in this video we are going to talk about the concepts of genes alleles genotypes and phenotypes so let's say we have a deployed individual and that means this individual has two of each type of chromosome one from the father and the other one from the mother now a gene is just a section of this chromosome now the the the section of the chromosome represents some sequence of dna so if we take this gene there will be some sequence of dna that corresponds to the gene and let's recall here the central dogma which says that this dna is transcribed into rna and the rna is translated into protein and this protein determines phenotypes phenotypes are observable features of organisms such as the color of peas green or yellow in mental experiments or their texture wrinkled or smooth or um the coat color of dogs from gray to black to white to brown or whether their tail groups are not or human height for example anything that's observable about an organism is a phenotype to summarize the diploid individual has two chromosomes of each type the chromosome is a long dna molecule and stretches of that dna molecule are genes which are transcribed or read into rna and that rna is translated into protein which determine the phenotypes or observable features of these deployed individuals next let's discuss what alleles are let's say that mice can have two coat colors black and white and so these are your phenotypes black and white there are two versions or alleles of the code color code color gene b1 and b2 i will represent b1 by the red color and so there is a chromosome red in color which has this gene b1 or this allele rather the version of this gene and i'm going to represent the other version or allele of this gene with b2 so there is this different version of the chromosome that carries the b2 allele so what's different about them well what's different about them is the dna sequence so perhaps the b1 allele is ag tta cc while the b2 allele is eg a t acc and so the dna sequence of the stretch of the chromosome that corresponds to this the gene is different and that gives rise to different versions or alleles of the genes and one allele is b1 the other allele is b2 so now what is the genotype of these two individuals well the first individual has both b1 alleles and so we can write that the genotype is b1 b1 or b1 forward slash b1 and the other individual has both b2 alleles and we can say that the genotype is b2 b2 or b2 forward slash b2 i would like you to get used to the second this way of writing genotypes because it's much clearer and will come in handy when we start dealing with multiple chromosomes all right so what is the phenotype of these two individuals which means the observable features of the organism or in this case the mice and well we said that there are two coat colors black and white and they are determined by two genes or two alleles b1 and b2 in this case the phenotype of this first individual is going to be black and the second individual will have the phenotype of white and the other thing that we can say about these two individuals is that they have the same alleles they have both copies of the alleles are the same so they are homo which means same zygus the first individual is homozygous because they are b1 b1 the both alleles are the same and the second individual is also homozygous since they are b2 b2 so the question arises what happens when you have an individual who has a mixture of alleles or different alleles for example an individual who has a red chromosome with the b1 allele and a blue chromosome with the b2 allele well first of all this individual would be called heterozygous and the second question arises which is what is the genotype of this individual well following our previous convention the genotype of this individual is going to be b1 b2 or b1 forward slash b2 and the third question arises what is going to be the phenotype of this individual well we know that if you're homozygous for b1b1 you are black whereas if you are homozygous for b2b2 you are white so there are only two possibilities here you could either be black or you could be white if you are black that means that the b1 allele is dominant over the b2 allele whereas if you are white that means b2 is dominant over the b1 allele and the crucial point here is that dominance or recessiveness can only be decided by looking at what the phenotype of the heterozygous individual is if the phenotype of the heterozygous individual follows the phenotype of the first allele let's say b1 then b1 is dominant whereas if the phenotype of the heterozygous individual follows the phenotype of the other allele let's say b2 then b2 is dominant and b1 is recessive to summarize we have discussed what genes are genes are stretches of dna along our chromosomes that get transcribed into rna and that rna gets translated into protein and the protein goes on to do things that determine phenotypes which are observable features of organisms such as the color of flowers human height pigmentation and so on genes can come in different versions or alleles so alleles are versions of genes and what's different about them is that the sequence of dna is different on different chromosomes and if you have the same versions or same alleles of genes in a diploid individual that individual is called homozygous whereas if you have different versions or different alleles in the same individual that individual is called heterozygous and if the heterozygous individual follows the phenotype has the same phenotype as one allele then that allele is considered dominant and the other allele is considered recessive