Mendel studied seven of their characteristics:<\/p>\n\n\n\n
- The shape of the pod<\/strong><\/li>
- The colour of the pod<\/strong><\/li>
- The shape of the seed<\/strong><\/li>
- The colour of the seed<\/strong><\/li>
- The colour of the flower<\/strong><\/li>
- The position of the pod<\/strong> and the flower<\/strong><\/li>
- The length of the stem<\/strong><\/li><\/ul>\n\n\n\n
Important definitions in Genetics<\/h2>\n<\/span>\n\n\n
Before we get into the details of Mendelian Genetics, let us point out and explain important terms that will be used repetitively throughout this course.<\/p>\n\n\n\n
- Gene<\/strong>: a segment of the DNA that codes for a protein and\/or RNA<\/li>
- Locus<\/strong>: the location of a particular gene on a chromosome<\/li>
- Allele<\/strong>: An allele is a variant of a gene. It can be dominant or recessive<\/li>
- Dominant allele<\/strong>: this is an allele that will be expressed as a dominant trait. A dominant trait is one that is expressed always if it is present<\/li>
- Recessive allele<\/strong>: this is an allele that will be expressed as a recessive trait. A recessive trait will only be expressed in the absence of a dominant trait.<\/li>
- Co dominance<\/strong>: this is an exception to the ‘dominant-recessive allele’ theory. In this case, the two alleles present are both dominant and both of them will be expressed in the cell individually. For example, a person with blood group AB has the genes for both A and B antigens. As they are both dominant genes, both the ‘A’ antigen and ‘B’ antigen will be expressed on the surface of the red blood cell (see illustration below). Details on blood groups are<\/em> discussed in detail in the Polymorphisms: Blood Groups course<\/em><\/li>
- \u00a0Incomplete dominance<\/strong>: this is an exception to the ‘dominant-recessive allele’ theory. In this case, the two alleles are both dominant but unlike codominance<\/strong>, the traits will form a new, intermediate phenotype instead of being expressed individually. For example, if a red flower mates with a white flower and a pink flower is produced. Both the red trait and the white trait are dominant so they will ‘co-operate’ and create a new phenotype that is an intermediate of both of them<\/li>
- Homozygous:<\/strong> having the same allele. Eg TT or tt<\/li>
- Heterozygous<\/strong>: having two different alleles. E.g Tt<\/li>
- Compound heterozygote<\/strong>: if there is the dominant gene (T) and the recessive one(t), it is possible that there are various forms of the recessive one(this is known as polymorphism<\/strong> and is discussed in detail in the Polymorphisms: Introduction course)<\/em> such as t1,t2,t3,t4, etc but they are all recessive. A person having a homozygous genotype such as t1t1 and another with a heterozygous genotype such as t2t4 will have the same phenotype(because they are both recessive alleles). The person with t2t4 is a compound heterozygote<\/strong> because although they are heterozygotes, they are heterozygotes for the recessive gene.<\/li>
- Hemizygote:<\/strong> a hemizygote is a diploid organism that has only one copy of a certain gene. For example, as human males have only one copy of the X and Y chromosome, they are hemizygous for most of the genes on both chromosomes.<\/li>
- Genome<\/strong>: the genome is the total amount of genes (DNA) present in a person<\/li>
- Genotype<\/strong>: this involves the type of genes present in a person. Eg TT<\/li>
- Phenotype<\/strong>: this involves the physical expression of the genotype. It is multi-factorial i.e it is influenced by the genotype and the environment. Eg, a person with homozygous dominant genotype; TT, should be tall(phenotype) but if they are malnourished, they will not be tall<\/li>
- Monohybrid cross<\/strong>: this is a cross where the organisms being crossed(male and female) differ in only one phenotypic trait. E.g Mendel crossed plants with round seeds with those with wrinkled seeds, everything else was the same<\/li>
- Dihybrid cross<\/strong>: this is a cross where the organisms being crossed(male and female) differ in two phenotypic traits. E.g Mendel crossed plants with Round and Yellow seeds with those with Wrinkled and Green seeds. Everything else was the same<\/li>
- True-breeding<\/strong>: this is when parents produce offspring exactly like themselves by mating with each other<\/li>
- Test-cross<\/strong>: this is when a dominant phenotype organism( Homozygous dominant or heterozygous) is crossed with a homozygous recessive organism<\/li>
- Backcross<\/strong>: this is when the offspring is crossed with the parent. This is different from true-breeding in the sense that in true-breeding, parents and offspring have the same genotype and phenotype but in backcross, it can be the same or different<\/li>
- Homoplasmy:<\/strong> this is when a person has only one type of mitochondrial DNA present in their mitochondria<\/li>
- Heteroplasmy:\u00a0<\/strong>this is when a person has more than one type of mitochondrial DNA present in their mitochondria<\/li>
- Linkage disequilibrium<\/strong>: this is when the probability of a certain gene or group of genes to be expressed is different from the theoretical number if the genes were inherited independently. Let us take for example the probability of gene A to be expressed in the population is o.3% and the probability of gene B is 0.1%, the probability of AB to be expressed should be o.o3%. If this is more or less for any reason e.g linked genes, this is known as linkage disequilibrium<\/strong>.<\/li>
- Loss Of Heterozygosity:<\/strong> as the word says, it is when a heterozygous individual loses one of his\/her chromosomes due to epigenetics silencing, mutations, somatic recombination and loss of chromosome.<\/li>
- Penetrance: <\/strong>this is how much a certain genotype is expressed in the phenotype of individuals who carry that genotype.\u00a0<\/strong>It is expressed in percentage and can also be described as the chance of the genotype to be expressed phenotypically.<\/li>
- Expressivity:\u00a0<\/strong>this is a measure of the degree of expression of the phenotype.<\/li><\/ol>\n\n\n\n
- Locus<\/strong>: the location of a particular gene on a chromosome<\/li>
- The colour of the pod<\/strong><\/li>
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