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The branch of biology that is concerned with the study of heredity, its biological processes, the study of genes, genome, cell cycle, heredity, inherits genes, and associated branches, is broadly referred to as Genetics. In this article, we will discuss, in detail, Genetics Class 10 ICSE notes, compiled by Superprof.
Understanding Genetics
Genetics explores the working and major codes of variation and heredity. The study of genetics is based on the principles of inheritance. Inheritance is the procedure by which characteristics are handed down from one generation to the other. Gregor Johann Mendel is known as the “Father of Modern Genetics” for his discoveries on the basic principles of heredity.
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Variation, as the name suggests is the amount of dissimilarity that exists between children and their parents. It is determined by considering the behaviouristic, cytological, physiological, and morphological characters of individuals fitting into similar species.
Some of the major reasons that variation occurs include:
- Genetic/chromosomal rearrangement
- Mutated genes due to the influence of the ecosystem
- Crossing over
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Key Terms of Genetics
- Gene: A region of DNA that is made up of nucleotides. A gene is the molecular unit of heredity. The transmission of genes to an organism's offspring is the basis of the inheritance of phenotypic traits.
- Allele: Every gene has 2 alternative forms for a character producing different effects. These alternative forms of a gene are called alleles and are present in the same position on homologous chromosomes.
- Variation: These are small differences in individuals due to inheritance leading to differences in the phenotype (observable physical and behavioural characteristics).
- Mutation: These are sudden changes in one or more genes or in the number and structure of chromosomes in the progeny. They cause a change in the genotype and phenotype and are inheritable.
- Autosomes: Also called somatic chromosomes, autosomes carry genes that determine the somatic characteristics and do not have any influence on determining the sex of the organism. They are homologous by nature.
- Allosomes: These are sex chromosomes that carry the genes responsible for sexual characteristics and as such have a significant role in the determination of sex. They may be homologous (XX) in the case of females and heterologous (XY) in the case of males.
- Haploid cells: These are cells that contain a single set of chromosomes. The term haploid can also refer to the number of chromosomes in egg or sperm cells, which are also called gametes.
- Diploid cells: Most mammals are diploid organisms, which means they have two homologous copies of each chromosome in the cells. In humans, there are 46 chromosomes. In most diploid organisms, every cell except for gametes will be diploid and contain both sets of chromosomes.
- Criss-cross inheritance: The genes which are located on the X chromosome (sex chromosome) are called sex-linked genes. These genes show criss-cross inheritance.
Mendelian Laws of Inheritance
Mendel experimented on a Garden Pea (Pisum Sativum) plant for 7 years. At the end of his experiment, he proposed the laws of inheritance in living organisms. Mendel carefully chose 7 distinct characteristics of Pisum Sativum for the investigation concerning hybridisation. He used true-breeding lines i.e. those that go through constant self-pollination and display steady characteristic inheritance.
Monohybrid Cross
The mating between 2 individuals with different alleles at one genetic locus of interest is known as a monohybrid cross. The character(s) being studied in a monohybrid cross are governed by 2 or multiple alleles for a single locus. A cross between 2 parents possessing a pair of contrasting characters is known as a monohybrid cross.
To carry out such a cross, each parent is chosen to be homozygous or true breeding for a given trait (locus). When a cross satisfies the conditions for a monohybrid cross, it is usually detected by a characteristic distribution of second-generation (F2) offspring that is sometimes called the monohybrid ratio. The ratio obtained when one pair of contrasting characters are considered. The monohybrid ratio is 3:1.
Dihybrid Cross
A dihybrid cross is a cross between 2 different lines (varieties, strains) that differ in 2 observed traits. In the Mendelian sense, between the alleles of both these loci, there is a relationship of complete dominance – recessive.
In the name “Dihybrid cross”, the “di” indicates that there are 2 traits involved (e.g. R and Y), the “hybrid” means that each trait has two different alleles (e.g. R and r, or Y and y), and “cross” means that there are 2 individuals (usually a mother and father) who are combining or “crossing” their genetic information. The ratio obtained when two sets of contrasting characters are considered. The dihybrid ratio is 9:3:3:1.
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Incomplete Dominance
Incomplete dominance was discovered after Mendel’s work. Incomplete dominance is the situation in which both the alleles do not display a dominant trait resulting in a fine combination or a midway amid the characteristics of the alleles.
Codominance
When 2 alleles lack the dominant-recessive association and, so, the duo affects the creature together.
Law of Independent Assortment
Separation of 1 set of the characteristic is independent of the other set of the characters when they are pooled in a hybrid.
The Chromosomal Theory of Inheritance
Both genes and chromosomes exist in sets of 2. The homologous chromosome contains the 2 alleles of a gene pair in the homologous sites. The coupling and split of a set of chromosomes cause a split in the set of genes (factor) they carry. This united knowledge is termed the Chromosomal Theory of Inheritance.
Sex Determination
Henking perceived a unique nuclear arrangement that was found in only fifty percent of sperms. He termed this body as x. Later, it became clear that those ova that obtain only the x chromosome are born female and those that don’t are born male. So, the X- chromosome was termed as the sex chromosome and the remaining ones were termed as autosomes.
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Mutation
The occurrence due to which a modification in DNA occurs and causes a variation in the phenotype and genotype of a creature is termed a mutation.
Principles of Inheritance
Law of Dominance
Distinct elements termed as factors control the characteristics. These factors at all times exist as a couple. One of the constituent genes of the couple dominates over the former.
Law of Segregation
Alleles don’t blend and the 2 characteristics are recuperated all through the gamete formation (in the F2 generation). The characters apart from each other and pass on to diverse gametes. Comparable types of gametes are produced by homozygous and heterozygous produces diverse sorts of a gamete with varied characteristics.
Genetic Disorders
Disorders of a Mendelian nature include:
- Haemophilia
- Sickle cell anemia
- Phenylketonuria
Disorders of a chromosomal nature include:
- Down’s syndrome
- Klinefelter’s syndrome
- Turners syndrome
Sex-linked Inheritance of Haemophilia
Sex-linked diseases are inherited through one of the sex chromosomes (the X or Y chromosome). X-linked recessive characters follow the crisscross pattern of inheritance or skip-generation inheritance. The latter refers to the inheritance of sex-linked characters transmitted from fathers to daughters or from mothers to sons
Haemophilia is usually an inherited genetic disorder that impairs the body's ability to make blood clots, a process needed to stop bleeding.
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The Process
A heterozygous carrier mother and a normal father pass the gene for hemophilia on to possibly one-half of their children. Half the daughters will be carriers and half the sons will be hemophilic. The rest of the siblings will be normal. Daughters can only be carriers.
The normal gene on the second X chromosome counteracts the defect and the daughters do not suffer from the trait. If a son receives the defective gene from his mother, he will be hemophilic because the Y chromosome cannot counteract the defective gene located on his X chromosome.
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Inheritance of Colour Blindness
When a normal woman is married to a colour-blind man, all her sons and daughters have normal vision. But when her daughters are married to a man with normal colour vision, some colour-blind sons are formed.
It means that a woman with a normal colour vision, whose father is colour-blind, gives birth to children of whom about half of the sons are colour-blind and the other half have normal colour vision.
A woman will be colour-blind only when she possesses a gene for colour-blindness in both the X-chromosomes, whereas a man will be colour-blind even when this gene is present singly.
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