Genetics Class 12 Biology Revision Notes
Class 12 Biology students should refer to the following concepts and notes for Genetics in standard 12. These exam notes for Grade 12 Biology will be very useful for upcoming class tests and examinations and help you to score good marks
Genetics Notes Class 12 Biology
Mendel's first law ( Law of dominance )characters are controlled by discrete units called genes (allele) which occur in pair. In heterozygous condition only one gene that is dominant can express itself. (Can be explained by monohybrid cross)
Mendel's second law (Law of segregation): The two alleles received, one from each parent, segregate independently in gamete formation, so that each gamete receives one or the other with equal probability. (Can be explained by monohybrid cross)
Mendel's third law (Law of recombination): Two characters determined by two unlinked genes are recombined at random in gamete formation, so that they segregate independently of each other, each according to the first law (note that recombination here is not used to mean crossing-over in meiosis). (Can be explained by dihybrid cross)
This is what Mendel said (summary) :
1) Dominant alleles overpower recessive alleles. Dominant traits overpower recessive traits.
2) Rule of segregation (Separation): Gametes (sex cells) only receive one allele from the original gene.
3) Rule of Independent assortment: One trait will not determine the random selection of another. Incomplete dominance: When one allele of a gene is not completely dominant over the other and the F1 hybrids are intermediate between two parents. The phenotypic and genotypic ratio is same.1:2:1 in F2 generation. E.g. Snapdragon or Antirrhinum majus Co dominance: Two alleles of a gene are equally expressive and dominant in a generation eg. Human blood group
( Note : Human blood group is also an example for multiple allelisim i.e when a gene exists in more than two allelic forms)
Basic outline of Mendels cross
1. Pure breeding parents for a pair of contrasting character (allelic pair) is taken
Eg.Tall pure-bred pea plants (TT) & short pure-bred pea plants (tt)
2. Gamete formation (Meiosis)
3. Hybridization (crossing is done)
4. F1 generation - the product of the above cross (are called hybrids)
5. Selfing (allowed to self fertilize / self breeding)
6. Gamete formation (Meiosis)
7. F2 generation - the product of the above selfing
8. Analysis of result (Phenotype and Genotype)
Tendency of genes on same chromosome to remain together
Such genes are called – linked genes.
Linked genes present only parental types
Figure Schematic of Genetic Linkage and Recombination
(A) Two homologous chromosomes: blue (paternal) and orange (maternal). Three genes with separate alleles and linkage " noted (A,a; B,b; C,c;).
(B) Crossing over during meiosis. (chiasma formation)
(C) Two alleles and their linked genes have switched locations via recombination. Four additional alleles and their associated (A,a; B,b;) have not switched and are considered linked.
(D) Recombined haploid chromosomes segregate separately during meiosis as gametes before fertilization.
(E) Sample recombination frequencies between genes demonstrating higher rates of recombination for genes further apart.
Sex determination and sex chromosome
Pedigree is a chart of graphic representation of record of inheritance of a trait through several
generations in a family
Symbols used:- refer NCERT Text Book
Four patterns of inheritance
Please click the link below to download pdf file for CBSE Class 12 Biology - Genetics.
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