(3.1) Introduction to genetics and inheritance of characteristics and technical terms explained

Reminders: A chromosome as a thread-like structure of DNA, carrying genetic information in the form of genes.

A gene is a length of DNA that codes for a protein.

An allele as a version of a gene.

Reminder that in the biological science of genetics, inheritance is the transmission of genetic information from one generation to the next generation by chromosomes of DNA, the traits passed on are usually desirable, but not all traits passed on are desirable.

 

This section will help you answer questions such as ...  What is the study of genetics?  How are characteristics inherited?   What is dominant gene? What is a recessive gene?   What have alleles got to do with inheritance?  What do the terms homozygous and heterozygous mean? How to explain the terms genotype and phenotype? What do we mean by gene expression?

 

Summary of some definitions for genetics

All body cells in an organism contain the same genes, but many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs to fulfil its specific function.

A haploid nucleus is a nucleus containing a single set of unpaired chromosomes, e.g. in gametes (sex cells).

A diploid nucleus is a nucleus containing two sets of chromosomes, e.g. in human body cells, which contain a pair of each type of chromosome, so the human diploid cell has 23 pairs of chromosomes.

Genetics is the study of heredity and the variation of inherited characteristics.

Genes, sections of DNA, are the means by which characteristics are passed on from one generation to the next in both plants and animals.

In other words, the genes you inherit from your parents control the characteristics (phenotypes) you develop. You can use simple genetic diagrams can be used to show this (see Basic Genetics Part 2).

A single gene can code for a single characteristic, but quite often several genes are responsible for a characteristic of an organism - it can get very complex!

Gametes (sex cells) only have one allele per gene, but all the other cells in an organism have two alleles per gene.

Our knowledge of genetics enables us to treat certain medical conditions but there are ethical considerations in treating genetic disorders.

A gene is a shorter section of the huge DNA coiled up molecules that make up chromosomes.

Genes exist in alternative forms called alleles which give rise to differences in inherited characteristics.

Particular genes control specific characteristics e.g. most characteristics are controlled by the coordination (interaction) of several genes but some are controlled by one gene e.g. fur colour of mice, red-green colour blindness in humans.

 

In sexual reproduction, the parents (mother and father) produce gametes (egg and sperm reproductive cells).

Each gamete only has one copy of each chromosome, unlike pairs of chromosomes in all other cells.

Therefore the gametes have only one version of each gene, i.e. one allele per gene.

This is because we inherit half of our genes from our mother and the other half from our father.

In producing offspring from fertilisation, the chromosomes from a male gamete (sperm) mix with the chromosomes from the female gamete (egg) to produce the full compliment of pairs of chromosomes - two alleles for each gene.

 

Alleles are essentially two versions of the same gene.

Usually you have two copies of the same gene (two alleles), one from each parent.

Therefore eg in humans, between the two copies of the chromosomes you can have two alleles the same (homozygous) or different (heterozygous) for a particular gene.

Individual alleles can be 'dominant' or 'recessive' in character and are represented in genetic diagrams or charts by upper case letters e.g. D for a dominant gene or a lower case letter e.g. d for a recessive gene.

Remember alleles are versions of the same gene and are represented by single letters in genetic diagrams.

 

Humans have two alleles, different versions, of every gene in the chromosomes of your body.

If you have two alleles for a particular gene that are the same e.g. DD or dd, then it is homozygous for that characteristic trait.

If two alleles for a specific gene are different, then they are heterozygous for that characteristic trait e.g. Dd.

This means you have instructions for two different versions of a characteristic trait, but you will only display one version of the two (only one of the two possible phenotypes).

As we have said, if the two alleles for a gene are different (heterozygous e.g. Dd), only one can determine the characteristic trait. The allele for that characteristic phenotype observed (gene expression) is called the dominant allele (denoted by a capital letter - upper case e.g. D).

The other allele (denoted by a small letter - lower case) is described as a recessive allele e.g. d.

Note that D overrides d, i.e. a dominant allele overrides a recessive allele in all heterozygous organisms.

So, a pair of homozygous alleles e.g. DD, or heterozygous alleles Dd, will both produce the dominant gene trait, BUT, a  pair of homozygous recessive alleles e.g. dd, will produce the recessive gene trait.

In order to display a characteristic caused by a recessive allele, both alleles must be recessive e.g. dd.

So DD or Dd allele pairs lead to a dominant phenotype and a dd allele pair produces the recessive phenotype.

In total, your genotype is a combination of all the genes-alleles you have in your chromosomes.

In your body's biochemistry, your alleles are functioning at a molecular level (DNA/RNA) to determine the characteristics you display - described as phenotypes - the results of your gene-allele expressions, which can be either dominant or recessive.

Many characteristics are controlled by a single gene, known as single gene inheritance.

 

Summary of some important terms to know the meaning of, and use appropriately in the correct context.

genotype - a 'bit of genetic code' pairs of or individual alleles eg XX, XY, X, Y (and it is the genotype pairs that give rise to the phenotype you observe in the organism.

Watch out for the different allele genotypes in parents e.g. Dd, but in gametes this becomes D  and d, (separated alleles), this is rather important when working out the genotypes, and hence phenotypes, of offspring.

 

dominant (D) - if two alleles for a characteristic are different (heterozygous) then only one of the alleles can determine the nature of the characteristic - know as the dominant allele (usually shown as a capital/upper case letter) eg a gene for height might be H, so HH or Hh genotypes will give a tall organism. A dominant allele will override a recessive allele.

 

recessive (d) - if an allele is not dominant, it is described as recessive (small/lower case letter), and, in order for the recessive allele to be expressed in the phenotype observed.

You must have a double recessive allele eg homozygous genotype hh will give rise to a recessive phenotype.

 

homozygous - if a pair alleles for a characteristic are the same on a gene eg genotype XX for phenotype female.

Homozygous alleles can be dominant or recessive e.g. DD or dd.

 

heterozygous - if a pair of alleles for a characteristic are different on a gene eg genotype XY for phenotype male.

These are typically denoted in genetics using upper case (dominant) and lower case (recessive) letters e.g. Aa, Dd or Pp.

 

phenotype - the result of 'gene expression' - the nature of the characteristic you see eg tall, blue eyes, male etc.

 

gene expression - the process from the genotypes to the observed phenotypes - the genetic results!

 

gamete cells are sex cells (gametes).

 

You need to be able to analyse and interpret patterns of monohybrid inheritance using a genetic outcome diagram, Punnett squares and family trees and family pedigrees

and be able to calculate and analyse outcomes (using probabilities, ratios and percentages) from monohybrid crosses.

Example of a family tree

 

Parent genotypes: PP x pp
Gametes: P, P, p and p (alleles)
Genotypes of plants - gametes - alleles	P	P
p	Pp	Pp
p	Pp	Pp

 Example of a Punnett square

 

Example of a genetic diagram - contains the same information as a Punnett square

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Summary of learning objectives and key words or phrases

Introduction to genetics and inheritance of characteristics technical terms explained.

Know the meaning of the keywords and phrases: genetics. inheritance of characteristics. dominant genes, recessive genes, alleles, homozygous genotype, heterozygous genotype, phenotype, gene expression, monohybrid, genetic diagram, family tree, family pedigree, and Punnett square

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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points of an introduction to the genetics of inheritance

Introduction to the Genetics of Inheritance

Inheritance is the process by which genetic information is passed from parents to offspring.

Understanding genetics is crucial in studying heredity, variation, and evolution.

The principles of inheritance explain how traits are transmitted and how genetic disorders arise.

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Key Concepts in Inheritance

1. Genes and Alleles

• A gene is a segment of DNA that codes for a specific trait or protein.

• Alleles are different versions of a gene. They can be dominant or recessive, affecting how traits appear.

2. Chromosomes and DNA

• Humans have 46 chromosomes (23 pairs), with one set inherited from each parent.

• Chromosomes contain DNA, which carries genetic instructions for development and function.

3. Genotype and Phenotype

• Genotype: The genetic makeup of an organism, defined by the alleles present (e.g., BB, Bb, or bb).

• Phenotype: The observable characteristics, influenced by the genotype and environment (e.g., eye color).

4. Dominant and Recessive Inheritance

• Dominant Alleles express traits when present (e.g., Huntington’s disease).

• Recessive Alleles require both copies to be expressed (e.g., cystic fibrosis).

5. Homozygous and Heterozygous

• Homozygous: An individual has two identical alleles (BB or bb).

• Heterozygous: An individual has two different alleles (Bb).

6. Punnett Squares

A Punnett Square predicts the probability of inheriting certain traits by showing possible allele combinations.

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Importance of Studying Human Genetics

• Understanding Genetic Disorders: Helps in diagnosing and treating inherited diseases such as sickle cell anemia and cystic fibrosis.

• Medical Advancements: Supports genetic research in personalized medicine, gene therapy, and cancer treatments.

• Evolutionary Studies: Explains how species adapt and how genetic variation contributes to biodiversity.

• Selective Breeding & Biotechnology: Used in agriculture and medicine to improve traits and develop treatments.

Genetics plays a vital role in biology and medicine, shaping our understanding of human health and evolution.

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