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Principles of Inheritance and Variation Notes

Inheritance: The process by which characters are transferred from one generation to the next generation is called inheritance / heredity. Inheritance is the basis of heredity and by this process, traits are passed on from the parents to the offspring’s. Continuity of the gene pool is maintained by the process of inheritance.

Variations: The differences in traits of individuals of a progeny, from each other and from their parents are called variations. Variation exists among individuals of one species. Variation is due to crossing over, recombination, mutation and environmental effects on the expression of genes present on chromosomes.

Heredity: It is a process of transmission of heritable traits from parents to their offspring’s.

Genetics: The branch of science which deals with the principles and mechanism of inheritance and variation is called genetics.

Genes: Genes are the basic unit of inheritance and located on chromosomes.

Mendel’s Laws of Inheritance

• Gregor Johann Mendel (1822–1884) is known as “Father of Genetics”.
• In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants.
• He called genes as “factors”, which are passed from parents to offspring’s.
• Genes, that code for a pair of opposite traits are called “alleles”. He conducted artificial pollination / cross-pollination experiments using several true-breeding varieties having contrasting traits.

Mendel’s Experimental Plant

• Mendel selected garden pea as his experimental material.
• Phenotype: Visible expression of genetic constitution e.g., Tall/dwarf
• Genotype: Genetic constitution of individual e.g., TT, Tt, tt.

Mendel’s Observations

Monohybrid Cross: Cross involving study of inheritance of one character, e.g., height of plant.
Dihybrid Cross: Cross between plants differing in two traits/cross involving study of inheritance of 2 genes or characters.
Homozygous: The individual carrying similar alleles for a trait e.g., TT or tt.
Heterozygous: Individual carrying different alleles for a trait e.g., Tt

Mendel’s Laws of Inheritance

1. LAW OF DOMINANCE

This law states that when two alternative form of a trait or character (genes or alleles) are present in an organism, only one factor expresses itself in F1 progeny and is called dominant while the other that remains masked is called recessive.

Monohybrid Cross

2. LAW OF INDEPENDENT ASSORTMENT

According to this law the two factor of each character assort or separate out independent of the factors of other characters at the time of gamete formation and get randomly rearranged in the offspring producing both parental and new combinations of characters.

3. LAW OF SEGREGATION

This law states that the factors or alleles of the pair segregate from each other during gamete formation, such that a gamete receives only one of the two factors. They do not show any blending.

Incomplete Dominance

• When neither of the two alleles is dominant and the phenotype of the heterozygote does not resemble any of the parents. The heterozygote expresses intermediate or a mixture of two parents’ traits
• Example: The flower colour inheritance of snapdragon. On crossing true breeding red (RR) and white flower (rr), we get all pink colour flowers in the F₁ generation, which on self-pollination give red: pink: white flowers in the ratio 1:2:1 in the F₂ generation.

Co-dominance

The alleles which are able to express themselves independently, even when present together are called co-dominant alleles and this biological phenomenon is called co-dominance.

Test Cross

It is a method devised by Mendel to determine the genotype of an organism. In this cross, the organism with dominant phenotype (but unknown genotype) is crossed with the recessive individual.

Pleiotropy

• It is the phenomenon in which a single gene exhibits multiple phenotype expressions.
• The pleiotropic gene affects the metabolic pathways, resulting in different phenotypes.
• Example: a single gene mutation in the gene coding for the enzyme phenylalanine hydroxylase results in the disease known as phenylketonuria, which is characterised by mental retardation, reduced hair and skin pigmentation.

Polygenic Inheritance

• It is a type of inheritance, in which a trait is controlled by three or more genes. Some traits are called polygenic traits.
• The phenotype reflects contribution of each allele and is also influenced by the environment.
• Example: eye colour, skin pigmentation, height, hair colour, etc.

Chromosomal Theory of Inheritance

• The chromosomal theory of inheritance was proposed independently by Walter Sutton and Theodore Boveri in 1902.
• They stated that behavior of chromosomes was parallel to behavior of genes and used chromosome movement to explain Mendel’s laws.
• The hereditary factors are carried in the nucleus.
• Like the Mendelian alleles, chromosomes are also found in pairs.
• The sperm and eggs having haploid sets of chromosomes fuse to re-establish the diploid state.
• Morgan extensively worked on fruit flies, Drosophila melanogaster and provided experimental evidence to support the chromosomal theory of inheritance.

Comparison between the behavior of Genes and Chromosomes

Genes	Chromosomes
Occurs in Pairs.	Occurs in Pairs.
Segregate at the time of gamete formation such that only one of each pair is transmitted to a gamete.	Segregate at gamete formation and only one of each pair is transmitted to a gamete.
It has independent pairs segregate independently of each other.	It has only pairs segregates independently of another pair.

Linkage and Recombination

T.H. Morgan carried out several dihybrid crosses in Drosophila to study the genes that are sex – linked. He observed that when the two genes in a dihybrid cross are located on the same chromosome, the proportion of parental gene combinations in the progeny was much higher than the non-parental or recombination of genes.

• Physical association of genes located on a chromosome is known as linkage.
• In a dihybrid cross, if the two genes are tightly linked or present on the same chromosome, the parental combination is more prevalent than non-parental combinations or recombinants.
• The linkage and recombination are directly dependent on the distance between a pair of genes. More the distance, greater is the probability of recombination.

Sex Determination Mechanism

• Finalization of sex at the time of zygote formation is called sex determination.
• Two types of chromosomes are present in individuals – sex chromosomes (which determine the sex of individuals) and autosomes.

1. XX-XY TYPE

Seen in many insects and mammals including humans, Drosophila melanogaster. Males have X and Y chromosomes along with autosomes [A] and females have a pair of X chromosomes.

2. XX-XO TYPE

Seen in grasshopper. Males have only one X chromosomes besides autosomes and females have a pair of X chromosomes.

3. ZZ-ZW TYPE

Seen in birds, fowl and fishes. Females have one Z and one W chromosomes whereas males have a pair of Z chromosomes.

4. XX-XY TYPE

Seen in cockroaches. Females have only one Z chromosomes besides autosomes and males have a pair of Z chromosomes.

Sex Determination in Humans

The mechanisms for sex determination are genotypic sex determination systems, in which sex is governed by the genotype of the zygote and environmental sex determination systems in which sex is governed by internal and external environmental conditions. In genotypic sex determination the sex chromosomes play a decisive role in the inheritance and determination of sex.

• Chromosomal sex is determined at fertilization.
• Sexual differences begin in the 7th week.
• Sex is influenced by genetic and environmental factors.
• Females (generally XX) do not have a Y chromosome.
• Males (generally XY) have a Y chromosome.

• Sex determination mechanism is humans is XY type.
• 22 pairs are similar in male & females whereas presence of X/Y chromosomes determine human sex.
• In spermatogenesis male produce 50% of sperms with X-Chromosomes and the other 50% with Y-Chromosomes.
• Female have only X-Chromosomes.
• It is hence a matter of chance, since there is equal probability of getting fertilized with sperm carrying X or Y chromosomes.
  • If X chromosome gets fertilized – XX (Female)
  • If Y chromosome gets fertilized – XY (Male)

 

Sex Determination in Honeybees

• An offspring formed from the union of a sperm and an egg develops into queen/worker.
• The unfertilized egg develops as a male (drone) by parthenogenesis.
• Males thus have half the number of chromosomes than female.
• Females have 32 chromosomes and male have 16 chromosomes.
• It is called as haplodiploid sex determination system and has its own importance such as male produce sperm by mitosis.

Mutation

Mutation is the phenomenon which results in alteration of DNA sequence and consequently results in changes in the genotype and phenotype of an organism. There are two types of genetic mutation:

• Point mutation: There is a substitution in the single base pair of DNA, e.g. in the sickle cell anaemia. The 6th codon of the gene coding for the 𝛃-globin chain of haemoglobin changes from GAG to GUG, resulting in the substitution of glutamic acid by Valine.
• Frameshift mutation: It results from the insertion or deletion of one or more pairs of bases in DNA. it changes the reading frame of triplet codons, that code for certain amino acids of the protein.

Genetic Disorders

There are many disorders in the human being that are inherited and caused due to mutation in the gene or alteration in chromosomes.

Pedigree Analysis

It helps in determining the risk of getting a genetic disorder in the offspring by studying the inheritance pattern of a particular trait present in various generations of an individual.

Types of Genetic disorders

Genetic disorders can be grouped into two types:

Mendelian Disorders

• Mendelian disorders are mainly determined by alteration or mutation in the single gene.
• These disorders are transmitted to the offspring on the same lines as we have studied in the principle of inheritance.
• Most common and prevalent Mendelian disorders are Hemophilia, Cystic fibrosis, Sickle-cell anemia, Colour blindness, Phenylketonuria, Thalassemia, etc.
• It is evident that this X-linked recessive trait shows transmission from carrier female to male progeny.

Examples:

Name	Genetic Trait	Cause	Effects	Inheritance pattern
Sickle cell anaemia	Autosome-linked recessive	A single point mutation in the beta-globin chain of haemoglobin	Anaemia	Offsprings may get the disease when both the parents are a carrier (heterozygote)
Thalassemia	Autosome-linked recessive	Mutation in the genes HBA1 and HBA2 present on the chromosome 16	Formation of abnormal haemoglobin molecule resulting in anaemia	Offsprings may get the disease when both the parents are a carrier (heterozygote)
Phenylketonuria	Autosome-linked recessive	Lack of an enzyme that converts phenylalanine to tyrosine	Mental retardation. Accumulation and excretion of phenylalanine and its derivatives in urine	Offsprings may get the disease when both the parents are a carrier (heterozygote)
Colour blindness	X-linked recessive	Defect in the green or red cone of the eye	Unable to discriminate between red and green colour	A daughter will be colour blind only if the father is colour blind There is a 50 percent probability of a carrier female to transfer the disease to sons
Haemophilia	X-linked recessive	Defect in one protein involved in the clotting of blood	Continuous bleeding from wounds	A daughter will be colour blind only if the father is colour blind There is a 50 percent probability of a carrier female to transfer the disease to sons

Chromosomal Disorders

• The chromosomal disorders on the other hand are caused due to absence or excess or abnormal arrangement of one or more sex chromosomes.
• Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy.
• Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and, this phenomenon is known as polyploidy.

Examples:

• Down’s syndrome- Trisomy of chromosome 21. Symptoms include mental retardation, short stature, furrowed tongue, partially opened mouth.
• Klinefelter’s syndrome- Total 47 chromosomes with one extra X chromosome, i.e. XXY, They are sterile, tall, overall masculine with feminine characteristics such as breast development (gynecomastia).
• Turner’s syndrome- Total 45 chromosomes. One X chromosome is missing, i.e. XO. females are sterile, short stature and under-developed sexual characters.

 

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