Genetics Explained: Inheritance, Linkage, Sex Determination & Disorders

children look different from their parents and Sue do the young ones of other living organisms what are the

reasons for such variations to find the answer to this question Thomas Hunt Morgan and his colleagues

tried to verify the chromosomal theory of inheritance through various experiments

Morgan chose the tiny fruit fly Drosophila Augusta

for his research there were several reasons for this first these flies could be easily grown

in a simple synthetic medium in a laboratory morgen also found that a single mating

in these flies produced a large number of offspring moreover

the male and the female flies were easily distinguishable and [Music]

had only four pairs of chromosomes making their analysis easier another reason for Morgan selecting

these flies was that they showed several headed it variations which could be easily seen through

low-power microscopes finally these flies had a lifecycle of just two weeks which would help study

several generations within a short span of time Morgan carried out several dihybrid

cross experiments with Drosophila to study sex linked genes example

he crossed yellow bodied the white eyed females with the genotype small wise small W with brown bodied red eyed males

with the genotype small y+ and small W class to produce the f1 generation yeah

[Music] in the superscript represents the dominant wild-type alleles Morgan then

inter cross to the f1 generation to obtain the f2 generation and observed that the jeans did not

separate independently which was an exception to Mendel's law of independent assortment

and his colleagues knew that only the x-chromosome bear genes furthermore they noticed that during a dihybrid cross

if two genes lie on the same chromosome then the proportion of parental gene combinations

that is small W and small Y plus small W plus in this case

was much higher than non-parental gene combinations for the recombinant types that is

small y+ small W and small Y small w+ Morgan found that this was due to physical association or linkage

which describes the tendency of certain genes to be inherited together and their ability to retain their physical

combination in progeny genes which are located on the same chromosome and that are inherited

together are known as linked genes for example it is the linked genes that account for a phenomenon like red hair

being strongly associated with light-colored skin in human beings if an individual inherits one of these

traits he or she is most likely to inherit the other traits too

this phenomenon of linkage can be better understood through a process known as crossing over

which occurs during meiosis during the pachitea stage of meiosis one sections of a chromosome usually

intertwine and exchange genetic material between non sister chromatids of homologous chromosomes

this process where chromatids of a homologous pair exchange segments between them is known as crossing over

crossing over leads to recombination or the creation of different combinations of alleles that do not exist in either

parents and hence it is one of the most important events that lead to variations

in offspring factor discovered by Morgan and his colleagues was that the likelihood of

two genes separating during crossing over was proportional to the distance between them

in other words the closer the jeans the more likely that they will stay

together and the further they are the more likely that they will separate

now let's consider another experiment conducted by Morgan where he crossed white eyed miniature

winged females with the genotype small W small M with red-eyed normal winged males with the genotype small W plus

small M plus from his experiments Morgan noticed that tightly linked genes or genes nearer to

each other showed very little recombination while loosely links genes or genes far

from each other showed higher recombination for example

the yellow and white jeans showed a recombination of just 1.3 percent as these jeans were tightly linked

while the white and miniature jeans showed a recombination of 37 points 2% as these jeans were loosely linked

this linkage and the frequency of recombination of genes on the same chromosome were used by Morgan's student

Alfred Sturtevant as a measure of the distance between genes to map their position on the chromosome

for instance in the experiment we just saw it is only through mapping that we learnt that genes small Y and small W

were located close to each other and genes small Y and small M were farther apart

today genetic maps are being extensively used for linkage studies and analysis and are a key tool for genome sequencing

or discovering the location and identity of genes thus

Morgan's experiments on linkage ruled out Mendel's law of independent assortment when linked genes are

considered people are usual eager to know the sex of their unborn

child today advancements in genetics have made it possible to determine the sex of an unborn child though the

practice is illegal in many countries including India [Music]

however determining the sex of an unborn child wasn't easy in earlier times and the mechanism of sex determination

puzzled geneticists for a long time early research in the field of genetics was carried out using insects

in fact the development of the chromosomal theory of sex determination took place through psychological studies

carried out on insects today sex determination in human beings and several other animals is done with the

help of specific chromosomes these sex-determining chromosomes are called sex chromosomes while the rest of the

chromosomes are called autosomes [Music] for example

human beings have 23 pairs of chromosomes [Music]

of these just one pair is the sex chromosome while the remaining 22 pairs are autosomes the very first indication

that sex chromosomes were different from autosomes came from the experiments conducted by German biologist Hermann

henking in 1891 [Music] henking noticed a specific nuclear

structure in a few insects during spermatogenesis however he also noticed that only 50% of the sperms received

this structure henking named the structure the X body however he couldn't explain the significance of this

structure further research by several other scientists led to the conclusion that the structure was actually a

chromosome and they named it the X chromosome let's now take a look at a few sex determination systems which are

actually biological systems that help determine the development of sexual characteristics in an organism in human

beings and several other mammals and insects the sex determination system is of the X X X Y type

[Music] human beings have 23 pairs of chromosomes no fees the 22 autosomes are

identical in both females and males while the sex chromosomes are different the sex chromosomes and females are a

pair of X chromosomes while cows and males are an X chromosome and a y chromosome in other words the

ovum produced in females contains only X chromosome while in males two types of gametes x and y are produced during the

process of spermatogenesis of the total sperms produced during spermatogenesis 50% carry X chromosomes

while the remaining 50% gary y chromosomes and there is an equal probability of fertilization of an ovum

with a sperm carrying either of the chromosomes so when an ovum carrying the X chromosome unites with a sperm

carrying the X chromosome the resultant x x z goat develops into a female offspring

on the other hand when an ovum carrying the X chromosome unites with a sperm carrying the Y

chromosome the resultant XY zygote develops into a male offspring therefore it is obvious that the sex of

the offspring is determined by the genetic makeup of the sperm [Music]

moreover it is quite evident that the probability of a pregnant woman delivering either a male or a female

offspring is always 50% another mechanism of sex determination is the XX Expo type which is applicable

to several insects such as grasshoppers cockroaches and bugs in this type of sex determination females have two identical

homologous sex chromosomes designated as xx while the male's have just one sex

chromosome designated as Expo here the O denotes the absence of a second sex chromosome now when an ovum carrying the

X chromosome unites with a sperm carrying the X chromosome the resultant xx zygote develops into a female

offspring conversely when an ovum carrying the X chromosome unites with a sperm carrying

no chromosome the resultant EXO zygote develops into a male offspring therefore we have seen that in both the X X X Y

and the xx x o mechanism of sex determination it is the genetic makeup of the sperm that determines the sex of

the offspring such a system of sex determination is known as male hetero gamma T however there are also several

organisms in which the mechanism of sex determination is female heterogeneity that is two different types of gametes

pertaining to the sex chromosome are produced by the female an example of such a sex determination

system is the ZW Z Z type and it occurs in birds reptiles fish and some insects like butterflies and moths in this type

of sex determination females have two different types of sex chromosomes designated as Z W apart from the

autosomes [Music] males on the other hand have identical

sex chromosomes designated as Z Zed apart from the autosomes now if an ovum carrying a said

chromosome unites with a sperm carrying the Z chromosome then the Z Zed zygote is formed and it develops into a male

offspring however if an ovum carrying a W chromosome unites with a sperm carrying the Zed chromosome a ZW zygote

is formed and it develops into a female offspring another example of female heterogeneity

is the Z dou Z Z type of sex determination seen in some butterflies and moths

in this type of sex determination females have only one sex chromosome designated as z vu here the o denotes

the absence of a second sex chromosome [Music] males on the other hand have two

identical sex chromosomes designated as Z Z therefore if an ovum carrying the z chromosome unites with a sperm carrying

the Z chromosome the resultant Z Zed zygote develops into a male offspring however if an ovum containing no

chromosome unites with a sperm carrying the Z chromosome the resultant said OHS I goat develops into a female offspring

[Music] thus as soon as the zygote is formed the sex of an organism gets fixed and it can

be determined by analyzing the composition of the zygote these are a few images of people

suffering from various headed eteri disorders the hereditary disorder is a condition that is genetically passed on

to one's offspring for a long time human beings have been aware that

certain disorders are passed on from one generation to another yes it was primarily after the

rediscovery of Mendel's work that studies pertaining to the inheritance pattern of traits in human beings began

however due to certain sand and ethical reasons it was not possible to carry out controlled crosses in human

beings as in certain plants and animals duration the only alternative was to study a

family's history to understand the inheritance of a particular trait such a genetic analysis of a trait that

is traced through several generations of a family is known as pedigree analysis this analysis plays a very important

role in tracing the inheritance of abnormal traits Oh pedigree analysis involves using a

pedigree chart with standard symbols to represent the inheritance of a particular trait

a few of these symbols are as shown the standard symbol representing a male is a square and that for a female is a

circle rhombus is used in case the sex is unspecified

and a filled square circle or rhombus is used to represent an affected individual mating is indicated by a single line and

mating between relatives by a double line parents are placed above and children

below as shown however a parent with a disease affected male child is represented differently

and five unaffected offspring are represented by a number within the rhombus

identical twins and non identical twins are represented as shown [Music]

whereas a heterozygous male is represented by a half-filled square and a heterozygous female by a half-filled

circle a circle with a dot in the center represents a carrier and the death of a

male or female is represented as shown pedigree analysis helps trace alterations in genes

let us learn about these alterations and the reasons for the same it is the genes in the DNA helix of a

chromosome that contain information regarding inheritance or in other words information such as height color and

even diseases that an individual is prone to this DNA helix is in a highly super

coiled form and runs continuously from one end of a chromatid to the other also each DNA helix contains the genetic

information required for the functioning and development of all living organisms and is thus passed from one generation

to the next however occasions such as recombination and

mutation changes or alterations do occur in the genetic material which alter the chromosome

mutation in genetic material leads to changes in the genotype and phenotype of an organism

stations in the genetic material can be of two types gene mutation and chromosomal mutation

further gene mutations can be of different types such as substitution deletion and insertion

in substitution mutation normal base pairs of DNA are substituted by other base pairs

a type of substitution mutation is points mutation where a single base pair of DNA is substituted by another base

pair for example in sickle cell anemia mutation occurs due to the substitution

of a single base pair deletions on the other hand occur when a sequence in a gene is deleted

for example deletion of a sequence of gene in the APC gene which is a tumor suppressor gene

results in a non-functional protein which leads to cancer insertions occur when a sequence in a

gene is added for example in fragile X syndrome the additions of base pairs in the X

chromosome make it fragile leading to mental retardation however when deletions and insertions of

several base pairs of DNA occur it results in a frameshift mutation for example

disease which causes mental and physical disabilities is due to frameshift mutation

mutation chromosomal mutations occur when the number of chromosomes changes or when there is a change in the

structure of a chromosome chromosomal mutations can occur during the formation of a zygote which

can lead to an increase or a decrease in the chromosomal number or structure Down's syndrome and Jacobson's syndrome

are some disorders caused by chromosomal mutations Down's syndrome is caused by an

additional X chromosome in the 21st chromosome and Jacobson's syndrome due to deletion of a portion of the 11th

chromosome but why do genes and chromosomes undergo mutations there are several physical and chemical

factors that change the genetic structure or that induced mutations these factors are known as mutagens

UV radiation gamma rays alpha particles bromine and nitrous acid are some nutrition's that caused changes in the

genetic structure therefore mutations and genes and chromosomes lead to several disorders and the inheritance

pattern of these disorders can be studied through pedigree analysis all of us are carriers of potentially hazardous

genes while some of these genes lie hidden in a recessive form all through our lives

there are others that show their influence later in our lives when triggered by environmental factors

there are also a few genes that show their presence immediately after birth or even before we are born in the form

of genetic disorders we can broadly classify genetic disorders into two types Mendelian and

chromosomal Mendelian disorders include genetic disorders caused by alterations or

mutations in a single gene as in thalassemia and sickle-cell anemia while chromosomal disorders include

genetic disorders caused by an excess [Music] normal arrangement of chromosomes as in

Down's syndrome and Turner's syndrome let's now learn about Minh disorders in greater detail

the mutated genes that cause Mendelian disorders follow the laws of Mendelian inheritance

these disorders can autosomal recessive autosomal-dominant

[Music] x-linked dominant or x-linked recessive some Mendelian disorders are cystic

fibrosis hemophilia thalassemia

sickle-cell anemia and phenylketonuria let's study more about some of these

Mendelian disorders hemophilia is a genetic disorder that impairs the body's ability to coagulate

or cut blood when a blood vessel breaks this disorder occurs when a single factor from the several factors involved

in the process of coagulation is affected it is a sex-linked recessive disorder

inherited through the X chromosome and the possibility of a male offspring inheriting this disorder is greater than

in females a male offspring inherits this disorder from an unaffected carrier or a

heterozygous female parent and these males are generally in viable or infertile in later stages of their lives

[Music] the possibility of a female offspring being hemophilic is very rare and occurs

only if both the X chromosomes in the offspring are in recessive form that is

the father must be a hemophilic getting the recessive X chromosome the mother must be a carrier or a

hemophilic to did you know that haemophilia is also known as a royal disease as Queen

Victoria of England was a carrier of this disease and had passed it to her descendants

another widely studied Mendelian disorder is sickle-cell anemia which affects red blood cells or RBC's

it is a lifelong disorder we're the RBC's turn into a rigid sickle shape

which reduces their flexibility thereby leading to several health complications

[Music] this disorder also reduces the life expectancy of the affected individual

sickle-cell anemia is an autosome linked recessive trait or a trait inherited through non-sex chromosomes and

it is passed on to the progeny when both parents are carriers of the gene which means they are heterozygous for the

trait the disorder is controlled by the two alleles

B a and HB s these alleles can form three genotypes namely h ba h ba

eh be a HBS and HBS HBS of these homozygous individuals with genotype h

ba h ba are neither carriers nor sufferers heterozygous individuals on the other

hand have the genotype HBA HBS and are unaffected by the disorder but are

carriers and have a 50% probability of passing the disorder to the progeny it is homozygous individuals with

genotype HBS HBS that have a phenotype displaying the disease sickle-cell anemia is caused when

glutamic acid is substituted with valine at the sixth position of the beta globin chain of a hemoglobin molecule the code

for amino acid is present on the beta globin gene where g AG coding for glutamic acid in a normal gene is

substituted with GU g coding for valine in a sickle cell gene though

under low oxygen tension the mutant hemoglobin molecule undergoes polymerization

which changes the shape of the RBC from a biconcave disc shaped structure to an elongated sickle shaped structure

sickling of RBC's obstructs the blood vessels and reduces blood flow to different organs of the body

condition like sickle-cell anemia phenylketonuria is also an autosomal

recessive trait it is an inborn error of metabolism inherited during birth and is

characterized by the deficiency of the enzyme phenylalanine hydroxylase is required to convert the amino acid

phenylalanine into tyrosine as a result phenylalanine accumulates in the body

and gets converted into fennel pyruvic acid and other derivatives

Fennell pyruvic acid is even detected in the urine as the kidneys failed to absorb it

if fennel pyruvic acid and its derivatives accumulate in the brain it could lead to mental retardation

to avoid such a situation a diet low in phenylalanine is usually recommended for patients suffering from phenylketonuria

thus Mendelian disorders occur due to a mutation in a single gene and a pedigree analysis helps determined the trait in

question take a look at this family photo can you see that the color of the child's eyes

is blue just like her parents now take a look at this picture here the child has light brown hair while that of his

parents is black what accounts for such similarities and differences the answer to this and

several related questions are dealt with in genetics the branch of biology that studies inheritance and variations and

characters among related organisms largely in their evolutionary aspects [Music]

inheritance is the basis of heredity it is the process of passing characters from parents to offspring variation on

the other hand is the degree by which an offspring differs from its parents human beings have been using their

knowledge of genetics since prehistoric times our ancestors knew that sexual reproduction was one of the causes of

variation based on this principle they practiced domestication and selective breeding of animals and plants

for example this evil cow found in Punjab was obtained through artificial selection and the domestication of

ancestral wild cows [Music] however our ancestors had very little

knowledge about the scientific basis of genetics it was only in the mid 19th century that

advances were made in the field of genetics by Gregor Johann Mendel Mendel was an Augustinian priest and scientist

who gained fame posthumously prospering the science of genetics he proposed several laws of inheritance by

conducting extensive research and hybridization experiments on garden peas or by some satyam for about seven years

hybridization is the process of crossing two individuals deferring in at least one character

[Music] resulting in a hybrid individual the reason for Mendel choosing common pea

plants for his research was that the plants had various contrasting characters such as length of the plant

color of the pod and shape of the seed that were visible to the naked eye [Music]

borova self pollination as well as cross pollination was possible in these plants and crossing between these plants could

be controlled in addition these plants grow quickly and produce a large number of offspring Mendel for the first time

had applied statistical analysis and mathematical logic to problems in biology

he chose 14 pairs of true breeding felines similar in all aspects except for one character to conduct cross

pollination or artificial pollination experiments a true breeding line exhibits stable trait inheritance and

expression for many generations due to continuous self pollination [Music]

round or wrinkled seeds yellow or green seeds green or yellow pods and tall or dwarf plants were some of the

contrasting traits chosen by mental the use of true breeding lines helped mend will create a basic framework of rules

relating to inheritance which were later expanded as scientists discovered new information pertaining to genetics

[Music] also the data collected by Mendel was quite credible as he had used a large

sampling size for his experiments [Music] moreover as he conducted experiments on

several generations of test plants it helped him confirm his influences and prove that his rules were not

unconfirmed ideas but general rules of inheritance in all living organisms however the world had to wait till the

end of the 20th century to understand the significance of Mendel's discoveries due to his valuable research in the

field of genetics Mendel came to be known as the father of genetics and the rediscovery of his laws laid the

foundation for the modern science of genetics now let's take a look at the different terms that we come across

while studying genetics [Music] a gene or a factor is the functional

unit of heredity material that is DNA in other words it is a unit of DNA that is responsible for the appearance and

inheritance of a character the alternative form of a gene located at a specific position on a specific

chromosome is known as the allele it governs the trait of an individual and is generally represented by a letter the

trait can be defined as a distinct variant of an organism's phenotypic character which may be inherited or

environmentally determined for example the color of petals is a character or an attribute while the colors pink violet

and white are traits the genetic makeup of an organism is designated by the term genotype while

the appearance of the organism is designated by the term phenotype also every organism possesses two alleles for

each character if these two alleles for a particular character are identical the gene is said to be homozygous or pure

and the individual is called a homozygote on the other hand if an organism possesses two contrasting or

different alleles in a pair the gene is called heterozygous and the individual is called the heterozygote in a

heterozygote only one of the contrasting genes is able to express itself while the other gene remains hidden this gene

which expresses itself in the f1 hybrid is called the dominant gene while the gene that is unable to express itself is

called the recessive gene f1 hybrid of first filial generation is the first generation of plants or animals obtained

from cross mating distinctly different parent types when two individuals of the first filial generation are self

fertilized the resulting progeny is called the second failure generation or f2 hybrid self fertilization is the term

used when fertilization takes place between male and female gametes from the same individual

the study of inheritance of one pair of contrasting characters is known as a monohybrid cross for example the study

of inheritance of tall and dwarf plants while the study of inheritance of two pairs of contrasting characters is known

as a dihybrid cross for example the study of inheritance of round and yellow seeds and wrinkled and green seeds to

determine if an individual is homozygous dominant heterozygous or homozygous recessive the back cross or a test cross

is carried out the back cross is a cross between the f1 generation progeny and any of its parents while a test cross is

a part of a back cross between f1 individuals with a homozygous recessive parent and it is performed to determine

the unknown genotype thus genetics explains how traits are passed on from parents to the young ones and the

principles governing genetics were discovered by Gregor Johann Mendel the father of genetics

[Music] [Applause] [Music]

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