UP GIC Biology 2026 Genetics Notes: Sex Determination, Mendelian Genetics, Polygenic Inheritance & Chromosomal Disorders

Genetics is one of the most important units in Biology for the UP GIC Lecturer Biology 2026 examination. Questions from Mendelian genetics, sex determination, polygenic inheritance, chromosomal disorders, and post-Mendelian inheritance are frequently asked in teaching and government recruitment exams. 

Candidates preparing for UP GIC Lecturer, TGT, PGT, BPSC, and similar examinations should build strong conceptual clarity in this topic. Here, we cover the important concepts of Genetics in a simple and structured manner for effective revision.

What is Genetics?

Genetics is the branch of biology that studies heredity and variation. It explains how characters are transferred from parents to offspring through genes.

The foundation of genetics was laid by Gregor Mendel through experiments on garden pea plants. His laws of inheritance became the basis of modern genetics.

Important areas of genetics include:

• Mendelian inheritance

• Sex determination

• Polygenic inheritance

• Chromosomal disorders

• Incomplete dominance

• Codominance

Sex Determination

Sex determination is the biological process that decides whether an individual becomes male or female.

The concept of sex chromosomes was first studied by Henking in 1861. He discovered a structure called the X-body, which was later identified as the X chromosome.

Later, Nettie Stevens discovered the Y chromosome.

The chromosomal theory of sex determination was proposed by Edmund Beecher Wilson and Nettie Stevens.

XX-XY System (Lyon Type)

In this system:

• Female = XX

• Male = XY

This type of sex determination is found in:

• Humans

• Mammals

• Drosophila

• Many animals

The Y chromosome carries special genes called holandric genes.

The most important gene present on the Y chromosome is the SRY gene.

The SRY gene is also called:

• Sex Determining Region Y

• Testis Determining Factor (TDF)

This gene plays an important role in male development.

XX-X0 System (Protenor Type)

In this system:

• Female = XX

• Male = X0

The male lacks one sex chromosome.

This system is found in:

• Grasshopper

• Cockroach

• Vallisneria

• Dioscorea

This system is also called the Grasshopper type of sex determination.

ZW-ZZ System

In this system:

• Female = ZW

• Male = ZZ

This system is present in:

• Birds

• Reptiles

• Some fishes

Here, females are heterogametic.

Haplodiploidy System

This system is found in honeybees.

In honeybees:

• Female bees are diploid

• Male drones are haploid

Chromosome number:

• Female = 32

• Male = 16

Male bees develop from unfertilized eggs. This process is called arrhenotokous parthenogenesis.

Polygenic Inheritance

Polygenic inheritance refers to a condition where one character is controlled by multiple genes. It is also called:

• Quantitative inheritance

• Multiple factor inheritance

• Biometric inheritance

Carl Rutter discovered polygenic inheritance in tobacco plants.

Features of Polygenic Inheritance

• Many genes control one trait

• Continuous variation is observed

• Mendel’s laws are not completely followed

• Environmental factors may influence traits

Examples of Polygenic Inheritance

Wheat Kernel Color

Herman Nilsson-Ehle and Edward Murray East explained polygenic inheritance through wheat grain color experiments. The trait is controlled by two pairs of genes.

Human Skin Color

Human skin color is controlled by three pairs of genes. This concept was explained by Charles Davenport.

Formula Used in Polygenic Inheritance

The formula used for calculating phenotypes is:

2n+1

Where:

• n = number of gene pairs

Example:

If n = 2

2(2)+1=

So, five phenotypes are formed.

This formula is important for numerical questions in competitive exams.

Mendel’s Experiments on the Pea Plant

Gregor Mendel performed experiments on garden pea plants (Pisum sativum). He studied seven pairs of contrasting characters. These traits were located on chromosomes:

• 1

• 4

• 5

• 7

Students can remember this using the mnemonic: 1457

One important exam question is: “Which chromosome carries the gene for plant height?”

Answer: Chromosome 4.

Mendel’s Work on Other Plants

Mendel also performed experiments on:

• Hawkweed (Hieracium)

• Lovage (Levisticum)

However, he did not achieve successful results in these plants.

Other important scientists include:

• Carl Correns worked on maize

• Hugo de Vries worked on evening primrose

These facts are important for objective questions.

Post-Mendelian Inheritance

Post-Mendelian inheritance includes exceptions to Mendel’s law of dominance.

The two major exceptions are:

• Incomplete dominance

• Codominance

Incomplete Dominance

Incomplete dominance occurs when neither allele completely dominates the other allele. The offspring shows an intermediate character. It is also called blending inheritance.

Example of Incomplete Dominance

Mirabilis jalapa

In four o’clock plants:

• Red flower × White flower

• Offspring = Pink flower

Phenotypic ratio:
1 : 2 : 1

Other examples:

• Antirrhinum majus

• Andalusian chicken feather color

Codominance

Codominance occurs when both alleles express equally in the offspring. It is also called mosaic inheritance.

Examples of Codominance

• AB blood group in humans

• Coat color in cattle

• Sickle cell trait

In codominance, both traits appear together without blending.

Chromosomal Disorders

Questions related to genetic disorders and chromosome numbers are frequently asked in the UP GIC Lecturer Biology examination.

Huntington’s Disease

Huntington’s disease is linked to the short arm of chromosome 4. It is an autosomal dominant disorder. This topic is important for direct factual questions.

Important Points for UP GIC Lecturer Biology 2026

Students should remember the following points carefully:

• Henking discovered the X chromosome

• Stevens discovered the Y chromosome

• Wilson and Stevens proposed chromosomal sex determination

• SRY gene is present on the Y chromosome

• Honeybee males are haploid

• Polygenic inheritance involves many genes

• Formula for phenotypes = 2n + 1

• Mendel worked on Pisum sativum

• Incomplete dominance shows intermediate characters

• Codominance shows expression of both alleles

• Huntington’s disease is linked to chromosome 4

Preparation Tips for Genetics

Genetics includes both conceptual and factual topics. Regular revision and proper practice can help candidates improve accuracy and confidence for the UP GIC Lecturer Biology 2026 examination. 

• Focus on Diagrams and Tables: Genetics contains many factual concepts. Tables and short notes improve revision speed.

• Practice Objective Questions: Most genetics questions in competitive exams are fact-based. Daily MCQ practice improves accuracy.

• Revise Chromosome Numbers: Questions related to chromosomal disorders are common in teaching exams.

• Learn Scientist Names: Important discoveries and scientist names should be memorized carefully.

Genetics is an important unit for the UP GIC Lecturer Biology 2026 examination. Topics like sex determination, polygenic inheritance, Mendelian genetics, chromosomal disorders, incomplete dominance, and codominance are highly scoring areas.

Candidates should revise important formulas, chromosome numbers, examples, and scientist names regularly. Consistent practice and short note revision can help improve performance in Biology sections of competitive examinations.