ICSE Class 10 Biology Chapter 03 Genetics Some Basic Fundamentals

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Genetics - Some Basic Fundamentals

Syllabus

Genetics: Mendel's laws of inheritance and sex-linked inheritance of diseases.

Scope of Syllabus: Monohybrid cross, dihybrid cross. The following terms to be covered: gene, allele, heterozygous, homozygous, dominant, recessive, mutation, variation, phenotype, genotype. Sex determination in human beings. Sex-linked inheritance of diseases to include haemophilia and colour blindness (only criss cross inheritance).

Whenever a child is born the family members usually start comparing his appearance as to whom he/she resembles more. All this, although a matter of chance, is governed by certain laws. This chapter presents some fundamentals of Genetics including Mendel's laws of inheritance as envisaged in the syllabus. In order to properly understand Mendel's laws of inheritance (page 29) and the sex-linked inheritance (page 26), it is essential to know beforehand, some basic aspects of genetics. You will learn about the chromosomes, the genes and some very common hereditary traits in humans. You can do several investigatory projects, if desired.

3.1 WHAT IS GENETICS?

Genetics is the study of transmission of body features (both similarities and differences) from parents to offspring and the laws relating to such transmission.

GREGOR MENDEL - An Austrian Monk and Father of Genetics

Gregor Mendel (1822-84) was born in a peasant family. He had his early education in a monastery and later he studied Science and Mathematics at University of Vienna. He wanted to be a teacher but luck did not favour him, and he failed in the examination of teaching certificate. But what he discovered later as a craze, has made him a "teacher of teachers". He had returned to his monastery and spent the rest of his life as an abbot. The monastery had a lovely garden which satisfied his urge to understand some secrets of life regarding the inheritance of certain features in garden pea. His findings are now called Mendel's laws of inheritance.

Two modern applications of genetics

Genetic engineering is the technique in which the genetic constitution of an organism (bacterium) is altered by introducing new genes into its chromosomes. The genetically modified organism (GMO) thus produced is grown to multiply fast and the gene product is obtained in large quantities. The hormone insulin was the first such product. The insulin producing gene of mammals has been successfully introduced in certain bacteria which have been made to produce it.

Genetic counselling is yet another practical aspect. Newly married couples are advised to consult a specialist regarding the possibilities of any undesirable trait which their children might inherit. Diseases like haemophilia (bleeder's disease with a tendency to bleed freely from even a slight wound), thalassaemia and sickle cell anaemia with defective haemoglobin are examples of genetic diseases which can be prevented to some extent by proper genetic matching of the prospective parents.

Heredity

The term heredity may be defined as "transmission of genetically based characteristics from parents to offspring."

The term heredity is rather old. For centuries, man has known that many characteristics are inherited in animals (domesticated ones) as well as in humans, and that he could breed varieties of animals and plants with desirable qualities.

Like begets like

It means that young ones look like their parents.

• Cats produce cats and not dogs.
• A mango seed germinates into a mango tree.
• Humans give birth only to humans and not to apes.
• Even the curd bacteria which grow in milk undergo hundreds of generations each day, and continue to produce the same type of bacteria and not of any different type.

Like Begets Like and Yet the variations

All organisms - whether animals, plants or microorganisms, produce their own kind through reproduction. But the offspring are never identical to their parents; some difference, how-so-ever small it may be, is found in them.

3.2 VARIATIONS IN POPULATION

Human beings as a species share many main characters or traits among themselves which identify the species Homo sapiens. Yet the various races or tribes look different in several features. Even within the same race or tribe, the individual members in the population show differences. Further, the smallest unit of population is the family and there too, members show differences in body features. These small differences among individuals are called variations.

The same is true for animals also. We keep pets, like dog or cat and domesticate animals, like cow or buffalo. There are so many breeds of each one of them and even in the same breed, there are minor differences, and variations among individuals.

CHARACTER AND TRAITS

Any inheritable feature is a character. The alternative forms of a character are called traits.

INHERITANCE IN HUMANS

We inherit thousands of characters from our parents (father and mother) who in turn, had inherited them from their parents. Thus the family members - brothers, sisters, cousins - tend to resemble one another. Here is a list of some such characters and their traits :-

1. Colour of the eyes - brown or blue

2. Hair shape - Curly or straight

3. Eyebrows - Heavy bushy or thin

4. Hair on the middle joint of fingers - Growth or no growth

5. Colour vision - Normal or red-green colour blindness

6. Tongue rolling - Rolling of tongue into U-shape when extended out from the mouth or no rolling

7. Hand use - Right-handedness or left-handedness

8. Skin colour - Albinism (total absence of pigment in skin) or normal (light or dark) pigment.

9. Ear lobe - Free or attached

10. Lips - Thick or thin

11. Rh Blood group - Rh positive or Rh negative.

PROGRESS CHECK

1. Mention if the following statements are True (T) or False (F) :

(i) Genetics and heredity are the same thing.

(ii) "Like begets like", this applies only to animals.

(iii) The entire human population shows variations.

2. Which of the following in humans are established genetic traits? (Tick-mark the correct ones) in the box provided.

(i) Capacity to be a good cricketer

(ii) Curly hair

(iii) Left-handedness

(iv) Quality of voice

(v) Red-green colour blindness

3.3 CHROMOSOMES - THE CARRIERS OF HEREDITY

Chromosomes are only visible when a cell nucleus is about to divide. Photographs of the dividing cell nucleus can be taken through a high powered light microscope. These photographs are used for artificially arranging the chromosomes according to their size and shape on a chart (karyotype). One such human karyotype is shown below in Fig. 3.1

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