[Music] cells contain DNA DNA is arranged on chromosomes if we unravel a chromosome it is basically just a long string of DNA which contains instructions for the features that make you who you are one individual set of instructions is known as a gene there are genes that determine eye color whether you'll have straight or curly hair and what color it is there's even some evidence that being good at writing music might be affected by specific genes everybody has the same genes there are a few differences between men and women but apart from those you have exactly the same genes as every other human on the planet the reason that you are unique the reason that people are different from each other despite having the same genes is that genes come in different versions a version of a gene is known as an allele for example there are several genes that determine what eye color you will have if you have one set of alleles for those genes then you would have one eye color but if you have different alleles for those same genes then you would have a different eye color it's the same genes for everyone that determine eye color but different people have different versions of those genes hence different features humans have 23 pairs of chromosomes totaling 46 chromosomes in their cells they get 23 chromosomes from their mother and 23 from their father since each person inherits two sets of chromosomes one from each parent this means they inherit two of every Gene when reproductive cells are formed for example egg cells some of the parent cells with 46 chromosomes divide in a way to provide reproductive cells each with 23 chromosomes in them each sperm cell and each egg cell contain 23 chromosomes so that when they combine they produce a new cell the new cell has the correct number of chromosomes and that cell divides and develops to become a new human when reproductive cells are produced in the parents exactly which one of the pair of chromosomes will be passed on Is Random remember that chromosomes contain genes for every Gene a parent has one of their two versions of it is randomly passed on to their child let's look at the inheritance of a specific trait to understand what this might mean in real terms there is a genetic disease called cystic fibrosis that causes the body to produce sticky mucus in the digestive system and lungs this causes problems with digestion as well as lung infections there is a gene found on chromosome 7 which codes for cystic fibrosis and most people Express a version or allele for this Gene that is normal meaning they don't have cystic fibrosis remember that a baby will inherit two versions of chromosome seven one from each parent just like all chromosomes and therefore will inherit two copies of this cystic fibrosis Gene let's focus just on this Gene within a person's cells and let's use some symbols for it we usually use letters of the alphabet so let's go with f for fibrosis we'll say that uppercase F represents the normal version of the Gene and lowercase f represents the version of the gene that codes for Cystic Fibrosis most people have two normal alleles of this Gene and so whichever one of their two alleles each parent passes on their baby will have two normal alleles but let's look at other possibilities let's say both parents have one normal allele and one cystic fibrosis allele it's possible the baby May by chance inherit this version from their father and this one from their mother they may instead inherit this version from their mother and have one normal version and one cystic fibrosis version they may instead inherit a normal version from their mother and a cystic fibrosis version from their father or two cystic fibrosis versions one from each parent as you can see there are four different possibilities of inheritance by the way if we look at this possibility we tend to write the uppercase letter first so this combination would usually be written like this so what do these combinations mean in practice for this baby remember that uppercase F is our symbol for normal or doesn't have cystic fibrosis so if the baby were by chance to inherit this combination they would not have cystic fibrosis lowercase f represents the cystic fibrosis version and if the baby inherited this combination they would suffer from cystic fibrosis but what about a child who inherits both alleles there are only two possibilities here the child either Hazel does not have cystic fibrosis so which one is it well in the case of this Gene like with many genes one version of it is kind of stronger than the other one stronger is not a particularly scientific word to use alleles don't have muscles and they don't lift things we actually say that one allele is dominant to the other and the other one is said to be recessive if a person inherits both the dominant and the recessive one only the dominant one will have any effect we would say that only the dominant one is expressed and by the way we always use the uppercase letter to represent the dominant allele and the lowercase letter to represent the recessive allele so if a person inherits two alleles only the dominant one will be expressed babies that inherit these alleles would not have cystic fibrosis if these two parents with these particular alleles were to have a baby there is a one out of four or twenty five percent chance that their baby would have cystic fibrosis it's really important that you're able to draw genetic diagrams to determine these kinds of probabilities with inheritance how to do that is all covered on this next video along with some really really important vocabulary I'll see you guys there [Music] [Applause]