3.2.1 Key Definitions · 3. Genetics | Edexcel GCSE Combined Science 1SC0: Biology

Section 1 of 9Genes, alleles and chromosomes

Homologous Chromosomes — Step-by-Step Simulation

Homologous Chromosome Pair — Step-by-Step Mechanism

DNA, chromosome, gene and cell relationship

Diploid chromosome pairs: homozygous and heterozygous alleles

From DNA to chromosome — interactive teaching console

A chromosome is a long molecule of DNA, packaged with proteins, found in the nucleus.

Gene

A gene is a short section of DNA that codes for a particular protein and so influences a characteristic.

Humans have 23 pairs of chromosomes, one of each pair from each parent, so each person has two copies of most genes.

Alleles

Different versions of the same gene are called alleles.

For example, a gene for pea height has an allele for tall and an allele for short.

Alleles are found at the same position, or locus, on homologous chromosomes.

The set of all the genetic material is the genome, and many genes interact with the environment to produce the final features of the organism.

Section 2 of 9Genotype and phenotype

ABO Blood Groups: Genotype versus Phenotype

The genotype is the combination of alleles an individual has for a gene, written as letters.

Genotype and phenotype

Phenotype

The phenotype is the observable characteristic that results, such as tall stems or blood group.

The phenotype is produced by the genotype together with the environment.

Two plants with the same genotype for height may grow to different heights if one has less light or water.

Section 3 of 9Dominant and recessive

Genetics — TT × tt — One-Plate Animated Infographic

A dominant allele produces its feature whether there are one or two copies, and is written as a capital letter (T).

Recessive

A recessive allele only produces its feature when two copies are present and no dominant allele is present,

and is written as a lower-case version of the same letter (t).

Dominant does not mean 'more common' or 'better'; it only describes how an allele is expressed.

Some dominant alleles cause disease (Huntington's), and some recessive alleles are very common in populations.

Section 4 of 9Homozygous and heterozygous

Genes, Alleles, and Homologous Chromosomes

Homologous chromosome pairs: homozygous and heterozygous

Term	Meaning	Example	Phenotype (T = tall dominant)
Homozygous dominant	Two identical dominant alleles	TT	Tall
Homozygous recessive	Two identical recessive alleles	tt	Short
Heterozygous	Two different alleles	Tt	Tall (carries t)

A heterozygous person who has a recessive allele for a disease but does not show it is called a carrier if the allele would cause the disease in homozygous form (cystic fibrosis, for example).

Section 5 of 9Using the terms accurately

Mistakes with these terms lose marks.

Alleles are not the same as genes: a gene is a section of DNA, and alleles are the forms of the gene.

Genotype describes the alleles, while phenotype describes what you can see or measure.

An individual with the phenotype 'tall' could have the genotype TT or Tt, and the genotype cannot be known from appearance alone.

Test cross

A test cross can determine it: breeding with a homozygous recessive (tt) individual reveals the unknown genotype.

If any short offspring appear, the unknown parent was Tt; if all are tall, it was probably TT.

Gametes carry only one allele for each gene because the two chromosomes of a pair separate in meiosis. A Tt parent therefore produces two types of gamete, T and t, in equal numbers. These ideas are used in the next sub-topic to predict the results of crosses.

Section 6 of 9Worked ideas to practise the vocabulary

Take a gene for flower colour in which R is red and r is white, with R dominant.

1

A plant with genotype Rr is heterozygous and has the phenotype red.

2

A white plant must have the genotype rr, since the only way a recessive feature appears is with two recessive alleles.

3

Its gametes all carry r.

4

A red plant could be RR or Rr, so it may produce gametes of one type (R) or two types (R and r).

If a question gives the phenotypes of parents and offspring, work backwards.

Worked ideas to practise the vocabulary

Two red parents who have a white offspring must both be Rr: the white offspring got an r from each parent.

Organisms of one species have the same genes in the same places, but they may have different alleles, and this is the reason for variation.

When explaining, use precise vocabulary: do not say the gene is dominant when you mean the allele is dominant, as examiners will penalise this.

As a last check on vocabulary, note that a person with cystic fibrosis is homozygous recessive, a carrier is heterozygous, and a person without the allele is homozygous dominant. All three have different genotypes, but the carrier and the homozygous dominant person have the same phenotype.

Section 7 of 9Exam-style questions

Explain the difference between a gene and an allele. [2]2 marks
A plant has the genotype Rr, where R (round) is dominant to r (wrinkled). State its phenotype and say whether it is homozygous or heterozygous. [2]2 marks
A gardener has a tall plant of unknown genotype. Describe how a test cross could show whether it is TT or Tt. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Gene	A section of DNA that codes for a protein.
Allele	A version of a gene.
Genotype	The combination of alleles an organism has.
Phenotype	The observable characteristics of an organism.
Dominant allele	An allele expressed when present in one or two copies.
Recessive allele	An allele expressed only when two copies are present.
Homozygous	Having two identical alleles of a gene.
Heterozygous	Having two different alleles of a gene.
Carrier	A heterozygous individual with a recessive allele that is not expressed.

Section 9 of 9Exam technique

1

Using gene and allele as if they mean the same thing.

2

Saying a heterozygous individual shows both features (that is codominance, not complete dominance).

3

Writing alleles as different letters rather than upper and lower case of the same letter.

4

Believing dominant alleles are always the most common.

5

Mixing up genotype (alleles) and phenotype (appearance).