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Figure 3 illustrates the structure of a chromosome. Genes and alleles
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Genes consist of lengths of DNA that control a particular characteristic, such as fur colour or seed shape. Genes are found on chromosomes. The position of a gene on a chromosome is called a locus .
Different forms of a gene are called alleles . Alleles carry different information about the same characteristic. For example, if the characteristic is fur colour, there may be an allele for brown fur colour and an allele for white fur colour.
centromere chromosome
coiled DNA
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histone
Figure 3 Structure of a chromosome
Organisms have two alleles for each characteristic. Because of meiosis, each gamete (sperm and ovum) will carry only one allele for each characteristic. The gametes combine during fertilisation and each gamete contributes its alleles to the offspring. Therefore, the offspring will carry two alleles for each characteristic – one inherited from the mother and the other inherited from the father.
Homozygous and heterozygous RESOURCES Dictionaries see pages 100–101
If a gene and its allele are identical for a characteristic, then the individual is said to be homozygous for that characteristic. For example, both genes code for brown fur.
If a gene and its allele are non-identical for a characteristic, then the individual is said to be heterozygous for that characteristic. For example, one gene codes for brown fur and the other for white fur.
A homozygous individual is said to be pure-bred for a characteristic. dual is said to be a
dual is said to be pure-bred for a characteristic.
A heterozygous individual is said to be a hybrid for that characteristic, that is, of two non-identical alleles for the characteristic.
it has a combination of two non-identical alleles for the characteristic. ssive genes
Dominant and reces
In heterozygous individuals, one allele of a pair of genes is often dominant over the other.
The dominant allele blocks the action of the other allele (called the recessive allele ).
ele blocks the action of the other allele (called the
Although there may be two different alleles present for a characteristic, only the dominant characteristic is physically expressed. For example, if the allele for brown fur is dominant over the allele for white fur, a heterozygous
be two different alleles present for a characteristic, characteristic is physically expressed. For example, if the s dominant over the allele for white fur
viduals, one allele of a pair of genes is often dominant
Know the difference
the
The prefix homo- means the same.
The prefix hetero- means different.
for that characteristic, that is,
Figure 4 This mare and her foal obviously share the same genes .
Website
Helpful animations on various genetic principles: www. dnaftb. org
New word New word
hybrid: another term for heterozygous
another term for
Unit 1 Genetic concepts
235 2013/06/25 4:53 PM
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individual would have brown fur. The individual would have to inherit two alleles for white fur in order for it to physically have white fur.
Genotype and phenotype The genetic make-up of an organism is called its genotype. Note
The dominant allele is always placed first.
The physical appearance of an organism due to its genetic make-up or genotype is called the phenotype.
Table 1 Representing genotype and interpreting phenotype
Genotype Homozygous/ heterozygous
TT Tt tt
Phenotype
Homozygous Tall Heterozygous Tall Homozygous Dwarf
We use letters of the alphabet to show an individual’s genotype for a particular gene. Alleles of a gene are represented by capital and small letters. The first letter of the dominant trait is often, but not always, chosen as the symbol, for example, T for tallness.
The allele for the dominant trait is represented by a capital letter, in other words, T for tall plant.
The allele for the recessive trait is represented by a small letter, in other words, t for dwarf plant.
Using T for tall and t for dwarf, Table 1 shows how we can interpret genotype codes into phenotypes.
Activity 1 Select genetic terms
Genetic terms genotype
heterozygous dominant gene alleles
phenotype
From the box on the left, select the correct genetic term that matches each of the following descriptions: 1 A plant with one gene for yellow seeds and one gene for green seeds 2 Different forms of the genes for a trait such as seed colour 3
4 5
The type of gene that makes a plant with one gene for yellow seeds and one gene for green seeds produce only green seeds The genetic make-up of an individual The physical appearance of an individual.
Activity 2 Determine genotypes and breeding possibilities
Black- headed ram
White- headed ewe
A farmer has a ram with a black head that she uses for breeding. In her stud, black heads (B) are dominant over white heads (b). A white-headed ewe mates twice with the same black-headed ram. She produces a white-headed lamb on the first occasion and a black-headed lamb on the second occasion. The diagram on the left shows the different possibilities of the offspring. 1 Use the letters B and b to write down the genotypes of the following: 1.1 White-headed ewe 1.2 Black-headed ram
1.3 White-headed lamb 1.4 Black-headed lamb.
White- headed lamb
Black- headed lamb
2 Give the percentage of the ewe’s offspring that have the heterozygotic characteristic for black coat.
236 Module 5 Basic agricultural genetics OS_AS_Gr12_LB_Module5 (Eng).indb 236
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MODULE 5
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