Gr 11 Genes and Genetics

Back to Inheritance

A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic
A gene occupies a specific position on a chromosome
The various specific forms of a gene are alleles
Alleles differ from each other by one or only a few bases
New alleles are formed by mutation
The genome is the whole of the genetic information of an organism
The entire base sequence of human genes was sequenced in the Human Genome Project

DNA is the genetic blueprint which codes for, and determines, the characteristics of an organism

This includes the physical, behavioural and physiological features of the organism

DNA is packaged and organised into discrete structures called chromosomes

A gene is a sequence of DNA that encodes for a specific trait (traits may also be influenced by multiple genes)
The position of a gene on a particular chromosome is called the locus (plural = loci)

• The various specific forms of a gene are alleles

• Alleles differ from each other by one or only a few bases

A gene is a length of DNA that codes for the production of a polypeptide molecule. The code is held in the sequence of nucleotide bases in the DNA. A triplet of three bases codes for one amino acid in the polypeptide that will be constructed on the ribosomes in the cell. One chromosome contains enough DNA to code for many polypeptides.

Alleles

The gene that codes for the production of the β-globin polypeptide of the haemoglobin molecule (pages 42–43) is on chromosome 11. Each cell contains two copies of this gene, one maternal in origin (from the mother) and one paternal (from the father).

There are several forms or varieties of this gene. One variety contains the base sequence GGACTTCTC and codes for the normal β-globin polypeptide. Another variety contains the base sequence GGACATCTC and codes for a different sequence of amino acids that forms a variant of the β-globin polypeptide known as the sickle cell β-globin polypeptide. Different varieties of the same gene are called alleles.

A genotype in which the two alleles of a gene are the same – for example, HbAHbA – is said to be homozygous for that particular gene. A genotype in which the two alleles of a gene are different – for example, HbAHbS – is said to be heterozygous for that gene. The organism can also be described as homozygous or heterozygous for that characteristic.

Genotype

Most genes, including the β-globin polypeptide gene, have several different alleles. For the moment, we will consider only the above two alleles of this gene. For simplicity, the different alleles of a gene can be represented by symbols. In this case, they can be represented as follows:

HbA = the allele for the normal β-globin polypeptide

HbS = the allele for the sickle cell β-globin polypeptide

The letters Hb stand for the locus of the haemoglobin gene, whereas the superscripts A and S stand for particular alleles of the gene.

In a human cell, which is diploid, there are two copies of the β-globin polypeptide gene. The two copies might be:

HbA HbA or

HbS HbS or

HbA HbS.

The alleles that an organism has form its genotype In this case, where we are considering just two different alleles, there are three possible genotypes.

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