Gregor Mendel's Laws of Heredity and Chromosomal Disorders Explained

Gregor Johann Mendel played a vital role in discovering the laws of heredity he carried out several hybridization

experiments with true grading P lines which had sharply contrasting characters one such experiment was the crossing of

a tall and a dwarf pea plant since the cross was between plants that deferred in a single trait that is height in this

case Mendel called it a monohybrid cross furthermore Mendel called the tall and the dwarf plants in this experiment the

parental or the pea generation after the cross between the tall and dwarf plants Mendel collected the seeds

and planted them to produce the first hybrid called the f1 generation or the first filial of plants he found that all

the plants in the f1 generation were always tall like one of the parents and the dwarf mass had disappeared Mendel

conducted similar experiments with other pairs of traits and made certain observations every single time he found

that the f1 hybrid resembled only one of the parents while the other parents trait was missing to explore this

further Mendel self pollinated the f1 hybrids to produce a second hybrid generation of plants known as the f2

generation or second filial of plants interestingly this time the progeny included both tall and dwarf plants in a

definite ratio 3/4 of the plants in the f2 hybrid were tall while 1/4 was dwarf

mendel also noticed that the clowns did not show blending or mixing that is none of them were of in-between height once

again to confirm his observations Mendel conducted experiments with other traits his results were confirmed when he found

that both the parent all traits were expressed in the f2 hybrid in the proportion 3 s 2 1 moreover the

contrasting traits did not show any blending in either the f1 or f2 stages from his experiments Mendel inferred

that something was being stable he passed on from parents to progeny over successive generations this was the

reason the dwarf Lestat had disappeared in the f1 hybrid had reappeared in the f2 hybrid Mendel called this something

factors which are known as genes today genes are represented by letters per our convenience

the trait expressed in the f1 hybrid is denoted by a capital letter while the other trait is denoted by a small letter

as all plants obtained in the cross between tall and dwarf plants were tall in the f1 hybrid the tall trait is

denoted by capital T and the dwarf by small T here capital T and small T are alleles of each other that is they are a

pair of genes representing two alternative forms of the same character namely the height of the plant there

four pairs of alleles for height in plants would be capital T capital T capital T small T and small T small T

here capital T capital T capital T small T and small T small T are genotypes of the plant and the descriptive terms tall

and dwarf are the phenotypes the alleles in the genotype capital T capital T and small T small T are

similar or homozygous while the alleles in the genotype capital T small T are dissimilar or heterozygous

moreover Mendel also called the character that expresses itself in the f1 hybrid dominant and the one that

failed to express itself recessive therefore through his experiments with tall and short B plants Mendel found

that the character of tallness was more potent over the character of dwarf nur's in the f1 hybrid and that both tallness

and dwarf nos appeared in a three-to-one ratio in the f2 hybrid Mendel called this the law of dominance

[Music] the law states that in a monohybrid pure line cross between parents with

contrasting traits only one form of the trait that is the dominant form appears in the f1 generation and both forms

appear in a ratio of 3 to 1 in the f2 generation also from his experiments Mendel observed that the recessive

parent will trait was expressed without any blending in the f2 hybrid after verifying the results of the crossings

it was found that the segregation of alleles is a random process and the chances of a gamete containing either

allele is 50% this is known as the law of segregation and it states that the leaves do not show any blending but

instead segregate from each other during gamete formation into different gametes both the characters governed by these

alleles are recovered in the f2 generation though one of the characters is not displayed in the f1 generation a

diagram that is very useful in predicting the outcome of a particular cross or breeding experiment is piu net

square which was devised by British geneticist Reginald seed pew net pew net Square is a graphical representation

that helps calculate the probability of all possible genotypes of the progeny in a genetic cross let's take a look at how

Mendel's experiments are denoted using the pew net square in the f1 generation Mendel crossed homozygous dominant

capital T capital T parent with a homozygous recessive small T small T parent the gametes produced by the

parents are written on two sides of the square and the capital T small T progeny produced by the fertilization of the

gametes is written within the square though the genotype of the f1 hybrid is capital T small T its phenotypic

character is tall plants of the genotype capital T small T produced by the f1 hybrid are then self pollinated to

produce gametes of the genotype capital T and small T in equal proportion from the square you can easily see that the

pollen grains of genotype capital T have a 50% chance of pollinating the eggs of the genotype capital T as well as small

T during fertilization similarly pollen grains of genotype small T have a 50% chance of pollinating

eggs of genotype capital T as well as small T during fertilization thus

random fertilization results in zygotes that out of genotypes capital T capital T

capital T small T and small T small T square you can also see that one-fourth of the random fertilizations have led to

capital T capital T half has left a capital T small T and finally

1/4 has led to small T small T therefore the genotypic ratio is 1 is to 2 is to 1

and as 3/4 of the plans are tall and 1/4 dwarf the phenotypic ratio is 3 is to 1 the

ratio 1 is to 2 is to 1 or 1 by 4 is to 1 by 2 is to 1 by 4 of capital T capital T is to capital T small T is to small T

small T is condensable to a binomial expression of the form ax plus B Y the whole square the expression has the

gametes bearing the genes capital T or small T in equal frequency of 1 by 2 this expression can be expanded as shown

thus we can calculate genotypic ratios using mathematical probability however by just looking at the phenotype of a

plant it is not possible to predict its genotype for example you cannot predict whether a tall plant from the f1 or f2

hybrid is homozygous dominant that is capital T capital T or is heterozygous dominant that is capital T small T

therefore a test cross or a cross between individuals of the f1 generation with a homozygous recessive parent is

carried out to determine whether the genotype of the individual in question is homozygous dominant or heterozygous

dominant and thus through his mana hybrid cross experiments Mendel proposed the laws of

dominance and segregation this is little Sam can you see his face is swollen

moreover his eyes are slanting and bulging and he has a small mouth with a

protruding tongue these are a few symptoms of a chromosomal disorder known as Down's

syndrome normal human beings have 46 chromosomes arranged in 23 pairs

however even a slight variation from this pattern causes abnormalities or disorders

chromosomal disorders are caused either due to changes in chromosomal number or changes in chromosomal structure

changes in chromosomal number occur due to non disjunction of chromosomes failure of chromatids to disjoin during

cell division leading to either aneuploidy or you ploidy

aneuploidy is a condition where one or more chromosomes are either gained or lost

aneuploidy is of two types try so me and Moroso me a normal deployed individual has to n

number of chromosomes however when an extra chromosome is added the condition is known as trisomy

that is a 2n plus one condition on the other hand when a chromosome is

lost or when a chromosome of any pair is absent the condition is known as monosomy

that is a 2n minus one condition now you ploidy is a polyploidy condition

more than two haploid sets of chromosomes are formed due to the failure of cytokinesis after the

telophase stage the number of chromosomal sets added you ploidy can be triploidy or a tree end

condition tetraploid e or a foreign condition deploy d or a 5n condition and so on

let's now take a look at some chromosomal disorders that occur due to changes in chromosomal number

Down's syndrome or trisomy 21 is caused due to the presence of an extra copy of the 21st chromosome

it was dr. Langdon dung a British doctor who first described this disorder and hence the name Down's syndrome

the symptoms are a swollen face bulging and slanting eyes a smallmouth

and a protruding and furrowed tongue some other common symptoms of this disorder are short stature

small round hem a broad palm with a single crease and physical psychomotor and mental

development another disorder caused by changes in chromosomal lumber is Klinefelter's

syndrome it is a condition where males have an additional X chromosome

which results in a karyotype of 47 that is XXY

Klinefelter's syndrome is also known as XXY syndrome and is named after dr. Harry Klinefelter who was the first to

describe the disorder it is one of the most common sex chromosome disorders in males and shows

symptoms such as poor beard growth narrow shoulders gynecomastia or breast development

and underdeveloped testes moreover such males are sterile yet another disorder due to changes in

chromosomal number is Turner's syndrome we're one of the X chromosomes is absent in females chromosomes in these females

will be 45 with XO the syndrome is named after Henry Hubert Turner and endocrinologist who first described

it females with this syndrome show symptoms like a webbed neck

constriction of the iota poor breast development underdeveloped ovaries

and short stature moreover such females are usually sterile and have health problems like

diabetes earring and vision problems and hypothyroidism

[Music] just like changes in chromosomal number any change in the structure or

arrangement of genes can also lead to chromosomal disorders changes in chromosomal structure can

take several forms some of these changes include a portion of a chromosome getting deleted

you placated or transferred to another chromosome while some of these structural changes

in a chromosome are inherited others take place accidentally when reproductive cells are being formed or

during early fetal development Jacobson syndrome and tried a cat syndrome are some disorders caused due

to changes in chromosomal structure therefore chromosomes hold the genetic keys to all the functions of our body

and any change in the number or structure of a chromosome can lead to chromosomal disorders

and these flowers beautiful these are snapdragon flowers and they come in so many different colors

hybridization experiments conducted on pea plants were conducted on Snapdragon plants to

[Music] plants experiments were conducted on birds like pigeons and poultry and on

several animals like mice rabbits cats guinea pigs and even man however it was discovered that the law

of dominance didn't hold true in all cases that is Mendel's laws were not Universal in

occurrence to understand this better let's further study the experiment conducted on the

slab dragon plants when a true breeding or a pure line red flowered Snapdragon with genotype capital R capital R was

crossed with a true breeding quite flowered Snapdragon with genotype small R small R the results were quite

different from what was seen in the case of the peak lands

the flowers of the f1 hybrid were pink and the genotype was capital are smaller that is the f1 hybrid flowers had a

phenotype that resembled neither of the parents but was in between the two now the flowers of the f1 hybrid

were self-pollinated and the genotypic ratios obtained in the f2 hybrid were 1 is to 2 is to 1

the same result obtained by Mendel in his experiments on peers however the phenotypic ratios obtained

in the Snapdragon plant you can see is also 1 is to 2 is to 1 which is different from the ratio 3 is

to 1 dominant is two recessive ratio in the P plant so in the case of incomplete dominance

both genotypic and phenotypic ratios are the same that is 1 is to 2 is to 1 thus when a dominant allele does not

completely mask the phenotypic expression of the recessive allele in a heterozygote a blending of both dominant

and recessive traits takes place in the f1 and f2 heterozygotes this phenomenon is called incomplete

dominance this is quite the phenotypic ratios obtained in the experiment on snapdragon

plants different from those in the pea plants it is the gene for flower color that

controls the amount of pigment in petals each allele is a code for a trait that is specific amount of pigment

when both alleles for pigment are present the petals are dark red due to heavy pigment production

on the other hand if none of the alleles for pigment exists the flower is white however when only one of the alleles is

present only half the pigment is produced creating a pink shade

the alleles are therefore exhibiting incomplete dominance which is a form of intermediate inheritance where one

allele for a specific trait is not completely dominant over the other allele

but why is it that some alleles are dominant while others are recessive let's understand this by studying the

functioning of a gene in greater detail as you know a gene is the unit of heredity that occupies a fixed position

on a chromosome called the locus a gene generally has two forms called alleles

in homozygotes the alleles are the same while in heterozygotes they are not identical

an allele may be different due to certain changes it has undergone which modify the information contained by that

particular allele for example let's consider a gene that contains information for producing an

enzyme this gene contains two copies which are the two allelic forms

now let's assume that the normal allele produces the normal enzyme that is required to transform a substrate s

according to assumed facts the modified allele could be responsible for producing a normal enzyme a less

efficient enzyme unfunctional enzyme or no enzyme at all now if the modified allele produces a

normal or a less efficient enzyme we can say that the modified allele is equivalent to the unmodified or normal

allele that is the modified allele will produce the same phenotype or trait or in other

words we'll bring about a transformation in the substrate s

such equivalent allele pairs are quite common however if the modified allele produces

a non-functional enzyme or no enzyme then the phenotype is dependent on the functioning of the unmodified or the

normal allele here the unmodified functioning allele or the normal allele that represents the

original phenotype is the dominant allele and the modified allele is generally the recessive allele

so in the example we just saw the recessive trait was due to a non functional enzyme or because no enzyme

was produced therefore incomplete dominance causes a distortion of the normal phenotypic

ratio and results in offspring with a combined phenotype this is a homozygous bull white in color with a genotype

capital W capital W and this is a homozygous go [Music]

in color with a genotype capital R capital R what color will their cough be in the f1

generation well it isn't white or red and neither is it a color in between

the gaff is wrong colored it's goat is a mix of red and white hair therefore in this case the f1 generation

doesn't resemble either of the parents as in dominance no is it in between as in incomplete dominance

displayed was codominance Guu dominance refers to a situation where a heterozygous organism has a

phenotype that demonstrates traits from both dominant as well as recessive genes a cou dominant trait is not blended but

is independently and equally expressed another widely known example of codominance

in human beings is the blood system which is used to determine human blood type

the plasma membrane of the red blood cells or the RBC is present in human blood has sugar polymers that protrude

from its surface it is these sugar polymers that determined the specificity of the major blood types for example the

difference between blood types a and B lie in a single sugar unit that protrudes from the end of a carbohydrate

chain of a glycoprotein or glycol lipid on the plasma membrane of an RBC it is the gene I that is responsible for

determining the type of sugar produced by the RBC's this gene has three different alleles namely capital I a

capital IB and small I [Music] of these believes capital I a and

capital IB produce sugars that are slightly different from each other while the allele small I doesn't produce any

sugar here the alleles capital I a and capital IB are completely dominant over allele small I

now human beings are diploid organisms and hence possess only two of the three alleles or the gene I consequently as

you can see there are six possible combinations of alleles therefore if the allele from one of the

parents is capital I a and from the other is small I only capital I a is expressed in the offspring as small I

does not produce any sugar in this case the RBC's have only type a sugar in them and the genotype of the offspring is

capital I a small I and the phenotype or the blood group is a similarly when the allele from one of the parents is

capital IB and from the other is small I only capital IB is expressed in the offspring as the RBC's have only type B

sugar in them in this case the genotype is capital IB small I and the phenotype is B however when the allele from one

parent is capital I a and from the other is capital I be both of the alleles express their own types of sugar in the

offspring due to codominance in this case the RBC's have both a and B types of sugars in them and the genotype

of the offspring is capital I a capital IB and the phenotype or the blood group is a B now if the alleles from both

parents are capital I a once again the RBC's have only type a sugar in them and therefore the offspring has a genotype

capital I a capital I a and phenotype a likewise if the alleles from both parents are capital I be the are pcs

have only type B sugar and so the offspring has a genotype capital I P capital IB and phenotype B

however if the alleles from both parents are small I the RBC's have no sugar in them and so the genotype of the

offspring is small I small I and the phenotype is boo therefore as there are three different types of alleles there

are six possible genotypes and four different blood groups or phenotypes that exist apart from being a good

example for codominance the blood grouping system is also an ideal example of multiple alleles as we just saw the

gene I has three alleles capital I a capital IB and small I that govern the same character there is blood group

however since an individual can have only two alleles it is possible to learn about multiple alleles only when

detailed population studies are conducted according to the multiple alleles concept more than two alleles

govern the same character now let us study an experiment that shows how a single gene may sometimes produce more

than one effect these seeds have a single gene with two alleles capital B and small B that

controls starch synthesis in them in the experiments conducted the effective starch synthesis was seen in capital B

capital B homozygotes and the seeds produced were large for had a large amount of starch in them moreover the

pea seeds were round on the other hand small B small B homozygotes were not very efficient at synthesizing starch

and so the pea seeds produced were small or had less starch in them also the seeds were wrinkled moreover in the

experiments conducted the seeds or the starch grains produced by heterozygotes capital B small B one of intermediate

size but were round now in this experiment if we consider the start size as the phenotype then the alleles are

displaying incomplete dominance on the other hand if we consider the shape of the seed as the phenotype then the

alleles are displaying dominance as the seeds were round like one of the parents hence the experiment shows that

dominance is not an autonomous feature of a gene or gene product therefore dominance

incomplete dominance or codominance depends on gene product and the production of a particular phenotype

from this product it also depends on the phenotype that we wish to examine in case the same gene influences more than

one phenotype did you know that Gregor Mendel cultivated and tested about 29,000 people ants between 1856 and 1863

[Music] one such test was the crossing of pea plants with yellow and round seeds and

those with green and wrinkled seeds as you know such a cross between plants that differ in two traits or characters

is known as a dihybrid cross this experiment conducted by Mendel resulted in a pea plant that produced yellow and

round seeds in the f1 generation here and you guess the dominant shape and color obviously the round shape is

dominant over the wrinkled shape and the yellow color dominant over green Mendel noticed that these results were

identical to the results he had obtained when he had carried out mana hybrid crosses between plants with round and

wrinkled seeds and those with yellow and green seeds now in Mendel's dihybrid experiment let's denote the dominant

yellow seed color with the genotypic symbol Capital y and the recessive green seed color with small Y likewise let's

denote the round seed as capital R and the wrinkled seed as small R the genotype of the parents can therefore be

denoted as capital R capital R capital y capital y and small R small R small Y small Y

on crossing the Klan's the gametes capital r capital y and small our small Y unites to produce yellow and round

seeds with the genotype capital R small R Capital y small Y in the f1 generation Mendel further self pollinated the

plants obtained in the f1 generation to produce f2 generation plants this time he found that three-fourths of the seeds

were yellow while one-fourth was cream similarly he noticed that three-fourths of the seeds were round while one-fourth

was wrinkled therefore the yellow and green color had segregated in the ratio trees to one similar to the results

obtained in a monohybrid cross and the round and wrinkled shape had segregated in the same ratio 3 is to 1 Mendel also

found that the phenotypic ratio in the f2 hybrid was 9 is to 3 is to 3 is to 1 that is there were 9 round yellow seeds

3 round green seeds 3 wrinkled yellow seeds and 1 wrinkled green seed Mendel had got the same phenotypic ratio

in several dihybrid experiments that he had conducted this ratio nine is to 3 is to 3 is to 1 can be derived as a

combination series of 3 yellow is to 1 green with 3 round is to 1 wrinkled and further this derivation can be written

as shown Jason's on dihybrid crosses Mendel proposed a new set of generalizations

which was later called the law of independent assortment according to this law when two pairs of traits or

characters are united in a hybridization experiment the segregation of one pair of characters is independent of the

other pair of characters we can use the Pugh net Square to easily understand the independent segregation of the two genes

during meiosis and the production of pollen grains and eggs in the f1 generation for example in the dihybrid

cross experiment conducted by mental the genotype of the seed obtained in the f1 generation was capital R small R Capital

y small Y now let's consider the segregation of one pair of jeans capital R and small R here 50 percent of the

gametes have the capital R gene and the remaining 50 percent have the small R gene now apart from each gamete having

either capital R or small R it must also have either capital y or small Y here it is important to note that the

segregation of 50 percent capital R and 50% small R is different from the segregation of 50 percent capital y and

50 percent small y hence 50% of the gametes bearing small are has capital y and the remaining 50%

has small Y similarly 50% of the gametes bearing capital r has capital y and the remaining 50% has small Y therefore the

genotypes of the gametes not of four types namely capital R capital y capital R small Y small R Capital y and small R

small Y in other words there are four types of eggs and four types of pollen and as you

can see the frequency of these gametes is 25% or one-fourth of the total number of gametes produced now the allelic

composition of the zygotes of the f2 plants can be easily obtained using the peu net Square as shown

as you already saw there are four different types of phenotypes formed and the phenotypic ratio is 9 is to 3 is to

3 is to 1 however 9 different genotypes results from the f2 hybrid and the genotypic

ratio is 1 is to 2 is to 2 is to 1 is to 4 is to 2 is to 1 is to 2 is to 1 thus from the dihybrid cross experiment

Mendel found that the gene combination of the progeny the first on the parental gene combination

gregor mendel the father of modern genetics not only researched pea plants but also

carried out several experiments with honeybees in fact by 1865 Mendel had published his

work on inheritance of characters however his work did not receive due recognition until the beginning of the

20th century due to several reasons firstly Mendel's work couldn't be widely publicized because means of

communication in those days were not well developed as they are today secondly Mendel had considered genes or

factors as discrete and stable units responsible for controlling the expression of traits

he had also explained the concept of alleles and how they were responsible for the continuous variations observed

in nature however his concepts about genes were not readily accepted by his colleagues

thirdly Mendel for the first time had applied statistical analysis and mathematical logic to problems in

biology which was objected to by many biologists of his time finally Mendel could neither explain

what genes were made of nor could he provide any physical proof of their existence

[Music] Mendel's work got dear credence only in 1900

on the inheritance of characters were independently rediscovered by three scientists Hugo DeVries called

currents and erich von share mark also both chromosomes and genes segregate during the process of gamete

formation and only one of each pair is passed on to a gamete however they defer in the way they

segregate in jeans independence pairs segregate independently of each other while in

chromosomes one pair segregates independently of another pair

let's now study the behavior of chromosomes during cell division in the anaphase stage of meiosis one

a set of chromosomes aligns along the metaphase plate here you can see a long orange and a

short green chromosome as well as a long yellow and a short red chromosome aligned at the same pole

now consider another set of chromosomes aligned along the metaphase plate during the anaphase stage of meiosis one

here there is a long orange and a short red chromosome and a long yellow and a short green chromosome arranged at the

same bone now during the anaphase stage of meiosis two

you can see the orange and green chromosomes segregating together in the cell on the left

while the orange and red chromosomes segregate together in the cell on the right

Sutton analyzed this segregation of chromosomes during meiosis and concluded that chromosomes have individuality and

corine bears with members of each bear contributed by each parent he also concluded that the paired

chromosomes separate from each other during meiosis and the distribution of the paternal and maternal chromosomes in

each homologous pair is independent of each other both Sutton and vivere were of the

opinion that the pairing and separation of a pair of chromosomes would result in the segregation of a pair of genes or

factors carried by them their hypothesis on chromosomal behavior came to be known as the bow very certain

chromosome theory in fact Sutton combined this hypothesis of chromosomal segregation and Mendelian

principles and called it the chromosomal theory of inheritance [Music]

[Applause] [Music] [Music]

you