Human Pedigree Analysis | Principles of Inheritance & Variation ​

hi friends welcome to biologyexams4u.com. Today the the topic of our discussion is understanding pedigree analysis we will be discussing what is pedigree chart? what are the different inheritance pattern and how can we easily solve pedigree analysis problems? if you are new to this channel please subscribe and support this channel. please watch till the end for complete understanding. Let's begin starting with why we need a pedigree chart? A pedigree chart is a diagrammatic representation that s

hows the presence or absence of a trait a character or a disorder in a family over several generations. Genetic counselors use this to track genetic diseases. Let's begin with some important symbols to understand pedigree chart, this circle represent female, square represents male if it is filled that indicates affected individuals, a horizontal line between male and female indicates parents unrelated, two lines horizontal lines indicates related patterns or consanguineous parents and these repr

esent the offsprings 1 2 3 4 starting from eldest to the youngest. so these are the major symbols used in pedigree chart. let's begin with some background concepts for better understanding and also for solving pedigree analysis problems in an easier way. Affected individuals is denoted in filled square or circle. suppose the allele is D, then there are two probabilities it can be a homozygote like this capital D capital D or small d small d, may be affected, the second possibility is hetero zygo

te capital D small d and there are two possibilities sometimes maybe affected sometimes it'd become a carrier depending on the nature of the allele, if it is a dominant allele then this heterozygote condition, that person will be affected if it is a recessive allele, that person will become a carrier. this is the first concept. the second concept this is very important in solving problems related to sex-linked inheritance. Point number one is in the case of males, the X chromosome is received fr

om the mother and Y chromosome is received from the father where as in the case of females, one of the X chromosome is received from the father and one of the X chromosome is received from the mother. ..... this understanding is very important for solving problems related to sex-linked inheritance. Now starting with the simplest inheritance pattern and these are wonderful pictures for easy understanding and this is from NIH and I have given the link and I will be dropping the link in the descrip

tion also for your reference. so in the case of Y linked inheritance, Y chromosome as it is present only in males it is passed from father to son. affected men passed the trait to all sons. you can see the Son is affected and this Son is also affected as this y chromosome is transferred from father to the son, all sons. Daughters are not affected as daughters doesn't havef Y chromosome. The trait doesn't skip generations. Examples include hairy ears retinitis pigmentosa etc Now let us see a pedi

gree chart for better understanding so this is a typical pedigree chart of y-linked inheritance as you can see this filled indicates that this father is affected and you can see in this, next generation also all the males are affected this male 4 is affected and this male 8 is affected from that also in the next generation also this 10 and 11 is affected. So the typical characteristics of Y linked inheritance are point number 1, all males are affected, point number 2 only males are affected as

you can see daughters are not affected at all. point number 3 these type of inheritance doesn't skip generations, as you can see this in this generation first generation second generation also there is individuals with a disorder and in the third generation also there is individuals with that disorder. Hope this is fine. now moving into the second pattern that is X linked dominant traits, x-linked dominant traits are more in females than males as you know there are two x chromosomes in females.

let's take the case one. if father is affected, then father will pass that affected x chromosomes to all daughters so as it is a dominant trait all daughters will get affected also. There is no male-to-male transmission as this father gives only the Y chromosome to the sons. so in the case of x-linked dominant trait if the father is affected all daughters become affected, as the X chromosome which is dominant the disorder is dominant is received from the father. Case number two, if mother is aff

ected .. if mother is affected both daughters and sons are affected, often 50% chance as you can see this is a dominant allele so one of the X chromosome, the chance of getting is 50% if there are 4 children. This trait also as it is dominant doesn't skip generations. Examples include fragile X syndrome so in these son as you can see this affected X chromosome is transferred from the mother and this Y chromosome from the father, so here there is one chromosome which is not affected and that is t

ransferred to this unaffected son, therefore that particular son is not affected. Now let us see the pedigree chart as you can see in X-linked dominant trait, affected mother of the 4 offspring's 50% are affected it can both males and females are affected, if the mother is affected. In the case of males, as you can see this is the affected father, all daughters are affected as you can see 8 and 10 both daughters are affected indicating that this is an X linked dominant trait. so point number on

e if the father is affected all daughters will also get affected, if the mother is affected 50% of the progenies will be affected. Both males and females has the same chance of getting the disorder. Moving into the third pattern that is X-linked recessive disorder it is more in males than females. if the father is affected there will be carrier daughters, as you can see one of the x-chromosome will be transferred to this daughter and other X chromosome one of the X chromosome is transferred to t

his daughter, so daughter's become carriers there is no male-to-male transmission as father only passes y chromosome to the sons. if mother is a carrier, the second option is if mother is a carrier then one by four of the children gets affected and only males are affected, whereas the daughter becomes the carrier as this is a recessive trait. In the case of females, there are two x chromosomes so a recessive trait will not express in heterozygous condition in females, so females become carriers

whereas in the case of son or males there is only single X chromosome if that recessive X chromosome is received that by that particular person or particular child that child gets affected. then the third possibility, mother is affected as a recessive disorder it should be small d small d or small x small x, then all sons becomes affected as son will receive the X chromosome from the mother so all sons get affected. here as it is a recessive trait there are carriers in between therefore the tra

it skips one or more generations. Examples include hemophilia, Fabry disease etc. Let's look into a pedigree chart of x-linked recessive trait, as you can see this is the affected mother affected mother then as you can see for offsprings, 4 5 6 & 7 and all males are affected, all sons are affected. then here you can see carrier mother, then out of the four offsprings one male is affected so one out of four gets the disorder. here also you can see in this pattern, males are more affected, as male

s are having a single X chromosome therefore recessive trait will express easily as there is only one X chromosome. If both the parents has the disorder then all the children will get the disorder. Now moving into fourth, that is autosomal dominant disorder, so previously one two three we have discussed disorders that is happening in sex chromosomes that is x and y chromosomes so such disorders are called as sex-linked inheritance or sex linked disorders where mutation occurs in sex chromosomes.

Now autosomal dominant disorders, autosomal disorders are disorders that is happening in all chromosomes other than sex chromosomes. In the case of autosomal dominant disorder ,affected children are born to affected parents. so this particular father is... this father is affected so the chance is both the males and females are affected in equal frequency and 50% children affected from affected parents. Of the four offsprings two will be getting the disease. here also as it is a dominant disorde

r this trait doesn't skip generations. Examples include Huntington disease Marfan syndrome, polydactyly neurofibromatosis type 1, achondroplasia etc so as you can see if the parents are heterozygous then as this is a dominant trait out of four three will be affected and one will remain unaffected. Let's take a pedigree chart for better understanding, so this is a typical of autosomal dominant trait as you can see affected parents then the chance is three out of four will be affected. here also o

ut of three, two are affected. If both parents are affected as it is a dominant trait if there are four offsprings, 75% will be affected. If one parent is affected, then 50% of the offsprings will get the disorder as you can see out of two offsprings one is affected and one is unaffected and this will not skip generation. These ten eleven and twelve are not affected as the parents are not affected. Let me repeat once more, in the case of autosomal dominant trait if both parents are affected, th

ree out of four of the offsprings will get the disorder. If one of the parent is affected, then 50% of the offspring will get the disorder. Only affected parents give rise to affected offsprings. Moving into the next one and the final one that is autosomal recessive disorder, here also affected children are born to affected or carrier parent. Both males and females are having equal chance of getting this disorder. if both parents are carriers, this take this case autosomal recessive, if both par

ents are carriers, then one out of four will get the disorder. one out of four will get the disorder this is a recessive trait both the recessive alleles should come together. Maybe take the case as small d small d, the trait often skip generation as it is a recessive trait, there are carriers in between. These autosomal recessive disorders are mostly seen in progeny of related individuals or consanguineous mating. Example include cystic fibrosis sickle-cell disease etc here also as you can see,

this is heterozygous carrier, the father is also heterozygous carrier then one will be affected and two will be carriers and one will be unaffected. In total, 25 percent of the offspring gets the disorder. Now let's take a pedigree chart this is an autosomal recessive pedigree chart as you can see this father is affected as it is a recessive trait the individuals in this generation there is no disorder indicating a recessive trait. One parent is affected, then one by fourth, out of one, two, th

ree, nine, ten and twelve, three offspring one is affected. If both are affected, as this is a recessive trait if both parents are affected all the offsprings will also get the disorder. here you can see this type of inheritance pattern often skips generation, in first generation there is an affected individual and here there is.. the offsprings are fine. then in the next generation there may be carriers in the next generation then again comes offsprings with the disease... that indicates that t

his is an autosomal recessive trait. hope you are fine with this concept. Just want to add one more point autosomal recessive traits usually appear equally in males and females that we have discussed, there may be unaffected individuals, as this one one or two may be a carrier that can lead may be heterozygous this is heterozygous carrier that lead to the disease in this generation, then the chance of getting this autosomal recessive trait is often with progenies of related individuals these two

horizontal lines indicates related marriage or consanguineous marriage then the chance of autosomal recessive traits are high as you can see, then the reappearance of affected individuals... so it often skips generation in the second generation .... there are no affected individual, then there may be carrier often there will be carriers, this three and four will definitely be a carrier and that lead to the reappearance of affected individuals in the next generation.. this 2 is affected. Hope yo

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