Page 12 - CB1.3 Genetic Varation sciPAD

PHOTOCOPYING PROHIBITED
© Silverback Academic Media
12 CB 1.3 GENETIC VARIATION
PRINCIPLES OF GENETIC VARIATION
Genes and Alleles
The base sequence of DNA carries the instructions that make proteins. Proteins are the ‘building
blocks’ for all organisms and carry out a number of important biochemical functions. A section of
DNA that carries the code for making a specific protein is called a gene. A single gene may be
thousands of base pairs long.
Each pair of homologous chromosomes carries the same, but not necessarily identical, genes.
For example, both chromosomes of a particular homologous pair might contain the gene for eye
colour, but one can be a ‘brown-eyed’ version of the gene and the other can be ‘blue-eyed’.
Alternative versions of genes are called ‘alleles’ (pronounced ‘el-eels’).
1. Distinguish between:
(a) ‘DNA’ and a ‘gene’. 




(b) a ‘gene’ and an ‘allele’. 




2. Explain why:
(a) alleles occur in pairs. 


(b) someone could have two different alleles for the same gene. 



PAIR 23
PAIR 22
PAIR 21
PAIR 16 PAIR 17 PAIR 18 PAIR 19 PAIR 20
PAIR 11 PAIR 12 PAIR 13 PAIR 14 PAIR 15
PAIR 6 PAIR 7 PAIR 8 PAIR 9 PAIR 10
PAIR 1 PAIR 2 PAIR 3 PAIR 4 PAIR 5
Because you inherit half of your genetic information from your
mother, and the other half from your father, you may have
different forms of a gene (called ‘alleles’).
Homologous Pair 15
An eye colour gene
from your mother, e.g.
the brown-eyed allele.
An eye colour gene
from your father, e.g.
the blue-eyed allele.
Alleles have a slightly different order of bases, so the protein,
and therefore the trait (characteristic), they produce is different.
46 Human Chromosomes
Unchecked Sample Pages
For Review Purposes Only - Photocopying Prohibited
www.scipad.co.nz