6.2.1 Key Terms in Genetics Flashcards
What is a gene?
The DNA contained within chromosomes is essential for cell survival
Every chromosome consists of a long DNA molecule which codes for several different proteins
A length of DNA that codes for a single polypeptide or protein is called a gene
What is a locus?
The position of a gene on a chromosome is its locus (plural: loci)
What is an allele?
Each gene can exist in two or more different forms called alleles
Different alleles of a gene have slightly different nucleotide sequences but they still occupy the same position (locus) on the chromosome
Examples of alleles:
One of the genes for coat colour in horses is Agouti
This gene for coat colour is found on the same position on the same chromosome for all horses
Hypothetically there are two different forms (alleles) of that gene found in horses: A and a
Each allele can produce a different coat colour:
Allele A → black coat
Allele a → chestnut coat
What are homologous pairs?
The chromosomes of eukaryotic cells occur in homologous pairs (there are two copies of each chromosome)
As a result cells have two copies of every gene
As there are two copies of a gene present in an individual that means there can be different allele combinations within an individual
What is a genotype?
The genotype of an organism refers to the alleles of a gene possessed by that individual. The different alleles can be represented by letters
When the two allele copies are identical in an individual they are said to be homozygous
When the two allele copies are different in an individual they are said to be heterozygous
The genotype of an individual affects their phenotype
What is the phenotype?
A phenotype is the observable characteristics of an organism
Example of genotype and phenotype:
Every horse has two copies of the coat colour gene in all of their cells
A horse that has two black coat alleles A has the genotype AA and is homozygous. The phenotype of this horse would be a black coat
In contrast a horse that has one black coat allele A and one chestnut coat allele a would have the genotype Aa and is heterozygous
What is dominance?
Not all alleles affect the phenotype in the same way
Some alleles are dominant: they are always expressed in the phenotype
This means they are expressed in both heterozygous and homozygous individuals
What are recessive alleles?
Others are recessive: they are only expressed in the phenotype if no dominant allele is present
This means that it is only expressed when present in a homozygous individual
Example of dominance:
If for horses the allele A for a black coat is dominant and the allele a for a chestnut coat is recessive the following genotypes and phenotypes occur:
Genotype AA → black coat
Genotype Aa → black coat
Genotype aa → chestnut coat
What is codominance?
Sometimes both alleles can be expressed in the phenotype at the same time
This is known as codominance
When an individual is heterozygous they will express both alleles in their phenotype
When writing the genotype for codominance the gene is symbolised as the capital letter and the alleles are represented by different superscript letters, for example IA
Example of codominance:
A good example of codominance can be seen in human blood types
The gene for blood types is represented in the genotype by I and the three alleles for human blood types are represented by A, B and O
Allele A results in blood type A (IAIA or IAIO) and allele B results in blood type B (IBIB or IBIO)
If both allele A and allele B are present in a heterozygous individual they will have blood type AB (IAIB)
Blood type O (IOIO) is recessive to both group A and group B alleles
What are F1. F2 and test crosses?
When a homozygous dominant individual is crossed with a homozygous recessive individual the offspring are called the F1 generation
All of the F1 generation are heterozygous
If two individuals from the F1 generation are then crossed, the offspring they produce are called the F2 generation
A test cross can be used to try and deduce the genotype of an unknown individual that is expressing a dominant phenotype
The individual in question is crossed with an individual that is expressing the recessive phenotype
The resulting phenotypes of the offspring provide sufficient information to suggest the genotype of the unknown individual
If there are any offspring expressing the recessive phenotype then the unknown individual must have a heterozygous genotype
What is sex linkage?
There are two sex chromosomes: X and Y
Women have two copies of the X chromosome (XX) whereas men have one X chromosome and one shorter Y chromosome (XY)
Some genes are found on a region of a sex chromosome that is not present on the other sex chromosome
As the inheritance of these genes is dependent on the sex of the individual they are called sex-linked genes
Most often sex-linked genes are found on the longer X chromosome
Haemophilia is well known example of a sex-linked disease
Sex-linked genes are represented in the genotype by writing the alleles as superscript next to the sex chromosome. For example a particular gene that is found only on the X chromosome has two alleles G and g. The genotype of a heterozygous female would be written as XGXg. A males genotype would be written as XGY
