Principles Of Inheritance And Variation

Principles Of Inheritance And Variation

Mendel’s laws for inheritance of traits

Mendel has put forth three laws governing inheritance:
1. Law of dominance: Out of a pair of contrasting characters present together, only one is able to express itself while the other remains suppressed.
2. Law of segregation of characters: The two members of a pair of factor separate during the formation of gametes.
3. Law of independent assortment: When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair.

Mendel’s monohybrid cross

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1. Mendel crossed true-breeding plants that differed for a given character
2. A monohybrid cross involves one (mono) character and different (hybrid) traits.
3. Pollen from true-breeding pea plants with purple flowers (one trait) was placed on stigmas of true-breeding plants with white flowers (another trait).
4. The F1 seeds were all purple; the white flower trait failed to appear at all.
5. Because the purple flower trait completely masks the white flower trait when true-breeding plants are crossed, the purple flower trait is called dominant, and the white flower trait is called recessive.
6. The F1 plants were allowed to self-pollinate.
7. This step was the monohybrid cross. (or the F1 cross).
8. The progeny, called F2, were examined: roughly 1/4 were white, and 3/4 were purple.


Mendel’s dihybrid cross

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1. A dihybrid is an individual that is a double heterozygote (e.g., with the genotype RrYy – round seed, yellow seed).
2. Mendel’s second law states that the Rr alleles assort into gametes independently of the Yy alleles.
3. The dihybrid, RrYy, produces gametes that have one allele of each gene.
4. Four different gametes are possible and will be produced in equal proportions: RY, Ry, rY, and ry.
5. Random fertilization of gametes yields the outcome visible in the Punnett square. Note its 4×4 table construction to accommodate 16 possible phenotypes.
6. Filling in the table and adding the like cells reveals a 9:3:3:1 ratio of the four possible phenotypes: (Round, Yellow – Round, Green – Wrinkled, Yellow – Wrinkled, Green.)

Chromosomal theory of inheritance

Salient features of chomosomal theory of inheritance are as follows:

1. Bridge between one generation and the next is through sperm and ovum. The two must carry all the hereditary characters.

2. Both the sperm and egg contribute equally in the heredity of the offspring. The sperm provides only nuclear part to the zygote. As such hereditary characters must be carried by nuclear materials. There is fusion of the sperm and egg nuclei during fertilization.

3. Nucleus contains chromosomes. Therefore, chromosomes must carry the hereditary traits.

4. Every chromosome or chromosome pair has a definite role in the development of an individual. Loss of a complete or part of the chromosome produces structural and functional deficiency in the organism.

5. Like the hereditary traits the chromosomes retain their number, structure and individuality throughout the life of an organism and from generation to generation. The two neither get lost nor mixed up. They behave as units.

6. Both chromosomes as well as genes occur in pairs in the somatic or diploid cells.

7. A gamete contains only one chromosome of a type and only one of the two alleles of a character.

8. The paired condition of both chromosomes as well as Mendelian factors is restored during fertilization.

9. Genetic homogeneity and heterogeneity, dominance and recessiveness can be suggested by chromosomal type and behaviour.

10. In many organisms, sex of an individual is determined by specific chromosomes called sex chromosomes.

11. Homologous chromosomes synapse during meiosis and then separate or segregate independently into different cells which establishes the quantitative basis for segregation and independent assortment of hereditary factors.

Types of linkage

Complete linkage
1. The genes located on the same chromosome do not separate and are inherited together over the generations due to the absence of crossing over. 
2. Complete linkage allows the combination of parental traits to be inherited as such. 
3. It is rare but has been reported in male Drosophila and some other heterogametic organisms.

Incomplete linkage
1. Genes present in the same chromosome have a tendency to separate due to crossing over and hence produce recombinant progeny besides the parental type. 
2. The number of recombinant individuals is usually less than the number expected in independent assortment.

Sex determination in human beings

1. Sex determination is a mechanism which determines the individual to be a male or a female based on the sex chromosomes present in it.
2. Sex chromosomes in sperms determine the babys gender. Humans have 23 pairs of chromosomes in each cell. A pair of chromosome forms the sex chromosomes. Males carry XY and females carry XX chromosomes. Reproductive cells give rise to gametes by the division of meiosis. A gamete is a mature reproductive cell – a sperm or an egg.
3. If a sperm carrying X fertilises the egg with X chromosome, then the resulting baby is a girl.
4. If a sperm carrying Y chromosome fertilises the egg with X chromosome, then the resulting baby is a boy.
5. Hence, males are responsible for the gender of the newly formed babies.

XX-X0 types

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1. In roundworms and some insects (true bugs, grasshoppers, cockroaches), the females have two sex chromosomes, XX, while the males have only one sex chromosome, X. 

2. The males are heterogametic with half the male gametes (gynosperms) carrying X-chromosome (A+X) while the other half (androsperms) being devoid of it (A + 0). The sex ratio produced in the progeny is 1: 1

3. There is no second sex chromosome. Therefore, the males are designated as X0. 

4. The females are homogametic because they produce only one type of eggs (A+X).


1. When both the alleles of a gene express themselves simultaneously in a heterozygote, this condition is called codominance.
2. An example in humans would be the ABO blood group, where alleles A and alleles B are both expressed.

Pedigree analysis

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A chart which displays the affected members of a family by genetic diseases in the form of a family tree is called pedigree chart. The study of such a chart to detect genetic diseases in a family is called pedigree analysis. The diagram gives some common symbols in pedigree analysis.

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