Principles or laws of Inheritance
(I) Law of Dominance
When two homozygous individuals with a contrasting characters are crossed the character that appear in the F, hybrids is dominant character and that does not appear in F, is recessive character.
(II) Law of segregation
The law of segregation states that when a pair of contrasting factors or genes or allelomorphs are brought together in a heterozygote (hybrid) the two members of the allelic pair remain together without being contaminated and when gametes are formed from the hybrid, the two separate out from each other and only one enters each gamete.
It is also known as “Law of purity of gametes” because each gamete is pure in itself i.e., having either dominent or recessive -allele.
There are some limitations of law of segregation:
(i) The law applies only to diploid organisms that form haploid gametes to reproduce sexually.
(ii) The law only applies to traits controlled exclusively by a single gene pair in which one of the two alleles is dominant over the other.
(iii) The law does not apply to :
– Alleles that are incompletely dominant or codominant.
– Genes that are pleiotropic, complementary or epistatic.
– Traits caused by many gene pairs.
To verify his results of monohybrid crosses, Mendel also crossed pea plants differing in two characters (dihybrid cross) to verify the laws of inheritance.
• This helped him to understand inheritance of two genes (i.e., two pairs of alleles) at a time.
• It was found that inheritance of one pair of allele (one trait) does not interfere in the inheritance of other pair of allele (second trait).
• For example when a dihybrid cross between pea variety having yellow cotyledons and round (smooth) seeds with another variety having green cotyledons and wrinkled seeds was done, the F, seeds were yellow and round. When F, seeds were grown into plants and crossed with each other, the F2 seeds obtained showed all possible combinations i.e., yellow and round seeds, yellow and wrinkled seeds, green and round seeds and green and wrinkled seeds. These four kinds of seeds were obtained in 9:3:3:1 ratio.
It is evident that inheritance of contrasting traits follows the “product law” of probabilities.
When two independent events occur simultaneously, the combined probability of the two outcomes is equal to the product of their individual probabilities of occurrence.
• For example, the probability of an F2 plant having yellow and round seeds is (3/4) (3/4) or 9/16, because 3/4 of all F2 plants should be yellow and 3/4 of all F2 plants should be round.
• In a like manner, the probabilities of the other three F2 phenotypes can be calculated: yellow (3/4) and wrinkled (1/4) are predicted to be present together 3/16 of the time; green (1/4) and round (3/4) are predicted 3/16 of the time; and green (1/4) and wrinkled (1/4) are predicted 1/16 of the time.
Mendel’s fourth postulate for Inheritance
• This postulate of Mendel was later made into law of independent assortment.
• This postulate stipulates that any pair of unit factor segregate independently of all other unit factors.
• As a result of segregation, each gamete receives one member of every pair of unit factors. For one pair, whichever unit factor is received does not influence the outcome of segregation of any other pair.
• Thus, according to the postulate of independent assortment, all possible combinations of gametes will be formed in equal frequency.
(Ill) Law of independent assortment
• The law states that “when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.”
• Independent assortment is not applicable for the genes located on the same chromosome, i.e., linked genes. It is applicable to only those factors or genes which are present on different chromosomes.
• Mendel’s gene pairs did not show linkage.
• The genes controlling the seven pea characters studied by Mendel are now known to be located on four chromosomes (1, 4, 5, 7).
• Mendelian recombination were mainly due to independent assortment in inheritance rules.
• Mendel’s 9:3:3:1 dihybrid ratio is an ideal ratio based on probability events involving segregation, independent assortment, and random fertilization.
Independent assortment in the inheritance laws leads to extensive genetic variation For any individual, the number of possible gametes, each with different chromosome compositions, is 2n, where n equals the haploid number. Thus, if a species has a haploid number of 4, then 24 or 16 different gamete combinations can be formed as a result of independent assortment. Although this number is not high, consider the human species, where n = 23. If we calculate 223, we find more than 8 x 106, or over 8 million, different types of gametes are possible. Because fertilization represents an event involving only one of approximately 8 x 106 possible gametes from each of two parents, each offspring represents only one of (8x 106)2or 64 x 1012 potential genetic combinations! This number of combinations of chromosomes is far greater than the number of humans who have ever lived on Earth! It is no wonder that, except for identical twins, each member of the human species in a family inheritance demonstrates such as distinctive appearance and individuality.