Inheritance

CHAPTER 5INHERITANCE

5.1 MENDEL EXPERIMENT

•The passing on genetic instructions from generation to generation is called inheritance.

•The scientific study of inheritance is called genetics.

•It was in the 1850’s that the study of inheritance began.

•Discoveries made by Gregor Mendel, an Austrian monk, contributed to the basis of modern genetics.

MENDEL EXPERIMENT• Mendel chose pure-breeding pea plants to study

the inheritance of several characteristics.• A pure-breeding plant is obtained after many

generations of self-pollination.• They produce identical offspring and the

offspring show the same traits as their parents.• Table 5.1 shows the difference in meaning

between characters and traits. Character Trait

Plant height Tall or short (dwarf)

Seed colour Green or yellow

Seed shape Round or wrinkled

MENDEL’S RESULT• In one of his experiment, Mendel chose two

parent plants, one a pure-breeding tall plant and other a pure breeding short plant.

•He called this generation the parental generation or P generation.

•He carried out cross-pollination on the two plants by transferring the pollen grains from the tall plant onto the stigma of the short plant.

•He collected the seed, planted them and found that all grew to become tall plants.

•The results of the parental cross appeared in the first generation called the first filial generation or F1 generation.

Mendel’s experiment with tall and short pea plants

5.2 Schematic diagram of Mendel’s Experiment

• Mendel then allowed the plants of the F1

generation to self pollinate to produce the

second filial generation or F2 generation.

• He discovered that when he planted the seeds

from the F2 generation, about three quarters of

the offspring were tall and one quarter was short.

The ratio tall to short is 3:1.

• Mendel found similar results when he crossed

pure-breeding pea plants for each of the other

characteristics.

Character and traits of pea plants studied by Mendel

A pair of homologous chromosomes

From the results of his experiments, Mendel concluded that:

a) Inheritance depends on the transfer of heredity factors

from parents to offspring. There is a heredity factor that

determine a particular characteristics.

b) Each characteristic is controlled by a pair of factors.

c) The factors are passed from generation to generation

unaltered.

d) The factors may be dominant or recessive. Some factors

are not expressed in every generation.

e) The factors segregate or separate during gamete

formation so that each gamete contains only one pair of

factors for a given characteristic.

Genes and alleles

• Genes are the basic units of inheritance which occupy specific

positions on chromosomes.

• The position of the gene is called its locus.

• Gene determine specific characters in an organism.

• For example, in Mendel’s experiments there is a gene which

determine height of the pea plants.

• The height of the pea plant is expressed as tall or short.

• It is determined by factors called alleles.

• Alleles are different forms of the same gene for a trait and occupy

the same relative position on a pair of homologous chromosomes.

• For example, there is an allele for the tall and an allele for the short

trait.

• Alleles also appear in pairs.

Dominant & recessive alleles• Dominant allele will mask or cover the effect of

the recessive allele.• A recessive allele only express itself when the

dominant allele is absent.• A dominant allele is represented by a capital

letter, for example, T for tall.• A recessive allele is represented by a small

letter, for example, t for short.• If the organism is pure-breeding tall, we

represent it by TT.• If it is pure-breeding short, we represent it by tt.• Tt stands for hybrids of the F1 generation.• Hybrid are offspring o parents differing in at

least one cahracteristic.

Phenotype and genotype

•Phenotype is the observable characteristics of an organism like colour, size, form and structure.

•Example of phenotype are the various traits like tall and short plants, round and wrinkled, and yellow and green seeds.

•Genotype refers to the genetic composition of an organism and cannot be seen.

•The genotype TT and Tt indicate tall pea plants•The genotype tt shows the phenotype for short.

Homozygote and heterozygote

•Homozygote two same allele such as TT(tall) and tt(short)

•Heterozygote two different allele such as Tt (tall)

Importance of meiosis I in the segregation of allelles• In meiosis I, during metaphase I, the homologous

chromosomes are arranged on the equator.• During anaphase I, the homologous pairs separate

and each pair moves to the opposite poles.• On each pair of homologous chromosomes, there

are many pairs of alleles controlling different traits. • When the homologous chromosomes separate, the

pairs of alleles also segregate.• At the end of the meiosis process, only one allele for

each trait is found in each gamete.

Mendel’s First law of Inheritance

THE LAW OF SEGREGATIONThe members of each pair of alleles separate or segregate during the formation of gametes. Only one allele can be carried in a single gamete• Monohybrid cross : involve only one character

• Monohybrid inheritance : A cross which involves a single characteristic determined by one gene

• Dihybrid inheritance : A cross which involves two pairs two pair of allele determining two characteristics.

Dihybrid inheritance

Mendel’s Second Law of Inheritance

THE LAW OF INDEPENDENT ASSORTMENT

Two or more pairs of allele segregate independently of one another during the formation of gametes.

5.2 INHERITANCE OF TRAITS IN HUMAN•The ABO system•Autosome and sex chromosome•Different type human karyotypes•Sex determination in offspring•Sex-linked inheritance•Haemophilia•Colour blindness•Other heredity diseases

The ABO System• Some traits in human are controlled by more than two

alleles.

• The ABO system, is an example of a trait that is controlled by multiple allele.

• Multiple allele means there are more than two possible allele for a particular gene.

• There are four blood groups within the ABO system namely O, A, B and AB.

• These blood groups are determine by three alleles.

• They are allele A, allele B and allele O.

• Each individual carries only two of the three alleles.

• The three alleles can be written as IA,IB and IO.

• Alleles IA and IB are dominant to allele IO.

• Allelle IA and IB show codominance. This means when both alleles are present, they both have an effect.

Human blood groups and genotype

Blood Group (phenotype)

Possible genotype

Blood group A IAIA or IAIO

Blood group B IBIBor IBIO

Blood group AB IAIB

Blood group O IOIO

Use your knowledge on Mendel’s First Law to draw a schematic diagrams and show the inheritance of ABO blood group and the genotypes of offspring from the following:

•A father with blood group AB and a mother with blood group A (heterozygous)

•A father blood group is O and a mother with blood group AB.

Do you know that ?• If two different blood types are mixed together, the

blood cells may begin to clump together in the blood vessels, causing a potentially fatal situation. Therefore, it is important that blood types be matched before blood transfusions take place. In an emergency, type O blood can be given because it is most likely to be accepted by all blood types. However, there is still a risk involved.

• A person with type A blood can donate blood to a person with type A or type AB. A person with type B blood can donate blood to a person with type B or type AB. A person with type AB blood can donate blood to a person with type AB only. A person with type O blood can donate to anyone.

• A person with type A blood can receive blood from a person with type A or type O. A person with type B blood can receive blood from a person with type B or type O. A person with type AB blood can receive blood from anyone. A person with type O blood can receive blood from a person with type O.

• Because of these patterns, a person with type O blood is said to be a universal donor. A person with type AB blood is said to be a universal receiver.

The Rhesus factor

• The Rhesus factor is a group of antigens in red blood cells.

• This antigens will cause agglutination when it react with antibodies from individuals without this antigen.

• The Rhesus factor is controlled by a pair of alleles.• If an individual has the Rhesus factor, he is known as

Rh-positive.• If he does not have the Rhesus factor, he is known as

Rh-negative.• The Rh allele is dominant to the rh allele.• The inheritance of the Rhesus factor follows Mendel’s

Law.• An individual who is Rh-positive has the genotype Rh-

Rh or Rh-rh.• An individual who is Rh-negative has the genotype rh-

rh.

Inheritance of Rhesus factor

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

•For the first transfusion, there is no reaction.

•In subsequent transfusions, the recipient’s blood in the body of recipient reacts by producing Rh antibodies.

•This results in agglutination with the receiver’s blood.

Autosomes and sex chromosome • Each human somatic cell has 46 chromosomes.• In the human male:

The chromosome are arranges into 22 homologous pairs which are identical in size and shape.

In addition, there is one odd pair made up one larger chromosomes and one smaller chrmosome.

The larger chromosome is called the X chromosome, while the smaller chromosome is called the Y chromosome.

• In the human female:There are 22 homologous pairs of chromosomes.In addition, there is a pair of X chromosome

• The 22 homologous pairs are called the autosomes• The X and Y chromosome in the male, and the X and X

chromosomes in the female are called the sex chromosomes which determine the sex of an organism.

•When chromosomes are arranged and numbered by size, starting from the largest pair to the smallest pair their form what is called a karyotype for an individual.

•The autosome are numbered 1 – 22 and the sex chromosome are numbered 23.

The karyotype of a normal male

The karyotype of a normal human female

Observe the following human karyotype

Different human karyotypes•The cells of individuals with a genetic

disease or genetic disorder show a karyotype that is different from that of a normal human being.

•For example, an individual with Down’s syndrome shows a karyotype different from that a normal individual.

Sex determination in offspring•In somatic cells of a human male there are

44+XY chromosome.•In the female, there are 44+XX chromosome.•A male produces two types of sperms. One

carries a Y chromosome and the other an X chromosomes.

•The female only produces ova with X chromosomes.

•During fertilisation, when a sperm carrying a Y chromosome fuses with an ovum carrying an X chromosome, a male offspring is produced.

•However, when a sperm carrying an X chromosome fuses with an ovum carrying an X chromosome, a female offspring is produced.

•Draw a schematic diagram to show the sex determination in offspring.

Sex-linked inheritance

•Genes that control various traits which are found on the X and Y chromosome are called sex-linked gene.

•The Y chromosome is shorter than the X chromosome.

• It does not have many alleles that are found on the comparable portion of X chromosome.

•Hence, in the male, any trait caused by a dominant or recessive allele that is found on the X chromosome, will be expressed.

Haemophilia • Haemophilia is a condition which the blood

does not clot normally.• This condition is due to the lack of a protein

needed for normal blood clotting. • If often results in excessive bleeding.• The ability to produce a protein for the blood

to clot is due to a dominant allele.• An individual’s inability to produce the

protein is caused by a recessive allele found on the X chromosome.

• Individual who suffer from hemophilia are called hemophiliacs.

• Hemophiliacs can bleed to death from minor wounds.

•We can show the inheritance of haemophilia by using letters.

•To represent the normal dominant allele on the X chromosome, we use XH.

•The genotype for normal female is XH XH . The genotype of a heterozygous normal female is XH Xh . Heterozygous females are known as carrier of disease.

•A normal male XH Y and haemophiliac male is Xh Y.

•A sex-linked gene shows a criss-cross method of inheritance.

•This means the inheritance is from father to daughter and mother to son.

•This is due to the fact that the Y chromosome carries no such homologous allele on the same locus.

Schematic diagram of the sex-linked inheritance of haemophilia

• From the example, it can be seen that the haemophilia gene is passed from the mother to son.

• The probability of getting a normal son or a haemophiliac son is 0.5.

• The daughter are all normal but it is 50% likely that one of the daughters is carrier.

• The probability of the son to be a haemophiliac is higher compared to the daughter.

• This is because the recessive haemophilia gene is able to express itself in a male individual.

Colour blindness• Colour blindness is a condition in which

colours cannot be distinguished.• In most cases, a person who suffer from this

condition is unable to distinguish between red and green colours.

• Colour blindness is caused by a recessive allele carried on the X chromosome.

• The pattern of inheritance of this condition is similar to haemophilia.

• We can represent a normal vision gene as XC and a colour blind gene as Xc.

Thalassaemia •Thalassaemia is a childhood disease.•This disease is caused by a recessive gene

and affects the haemoglobin in the red blood cells.

•The red blood cells of a person with Thalassaemia are smaller.

•They may be deformed and pale in colour.•This is because the haemoglobin content

is less.•Thalassaemia is common among people in

Mediterranean and South-East Asian countries.

Huntington’s disease

•Huntington disease is caused by a single gene mutation which causes nerves to degenerate.

•This causes brain cells to die leading to behavioral changes and loss of mental powers.

5.3 GENES & CHROMOSME

•A gene is the basic unit of inheritance and has a specific location or locus on a chromosome.

•A chromosome is a thread-like structure found in the nucleus. Each chromosome is made up of a long DNA molecule coiled around protein molecules called histon.

•A DNA molecule contains thousands of genetic codes while the protein molecules do not carry any genetic information.

• The DNA molecule consists of two polynucleotide chains twisted about each other to form a double helix structure.

• Each polynucleotide chain is made up of many nucleotides through condensation.

• Each nucleotide is made up of a deoxyribose sugar, a nitrogenous base and a phosphate group.

• There are four different bases, thymine (T), adenine (A), cytosine (C) and guanine (G).

• Adenine is linked to thymine while cytosine to guanine by hydrogen bonds.

• The sequence of the bases forms the genetic codes.

• The genetic codes determine the characteristics of an organism through the synthesis of proteins.

A nucleotide

Structure of DNA

Application of knowledge in genetics

•Genetic knowledge and research are widely used in selective breeding and genetic engineering.

•DNA fingerprinting and the human genome project are two major fields involving genetic knowledge.

Selective breeding•Desireable characteristics in plants and

animals can be produced through selective breeding.

Genetic engineering•Genetic engineering or recombinant DNA

technology is the modification of the characteristics of an organism by manipulating its DNA.

•Genetic modified organisms (GMO), genetically modified food (GMF), gene therapy and production of medicine are applications of genetic engineering.

Genetic modified organisms (GMO)•Genetic codes of an organism can be

altered, added or taken out to produce new desirable characteristics.

•For example, the transfer of a gene from a firefly to a tobacco plant enables the plant to glow in the dark.

Genetically modified food (GMF)•Genetic codes from bacteria can be

transferred to crops such as maize plants to protect plants from pests.

•GMF gives higher yield, pest resistance, enhance quality, last longer and increases nutritional values.

Gene therapy

•Gene therapy is the insertion of genes into an individual’s cells to treat heredity disease such as sickle-cell anemia.

Production of medicine or drugs•A genetically modified form of insulin can

be produced by inserting human insulin gene into the E.coli bacteria.

DNA FINGERPRINTING

•DNA fingerprinting is a technique used to analyse a person’s DNA fragments.

•Certain nucleotide segments of the DNA do not code for proteins and exist as repeated short sequences of bases.

•DNA of a person is obtained from sample of semen, hair or blood and is cut into fragments using specific enzyme.

•The DNA fragments are separated and arranged to form specific band pattern.

•Each person has his own set of DNA except for identical twins.

Uses of DNA fingerprinting:•To identify criminals from samples of

blood, saliva, hair or semen collected at the scene of crime.

•To settle paternity disputes•To detect genetic diseases•To test compatibility of potential organ

donor with recipient•To confirm genotypes of animals and

plants

HUMAN GENOME PROJECT

• A genome is an organism’s complete set of genes.

• The project aims to map the position of the genes, to read and decode every code in the 46 human chromosome.

• Advantages of the human genome project:▫To understand the mechanism of a genetic

disease and find ways to prevent the disease▫To obtain information for diagnosis, treatment

and possible prevention of disorders or diseases such as cancer, diabetes and heart diseases.