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Leaving Cert Biology Revision: Introduction to Genetics

Introduction to Genetics

Genetics is the scientific study of genes, heredity, and the variation in living organisms. It is a fundamental branch of biology that helps us understand how traits and characteristics are passed down from generation to generation.

Genes and DNA

Genes are segments of DNA (deoxyribonucleic acid) that encode specific proteins, which determine an organism's physical and biochemical traits. DNA is the hereditary material in humans and almost all other organisms, carrying the instructions needed for an organism to grow, develop, survive, and reproduce.


Heredity is the process by which traits are passed from parents to their offspring. This transfer of genetic information ensures that offspring inherit characteristics from both parents, contributing to the genetic diversity within a species.


Variation refers to the differences in genetic makeup among individuals within a population. This genetic diversity is crucial for the survival and adaptation of species, as it allows populations to adapt to changing environments and resist diseases.

Variation Within a Species

Variation within a species refers to the differences in physical and genetic traits among individuals of the same species. These differences can be caused by a variety of factors, including genetic mutations, recombination of genes during sexual reproduction, and environmental influences. Variation is crucial for the survival and adaptability of a species, as it allows populations to respond to changing environments and resist diseases. 

For example, in a population of beetles, some may have green shells while others have brown shells. If the environment changes, such as the ground becoming darker, brown-shelled beetles might survive better, leading to natural selection. 

Overall, variation is the foundation of evolution and biodiversity.

Linkage - Genetics 3

Linkage refers to the tendency of genes that are close together on a chromosome to be inherited together.

This concept helps explain why some traits do not follow Mendel's Law of Independent Assortment.

Key Concepts

Chromosomes and Genes

Genes are located on chromosomes. Each chromosome carries many genes.

Linked Genes

Genes that are located close to each other on the same chromosome are called linked genes.

Discovery of Linkage

Observed by Thomas Hunt Morgan through his experiments with fruit flies.

He noticed that some traits were often inherited together, suggesting they were linked.

Crossing Over


During meiosis, homologous chromosomes can exchange pieces in a process called crossing over.

Effect on Linkage

 Crossing over can separate linked genes, leading to new combinations of traits.

Genetic Maps


Show the relative positions of genes on a chromosome.

Recombination Frequency

The frequency of crossing over between genes indicates their distance from each other.


- Two genes, A and B, are linked on the same chromosome.

- If crossing over occurs frequently between A and B, they are far apart. If it occurs rarely, they are close together.

Sex Determination - Genetics 4

  • Sex determination is the process by which an organism develops into a male or female.

  • Different species use different mechanisms to determine sex.

Sex Chromosomes

Chromosomes that determine the sex of an organism.


There are two types of sex chromosomes, X and Y.

  - Females: XX

  - Males: XY

Mechanisms of Sex Determination

XY System

  • Used by humans and most mammals.

  • The presence of Y chromosome determines the male sex.

  • Absence of Y (XX) results in female sex.

ZW System

  • Used by birds and some reptiles.

  • Females are ZW, males are ZZ.

Haplo-Diploid System

Used by bees, ants, and wasps.

  - Females develop from fertilized eggs (diploid).

  - Males develop from unfertilized eggs (haploid).

SRY Gene

- Location: Found on the Y chromosome.

- Function: Triggers development of male characteristics.

- Absence: Leads to female development.

Environmental Influence

- Temperature-Dependent: In some reptiles, sex is determined by the temperature at which eggs are incubated.

  - Warmer temperatures might produce females, cooler temperatures males, or vice versa.

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