Introduction
Q. What are Laws of Inheritance?
Ans: Based on his observations on monohybrid crosses Mendel proposed two general rules to consolidate his understanding of inheritance in monohybrid crosses. These rules are called the Principles or Laws of Inheritance, the First Law or Law of Dominance and the Second Law or Law of Segregation.
Laws
- 1. Characters are controlled by discrete units called factors.
2. Factors occur in pairs.
3. In a dissimilar pair of factors, one member of the pair dominates (dominant) the other (recessive).
Note: The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross [Mendel’s Experiments on Inheritance] in the F1 and the expression of both in the F2. It also explains the proportion of 3:1 obtained at the F2.
- This law is based on the fact that the alleles do not show any blending and that both the characters are recovered as such in the F2 generation through one of these is not seen at the F1 stage.
- Though the parents contain two alleles during gamete formation, the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors [either dominant or recessive].
- Of course, a homozygous parent produces all gametes that are similar while a heterozygous one produces two kinds of gametes each having one allele with equal proportion.
- When experiments on peas were repeated using other traits in other plants, it was found that sometimes the F 1 had a phenotype that did not resemble either of the two parents and was in between the two.
- The inheritance of flower colour in the dog flower (Snapdragon or Antirrhinum sp.) is a good example to understand incomplete dominance.
- In a cross between true-breeding red-flowered (RR) and true breeding white-flowered plants (rr), the F1 (Rr) was pink. When the F1 was self-pollinated the F 2 resulted in the following ratio 1 (RR) Red : 2 (Rr) Pink: 1 (rr) White.
- Here the genotype ratios were exactly as we would expect in any Mendelian monohybrid cross, but the phenotype ratios had changed from the 3:1 dominant : recessive ratio.
- What happened was that R was not completely dominant over r and this made it possible to distinguish Rr as pink from RR (red) and rr (white).
- Till now we were discussing crosses where the F1 resembled either of the two parents (dominance) or was in-between (incomplete dominance). But, in the case of co-dominance, the F 1 generation resembles both parents.
- A good example is different types of red blood cells that determine ABO blood grouping in human beings.
- ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene. The gene (I) has three alleles IA, IB and i.
- The alleles IA and IB produce a slightly different form of the sugar while allele i does not produce any sugar.
- Because humans are diploid organisms, each person possesses any two of the three I gene alleles.
- IA and IB are completely dominant over i, in other words when IA and i are present only IA expresses (because i does not produce any sugar), and when IB and i are present IB expresses.
- But when IA and IB are present together they both express their own types of sugars: this is because of co-dominance. Hence red blood cells have both A and B types of sugars.
- Since there are three different alleles, there are six different combinations of these three alleles that are possible, and therefore, a total of six different genotypes of the human ABO blood types.
| Allele from Parent I | Allele from Parent II | Genotype of Offspring | Blood type of Offspring |
|---|---|---|---|
| [latex]I^A[/latex] | [latex]I^A[/latex] | [latex]I^A I^A[/latex] | A |
| [latex]I^A[/latex] | [latex]I^B[/latex] | [latex]I^A I^B[/latex] | AB |
| [latex]I^A[/latex] | i | [latex]I^A i[/latex] | A |
| [latex]I^B[/latex] | [latex]I^A[/latex] | [latex]I^A I^B[/latex] | AB |
| [latex]I^B[/latex] | [latex]I^B[/latex] | [latex]I^B I^B[/latex] | B |
| [latex]I^B[/latex] | i | [latex]I^B[/latex] i | B |
| i | i | ii | O |
Note: There are 6 Genotypes and 4 Phenotypes [A, B, AB and O].
- Mendel also worked with and crossed pea plants that differed in two characters, as is seen in the cross between a pea plant that has seeds with yellow colour and round shape and one that had seeds of green colour and wrinkled shape.
- Yellow colour was dominant over green and round shape dominant over wrinkled.
- Let us use the genotypic symbols Y for dominant yellow seed colour and y for recessive green seed colour, R for round shaped seeds and r for wrinkled seed shape.
- The genotype of the parents can then be written as RRYY and rryy. The cross between the two plants can be written down as in Figure showing the genotypes of the parent plants.
- The gametes RY and ry unite on fertilisation to produce the F1 hybrid RrYy.
- When Mendel self-hybridised the F1 plants he found that 3/4th of F2 plants had yellow seeds and 1/4th had green.
- The yellow and green colour segregated in a 3:1 ratio. Round and wrinkled seed shape also segregated in a 3:1 ratio; just like in a monohybrid cross.
Image: Dihybrid cross
Source: NCERT Text Books
- In the dihybrid cross, the phenotypes round, yellow; wrinkled, yellow; round, green and wrinkled, green appeared in the ratio 9:3:3:1.
- Such a ratio was observed for several pairs of characters that Mendel studied. The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow: 1 green, with 3 round : 1 wrinkled. This derivation can be written as follows: (3 Round : 1 Wrinkled) (3 Yellow : 1 Green) = 9 Round, Yellow : 3 Wrinkled, Yellow: 3 Round, Green: 1 Wrinkled, Green.
- Based upon such observations on dihybrid crosses (crosses between plants differing in two traits) Mendel proposed a second set of generalisations that we call Mendel’s 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’.
- You can verify the law using The Punnett square above [Inheritance of Two Genes – dihybrid cross].
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