Genetics and Heredity: Key Concepts and Principles
1. Genetics Basics: Fill in the Blanks
- (a) The total number of chromosomes in a human cell is 46.
- (b) Phenylketonuria causes mental retardation or brain damage.
- (c) Genetic disorder due to the presence of an additional X-chromosome is Klinefelter’s syndrome.
- (d) The genetic constitution of an organism is called a genotype.
- (e) August Weismann proposed that nuclei are the bearers of hereditary characters.
- (f) Gregor Mendel is known as the father of genetics.
2. Fundamental Genetic Definitions
What is a frameshift mutation?
A frameshift mutation is a mutation caused by the insertion or deletion of nucleotides that shifts the reading frame of DNA.
What are alleles?
Alleles are alternative forms of the same gene present at the same locus on homologous chromosomes.
Define test cross
A test cross is a cross between a dominant phenotype organism and a homozygous recessive organism.
3. Inheritance and Mendel’s Laws
Mendel’s Laws of Inheritance
Mendel proposed three laws:
- Law of Dominance: The dominant trait appears in the F₁ generation.
- Law of Segregation: Alleles separate during gamete formation.
- Law of Independent Assortment: Genes assort independently during inheritance.
Chromosomal Theory of Inheritance
Proposed by Sutton and Boveri, this theory states that genes are located on chromosomes and chromosomes are the carriers of hereditary characters. Chromosome behavior during meiosis explains Mendel’s laws.
4. Genetic Disorders and Conditions
Haemophilia
Haemophilia is a sex-linked hereditary disorder in which blood does not clot normally due to the absence of clotting factors. It is more common in males and is inherited through the X-chromosome.
Sickle Cell Anaemia
A hereditary blood disorder caused by a mutation in the haemoglobin gene. Red blood cells become sickle-shaped, causing poor oxygen transport, pain, and anaemia. It is an autosomal recessive disorder.
Pedigree Analysis
Pedigree analysis is the study of the inheritance of traits or genetic disorders in a family over several generations using a family chart. It helps determine if a trait is dominant, recessive, or sex-linked and assists in genetic counseling.
5. Comparative Genetics
Genotype vs. Phenotype
- Genotype: The genetic makeup of an organism (e.g., TT, Tt, tt).
- Phenotype: The observable characters of an organism (e.g., Tall or dwarf).
Linkage vs. Recombination
- Linkage: Genes present on the same chromosome are inherited together; reduces variation.
- Recombination: Exchange of genes between homologous chromosomes during crossing over; produces new combinations and increases variation.
Aneuploidy vs. Polyploidy
- Aneuploidy: Loss or gain of one or a few chromosomes (e.g., Down’s syndrome).
- Polyploidy: Presence of more than two complete sets of chromosomes (e.g., 3n, 4n); common in plants.
6. Chromosomal Disorders
Chromosomal disorders occur due to an abnormal number or structure of chromosomes:
- Down’s Syndrome (Trisomy 21): Extra chromosome 21; causes mental retardation.
- Turner’s Syndrome (XO): Female with one X chromosome; causes sterility.
- Klinefelter’s Syndrome (XXY): Male with an extra X chromosome; causes sterility.
- Cri-du-chat Syndrome: Deletion of part of chromosome 5; causes cat-like crying and mental retardation.