Inheritance

Inheritance

• Inheritance – the passing down of genetic instruction from one generations to the next generations

• The scientific study of inheritance is called genetics

• Genetics is the study of how variations arises and how characters of individuals are passed on from one generations to the next

• Gregor Mendel – father of genetics

Monohybrid inheritance

• Mendel performed cross pollination with pure breeding pea plants

• Mendel choose two contrasting parent plants:– Pure breeding tall plant – Pure breeding short plant

short

T t T t

• Mendel concluded :– Inheritance depend on the transfer of

hereditary factors from parents to offspring– Each character is controlled by a pair of

factors

– These factors may be dominant or recessive – The hereditary factors that is described by

Mendel are called genes

The term used in the study of inheritance

Genes and alleles • Genes are basic unit of inheritance which

occupy specific position in chromosomes

• Gene occupies a specific site called locus on a chromosomes

• Alleles are alternate forms of the same gene occupying the same relative position on a certain pair of homologous chromosomes

– Example :– the gene for stem length has two alleles one

for a tall trait and other for short trait

G

R

S

T

g

r

s

t

Locus of genes for stem length

alleles

Phenotype and genotype • Phenotype – the traits of characters in an

organisms– Colour, size, structure

• Genotype – the genetic composition of an organism which is not manifested – Represented by the alleles present – TT, Tt

Dominant and recessive alleles • Dominant alleles – allele that can manifest

itself and cover the effect of the recessive allele

• Recessive alleles – allele that can manifest itself when there is no dominant allele

• Dominant allele is represented by capital letter – T

• Recessive allele is represented by a small letter - t

Homozygote and heterozygote • if both alleles at a give locus is the same

the genotype is called homozygous – TT

• If the alleles at a given locus are different, the genotype is called heterozygote

• TT – tall (homozygous domonant)• Tt – tall (heterozygous)• tt – short (homzygous recessive)

Mendel’s first law of segregation

• The characters of a diploid organisms are determined by alleles which occur in pairs

• The two alleles of a gene separate from each other during the formation of genes

• Only one allele is carried in a gamete and the gametes unite randomly during fertilisation

• Resultant offspring receive one allele from its male parent and one from its female parent

Both pea plants are pure breedingCarries homozygous alleles Alleles are located at the same position

Metaphase I – homologous Chromosomes are arranged at The equator

Anaphase I – homologous pair Separate and moving to opposite Poles

X

End of meiosis – only one allele for eachTrait is found in each gamete

After random fertilisation – F1 generationPlant has one pair of alleles of the same Length character

When the plants of F1 generation were allowed toSelf pollinate. It will result in F2 generations with differentGenotype combinations – TT, Tt, tt

Dihybrid inheritance

• Mendel continue his experiment by crossing pea plants to study the inheritance of two pairs of contrasting traits

• Mendel carried out dihybrid crosses between pure breeding pea plants

Pea plants seed A

Round yellow

RRYY

Pea plant seed B

Wrinkled green

rryy

Parental

Phenotype

RY ry

RrYy

All round and yellow seeds

Parental genotype

gametes

F1 genotype

F1 fenotype

F1 x F1 RrYy x RrYy

F1 x F1 RrYy x RrYy

RY Ry rY ry RY Ry rY rygametes

Gametes from one parentGametesFrom theOther parent RY Ry rY ry

RY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rY RrYY RrYy rrYY rrYy

ry RrYy Rryy rrYy rryy

Punnett square

• 9 : 3 : 3 : 1Round And yellow seeds

Round andGreen seeds

Wrinkled and Yellow seeds

Wrinkled andGreen seeds

• F2 genotype : RRYY, RRYy, RrYY, RyYy – round yellow RRyy, Rryy – round green rrYY, rrYy – wrinkled yellow rryy –wrinkled green

• Four different phenotypes are produced by the nine different combinations of the genotypes

Law of Independent Assortment

• Second law on inheritance • Two or more pairs of alleles segregate

independently of one another during the formation of gametes.

• Therefore traits are inherited by the offspring independent of one another

• Main concept of this law are:1.Segregation of alleles for the shape of the

seeds does not affect the segregation of alleles for the colour of the seeds

2. The alleles segregate independently because they are located on different chromosomes

3. the law explain the production of gametes with different allele combinations

new combinations – recombinations leads to genetic variations

• The result of dihybrid inheritance are explained in terms of the behaviour of the chromosomes during meiosis

Independent assortment produces four equal likely allele combinations during meiosis

The ABO blood group system in humans

• Multiple alleles means there are more than two possible alleles of a particular gene that control a specific character

• ABO blood group system in humans is an example of a character that is controlled by multiple alleles

• These blood groups are determined by three different alleles of a single gene called the I gene

• IA , IB, IO

Dominant

Recessive

• If both allleles IA and IB are present neither dominates the other (codominant)

Phenotype (blood group) Genotype

A IAIA, IAIO

B IBIB, IBIO

AB IAIB

O IOIO

• IA represent antigen A• IB represent antigen B• These antigens are secreted onto the

surface of the red blood cells

Antigen A

Blood type A

Anti-B antibody

• Antibodies are present in the blood serum of each blood group

Phenotype (blood group)Antigens on red blood cells

Antibodies presentin blood serum

Can donate blood to blood groupsCan receive blood from blood groups

A A Anti-B A, AB A, O

B B Anti-A B, AB B, O

AB A and B None AB AB, A, B, O

O None Anti-A, anti-B A,B,AB,O O

• Io – does not have any antigen

• Antibodies are present in each blood group

• Type A blood – has type B antibody (anti- B)

• Type B blood (antigen B) injected into a person with type A blood - anti-B in blood type A cause the blood to agglutinate

• Type AB blood has no antibodies - can receive blood from other blood groups - universal recipients

• Type O blood has no surface antigen - if injected into a person with blood group A, B or AB do not cause the type O blood to clump together

- type O – universal donor

The Rhesus factor

• Rhesus factor is an antigen present on the surface of red blood cells

• This antigen results in agglutination when it reacts with the antibodies from individuals without this antigen

• The Rhesus factor is controlled by a pair of alleles – Rh (dominant allele)– rh (recessive allele)

• Individual with Rhesus factor is known as Rh-positive (Rh+)

• Genotype of Rh-positive individual– Rh-Rh ( homozygous dominant)– Rh-rh (heterozygous )

• If a human does not have Rhesus factor – known as Rh-negative

• Rh-negative individuals are homozygous recessive (rh-rh)

• The inheritance of the Rhesus factor follows Mendels first law

Examples:

A man who is homozygous Rh-positive marries a woman who is Rh-negative. What are the chances of their children being Rh-negative

Phenotype Rh-positive Rh-negative

Parents : Rh-Rh X rh-rh genotypeGametes Rh rh

Rh-rhGenotype of offspring

Phenotype of : Heterozygous Rh-positive Offspring all children are Rh-positive

none are Rh-negative

Parents : Rh-positive Rh-negativePhenotype

Genotype Rh-rh x rh-rh

Gametes : Rh rh rh

Genotype off : Rh-rh rh-rh offspring

Phenotype of : Heterozygous Homozygous offspring

Rh-positive Rh-negative

50% chance of having a child who is Rh-negative

• Rhesus factor can be a problem when a Rhesus-negative person receives Rh-positive blood during blood transfusion

First transfusion – does not Result any reaction

Second transfusion – recipient Blood reacts by producing Rhesus antibodies

Agglutination of the donor’s blood- Lead to death

Pregnant mother (Rh-negative)

First baby has Rh-positive -Fragment of baby’s blood may enter the mother’s blood circulation

Mother’s immune system Produce Rhesus antibodies

Rhesus antibodies enterThe foetus’s blood circulatorysystem through the placenta

The antibodies is not sufficient To cause any effect on the firstborn

Second baby-If the foetus is Rhesus-positive-Antibodies from the mother can cause baby’s blood to agglutinate

Second baby will die

Treatment -Replace baby’s blood with Rh-negativeBlood- Injection of anti-Rhesus antibodies

Autosomes and sex chromosomes

• Autosomes – 22 homologous pairs in male and female – Control all characteristics of the somatic

cell– Do not carry genetic information for sex

determination

• Sex chromosomes carry genes that determine the sex of an organism

• Male – XY• Female - XX

• X chromosome is larger than the Y chromosome

• Y chromosome is much shorter than the X chromosome and it carries fewer genes

• Male – 44 + XY • Female – 44 + XX

Different human karyotypes

• When homologous chromosomes are arranged from the largest pair to the smallest pair and numbered according to size the form the karyotype of an individual

• Karyotypes are identical in all diploid cells of an organism

• Autosomes are numbered 1-22• Sex chromosomes – 23

• Cell of an individual with a genetic disease show different karyotype from the normal human being

• Down syndrome – 2n + 1– 45 + XX / 45 + XY

• Down’s syndrome karyotype

Extra chromosomes Number 21

• Phenotype :– Slanted eyes– Small nose– Large tongue– Short, wide arms– Low immunity– Mental retardation

Sex determination in offspring

• Sperm carries either Y chromosomes or an X chromosomes

• Meiosis produced :– 22 + X– 22 + Y

• A sperm with an X chromosomes (22 + X ) combines with an ovum (22 + X) the zygote that is produce contains XX chromosomes – Female offspring

• The sex of the offspring is determined by the male parent

• The probability having a boy is 50% and the probability of having a girl is also 50%

Parents : father mother

Parent's : 44 + XY 44 + XXgenotype

Gametes : 22 + X 22 + Y 22 + X 22 + X

Genotype of 44+XX 44+XX 44+XY 44+XYoffspring

female female male male

• All ova carry the X chromosomes

Sex-linked inheritance

• Sex-linked genes refers to the genes carried on the X chromosomes

• Y chromosomes does not carry sex linked genes

• Y chromosomes is shorter and carry less genes/allele

• In male any trait caused by a dominant or recessive allele present on the X chromosomes will be manifested fully

• Genes on the X chromosomes are present in two copies in females but only one copies in males– X Y

• Male offspring must inherit the Y chromosome from their father and, therefore always inherit only the maternal allele of any sex linked gene

• Disorder caused by recessive genes are linked to the sex chromosome X– Haemophilia – Colour blindness

Haemophilia

• A condition in which the blood cannot clot normally

• Due to lack of a protein needed for blood clotting

• Individual's inability to produce the protein is caused by recessive allele on the X chromsome

• Normal dominant gene – XH

• Female have a pair of alleles of the genes that controls the production of the clotting factor

• Male have only one allele • Females may be homozygous dominant

or heterozygous dominant

– XHXH

– XHXh

• Female with heterozygous dominant are the carriers of the disease

• A normal male – XHY• Homeophilic male – XhY

A heterzygous female married with a recessive allele for blood clotting (female carrier) marries a normal male

Phenotype of parents : Normal male Heterozygous female

genotype of parents : XHY X XHXh

gametes : XH Y XH Xh

Genotype : XHXH XHXh XHY XhYof offspring

phenotype : normal normal normal Haemophiliac of offspring female female male male carrier

• A female who receive one dominant allele and one recessive allele for blood clotting is a carrier

• A female can only be a haemophiliac if she has two recessive alleles on the X chromosomes

• A male who carries dominant allele on the X chromosome is normal

• A male who has recessive allele on the X chromosomes suffers from haemophilia because the Y chromosomes does not have a homologous allele at the same locus

Colour blindness A person cannot distinguish certain

colours Example : red green colour blindness Inability to differentiate between red and

green colours

Caused by a recessive allele on the X chromosome

Allele for normal colour vision represented by – B (dominant)

Allele for colour blindness – b (recessive)

A female with normal colour vision may have these genotype : Homozygous dominant (BB) Heterozygous dominant (Bb)

Genotype for a female who is colour blindness bb

XBXB - dominant homozygote XbXb - recessive heterozygote

Colour blind

XBXb - heterozygote Carrier

XBY – normal XbY – colour blind

A man with normal vision marries a woman with normalvision. The woman carries the colour bindness allele

phenotype : normal male heterozygous of parents female (carrier)

genotype of parents : XBY X XBXb

gametes : XB Y XB Xb

genotypeof offspring: XBXB XBXb XBY XbY

phenotype normal normal normal colour blindof offspring: female female male male

In order for a female to be colour blind, both her parents must carry the recessive allele

More male are colour blind because males inherit the X chromosome from their mother

Male have no other allele to assert dominance over the recessive allele

A colour blind man marries a homozygous normal vision woman

phenotype of parents : colour blind homozygous normal female

genotypeof parents : XbY X XBXB

Gametes : Xb Y XB XB

genotype : XBXb XBXb XBY XBYof offspring

phenotype female female normal normalof offspring carrier carrier male male

A man with normal vision marries a colour blind woman

phenotype : normal vision colour blind of parents male female

genotype of parents : XBY X XbXb

gametes : XB Y Xb Xb

genotypeof offspring: XBXb XBXb XbY XbY

phenotype female female colour blind colour bindof offspring: female female male male

Other hereditary disease A medical condition caused by an allele

inherited from the parents It is passed down from one generation to

the next

Huntington's disease Caused by mutation of an autosomal

dominant gene which is located on chromosome number 4

Neurological disorder which leads to the progressive degeneration of the nerve cells

Loss of motor coordination, behavioural changes , loss of mental power

Only appear between ages of 30 and 50 years

Sickle-cell Anaemia Caused by defective allele for

haemoglobin synthesis Autosomal gene which located on

chromosome number 11

When blood oxygen is low the red blood cell have the shape of a sickle

This is due to the clumping of the abnormal haemoglobin molecules in the red blood cell

They more likely to break, aggregate and clog the blood capillaries

Cystic fibrosis Caused by a lack of transport protein

which allows chloride ions to move across plasma membranes

Normally water will pass through the plasma membranes after the chloride ions passed

Affected persons – frequent respiratory infections

Caused by cystic fibrosis gene which loacted on chromosome number 7

Thalassaemia Number of different forms of anaemia

Caused by recessive gene which lead to the synthesis of abnormal haemoglobin in red blood cells

• Red blood cells cannot carry enough oxygen. Deficiency of iron.

Passed down by parents who carry thalassaemia gene in their cells

Symptoms : Appear healthy at birth After two years – become pale, listless,

fussy, poor appetite

Grow slowly Develop jaundice

Treatment : Frequent blood transfusion Bone marrow transplant

Gene and chromosomes Chromosome – thread like twisted

structure found in the nucleus

Gene – the basic unit of inheritance Has specific location on the chromosomes Control the various traits or chracteristics of

organisms Number of gene is depend on the size and

length of the chromosomes

The structure of DNA When a chromosome is uncoiled it forms

a very long thread that is made up of one DNA molecule and proteins s

The DNA is made up of units called nucleotides

Each nucleotides contains : Five carbon sugar Phosphate sugar A nitrogenous base

DNA molecules is made up of four different types of nucleotides which have varied nitrogenous base

Nitrogenous base : Adenine (A) Guanine (G) Thymine (T) Cytosine (C)

A G

C T

The deoxyribose of a nucleotide is linked to the phosphate group of and a nitrogenous base

The sequence of phosphate and sugar on the chain does not change

The sequence of bases differs from one DNA molecules to another

When the sequence of nitrogenous base is changed different sequences of nucleotides can obtained

DNA double helix Nucleotides are joined in a specific

sequence to form a polynucleotide

A DNA molecules consist of two polynucleotide chains that spiral and coil around each other to form a double helix

Two polynucleotides or strands are held together by hydrogen bonds between pairs of bases

How trait of an organism is manifested from the basic unit

of inheritance DNA double helix consist of many

genes, each located on a particular segment

The determinations of characteristics in organisms is controlled by the DNA through protein synthesis in the cells

Genes contain genetic code for the synthesis of polypeptides which make up part of an enzyme or protein

Genetic instructions is carried in the sequence of nitrogenous bases along the DNA molecules

It is coded by letters A, T, C, G

The nucleotide sequence in a segment of the DNA molecules determines the sequence of amino acids in the protein or enzymes to be synthesised

The flow of informations from a gene to a polypeptide or protein is based on the triplet codes

Different sequences of the three nucleotide bases are codes for different amino acids

Example : AAT – code for amino acid leucine AGT – code for amino acid serine

Protein function as the building blocks of an organism and control the chemical processes in an organisms

Importance of genetic research Manipulate genes for benefit for mankind

Combined genes from different species of organism

Identify specific genes that causes diseases and replace the defective genes

Forensic science – identify suspect in crimes

Genetic engineering bacteria – produce insulin

Agriculture – improved plant and animal product

Human genome project International research programme to

map all the human genes

To detect, map and determined the sequence of adenine, cytosine, guanine and thymine in all human genes

Benefit Identification of genes that cause disease Diagnoes, treatments and possible

prevention of many ailment

DNA samples – hair, saliva, blood, semen

Other applications : Screen genetic disorder Track genes that is responsible for certain

disease Test compatibility for potential organ donor

Advantages : Everyone has a different DNA fingerprint

except identical twins More useful than blood types forensic

because many people has the same blood type

- More information on a criminal identity

- very small quantities of DNA are required for test

- DNA samples last much longer than fingerprint

- DNA samples are much harder to clean up at crime scene

Disadvantages : Poor quality and poorly controlled testing

can lead to questionable result The origin of the DNA samples may be

question in courtroom

- difficult to analyse accurately blood that is mixed with wrong chemicals

Stem cell research Stem cells are undifferentiated cells that

can undergo unlimited division to form other cells

They can differentiate to form specialised functioning body cells Skin cells, red blood cells, nerve cells

Two types of stem cells :

1. embryonic stem cells

2. adult stem cells

Embryonic stem cells

- can be isolated from the embryos at the blastocyst stage

- can be derived from embryos that are created in vitro

Adult stem cells

-can divide but remain inactive until triggers prompt them to differentiate - injury

Only certain tissues have stem cells Brain tissues Skeletal muscle Liver tissue Blood vessel

Differentiate to become certain types of cells

Difficult to grow in a petri dish

Function of stem cells :

- treatment of injury or diseases

- develop ways to manipulate cells

Genetic engineering The gene manipulation and alteration of

genetic materials (DNA / RNA) of an organism to create new combinations of genes

Involves the transfer of genes on the DNA molecule from one living organism onto the DNA molecule of another organism

Genetic engineering involves :

- The transfer of genes produce a transgenic organism

- deletion or multiplication of genes within organism

- modification of existing genes or the construction of new one and the incorporation of the genes into a new organism

Original DNA combined with foreign genes – rDNA

Application of genetic engineering

- produce viral proteins that can be used to generate vaccine

- produce interferon – human protein which stop virus from multiplying

- produce growth hormone to treat abnormalities

- produce antibodies

- produce blood clotting factor

- produce enzyme to treat heart attack

Gene therapy The application of genetic engineering

techniques to alter or repace defective genes in human

Involves the insertion of genetic materials into a patient Its restore the function of the protein

Can be used for treatment of : Sickle cell anaemia Cystic fibriosis Cancer Heart problem

Genetically modified organism (GMO) Organisms whose genetic materials have

been altered

Benefits : Produce large quantities of safer drugs and

vaccines for humans and animals Example – mass production of human

insulin from genetically enginered bacteria to treat diabetis

Genetically modified food (GM food) The result of modifying organisms

genetically Transgenic plant – one or more genes

are added to a plant genome

Benefits : Improve surviving capability Greater resistance to pests and disease Improve nutritional values Increase immunity to certain herbicides Increase shelf life

Examples : wheat, soya, beans, tomatoes, maize, eggs

Transgenic animal – cloned DNA is injected into fertilised eggs The eggs are implanted in surrogate

mothers for development to take place

Benefits : Sheep – higher nutritional milk Tilapia – greater growth rate Cow – make the milk more suitable for

babies Salmon – grow faster

Controversies Genetic modifications is seen as

interfering with nature Genetic modification has not been

proven safe Widespread of pest resistant plant may

result in other plants to be resistant to the pest

Plant which are herbicides resistant may cross pollinate and make other plants become herbicides resistant

Unknown risk to human

Virus and bacteria with foreign genes may become dangerous pathogens

The use of discarded embryo in stem cells research is questionable because it is like killing lives