Inheritance of One and Two Genes — Revision Notes

⚡ 30-Second Revision

Monohybrid Cross: — Tracks 1 trait. F1: all dominant. F2: $3:1$ phenotypic, $1:2:1$ genotypic.
Law of Segregation: — Alleles for a trait separate during gamete formation. Each gamete gets one allele.
Test Cross: — Unknown dominant phenotype $imes$ homozygous recessive. Reveals unknown genotype.

- If all dominant offspring $ightarrow$ unknown is homozygous dominant. - If $1:1$ dominant:recessive offspring $ightarrow$ unknown is heterozygous.

Dihybrid Cross: — Tracks 2 traits. F1: all dominant for both. F2: $9:3:3:1$ phenotypic.
Law of Independent Assortment: — Alleles for different genes assort independently during gamete formation (if on different chromosomes or far apart).
Probability Method: — For independent events, $P(A \text{ and } B) = P(A) \times P(B)$.

2-Minute Revision

Inheritance of one and two genes forms the core of Mendelian genetics. A monohybrid cross focuses on a single trait, like height. Crossing pure tall (TT) with pure dwarf (tt) yields all tall F1 (Tt).

Self-pollinating F1 gives F2 with a 3 Tall : 1 Dwarf phenotypic ratio and a 1 TT : 2 Tt : 1 tt genotypic ratio. This demonstrates the Law of Segregation, where alleles (T and t) separate during gamete formation.

A test cross (unknown dominant $imes$ homozygous recessive) helps determine if a dominant individual is homozygous (all dominant offspring) or heterozygous (1:1 dominant:recessive offspring).

A dihybrid cross tracks two traits simultaneously, like seed shape (Round/Wrinkled) and color (Yellow/Green). Crossing pure Round Yellow (RRYY) with pure Wrinkled Green (rryy) gives all Round Yellow F1 (RrYy).

Self-pollinating F1 results in an F2 phenotypic ratio of 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green. This illustrates the Law of Independent Assortment, meaning alleles for different genes segregate independently during gamete formation, leading to new trait combinations.

Remember to use Punnett squares or the probability method for solving problems.

5-Minute Revision

To master the inheritance of one and two genes for NEET, first solidify your understanding of basic terminology: gene, allele, homozygous, heterozygous, dominant, recessive, genotype, and phenotype.

Monohybrid Cross (Inheritance of One Gene):

Concept: — Study of one contrasting trait (e.g., Tall vs. Dwarf).
P Generation: — Pure Tall (TT) $imes$ Pure Dwarf (tt).
F1 Generation: — All heterozygous Tall (Tt).
**F2 Generation (from F1 $imes$ F1):**

* Phenotypic Ratio: 3 Tall : 1 Dwarf * Genotypic Ratio: 1 TT : 2 Tt : 1 tt

Law Demonstrated: — Law of Segregation (Alleles separate during gamete formation).
Test Cross: — To find genotype of a dominant phenotype (e.g., Tall plant, T_).

* Cross with homozygous recessive (tt). * If all offspring are Tall $ightarrow$ parent was TT. * If offspring are 1 Tall : 1 Dwarf $ightarrow$ parent was Tt.

Dihybrid Cross (Inheritance of Two Genes):

Concept: — Study of two contrasting traits simultaneously (e.g., Round Yellow vs. Wrinkled Green).
P Generation: — Pure Round Yellow (RRYY) $imes$ Pure Wrinkled Green (rryy).
F1 Generation: — All heterozygous Round Yellow (RrYy).
**F2 Generation (from F1 $imes$ F1):**

* Phenotypic Ratio: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green. * Genotypic Ratio: More complex (1:2:1:2:4:2:1:2:1 for 9 genotypes).

Law Demonstrated: — Law of Independent Assortment (Alleles for different genes assort independently).

Problem-Solving Tips:

1
Punnett Square: — Useful for monohybrid and simple dihybrid crosses. Systematically lists gametes and offspring genotypes/phenotypes.
2
Probability Method: — Essential for dihybrid and polyhybrid crosses. For independent events, multiply individual probabilities. E.g., P(RrYy) from RrYy $imes$ RrYy = P(Rr from Rr $imes$ Rr) $imes$ P(Yy from Yy $imes$ Yy) = (1/2) $imes$ (1/2) = 1/4.

Example: What is the probability of getting a plant with genotype RrYY from a cross RrYy $imes$ RrYy?

For Rr from Rr $imes$ Rr: Probability = 1/2
For YY from Yy $imes$ Yy: Probability = 1/4
Combined Probability = (1/2) $imes$ (1/4) = 1/8.

Always practice identifying parental genotypes from offspring ratios and vice-versa. Understand the biological basis of allele segregation (meiosis I) and independent assortment (random alignment of homologous chromosomes at metaphase I).

Prelims Revision Notes

Inheritance of One and Two Genes: NEET Quick Revision

I. Basic Terminology:

Gene: — Unit of heredity.
Allele: — Alternative form of a gene (e.g., T/t).
Homozygous: — Identical alleles ($TT$, $tt$).
Heterozygous: — Different alleles ($Tt$).
Dominant: — Allele expressed in heterozygous state (e.g., T in $Tt$).
Recessive: — Allele masked in heterozygous state (e.g., t in $Tt$).
Genotype: — Genetic constitution (e.g., $TT$, $Tt$, $tt$).
Phenotype: — Observable trait (e.g., Tall, Dwarf).

II. Inheritance of One Gene (Monohybrid Cross):

Cross: — Pure Tall ($TT$) $imes$ Pure Dwarf ($tt$).
F1 Generation: — All heterozygous Tall ($Tt$).
**F2 Generation (from F1 $imes$ F1):**

* Phenotypic Ratio: $3:1$ (3 Tall : 1 Dwarf). * Genotypic Ratio: $1:2:1$ (1 $TT$ : 2 $Tt$ : 1 $tt$).

Mendel's Law: — Law of Segregation (Alleles separate during gamete formation; each gamete gets one allele).
Test Cross: — To determine genotype of dominant phenotype (T_).

* Cross with homozygous recessive ($tt$). * If all offspring T_ $ightarrow$ unknown is $TT$. * If offspring $1 T_ : 1 tt$ $ightarrow$ unknown is $Tt$.

III. Inheritance of Two Genes (Dihybrid Cross):

Cross: — Pure Round Yellow ($RRYY$) $imes$ Pure Wrinkled Green ($rryy$).
F1 Generation: — All heterozygous Round Yellow ($RrYy$).
**F2 Generation (from F1 $imes$ F1):**

* Phenotypic Ratio: $9:3:3:1$ (9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green). * Genotypic Ratio: $1:2:1:2:4:2:1:2:1$ (9 distinct genotypes).

Mendel's Law: — Law of Independent Assortment (Alleles for different genes assort independently during gamete formation, if on different chromosomes or far apart).

IV. Problem Solving Strategies:

Punnett Square: — Visual grid for predicting offspring. Effective for monohybrid and simpler dihybrid crosses.
Probability Method: — For complex crosses, multiply individual probabilities for each gene. E.g., $P(AABB) = P(AA) \times P(BB)$.
Key Skill: — Be able to deduce parental genotypes from offspring ratios and vice versa. Practice identifying the type of cross from the given information.

Vyyuha Quick Recall

Some Indian Doctors Remember Genetics:

Segregation (Monohybrid, 3:1 Phenotypic, 1:2:1 Genotypic)
Independent Assortment (Dihybrid, 9:3:3:1 Phenotypic)
Dominance (F1 shows dominant trait)
Recessive (Trait hidden in F1, reappears in F2)
Gametes (Pure, carry only one allele)