CHAPTER 3

CHAPTER 3HEREDITY AND VARIATION

3.1 CELL DIVISION

Genes Genes are the basic units of inheritance that determine the characteristics of the individual. Genes carry genetic information from one generation to another.Genes are located in long molecules known as deoxyribonucleic acid (DNA)

CHROMOSOMESAre located in the nucleus Carrying information that determines our characteristicEach chromosome is made up from DNA

Relationship between gene, chromosome, and DNA.

Type of Cell Division.the processes of reproduction and growth of any organism involve cell divisionthere are two types, which is :mitosismeiosis

Mitosis

Is a type of cell division which occurs in somatic cell.Each cell divides into two part identical daughter cellsOnly occurs in somatic cell but not in reproductive cells

Genetic content of the new cells is the same as that the parent cell.The new cells have the same number of chromosomes as the parent cell

the importance of mitosis :Increases the number of cells during growthReplace dead or damaged cellsRepairs injured cells

Meiosis

the process of cell division to produce gametes Contain half of the number of chromosomes of the parent cell.occurs only in the reproductive organs.

occurs in the testis of a male and the ovary of female who have attained sexual maturity.meiosis also occurs in the anther and ovary of plants.

the importance of meiosis :The daughter cell have the same number of chromosomes as parent cell after fertilization has taken placeGenetic variation between the offspring increases because of crossing-over between the chromatidsThe offspring produces will have different characteristic because of variation

Comparison between meiosis and mitosis

3.2: The principles and mechanism inheritance

Dominant genesare represented by the letter T

Recessive genesare represented by the letter t.

The dominant and recessive characteristic in humanDominant traitsRecessive traitsBlack hairBrown hairCurly hairStraight hairFree ear lobesAttached ear lobesAble to roll the tongueUnable to roll the tongueTallShortBlack/ brown eyesBlue eyesCheeks with dimplesNo dimples on the cheeks

The mechanism of trait inheritanceGregor Mendel (1822-1884) was an Austrian scientist known as the father of genetics because he discovered the principle of inheritance.

Genotype refers to the genetic information found in the genes of an organismPhenotype refers to the physical properties which can be seen in organism.The genotype of an organism determines the phenotype of the organism

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combination of gametes at the first filial generation, F1, can be shown in the Punnett Square.

GenTtTTTTttTttt

3.3 Sex Determination and the occurrences of twins in human being

Sex Determination.the sex of child is determined by the father according to the type of sperm that fertilises the ovum.if a sperm that carries the x chromosomes fertilises an ovum, a baby girl is born.the y chromosomes fertilises an ovum, a baby boy is born.the probability of having a male child or female child is the same> 50 : 50

Occurrence of twinsin humans, normally one ovum fertilised by one sperm to form one zygote which will develop to form an embryo.sometimes more than one embryo is produced in a pregnancy.twins - two babies born to a mother in a pregnancy and generally at the same time.there are two types of twins identical twins non-identical twins

Comparison between identical and non-identical twinsSimilaritiestwo babies born in a pregnancy

DifferencesIdentical TwinsNon-identical twinsFormationFormed when an ovum fertilised by a sperm divides into two to form two similar embryos.Formed when two ova are released by the ovary at the same time and fertilised separately by two sperms to form two different embryos.PlacentaThe two embryos formed will grow in one shared placenta in the mothers uterus.The two embryos formed when grow in separate placentas in the mothers uterus

Genetic make-upSame genetic make-up. Therefore, identical twins have the same appearance and the same sex.Different genetic make-up since they originate from two different ova and sperms. Therefore, non-identical twins have different appearances.SexThe sex of identical twins are the same.The sex of non-identical twins may be the same or different.

sometimes the division of the embryo is not complete, therefore Siamese twins are formed.Siamese twins may share certain organs like brain, stomach, or heart and maybe joined to one another at the head, abdomen, breast, or buttocks.they can be separated by surgery if they do not share important organs like brain, heart, kidneys, or lungs.

3.4 Mutationis the spontaneous change to the structure of genes or chromosomes of an organism.causes change of characteristics in a child.occurs in somatic cells or in gametes.the effect of mutation will be inherited by one generation to another.there are two types of mutation gene mutationschromosome mutations

Gene Mutationsinvolve changes in the structure of gene.gene mutations change or produce the new genes to replace normal genes.genes that have undergone mutation are called mutants.

Heredity disease caused by gene mutationBrief explanationAlbinismalbinisms is caused by the change in the gene that controls skin colour.the newly produced genes are unable to produce skin pigment.the skin and hair of albinos are white and their eyes are pink.

Albinism

Sickle cell anaemiasickle cell anaemia is a type of disease caused by the change in the genes that produce haemoglobin.haemoglobin that has undergone mutation is not efficient in transporting oxygen.

Sickle cell anaemia

Colour-blindnessis a sex-linked disease determined by a recessive gene on the X chromosome.a colour-blind person cannot see or differentiate the colours red and green.

Colour-blindness

Haemophiliacaused by the deficiency if a type of protein in the blood required for blood clotting.haemophilia is caused by the deficiency of a type of protein in the blood required for blood clotting.haemophilia is a sex-linked disease determined by a recessive gene on the X chromosome.a patient who has wound will experience continuous bleeding or will take a long time for the blood to clot thus this condition way lead to death.

Haemophilia

Chromosome Mutationsinvolve changes in the number of chromosomes or arrangement of genes in chromosomes.sometimes, a portion of a chromosome may break away and go missing, or become attached itself to other chromosomes.this causes the arrangement of a chromosome to change and differ from normal.the number of chromosomes can be increased or decreasedchromosome mutations can cause hereditary diseases like Downs syndrome, Klinefelters syndrome, and Turners syndrome

Downs syndromethis heredity disease is caused by the presence of one extra chromosome at chromosomes 21 in the somatic cells.incomplete separation of chromosomes during meiosis causes the zygote to have an extra chromosome : 47 in total.

characteristics of a downs syndrome patient :slanting eyesmetal retardationshort fingerswide distance between eyessmall mouth but big tongueprotruding tongue causing the mouth to remain partially open

Klinefelters syndromea person who suffers from this hereditary diseases has one extra X chromosome : XXY

Turners syndromea person who suffers from this hereditary lacks one X chromosome.people with Turner;s syndrome have only one sex chromosome which is XO.

Factors that cause mutation.mutation can occur naturally, caused by substances found in food or environment.agents causes mutations are called mutagens.chemical substances like pesticides, nicotine in cigarettes, drugs, nitrous acid and some preservatives, colouring and artificial sweetener can cause mutation.

radiation radioactive radiation can cause mutationgamma ray can affect growth and cell divisionultraviolet rays and X-ray rays can cause skin cells to mutate and this may cause cancer temperatures which are too high or too low can also cause mutation.

Advantages and disadvantages of mutation.

not all occurrences of mutation bring adverse effects to organisms.advantages of mutationmutation causes variations in organisms which allow them to adapt to the environment.species that are more resistant to disease, weather, and polluted environment can be produced.

disadvantages of mutationsome mutations cause diseases like colour-blindness and haemophilia cannot be cured.sometimes, disease like haemophilia and sickle cell anaemia can cause death.Klinefelters syndrome causes infertility.mutation causes physical, mental, or foestus retardation.