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3.1 Introduction  
What is the most important factor in determining how we develop as human beings from the time of conception until death? Is development determined by inherited or environmental factors? Which is more important in determining development – heredity or environment? Scientists have been debating this issue since the beginning of the study of developmental psychology. Scientists such as, John Locke, J. B. Watson and B. F. Skinner believed that there are no inborn predispositions to development. Their research tried to prove that life experiences are the only factors responsible for shaping human development. Other scientists have argued to the contrary, saying that hereditary factors are the most crucial in influencing the course of development. Most contemporary developmental psychologists are in consensus that the two factors interact to determine development.   
At the end of this topic you should be able to:  
Explain the process of genetic inheritance  
Explain the nature-nurture controversy  
Differentiate between autosomes and sex chromosomes.  
Explain the cause and symptoms of various genetic disorders.  
Differentiate between genotype and phenotype.  
Differentiate between dominant and recessive genes  


2.2 The Mechanisms of Heredity  
Human life begins with the joining of the mother’s egg cell with the father’s sperm cell. These cells (gametes) carry the instructions or the blueprints that determine what characteristics an individual will inherit. Genetics is the science that studies heredity.  
The basis of human life is the protein molecules whose function is determined by arrangement of the 20 or so amino acids of which they are composed. In turn, the arrangement of the   amino acids is determined by a specific genetic code contained in a sequence of deoxyribonucleic acid (DNA) molecules located on rod-like structures called chromosomes.   
3.2.1 The Chromosome  
A chromosome is a microscopic rod-like structure in the nucleus of a cell that contains genes which are the carriers of heredity. There are two types of cells in our bodies; body and sex cells. 
Body cells contain identical genetic information. Each of our body cells contains an identical component of 23 pairs of chromosomes (each of which contain an identical sequence of DNA).  
One member of each pair of chromosome is inherited from the mother and the other pair from the father. That is, we inherit 23 chromosomes from each parent. The division of body cells involves Mitosis - a processes that results in genetically identical pairs of cells. Unlike body cells, mature sex cells (sperm and ovum) each contain 23 chromosomes and not 23 pairs. The gametes result from a special kind of cell division called meiosis, which results in daughter cells that have only half the number of the parent cell.  

3.2.2 Sex Chromosome  
Out of the 23 chromosomes contained in each sperm and each ovum, one, termed the sex chromosome determines whether the offspring will be male or female. The other 22 are called autosomes (i.e., the 22 paired non-sex chromosomes). The sex chromosomes are X and Y. Only the male can produce a Y chromosome, whereas both females and males can produce X chromosomes. The presence of a Y chromosome in a fertilized egg determines that the offspring will be a male; two X chromosomes determine a female. Thus, the fertilized egg may contain XX or XY pair. The X chromosome that is contributed by the mother to her son is sometimes believed to be the carrier of sex-linked, predominantly male defects and illnesses such as colour blindness, heredity baldness, and    haemophili. .  
3.2.3 Genes  

Genhaemophilia carriers of heredity. Each of the 23 chromosomes is believed to contain between 40,000 and 100,000 genes. These genes either in pairs or complex combination of pairs, determine our inherited characteristics. For example, there are pairs of genes that correspond to eye color, baldness, hair characteristics and almost every other characteristic of the individual. In addition, combinations of genes appear to be related to personality characteristics such as intelligence, temperament, aggression etc. Traits that result from the combination of many genes are termed polygenetic traits. For example, skin color and height. Genes for eye color and other traits take on a variety of forms called alleles i.e., the alternative states of genes (allelomorph). Allelomorph is a term used to refer to the various states in which genes carry different traits. A person is homozygous for a trait if alleles in the inherited pair for a trait are identical. For example, the pair for eye color can be blue-blue.   A person is heterozygous if two different alleles form the pair of genes for a trait. For example, the pair for eye color may be brown–blue.      
3.3.4. Dominant and Recessive Genes  
Certain members of genes may be dominant over their corresponding member.  When a dominant gene is paired with a corresponding recessive (a gene whose characteristics are not manifested in an offspring) gene, the characteristic corresponding to the dominant gene will appear in the individual.  For example, in humans, the brown-eyed gene is dominant; (it will exert its effect regardless of whether the other member of the gene pair calls for brown or blue eyes) whereas the blue-eyed gene is recessive.  
Some traits that are caused by recessive genes include blue eyes, baldness, color blindness, hemophilia, and sickle cell anaemia. Some recessive genes are carried on the sex chromosome (Meece, 2002). The traits are called sex-linked characteristics, because they are carried on the female sex chromosome. For example, baldness, color blindness, and hemophilia are sex linked characteristics Females are the carriers of these disorders, but males have a higher likelihood of inheriting them. The vulnerability of males occurs because a recessive gene on the X chromosome has no matching dominant gene on the Y chromosome to mask its expression.   

Learning activities:  
i)  What is a dominant gene? ii)  What is a recessive gene?  Answers:  
A gene that is expressed when paired with any different or subordinate gene.   
A recessive gene is one that is expressed only when it is paired with  a similar gene  

The recessive blue-eyed gene will lead to blue eyes if only there is an identical blue-eyed gene.  


Bl            Bl  

Br  Br    
Br   Bl  
Br  Bl  

Br   Bl  
Br Bl  

All the offsprings in the above example will have brown eyes. The gene for normally pigmented skin is dominant over the gene for albinism (unpigmented skin).  
3.3.5 Phenotype versus Genotype  
Phenotype is the characteristic we observe in a given individual. It is the manifested characteristics. Genotype refers to one’s genetic makeup or our inherited chromosomal make up. It consists of the 46 chromosomes out of which 23 are inherited from each parent. For example, if you have blue eyes, then your eyes are part of your phenotype.

At the same time, the genes that correspond to blue eyes define your genotype. Two people who have brown eyes may have different genotypes for eye color; one may be Brown-Brown; the other Brown-Blue. The phenotypes are the same, but the genotypes are different, the Blue-eyed gene is masked by its Brown counterpart.   
3.3.6 Genetic and Chromosomal Abnormalities   
In most cases, genetic disorders are linked with recessive rather than dominant genes. This is because any abnormality that is linked with a dominant gene will be manifested in all carriers and will have relatively little chance of being passed on to an offspring, and especially if it leads to an early death. Abnormalities linked to recessive genes will be manifested only when the carrier has inherited the related recessive genes. Many individuals may be carriers for a single recessive gene of abnormality without manifesting the abnormality. An example of a genetic disorder linked with a dominant gene is Huntington’s chorea – a fatal neurological disorder. It manifests itself at the age of 30 or 40, and therefore it is hard to detect and keeps on being transmitted. Examples of various genetic    defects are discussed below:  
Sickle-cell Anaemia, a genetic disorder linked with a recessive gene. Effects of the defective gene are clearly discernible (noticeable) in abnormally shaped red blood cells (sickle-shaped rather than circular) which multiply as a function of lack of oxygen. These cells tend to clot or clog together, carrying less oxygen, thereby increasing in number and reducing oxygen even further. Thus, the cells can’t carry enough oxygen to the body. Individuals who are homozygous for the gene frequently die in childhood or are severely ill throughout life. Those who are heterozygous are ordinarily healthy except in conditions of low oxygen such as high altitude. Sickle-cell anemia is common among blacks in central African coastal areas.   
Down’s Syndrome or Mongolism, a condition in which victims have low IQ (20- 60), broad noses, square shaped ears, protruding tongues, small squarish  heads, defective hearts, eyes with oriental appearance hence the term Mongolism. About one out of every 600 children is born with this defect. This defect is caused by an extra chromosome (chromosome 21). The person will have three rather than a pair, hence the label, trisomy  
21. Most cases of Down’s syndrome are due to non chromosome disjunction (failure to separate) of the 21st chromosome pair during meiosis. A smaller number of cases are due to translocation of chromosome 21 material to another chromosome (i.e., part of one pair breaks away). In this case, the number is normal but chromosomal damage is present. Turner’s syndrome is a defect that affects female children. One sex chromosome is missing. The individual has only one sex chromosome (0X). Victims have underdeveloped secondary sex characteristics, are short, sterile, and have webbed necks (having loose folds of skin). Injection of female sex hormone, estrogen may enhance the development of feminine characteristics.   
Klinefelter’s Syndrome, a defect that involves an extra X chromosome in male children (47, XXY). Victims may have both male and female secondary sexual characteristics. The male secondary sexual characteristics may be underdeveloped. They may have low IQ, are tall, thin, have long arms and legs, and may also be sterile. Injection of the male sex hormone, testosterone may enhance the development of masculine characteristics. Phenylketonuria (PKU), a disorder that is related to two recessive genes. The disease is caused by a recessive allele that fails to produce an enzyme necessary to metabolize the protein phyenylaline. Failure to digest this protein results to accumulation of poison in the bloodstream causing brain damage. If it is not treated immediately after birth, PKU damages the nervous system and causes mental retardation and hyperactivity.  The Super male or XYY Syndrome, the victim may be tall, have low IQ, and has a strong tendency towards aggression and violence. XYY syndrome is prevalent among criminals. However, not all criminals have this syndrome.  
Tay sacs Disease is a genetic disorder linked to a recessive gene. Both parents must be carriers of the trait for the child to have it. The victim dies after a few years. The child may appear normal at birth, but fat begins to accumulate in the brain cells. This results to blindness, deafness, and mental retardation and finally death. It is   a disorder in which the nervous system also degenerates. It is common in Eastern Europe and among Jews. Haemophilia is a blood disorder characterized by poor blood clotting ability. It is associated with an X-linked recessive gene. It is treated by transfusions of clotting factors. New gene-spicing techniques make it possible to provide these factors without running the risk of blood-borne infections by donated blood products.   
Muscular dystrophy  
Is associated with a recessive gene and is characterized by a degenerative muscular disease.   
Albinism gene

3.3.7 Nature-Nurture Interactions.  
Development is influenced by both heredity (nature) and environment (nurture).  For a long time psychologists have debated on the relative contribution of nature and nurture in determining our development and behaviour.  Is our development decided upon by heredity or by environment?  
At the moment of conception an incredible number of personal characteristics and growth patterns are determined.  The heredity instructions are carried by the chromosomes and genes  

The individual inherits 23 chromosomes from each parent to make a pair of 23 (46 chromosomes in total).  Each chromosome contains thousands and thousands of genes which determine our inherited traits.  Heredity determines the sequence of growth, the timing of puberty, skin colour, hair colour, body size, vulnerability to some diseases (e.g. mental disorders, sickle-cell anaemia), body shape, height, intelligence, athletic potential, personality traits and other traits considerably.  
Both heredity and environment are important in determining development.  The two are inseparable.  As one grows, there is a constant interaction between the forces of nature and nurture.  Therefore, the total person is a product of hereditary and environmental factors.  Heredity shapes development by providing a framework of personal potentials and limits that are altered by environmental factors such as nutrition, culture, disease, learning, parents, peers, home, school, etc.  Researchers sometimes use the concept “reaction range” to understand genetic and environmental influences. The concept refers to a person’s unique, genetically determined response to environmental influences. Two people with the same set of genetic instructions if reared in vastly different environments can develop different characteristics. Our genes determine the lower and upper boundaries for our development, but the environment can facilitate or impede that development.     
The influence of environment starts after conception and continues until death.  Our heredity does not change.  Heredity equips the person with innate capacities and the environment determines whether he/she will reach full potential or not.    
Today psychologists agree that heredity and environment interact to determine development. From an educational and developmental point of view, the most important thing to learn is that many of our characteristics can be influenced by the environment.

  Although there is little we can do about heredity, much of the environment still remains under our control.  Therefore, we should improve the environment for our children to attain their full inherited potential.  
Conception occurs when the egg and sperm unite to form the zygote.  
The growth and development of the fertilized egg is guided by its unique genetic programming.  
Sex chromosomes determine gender and other sex-linked characteristics.  
At the time of conception, the genes from both parents determine the traits to be inherited.  These include personality patterns, intellectual potential, mental illness, height, certain diseases, and body shape.  From the moment of conception and throughout life, the environment influences genetic tendencies.  
Chromosomes are threadlike structures on which genes containing the genetic code are located.  
An allele is an alternate form of a gene; typically, a gene has two alleles, one inherited from the individual’s mother and the other one from the father. If the alleles are the same, the person is homozygous for that particular characteristic. If the alleles are different, the person is heterozygous.  
When a dominant gene is paired with a corresponding recessive (a gene whose characteristics are not manifested in an offspring) gene, the characteristic corresponding to the dominant gene will appear in the individual.  
Phenotype is the characteristic we observe in a given individual. It is the manifested characteristic. Genotype refers to one’s genetic makeup or our inherited chromosomal make up.  
Genetic disorders are linked with recessive rather than dominant genes .  
The environment and genetic code interact to produce most of the characteristics of human behaviour.  
Today psychologists agree that heredity and environment interact to determine development.  
Self-Assessment and Revision Questions  
Differentiate between the following concepts:  
Dominant and recessive genes  
Genotype and phenotype  
Mitosis and meiosis  
Sex chromosomes and autosomes  
Briefly explain the process of genetic inheritance  
Explain the causes and symptoms of four genetic disorders  
How many chromosomes are contained in a normal human zygote?  
What do you call a trait that has been determined by more than two pairs of genes?  

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