inheritance new

7/31/2019 Inheritance New

1/50

CHAPTER 5


2/50

Concept of Inheritance Based on

Mendels Experiment

Gregor Mendel (1822-1884).

An Austrian monk.

Known as father of genetics.

Observed garden pea plant (Pisum sativum) had

different characteristics: some had tall stems,

others had short stems; some had round seeds,

others had wrinkled seeds.Decided to carry out breeding experiment using

7 distinct characteristics of pea plants to find out

how these were transmitted from generation to

generation.


3/50


4/50

CHARACTERISTICS TRAITS

STEM LENGTH Tall Short

FLOWERPOSITION

Axial Terminal

FLOWER COLOUR Purple White

POD (FRUIT)SHAPE

Inflated Constricted

POD COLOUR Green Yellow

SEED SHAPE Round Wrinkled

SEED COLOUR Yellow Green

The seven garden pea characteristics studied by Gregor Mendels


5/50

Meaning of Inheritance,Characteristics and Trait

Inheritance: The transmission of particularcharacteristics from generation to generationby means of genetic code on the DNA of a

chromosome.Characteristic: A distinctive inherited

feature of an organism, such as height andcolour

Trait: Each variant for a specificcharacteristic. Eg: Seed colour ischaracteristic; yellow seed and green seed

are traits for this characteristic.


6/50

Monohybrid Inheritance

A genetic cross between two parents that

differed in only one characteristic known

as monohybrid inheritance.


7/50

Monohybrid inheritance

Mendel used pure breeds of tall and short

plants as the parents (P) and artificially cross-

pollinated them.

He observed that all the hybrid offspring of the

first filial generation (F1) were tall plants.

Mendel called the trait (tall stem) that showed

up in the F1 generation dominant and the trait(short stem) which did not show up

recessive.


8/50

He then, allowed the F1 plants to self-

pollinate. The seeds produced are planted

and produced the second filial generation

(F2). He recorded the number ofindividuals.

From 1064 plants, 787 were tall and 277

were short giving an approximate ratio 3:1.



9/50

Monohybrid Cross

ParentsPhenotypeGenotype

Tall X ShortT T t t

T TGametest t

Randomfertilisation

TtTtTtTt

All the first generation, F1, are tall plants and have the genotype Tt

When there is self-pollination of the F1 plants (Tt X Tt)

(continue next slide)


10/50

Monohybrid Cross

T t X T t

Gametes T t T t

Randomfertilisation

TT Tt Tt tt

In the second filial generation, F2, the ratio obtained is 3 tall plants : 1 short plant


11/50

whitepurple

x

all purple

705 purple 224 white

monohybrid ratio 3:1

pure-breedingparentsfirst filial (F1)

generation

second filial (F2)generation

Monohybrid Inheritance


12/50

From the results, Mendel made the following

deductions:

Within each organism are hereditary factors.

Each characteristic is determined by two hereditaryfactors in the chromosomes.

During the formation of gametes, the two factors are

separated and each gamete contains only one factor.

Fertilisation of gametes is at random; the zygotecontains two factors for a particular characteristic.

If the two factors differ, the factor that shows up its

effects is dominant while the other is recessive.



13/50

Mendel then proposed the First Law which

states that each hereditary characteristic is

controlled by the two factors.

During gamete formation, the two factors

separate or segregate and each gamete

will contain one factor.



14/50

Mendels First Law or Law of

Segregation states that each individualcharacteristic of an organism is

determined by a pair of alleles. The pairsof alleles segregate during meiosis and

only one of each pair of alleles can be

presented in a single gamete.



15/50

Terms used in genetics

Gene: A basic unit of inheritance thatdetermines a particular characteristic in an

organism. Consists of a segment of DNA

nucleotide on a specific locus of a chromosome.It controls a particular trait in an organism.

Allele: One of two alternative forms of a genethat can have the same locus on homologous

chromosome. One of each pair comes from themale parent and the other from the female

parent.


16/50


Locus: The location of a gene on thechromosome.

allele for dark skin colourallele for white skin colour


17/50


Dominant allele: the allele that produces thephenotype of the organism

Recessive allele: the allele that produces the

phenotype of the organism only when there is nodominant allele present.

Genotype: the genetic constituent of anorganism.

Phenotype: the observed characteristics of anorganism which is determined by the specificgenotype.


18/50


Homozygote: An organism where both the allelesof a particular gene of a pair of homologous

chromosomes are identical. Eg. TT, tt

Heterozygote: An organism where the alleles of aparticular gene on a pair of homologous

chromosomes are different. Eg. Tt

Pure line (pure breed): A population of organisms,

all having the same particular trait that has beengenetically unchanged through many generations.


19/50

Dihybrid Inheritance

Mendel then carried out dihybrid crosses

to explain how two different characteristics

were inherited.

Inheritance involving a cross between two

parents that differ in two characteristicsis known as dihybrid inheritance.


20/50

Dihybrid cross

Parent (P)PhenotypeGenotype

Tall, round seed

TTRR

X Short, wrinkled seed

ttrr

Gamete

Fertilization

All TR All tr

F1 GenerationGenotypePhenotype

TtRr

All tall, round seeds

Allow F1 plants to self-pollinate TtRr X TtRr

MeiosisTR Tr trtR trtRTrTR


21/50

Dihybrid cross

TR Tr tR tr

TR TTRR

Tall,round

TTRr

Tall,round

TtRR

Tall,round

TtRr

Tall,round

Tr TTRrTall,round

TTrrTall,wrinkled

TtRr

Tall,round

TtrrTall,wrinkled

tR TtRR

Tall,round

TtRr

Tall,round

ttRR

Short,round

ttRr

Short,round

tr TtRr

Tall,round

Ttrr

Tall,short

ttRr

Short,round

ttrr

Short,wrinkled

MalegameteFemale

Gamete

F2 generation

PhenotypeRatio

Tall plant, Tall plant, Short plant Short plant

round seed wrinkled seed round seed wrinkled seed9 : 3 : 3 : 1


22/50

Dihybrid cross

From the dihybrid cross, Mendelproposed his second hypothesis whichknown as Mendels Second Law or Lawof Independent Assortment

Mendels Second Law or Law ofIndependent Assortment states thatduring gamete formation, each member

of a pair of alleles may combinerandomly with either member ofanother pair of alleles.


23/50

ABO Blood Group System

The human ABO blood group is an example ofmultiple alleles and there are three alleles involved.

However, only two out of these three alleles can bepresented on the same gene locus of a pair of

homologous chromosomes in a single diploidorganism.

The alleles determine the type of antigens found onthe membrane of red blood cells.

AllelesIAand IB are dominant to allele IOwhich isrecessive.

Alleles IAand IB are codominant and areexpressed eqully in the phenotype of the

heterozygous offspring.


24/50

ABO Blood Group System

Genotype Phenotype

(Blood Group)

Types ofantigen on red

blood cell

Antibody inblood plasma

IA IAIA IO

Blood group A A Anti-B

IB IBIB IO

Blood group B B Anti-A

IA IB Blood group AB A and B None

IO IO Blood group O None Anti-A and Anti-B


25/50

Rhesus Factor in Human

The rhesus factor (Rh factor) is referring tothe antigen found on the surfacemembrane of some red blood cells.

People who have Rhesus factor are saidto be Rhesus positive (Rh+) and those whodo not are considered Rhesus negative(Rh-).

Allele Rhesus positive is dominant to alleleRhesus negative.


26/50


A Rhesus negative mother who becomes

pregnant by a Rhesus positive

heterozygous father has a 0.5 probability

of having a child who is Rhesus positive.

If the father is homozygous dominant for

Rhesus positive, then the child would be

Rhesus positive.


27/50


Problem will arise during pregnancy if themother is Rh neg and the child is Rh pos.

During late pregnancy or during birth,

some of the foetal blood cells may enterthe mothers blood.

This triggers the mothers blood to

produce anti-rhesus antibodies. Theharmful effects do not show in the firstpregnancy.


28/50


When the mother get second pregnancy, theantibodies level builds up in the maternalbloodstream.

Her anti-rhesus antibodies may cross theplacenta and cause agglutination andhaemolysis of the foetal red blood cells.

The condition is known as erythroblastosis

fetalis. The baby may suffers from jaundice,severe anemia and damage of the heart, liverand brain.


29/50

Human Autosomes and SexChromosomes

Each human somatic (body) cell contains46 chromosomes. There are 22homologous pairs of autosomes and

one pair of sex chromosomes.Autosomes are chromosomes that occur

in homologous pairs in both males andfemales. They do not contain gene thatcontrol sex determination. They haveidentical appearance in both male andfemale.


30/50


Sex chromosomes are the chromosomescontaining genes that determine the sex ofan organism.

Human females have two identical sexchromosomes called X chromosomes.

Human males have one X chromosome

and one smaller Y chromosome.The Y chromosome carries fewer genes

than the X chromosome.


31/50

Human Karyotype

Male Karyotype Female Karyotype


32/50


Downs syndrome is an example where anindividual has an abnormal number ofautosomes.

The individual has trisomy 21 that is threechromosome 21 instead of two chromosome21.

It is the result of non-disjunction during meiosis

where the two homologous chromosome 21 failto separate normally during anaphase I oranaphase II of meiosis.


33/50

Downs Syndrome

Individuals with Downs syndrome have flat, broad faces, slanted

eyes, a protruding eyes, protruding tongue, short palms and tend tobe mentally retarded.

The risk of having a Downs syndrome child increases with maternal

age.


34/50

Downs Syndrome

The karyotype of

a person with

DownsSyndrome

Trisomy 21


35/50

Sex Determination In Human

During the formation of gametes in thefemale, the sex chromosome segregate(separate), all the female eggs produced

contain 22 autosomes and one Xchromosome.

The human male produces two types ofsperms: sperms with 22 autosomes andone X chromosome and sperms with 22autosomes and one Y chromosome.


36/50

Sex Determination In Human

As the mothers egg (ovum) contains only

X chromosome, it is therefore the fathers

sperm that determines the sex of the child.

If the sperm carrying an X chromosome

fertilises the ovum, the child is female.

If the sperm carrying a Y chromosome

fertilises the ovum, the child is male.


37/50

Sex-linked Inheritance in Humans

Humans have a pair of sex chromosomes.

However, not all genes located on these

chromosomes are involved in sex determination.

These genes that are not involved in sexdetermination are called sex-linked genes.

Most sex-linked genes are carried by the Xchromosome because X chromosome largerthan the Y chromosome and can carry more

genes.


38/50

Examples of sex-linked disease

Haemophilia Hereditary sex-linked disease caused by recessive allele found

on the X chromosome.

When there is injury, the blood clots very slowly owing to thelack of blood-clotting factors

This leads to excessive loss of blood. Affects males more than females.

In females, if only one X chromosome carries the recessiveallele, its effect will be masked by the dominant allele on theother X chromosome. These heterozygous females (XHXh) arecarriers.

They do not suffer the disease but may pass the recessiveallele on to their offspring.

Female will suffer from the disease if only she inherits bothrecessive alleles ( XhXh) from her parents.

Males need to have only one recessive allele (XhY) to inheritthe disease.


39/50

Examples of sex-linked disease

Colour blindness

Unable to see the difference between all or

some colours.

Most common example is red-green colourblindness.

Caused by recessive allele located on the X

chromosome.

More common in males than females.


40/50

Hereditary diseases

Hereditary diseases are disorders that can beinherited.

Some hereditary diseases are caused bydefective genes found on the autosomes.

Eg: Cystic fibrosis, albinism, sickle cell anemiaand thalassemia.

Other hereditary diseases are sex-linked

commonly X-linked. Eg: red-greed colour blindness, haemophilia,

Duchenne muscular dystrophy.


41/50

Chromosomes and Genes

A gene is a basic unit of inheritance. The traits which you inherit from your parents

are controlled by genes found on thechromosomes in the nucleus.

Human somatic cell contains 23 pairs ofchromosomes.

Each chromosome is made up of a long DNA(deoxyribonucleic acid) molecule coiled around

protein molecules called histonesA DNA molecule contains thousands of genes

which code for the synthesis of specific proteins.


42/50


43/50


44/50

Structure of a DNA nucleotide


45/50

Chromosomes and Genes

A DNA molecule consists of two polynucleotide

strands coiled together to form a double helix.

The two strands are antiparallel that is facing the

opposite directions. The base adenine linked to thymine, and

cytosine to guanine by hydrogen bonds.

The sequence of nitrogenous bases forms the

genetic codes that determine the characteristics

of organisms.


46/50

Application of Knowledge in Geneticsto Mankind

Selective breeding

Apply in both animals and plants.

To produce offspring that possess desirable

characteristics of both parents.Eg: selection of suitable oil palm plants to

produce a hybrid plant with desirable

characteristics.


47/50

Application of Knowledge in Geneticsto Mankind

Genetic engineering

Also known as recombinant DNA technology.

Involves techniques used to alter characteristics of an

organism by introducing target genes from another

organism into its DNA.

This modified DNA is known as recombinant DNA.

The organism with the recombinant DNA is known as a

genetically modified organism (GMO).

Genetic engineering also used in gene therapy and

commercial production of drugs such as insulin and

vaccines for hepatitis B.


48/50


49/50

DNA Fingerprint


50/50

Human Genome Project

A genome is an organisms complete set of

genes made up of DNA nucleotide bases.

The human genome project was formed to:

Determine the sequence of all the base pairs found inthe DNA of human genome.

Make maps showing the exact locations of genes for

major sections in human chromosomes.

Produce linkage maps where inherited traits, forexample, genetic diseases can be tracked over

generations.

inheritance new

Documents