Knowledge-21

• Genotypes: is the genetic makeup of an organism. It describes all the alleles an organism possesses.
• The phenotype is the expression of the genotype and its interaction with the environment as identifiable characteristics.
• A gene is a section of DNA that determines the amino acid sequence in a polypeptide. genes are located at a specific loci on chromosomes.
• Genes occur in many forms known as alleles.
• the individuals in a species share the same gene but different combinations of alleles of these genes. An organism inherits alleles from its parents.
• In a diploid organism exist in homologous pairs. So at a particular locus each chromosome of the pair may carry the same or different alleles for that gene.
• if the alleles at a specific loci are the same, the organism is homozygous for that gene.
• if alleles at a specific loci are different then the organism is heterozygous for that gene.
• An allele that is always expressed is called dominant.
• An allele is recessive if it is only expressed when the phenotype is homozygous.
• If both alleles are expressed at a locus then they are co-dominant.
• filial 1 (f1) - is the first generation of a genetic cross.
• Monohybrid crosses investigated the inheritance of a single characteristic.
• Dihybrid crosses investigated the inheritance of two separate characteristics at the same time.
• The chi-squared test is used to compare observed results from genetic crosses with the observed outcomes.

  • chi-squared = sum of ( (observed -expected)^2 / expected )
• The first generation produced from a cross of two heterozygous plants:

  • 9 : 3 : 3 : 1
• Sex linkage occurs when the genes are on the sex chromosomes , X and Y.
• males only have one allele which will always be expressed in the phenotype.
• Chromosomes other than sex chromosomes are known as autosomes.
• Autosomal linkage of genes occurs when two genes are on the same chromosome.
• Autosomal linkage of genes occur when two genes are on the same chromosome.
• As the genes are on the same chromosome , independent assortment of homologous chromosomes in meiosis 1 cannot separate them.
• The two linked genes are thus passed into the gamete together and are inherited together. This reduces the possible number of genotypes in the offspring.
• Genotypic variation can only occur through crossing over.
• the closer together the loci are on the chromosome , the lower the chance of a chiasma forming between them and separating them.
• there are cases where genes at one locus interact with genes at another locus by masking or suppressing their expression. This is called epistasis.