expressed because there is no second X to mask it.
• Females are XX — they need two copies of the recessive allele (homozygous) to express the trait, which is statistically rarer.

Example — Haemophilia (X-linked recessive):

Let X^H = normal allele, X^h = haemophilia allele.

• Carrier mother (X^H X^h) × Normal father (X^H Y):

Outcomes:

• Daughters: 50% normal, 50% carriers (none affected)
• Sons: 50% normal, 50% affected (X^h Y)

Conclusion: Because males need only one recessive allele to be affected, X-linked recessive disorders like haemophilia appear far more frequently in males. Affected females occur only when an affected father mates with a carrier or affected mother.

Test Cross:

A test cross is a cross between an organism showing a dominant phenotype (whose genotype is unknown) and an organism that is homozygous recessive for that trait.

Purpose:

• To determine whether the dominant individual is homozygous (TT) or heterozygous (Tt).

Explanation with Example (Tall pea plant):

Case 1 — Unknown is Homozygous (TT):

• TT × tt → All Tt (tall)
• Result: 100% tall offspring.

Case 2 — Unknown is Heterozygous (Tt):

• Tt × tt → 1 Tt : 1 tt
• Result: 50% tall : 50% dwarf (1:1 ratio).

Interpretation:

• If all offspring show the dominant trait, the unknown parent is homozygous.
• If offspring appear in a 1:1 ratio of dominant to recessive, the unknown parent is heterozygous.

Significance: The test cross is a simple, reliable genetic tool to reveal hidden recessive alleles and confirm the genotype of dominant-phenotype individuals.

Linkage:

Linkage is the tendency of genes located close together on the same chromosome to be inherited together, rather than assorting independently.

• Linked genes do not follow Mendel's Law of Independent Assortment.
• The closer two genes are, the stronger the linkage.

Crossing Over:

Crossing over is the exchange of segments between homologous chromosomes during prophase I of meiosis.

• It occurs at points called chiasmata.
• It produces recombinant (new) combinations of alleles.
• The frequency of crossing over increases with the physical distance between genes.

Measuring Gene Distance (Genetic Mapping):

• The distance between genes is expressed in map units (centimorgans, cM).

Example: If two genes show 15% recombinants, they are 15 map units (15 cM) apart.

Significance: This principle, developed from Morgan's and Sturtevant's work, is the basis for constructing linkage (genetic) maps of chromosomes.

Autosomal Dominant Inheritance:

• Caused by a dominant allele on an autosome; a single copy is enough to express the trait.
• Characteristics:
  • Trait appears in every generation (vertical transmission).
  • Affected individuals usually have at least one affected parent.
  • Males and females affected equally.
  • Two affected heterozygous parents can have unaffected children.
• Examples: Huntington's disease, Achondroplasia, Marfan syndrome.

Autosomal Recessive Inheritance:

• Caused by a recessive allele; two copies needed to express the trait.
• Characteristics:
  • Trait may skip generations (horizontal pattern).
  • Affected individuals often have unaffected carrier parents (Aa × Aa).
  • Males and females affected equally.
  • More common in consanguineous (related) marriages.
• Examples: Cystic fibrosis, Sickle-cell anaemia, Albinism, Phenylketonuria.

Cross for Recessive Trait (Aa × Aa):

• Genotypic ratio: 1 AA : 2 Aa : 1 aa
• 25% affected (aa), 75% unaffected (of which 2/3 are carriers).

Key Difference: Dominant traits appear in every generation; recessive traits often skip generations and appear only in homozygotes.

Definition of Genetics:

Genetics is the branch of biology that deals with the study of heredity (the transmission of traits from parents to offspring) and variation (differences among individuals).

The term was coined by William Bateson in 1905. Gregor Johann Mendel is regarded as the Father of Genetics.

Major Branches of Genetics:

• Classical (Mendelian) Genetics: Study of inheritance patterns and gene transmission.
• Molecular Genetics: Study of DNA, RNA, gene structure, and expression.
• Cytogenetics: Study of chromosomes and their role in inheritance.
• Population Genetics: Study of allele frequencies and evolution in populations.
• Quantitative Genetics: Study of traits controlled by many genes.

Importance of Genetics:

• Helps understand inherited diseases and enables genetic counselling.
• Foundation for biotechnology and genetic engineering.
• Improves crops and livestock through selective breeding.
• Essential for forensic science, medicine, and evolutionary biology.

Conclusion: Genetics forms the core of life sciences, explaining how life's information is stored, passed on, and expressed.

In some cases, alleles do not follow the strict dominant-recessive relationship described by Mendel.

1. Incomplete Dominance:

When neither allele is completely dominant, the heterozygote shows an intermediate (blended) phenotype.

• Example: In Mirabilis jalapa (Four O'Clock plant):
  • Red (RR) × White (WW) → Pink (RW) in F.
  • F ratio: 1 Red : 2 Pink : 1 White (phenotype = genotype ratio).

2. Codominance:

When both alleles are fully and independently expressed in the heterozygote (no blending).

• Example: ABO blood group in humans:
  • Alleles and are codominant.
  • Genotype produces AB blood group, expressing both A and B antigens.
• Roan coat colour in cattle (both red and white hairs present).

Comparison with Complete Dominance:

Given traits:

• Tall (T) dominant over dwarf (t)
• Green pods (G) dominant over yellow pods (g)

Parental Cross:

• Tall, Green (TTGG) × Dwarf, Yellow (ttgg)

F Generation:

• Gametes: TG × tg
• All offspring: TtGg → Tall, Green pods

F Self-Cross (TtGg × TtGg):

• Each parent produces 4 gamete types: TG, Tg, tG, tg
• A 4×4 Punnett square gives 16 combinations.

F Phenotypic Ratio:

• 9 Tall, Green
• 3 Tall, Yellow
• 3 Dwarf, Green
• 1 Dwarf, Yellow

Thus, 9 : 3 : 3 : 1.

Verification using probability:

• Tall : Dwarf =
• Green : Yellow =
• Combined:

Conclusion: The result confirms Mendel's Law of Independent Assortment.

Sex Determination in Humans (XX–XY System):

• Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes.
• Females: XX (homogametic — produce only X-bearing eggs)
• Males: XY (heterogametic — produce X or Y-bearing sperm)

Sex Ratio:

• If sperm carries X → offspring is female (XX).
• If sperm carries Y → offspring is male (XY).
• Thus the father determines the sex of the child, giving a theoretical 1:1 ratio.

Inheritance of Sex-Linked Genes:

Genes present on the sex chromosomes are sex-linked.

• X-linked genes: Present on the X chromosome. Males (XY) are hemizygous, so a single X-linked recessive allele is expressed.
  • Examples: colour blindness, haemophilia.
• Y-linked (holandric) genes: Present on the Y chromosome; passed only from father to son.
  • Example: hairy ear pinnae.

Criss-Cross Inheritance:

• An X-linked trait passes from an affected father → carrier daughter → grandson.
• This zig-zag pattern is called criss-cross inheritance, a hallmark of X-linked traits.

Monohybrid Cross:

A cross between two parents differing in a single pair of contrasting traits.

Example (Plant height):

• P generation: Pure Tall (TT) × Pure Dwarf (tt)
• F generation: All Tall (Tt) — dwarf trait disappears.
• F generation (Tt × Tt self-cross):

• Genotypic ratio: 1 TT : 2 Tt : 1 tt
• Phenotypic ratio: 3 Tall : 1 Dwarf

Conclusion: Demonstrates the Law of Dominance and the Law of Segregation.

Why Mendel Chose the Garden Pea (Pisum sativum):

• Has clear-cut contrasting traits (e.g., tall/dwarf, round/wrinkled seeds).
• Short life cycle and produces many offspring.
• Naturally self-pollinating, allowing pure lines; can also be cross-pollinated manually.
• Easy to cultivate and maintain.
• Flowers have both male and female parts, giving control over breeding.

These features made the pea plant ideal for studying inheritance systematically.

Multiple Alleles:

When a single gene has more than two allelic forms in a population, they are called multiple alleles. (Although any individual carries only two of these alleles.)

ABO Blood Group System:

Controlled by a single gene I with three alleles:

• — produces antigen A
• — produces antigen B
• — produces no antigen (recessive)

Dominance Relationships:

• and are codominant to each other.
• Both and are dominant over .

Genotypes and Phenotypes:

Significance:

• Demonstrates both multiple alleles and codominance.
• Important in blood transfusion, paternity testing, and genetics of populations.

The Chromosomal Theory of Inheritance links Mendel's laws directly to chromosome behaviour during meiosis.

1. Law of Segregation and Meiosis:

• Each diploid cell has homologous chromosome pairs, each carrying one allele of a gene.
• During Anaphase I, homologous chromosomes separate and move to opposite poles.
• This physically segregates the two alleles into different gametes — exactly as Mendel proposed.

2. Law of Independent Assortment and Meiosis:

• During Metaphase I, homologous pairs align randomly at the equator.
• The orientation of one pair is independent of another pair.
• This random alignment causes independent assortment of genes on different chromosomes.
• Number of gamete combinations = (where = number of chromosome pairs).

3. Restoration of Diploidy:

• Fertilization combines two haploid gametes, restoring the diploid number and pairing of alleles.

Summary Table:

Conclusion: The parallel behaviour of genes and chromosomes provides the cytological proof of Mendel's principles.

Genotype vs Phenotype:

Homozygous vs Heterozygous:

Example:

• A plant with genotype TT is homozygous dominant → phenotype Tall.
• A plant with genotype Tt is heterozygous → phenotype Tall (due to dominance).
• A plant with genotype tt is homozygous recessive → phenotype Dwarf.

Key point: Two organisms (TT and Tt) can share the same phenotype but differ in genotype.

Red-Green Colour Blindness is an X-linked recessive disorder caused by a defective gene on the X chromosome.

Let X^C = normal vision allele, X^c = colour-blind allele.

Cross: Carrier Mother (X^C X^c) × Normal Father (X^C Y):

Results:

• Daughters: 50% normal (X^C X^C), 50% carriers (X^C X^c) — none colour blind.
• Sons: 50% normal (X^C Y), 50% colour blind (X^c Y).

Why more common in males:

• Males are XY (hemizygous) — they have only one X chromosome.
• A single recessive allele (X^c) is enough to express colour blindness because there is no second X to mask it.
• Females (XX) must inherit two defective alleles (X^c X^c) to be colour blind, which is much rarer.

Condition for affected female: A colour-blind father (X^c Y) must mate with a carrier or affected mother (X^c X^c or X^C X^c).

Conclusion: Because males need only one defective allele, colour blindness is far more frequent in men than in women.

Significance of Mendel's Work:

• Provided the first scientific explanation of heredity through discrete units (now called genes).
• Introduced the concept of dominant and recessive factors.
• Established the Laws of Segregation and Independent Assortment.
• Applied statistics and mathematics to biology for the first time.
• Formed the foundation of modern genetics.

Reasons His Work Was Ignored (1866–1900):

• Published in an obscure journal (Proceedings of the Natural History Society of Brünn).
• His mathematical/statistical approach was ahead of its time and poorly understood by biologists.
• The chromosomal basis of inheritance was not yet known, so his 'factors' had no physical explanation.
• Prevailing belief in blending inheritance contradicted his particulate model.

Rediscovery (1900):

• Independently rediscovered by Hugo de Vries, Carl Correns, and Erich von Tschermak, who confirmed his conclusions.

Later Criticisms:

• Some statisticians (e.g., R.A. Fisher) suggested his data were 'too good' (too close to expected ratios), hinting at possible bias in data selection.
• Mendel's laws do not explain linkage, incomplete dominance, or polygenic traits.

Conclusion: Despite limitations, Mendel's work remains the cornerstone of classical genetics.