Genetics and Inheritance

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INTRODUCTION ( key terms & notes )

1. A GENE is a sequence of bases on DNA that codes for one polypeptide.

2. ALLELES ARE ALTERNATIVE VERSIONS OF THE SAME GENE, which are positioned at the same place, or LOCUS on homologous chromosomes.

Individual cells only have 2 alleles for each gene, one on each of the homologous chromosomes.

MAY BE IDENTICAL ALLELES (HOMOZYGOUS) OR DIFFERENT (HETEROZYGOUS).

* Alleles for characteristic are the same, organism is HOMOZYGOUS for that characteristic. Organism is HOMOZYGOTE.

Alleles for a characteristic are different, organism is HETEROZYGOUS for that characteristic. Organism is a HETEROZYGOTE.

If allele is DOMINANT = it is expressed in the organisms phenotype, even if organism is heterozygous

If allele is RECESSIVE= it is only expressed in the phenotype when the organism in homozygous.*

3. GENOTYPE = Genetic constitution of organism , is determined by the combination of alleles that organism inherits form it's parent.

4. Expression of genotype and its interaction with environment = PHENOTYPE . Those used in A-level genetic questions have NO ENVIROMENTAL INPUT

5. Genetic problems try to predict which phenotypes will appear in offspring by carrying out PLANNED BREEDING EXPERIMENTS .

-----> Male & Female parents are chosen -usually pure