BIOLOGY FORM 5 NOTES CHAPTER 5 : INHERITANCE

CHAPTER 5: INHERITANCE

5.1   CONCEPT OF INHERITANCE BASED ON MENDEL’S EXPERIMENT

1. Genetic Science which is more systematic began in the middle of 19^th Century based on principles and evidence from experiments that conducted by Gregor Mendel.

2. Mendel’s experiments (Monohybrid Inheritance)
1. Mendel cross-pollinated the pea plants manually by brushing pollen from the tall pea plants onto the stigma of short pea plants. (Parental generation / P generation)
2. He collected the seeds and planted them.
3. All the seeds grew to become tall plants. (First filial generation / F₁ generation)
4. When he planted the seed of the F₁ generation, and the plants are allowed to self-pollinated.
5. The seeds are collected and produced the second filial generation, F₂ generation.
6. Three quarter of the offsprings were tall and one quarter were short. The ration of tall to short is 3:1

3. Conclusions from Mendel’s experiments:
1. There exist hereditary factors within each organism.
2. Each characteristic is controlled by a pair of factor.
3. During the formation of gametes, the two factors separate and each gamete contain only one factor.
4. These factors may be dominant or recessive.
5. If two factors differ, the factor that shows up its effect is dominant while the other is recessive.

      4.   Schematic diagram of a monohybrid cross.

 

      Punnet square: 

1. Mendel’s First Law or Law of segregation:

	The characters of a diploid organism are determined by alleles which occur in pairs. The two alleles of a gene separate or segregate from each other during the formation of gametes. Only one allele is carried in a gamete and the gametes unite randomly during fertilization. The resultant offspring receives one allele from its male parent and one from its female parent.

 

2. Terms used in genetic:

Term	Meaning
Characteristics	A distinctive feature of an organism that can be inherited from generation to generation. Example: height
Trait	The variant for a specific characteristic. Example: short or tall
Gene	The basic unit of inheritance that determine a particular characteristic in an organism
Allele	One of the two alternative forms of a gene that occupied the same locus on the homologous chromosomes.
Dominant allele	The allele that expresses itself and display the dominant trait of the characteristic when both alleles are dominant or at least one dominant allele is present.
Recessive allele	The allele that expresses itself in the absence of a dominant allele.
Phenotype	The physical appearance of an organism, which is an observable characteristic.
Genotype	The genetic composition or the genetic content of an organism.
Homozygote	The organism with two identical alleles for a particular characteristic.
Heterozygote	The organism with two different alleles for a particular characteristic
Homozygous dominant	Pair of identical dominant alleles
Homozygous recessive	Pair of identical recessive alleles
Heterozygous	Pair of different alleles, one dominant and one recessive
Purebred	A population of organisms which has a particular trait that remained unchanged for many generation.

3. Dihybrid Inheritance

(a)  A dihybrid inheritance is the inheritance involving a cross between two parents that differ in two characteristics.

(b)  Mendel crossed pure breeds of tall pea plants which produced round seeds with short pea plants which produced wrinkled seeds.

(c)  All the F₁ generation offsprings were tall plants with round seeds.

 

(d)  The F₁ hybrid plants were allowed to self-pollinated. The seeds formed were then planted. These produced the F₂ plants.

Phenotype ratio:

  Tall, round seeds:Tall, wrinkled seeds:Short, round seeds:Short, wrinkled seeds

=

Mendel’s Second Law Of Inheritance Or The Law Of Independent Assortment:

 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.

5.1   INHERITANCE OF TRAITS IN HUMANS

1.    Human blood groups and genotypes

Blood group (Phenotype)	Possible Genotype
Blood group A	IAIA or IAIO
Blood group B	IBIB or IBIO
Blood group AB	IAIB
Blood group O	IOIO

                        Allele I^A  and I^B are dominant. AlleleI^O is recessive.

                                Allele I^A  and I^B show codominance.

CASE 1: Father of blood group A (homozygous) married with mother of blood group B (homozygous)

 

If father’s blood group is A (homozygous), the genotype of the father is ____________. If mother’s  blood group is B (homozygous), the genotype of the mother is ____________. Gamete of the father contains _______ while gamete of the mother contains _______. If gamete of the father contains _______ fuses with gamete of the mother contains_______ during fertilization, the genotype of the offspring formed is ___________ and the blood group is ___________.

________% of the offspring formed have blood group ________.

CASE 2: Father of blood group A (heterozygous) married with mother of blood group B (heterozygous)

 

              If father’s blood group is A (heterozygous), the genotype of the father is ____________. If mother’s  blood group is B (heterozygous), the genotype of the mother is ____________. Gamete of the father may contain _______ or ________ while gamete of the mother may contain _______ or ________. If gamete of the father contains _______ fuses with gamete of the mother contains_______ during fertilization, the genotype of the offspring formed is ___________ and the blood group is ___________. If gamete of the father contains _______ fuses with gamete of the mother contains_______ during fertilization, the genotype of the offspring formed is ___________ and the blood group is ___________. If gamete of the father contains _______ fuses with gamete of the mother contains_______ during fertilization, the genotype of the offspring formed is ___________ and the blood group is ___________. If gamete of the father contains _______ fuses with gamete of the mother contains_______ during fertilization, the genotype of the offspring formed is ___________ and the blood group is ___________.

________% of the offspring formed have blood group ________.

   ________% of the offspring formed have blood group ________.

________% of the offspring formed have blood group ________.

________% of the offspring formed have blood group ________.

CASE 3: Father of blood group AB married with mother of blood group O

1.    The antigen and the antibodies present in the different blood groups

Phenotype (blood group)	Antigens on red blood cells	Antibodies present in the blood serum	Can donate blood to blood groups	Can receive blood from blood groups
A
B
AB
O

2.    The Rhesus Factor

-       The Rhesus factor is an antigen present on the surface of red blood cells.

-       The Rhesus factor is controlled by a pair of alleles: the Rh allele and the rh allele.

-       The Rh allele is dominant and the rh allele is recessive.

-       If an individual has the Rhesus factor, he is known as Rh-positive (Rh+). The genotype of a Rh-positive individual can be either homozygous dominant (Rh-Rh) or heterozygous (Rh-rh)

-       If an individual does not have the Rhesus factor, he is known as Rh-negative (Rh-). The genotype of a Rh-negative individual is homozygous recessive (rh-rh)

CASE 1: A man with homozygous Rh-positive marries a woman who is Rh-negative. What are the chances of their children being Rh-negative?

If father is homozygous Rh-positive, the genotype of the father is ____________. If mother is Rh-negative, the genotype of the mother is ____________. Gamete of the father contain _______ while gamete of the mother may contain _______ .If gamete of the father contains _______ fuses with gamete of the mother contains_______ during fertilization, the genotype of the offspring formed is ___________  and the offspring is ____________________.

________% of the offspring formed are  ____________________.

________% of the offspring formed are  ____________________.

CASE 2: A man with heterozygous Rh-positive marries a woman who is Rh-negative. What are the chances of their children being Rh-negative?

If father is heterozygous Rh-positive, the genotype of the father is ____________. If mother is Rh-negative, the genotype of the mother is ____________. Gamete of the father may contain _______ or _______ while gamete of the mother may contain _______ .If gamete of the father contains _______ fuses with gamete of the mother contains_______ during fertilization, the genotype of the offspring formed is ___________ and the offspring is ____________________. If gamete of the father contains _______ fuses with gamete of the mother contains_______ during fertilization, the genotype of the offspring formed is ___________and the offspring is ____________________.

________% of the offspring formed are  ____________________.

________% of the offspring formed are  ____________________.

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

-       Usually the first transfusion does not result in any reaction.

-       Is subsequent transfusion, the recipient’s blood reacts by producing Rhesus antibodies.

-       The Rhesus antibodies result in agglutination of the donor’s blood in the recipient and this may lead to death.

2.    The Rhesus factor can also be a problem when a Rh-negative mother has more than one Rh-positive baby.

-       During the later stages of the first pregnancy, fragments of the Rh-positive red blood cells of the foetus may enter the mother’s blood circulation.

-       This cause the mother’s immune system to react by producing Rhesus antibodies.

-       Normally, the amount of the antibodies formed is not sufficient to cause any effect on the first born.

-       However, if in a subsequent pregnancy, the foetus is also Rh-positive, the Rhesus antibodies of the mother may enter the foetus’s blood circulatory system and agglutinate its red blood cells.

-       The second baby will die if its blood is not replaced with Rh-negative blood in a blood transfusion or given an intravenous injection of anti-rhesus antibodies.

1.    Autosome and Sex Chromosomes

-       There are two types of chromosomes:

(c)  Autosomes: control characteristics of an organism except sex.

(d)  Sex chromosomes: determine the sex of an organism

-       In humans, there are 46 chromosomes, 44 of which are autosomes and two are sex chromosomes (XY for males and XX for females)

2.    Determination of the sex of a child

-       In human, the somatic cells of a male have 44 autosomes and sex chromosomes X and Y. During the formation of gamete, the number of chromosomes is halved. Every sperm has only 22 autosomes and one sex chromosome, either X or Y.

-       The somatic cells of a female have 44 autosomes and two sex chromosomes of X. During the formation of gamete, the number of chromosomes is halved. Every ovum has only 22 autosomes and one sex chromosome X.

 

-       During fertilization, if a sperm with ____________chromosomes fertilizes an ovum with ____________chromosomes, the child that is formed has _____________chromosomes, it is a ___________child.

-       If a sperm with ______________chromosomes fertilizes an ovum with ____________chromosomes, the child that is formed has __________ chromosomes, it is a _________ child.

-       The probability of having a male and a female child is _____________.

3.    Hereditary Disease

-       Hereditary diseases are genetic diseases that the offspring inherit from their parents.

-       There are hereditary diseases that are caused by defective genes

Ø  linked to X chromosomes

Ø  located on the autosomes

4.    Sex-linked inheritance

-       There are some characteristics that are controlled by genes located in the sex chromosomes, especially on the X chromosomes.

-       This characteristic is called sex-linked characteristic.

-       The inheritance of such characteristic is called sex-linked inheritance.

-       The genes on the sex chromosomes are celled sex-linked genes.

-       Examples of sex-linked diseases that can be transmitted from the parents to their offspring:

§  Haemophilia

§  Colour blindness

§  Muscular dystrophy

                                     

(c)  Haemophilia

-       It is a disease where the blood clots very slow when there is an injury.

-       This is because the patient is lack of blood-clotting factors, which will result in excessive blood loss.

-       Haemophilia is caused by a recessive allele (h) located on the X chromosome.

-       The dominant allele for normal bloot clotting = X^H

The recessive allele for haemopilia = X^h

-       The genotype for the blood clotting characteristic:

X^H X^H =  Female, homozygous dominant for normal blood clotting

X^H X^h =  Female, heterozygous dominant for normal blood clotting, but she is a carrier because there is one recessive allele on her X chromosome which can be inherited by her offspring.

X^h X^h = Female, homozygous recessive for haemophilia

X^H Y  = Male, with normal blood clotting

X^h Y  = Male, with haemophilia

CASE 1:  Male with normal blood-clotting married with female carrier of haemophilia

The genotype for male with normal blood-clotting is ____________.  The genotype for female carrier of haemopilia is ____________.  The gamete of the male with normal blood-clotting is __________ or __________. The gamete of the female with normal blood-clotting is __________ or __________. During fertilization, if gamete of the father contains _______ fuses with gamete of the mother contains_______, the genotype of the offspring formed is ___________ and the offspring is _______________________________________. If gamete of the father contains _______ fuses with gamete of the mother contains_______, the genotype of the offspring formed is ___________ and the offspring is _______________________________________. If gamete of the father contains _______ fuses with gamete of the mother contains_______, the genotype of the offspring formed is ___________ and the offspring is _______________________________________. If gamete of the father contains _______ fuses with gamete of the mother contains_______, the genotype of the offspring formed is ___________ and the offspring is _______________________________________.

The probability of having a female child who is normal for blood clotting is ________ %. The probability of having a male child who is normal for blood clotting is ________ % and a male haemopiliac child is ______ %.

CASE 2:   Male with normal blood-clotting married with female haemophiliac.

(b) Colour blindness

-       It is a condition in which a person cannot distinguish certain colours. The most common form of colour blindness is red-green colour blindness which is the inability to differentiate between red and green colours.

-       Colour blindness is caused by a recessive allele (b) located on the X chromosome.

-       The dominant allele for normal colour vision = X^B

The recessive allele for colour blindness = X^b

-       The genotype for the blood clotting characteristic:

X^B X^B =  Female, homozygous dominant for normal vision

X^B X^b =  Female, heterozygous dominant for normal vision, but she is a carrier because there is one recessive allele on her X chromosome which can be inherited by her offspring.

X^b X^b = Female, homozygous recessive for colour blind

X^B Y  = Male, with normal vision

X^b Y  = Male, with colour blind

CASE 1:  Male with colour blind married with female carrier of colour blindness

 

The genotype for male with colour blind is ____________.  The genotype for female carrier of colour blindness is ____________.  The gamete of the male with colour blind is __________ or __________. The gamete of the female carrier of colour blindness is __________ or __________. During fertilization, if gamete of the father contains _______ fuses with gamete of the mother contains_______, the genotype of the offspring formed is ___________ and the offspring is _______________________________________. If gamete of the father contains _______ fuses with gamete of the mother contains_______, the genotype of the offspring formed is ___________ and the offspring is _______________________________________. If gamete of the father contains _______ fuses with gamete of the mother contains_______, the genotype of the offspring formed is ___________ and the offspring is _______________________________________. If gamete of the father contains _______ fuses with gamete of the mother contains_______, the genotype of the offspring formed is ___________ and the offspring is _______________________________________.

The probability of having a female child who is normal for colour vision is ________ %. The probability of having a male child who is normal for colour vision is ________ % and a male colour-blind child is ______ %.

CASE 2:   Male with normal colour vision married with colour-blind female.

1.    Other hereditary disease

(c)      Albinism

-     Albinism is caused by a defective allele which is involved in the synthesis of melanin pigment.

-     The gene for melanin pigment synthesis is isolated in an autosome and is mutated.

-     The pigment cells from the skin, hair and iris are unable to synthesis melanin. Thus, these affected body parts lack melanin pigment.

Case: A man with albinism married a normal woman. What are the changes for them to have a child with albinism?

           A =

                 a =

(c)      Sickle cell anemia

-     This hereditary disease is caused by a defective allele which is involved in the synthesis of haemoglobin.

-     The gene of haemoglobin synthesis is located in an autosome and has undergoes mutation.

-     The red blood cells form defective haemoglobin that cause the red blood cells to be in the shape of a sickle.

-     The abnormal haemoglobin then causes the red blood cells to transport less oxygen, resulting anemia. This sickle-shaped red blood cells are more fragile and can break easily and then aggregate together to clog the blood capillaries.

(c)      Thalassaemia

-       Thalassaemia is a blood disorder that disrupts the production of haemoglobin in red blood cells.

-       The disease is caused by a recessive allele which causes abnormal haemoglobin to be produced.

-       Individuals with thalassaemia have either one or both parents suffering from thalassaemia. When one parent has the disease, the child will suffer from thalassaemia minor. When both parents have the disease, the child will suffer from thalassaemia major.

-       The symptoms range from mild to severe anemia. The most common symptom for thalassaemia minor is long-term anemia. Symptoms for   thalassaemia major include paleness, jaundice and enlarged spleen, liver and heart.

-       Thalassaemia can be diagnosed through blood test. Those with thalassaemia may need regular blood transfusions. The treatment generally will lead to iron overload. Consequently, a therapy is needed to reduce the excess iron in the body. Another treatment for thalassaemia major is a bone marrow transplant.

Case 1: A male with normal red blood cells married a woman with thalassaemia.

Case 2: A male with thalassaemia carrier married a woman with thalassaemia carrier

Case 3: A male with thalassaemia carrier married a woman with thalassaemia

5.1   GENES AND CHROMOSOMES

1.    Genes

-       A gene is a basic unit of inheritance.

-       A gene contains genetic information which determines a particular characteristic in an organism.

-       It can exist in different forms called alleles, that will determine the traits of an organism.

-       A gene occupies a specific position (locus) in a chromosome.

2.    Chromosomes

-       Chromosomes are threadlike structures in the nucleus of a cell

-       Each chromosome is make up of a long DNA molecule coiled around protein molecules called histones.

Genes are located in the DNA molecule. It is a sequence of nitrogenous base in the nucleotides of DNA which forms a particular sequence of genetic code.

1.    Structure of DNA

-       DNA is a type of nuclei acid.

-       A DNA molecule is made up of basic units called nucleotides.

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

-       There are four different types of bases: Adenine (A), Thymine (T), Cytosine (C) and Guanine (G). Hence, there are four different types of nucleotides.

-       Each nucleotide is joined to the next nucleotide through the phosphate group to form a long polynucleotide strand.

A DNA molecule consists of two polynucleotide strands which are linked together at the nitrogenous base by hydrogen bond.

-       The sequence of nitrogenous base in a DNA molecule is called a gene.

Based on the model of DNA molecule proposed by Watson and Crick, a DNA molecule consists of two polynucleotide strands coiled together, forming a double helix.

A schematic diagram to show how a trait is manifested from the basic unit of inheritance.