CHAPTER 5 & 6SPM 2015

AINA MARDHIA KHALID

5:1 Concept of inheritance nased on Mendel’s Experiment

1. The seven garden pea characteristics studied by Gregor Mendel:

2. Meaning of inheritance, characteristic and trait :a) Inheritance :

I. Is the transmission of particular characteristics from generation to generation by means of genetic code on the DNA of a chromosome

II. These characteristic are transferred to the offspring in the gametesb) Characteristics :

I. Distinctive inherited feature of an organism,II. Example height and color

c) Trait :I. Each variant for a specific characteristicII. Example Curly hair,straight hair

CHARACTERISTIC HEIGHT OF A PEA PLANTTRAIT

3. Monohybridd inheritance :a) Inheritance involved a genetic cross between two parents that differed in only one

characteristic

b) Mendel used pure breeds of tall and short plants as the parents (P) and artificially cross-pollinated them

I. The trait that showed up in the first filial generation ( F1)nis generation DOMINANT and the trait which did not Show up is RECESSIVE

II. F1 to self pollinate1) Produced the second filial generation (F2)2) From the 1064 plants,787 were tall and 277 were short,giving an

approximate ratio of 3 : 1

Figure 5.4 Monohybrid cross

C) Deductions made :

I. Within each organism are hereditary factors II. Each echaracteristics is detemined by 2 hereditary factorsIII. During the formation of gametes,the 2 factors are separated and each gamete

contains only one factorIV. Fertilization of gametes is at random ; the zygote contain 2 factors for a

particular characteristicV. If the 2 factors differ, the factor that shows up its effect is dominant while the

other is recessived) Mendel’s First Law / Law of segregation :

States that each individual characteristics of an organism is determined by a pair of alleles. The pairs of alleles segregate during meiosis and only one of each pair of alleles can be present in a single gamete

4. Terms used in genetics

Term MeaningGene

Allele

Dominant allele

Recessive allele

Genotype

Phenotype

Homozygous

Heterozygous

Pure line/ pure breed

5. Structure of a pair of homologous chromosomes:

6. Dihybrid inheritancea. The inheritance involving a cross between two parents that differ in Two

characteristicsb. Mendel crossed pure breed of tall pea plants which produced round seeds with

short pea plants which produced wrinkled seedsc. All the F1 generation offsprings were tall plants with round seedsd. The F1 hybrid self-pollinate and self-fertilizede. The F2 plants produced a phenotypic relationship of approximately 9:3:3:1

Known as dihybrid

f. Mendel’s Second Law /Law of Independent Assortment:

States that during gamete formation,each member of a pair of alleles may combine randomly with either member or another pair of alleles

V.2 Inheritance 1. ABO BLOOD GROUP SYSTEM

a. Example of multiple allelesi. 3 alleles involved

b. The alleles determine the type of antigens found on the membrane of RBCsc. Alleles IA and IB are dominant

i. Are codominantii. Expressed equally in the phenotype of the heterozygous offspring

d. Allele IO is recessive

e. Example :

2. Rhesus factor in humansa. Refer to the antigen found on the surface membrane of some RBCsb. People who have Rhesus factor – Rhesus positive (Rh+)--- DOMINANTc. People who do not have Rhesus factor—Rhesus negative (Rh-)---RECESSIVE

d. Problems will arise during pregnancy if mother is RH- and the child is RH+I. During late pregnance or during birth,some of the foetal blood cells may

enter the mother’s bloodII. This triggers the mother’s blood to produce anti-Rhesus antibodiesIII. The harmful effecrs do not show during the first pregnancy

IV. With successive pregnancies,the antibody level builds up in the maternal bloodstream

V. Her anti-Rhesus antibodies may cross the placenta and cause agglutination and haemolysis of the foetal RBC – Erythroblastosis fetalis (RBC PECAH)

VI. The foetus suffers from jaundice,severe anemia and damage to heart,liver and brain

VII. In severe cases the baby may stillborn or die in first few daysVIII. How to overcome??

1. The blood of the infant has to be changed from Rh- blood,a badly affected foetus may be treated in worb

2. Giving the monther anti-Rh globulin just after first pregnancy,preventing the formation of antibodies

3. The Rh-mother should seek medical advice to prevent Erythroblastosis fetalis in subsequent pregnancies

3. Human autosomes and sex chromosomes

a. Each human somatic cells contain 46 chromosomes

b. 22 homologoues pairs of autosomes and one pair of sex chromosomes

c. Autosomes;

a. Chromosomes that occur in homologous pairs in both males and femalesb. Do not contain genes that control sex determinationc. Have identical appearances in both males and females

Sex chromosome;

a. Chromosome containing genes that determine the sexb. Human females-1 X chromosome and 1 smaller Y chromosomec. Y chromosome –carry fewer genes

4. Human autosomes and sex chromosomesa. Each human somatic cell contain 46 chromosomesb. 22 homologous pairs of autosomes and one pair of sex chromosomesc. Autosomes:

i. Chromosomes that occur in homologous pairs in both males and femalesii. Do not contain genes that control sex determination

iii. Have identical appearances in both males and femalesiv. Sex chromosomes:

- Chromosomes containing genes that determine the sex- Human females – 2 identical X chromosome- Human males – 1 X chromosome and 1 smaller Y chromosome- Y chromosome – carry fewer genes

v. Human karyotype :vi. Karyotype of a person with Down’s syndrome:

vii. Down’s syndrome :- Abnormal number of autosomes- 3 chromosomes 21- Trisomy 21- Results of non-disjunction during meiosis where 2 Homologous

chromosomes 21 fail to separate normally during anaphase I / anaphase II of meiosis

- Have flat,broad faces,slanted eyes,a protruding tongue,short palms and tend to be mentally retarded.

- The risk of having a down’s syndrome child increases with maternal age.

5. Sex determination in humansa. All the female eggs produced contain 22 autosomes and one X – chromosomeb. Sperms – 2 types

i. 22 autosomes + 1X ii. 22 autosomes + 1Y

c. Therefore, the father’s sperm that determine the sex of the child

6. Sex – linked inheritance in humansa. Genes on the sex chromosomes which are not involved in sex determinantion are called

sex-linked genesb. Most sex-linked genes are carried by the X chromosome

i. Larger and can carry more genesC. Haemophilia

i. Hereditary sex-linked diseaseii. Caused by a recessive allele found on the X chromosome

iii. Lack of blood clotting factors-the blood clots very slowly-excessive loss of blood

iv. The X-linked trait affects male > femalesv. In female :

1. If only one X chromosome carries the recessive allele,its effects will be masked by the normal dominant allele on the other X chromosome

2. These heterozygous females female (XH Xh) are carriesi. DO NOT suffer from the disease but may pass the recessive

allele on their offspring3. Only inherits the disease if she inherits bothe the recessive alleles (XhXh) from

her parentsvi. In males :

1. Need to have only one recessive allele (Xh Y) to inherit haemophilia

d. Color blindnessI. Unable to see the difference between all or some colours

II. Most common –red-green colour blindness- Inability to ddistinguish between red and green colors

III. Caused by a recessive allele located on the X chromosomeIV. Most common in MALE then females

7. Hereditary diseasesa. Are disorders that can be inheritedb. Some are caused by defective genes found on the autosomesc. Examples :

-cystic fibrosis,albinism,sickle cell anemia,thalassaemiad. others hereditary sex linked diseases

- Red-green color blind,haemophilia,muscular dystrophy

V.3 : Understanding Genes and Chromosomes 1. Gene :

a. Basic unit of inheritanceb. Controls the traitc. Found on the chromosome in the nucleus

2. Human somatic cell:a. 23 pairs of chromosomesb. Each chromosome is made up of a long DNA (deoxyribonucleic acid) molecule coiled

around protein molecule called HISTONE

3. DNA molecules :a. Contain thousands of genes which code for the synthesis of specific proteinsb. Made up of basic units called nucleotides

I. Made up of a deoxyribose sugar,a nitrogenous base and a phosphate group.II. There are 4 different bases

- Adenine (A)- Thymine (T)- Cytosine (C)- Guanine (G)

III. There are 4 different nucleotides each containing one of the four nitrogenous bases

IV. A phosphate group of one nucleotide is linked to the deoxyribose sugar on the next nucleotide

- The nucleotides are joined together to form a long polynuceotide strand

c. Consists of 2 polynucleotide strands coiled together to form double helix- The 2 strands are antiparallel ; facing opposite directions

d. Adenine is linked to thymine and cytosine is linked to guanine- By hydrogen bonds

e. The sequence of nitrogenous bases forms the genetic codes that determine the characteristics of organisms

4. Each gene :

- Occupies a specific position caled a gene locus on a chromosome

- gene :

-Is a DNA segment containing a particular sequence of nucleotides that code for the synthesis of a specific polypeptide chain or protein in an organism

1. Proteins are needed to form enzymes and other important components in the cells.They determine the characteristics of organisms

5. How a trait is manifested from the basic unit of inheritance:

7. Application of knowledge in genetics to mankind :a. Selective breeding

I. Both animals and plantsII. To produce off speing that poses desirable characteristics of both parents

III. Example :

b. Genetic engineeringI. Recombinant DNA technology

II. Alter characteristics of an organism by introducing target genes from another organism to its DNA

III. The modified DNA – recombinant DNAIV. The organism with the recombinant DNA- genetically modified organism

(GMO)V. Food derived from these organism- genetically modified organusm)

VI. Application :

chromosomes in nucleus contain DNA

DNA contain nucleotides

A DNA segment contains specific sequence of nitrogenous bases

which acts as a gene

The genes codes for a specific polypeptide,protein or enzyme

produces the trait of an organism

- Gene therapy- Production of drugs

*insulin*vaccines for hepatitis B

c. DNA fingerprinting ( DNA profiling)

I. There are certain nucleotide segments in the DNA that do not code for proteinsII. These segments are the characteristics for each individual

III. The probability of 2 individuals having the same banding pattern is very small except for identical twins

IV. The DNA of an individual can be obtained from samples of hair,blood,semesV. The DNA is cut into into short fragments by specific enzymes

VI. The fragments produce specific banding patterns like that of fingerprintsVII. Used :

- Identification purposes in solving criminal cases- Paternity disputes- Detect human genetic diseases- Confirm the genotyples of animals and plants in agriculture.

f. Human Genome ProjectI. A genome :

-is an organisms complete set of genes made up of DNA nucleotide basesII. Was formed to :

- Determine the sequence of all the base pairs found in the DNA of the human genome

- Make maps showing the exact locations of genes for major sections of human chromosomes

- Produce linkage maps where inherited traits (genetic diseases)can be tracked over generations

8. Abuse of knowledge in genetics to mankinda. Genetic engineering

- Raise many questions on ethics and morality- There are unknown long-term harmful risks which may take

months or years before the harmful effects show up- Food previously considered safe for consumption may become

harmful if it has been genetically modified owing to production of allergens

- GMF may contain an animal gene which may be objectional to vegeterians

- There is a danger that a transplant gene may activate certain human genes to cause cancer

- Laboratary GM microorganisms may be accidentally released into the environment and spread certain harmful genes to plants,animals and humans