Maharashtra State Board · Class 12 · Biology · Chapter 3

Genetics and Evolution — Formula Sheet

Board Formulas

11 formulas

1.Monohybrid Cross Ratio★
$A a \times A a \to 1 AA : 2 A a : 1 aa (genotypic) = 3 : 1 (phenotypic)$
Basis of Mendel's law of segregation. Complete dominance assumed.
2.Dihybrid Cross Ratio★
$9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb$
Independent assortment of two genes on different chromosomes.
3.Test Cross★
$A a \times aa \to 1 A a : 1 aa$
Cross with homozygous recessive to determine genotype of the unknown.
4.Incomplete Dominance
$R R \times r r \to R r (intermediate phenotype)$
Neither allele fully dominant. Example: Mirabilis jalapa (4 o'clock plant).
5.Codominance
$I^{A} I^{B} \to AB blood group$
Both alleles fully expressed. ABO blood group in humans.
6.Multiple Alleles — ABO Blood Group★
$I^{A}, I^{B}, i; I^{A} = I^{B} > i$
Three alleles at one locus. Six genotypes; four phenotypes (A, B, AB, O).
7.Chromosomal Sex Determination★
$XY: mammals; XO: grasshopper; ZW: birds; ZO: moth$
In XY and XO systems, male is heterogametic. In ZW and ZO, female is heterogametic.
8.Sex-Linked Inheritance
$X-linked recessive: haemophilia, colour blindness$
Affects males more (only one X). Females can be carriers.
9.Hardy–Weinberg Equation★
$p^{2} + 2 pq + q^{2} = 1, p + q = 1$
Genotype frequencies in a stable population. Conditions: no mutation, no selection, no migration, random mating, large population.
10.Linkage Frequency
$Recombination frequency (RF) = \frac{recombinants}{total offspring} \times 100$
Used in gene mapping. 1% RF = 1 map unit (cM).
11.Pleiotropy
$One gene \to multiple phenotypic effects$
Example: sickle cell anaemia; phenylketonuria.