PRINCIPLES OF INHERITANCE AND VARIATION
List of the lesson
Mendelian experiments- Monohybrid and Dihybrid
Mendelian Laws
Dominance, co-dominance, incomplete-dominance, sex linked inheritance
Linkage and crossing over
Polygenic inheritance
Pleiotropy
Sex determination mechanisms
Pedigree analysis
Mendelian disorder: autosomal and sex-linked
Chromosomal disorders-Aneuploidy
SEX DETERMINATION
XX X0 XX XY ZZ ZW
Female Male Female Male Male Female
Grass Hopper Man Birds
MLL
Know the terms
Terms Meaning
Locus Location of a gene in a chromosome
Allele Alternative form of a gene
Homozygous Both alleles of a gene at a locus similar
Heterozygous Both alleles of a gene at a locus dissimilar
Homozygous dominant Both alleles of a gene at a locus similar & dominant
Homozygous recessive Both alleles of a gene at a locus similar & recessive
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 Independent assortment): The law states that when two pairs of traits are combined in a hybrid, segregation of on pair of characters is independent of the other pair of characters.
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 their product of the above selfing
8. Analysis of result (Phenotype and Genotype)
Linkage
Tendency of genes on same chromosome to remain together
Such genes are called linked genes.
Linked genes present only parental types
Co Dominance and multiple allelism
Blood Group Possible genotype
A |A|A OR |Ai
B |B|B OR |Bi
AB |A|B
O ii
Example for co-dominance: AB blood group in humans
Polygenic lnheritance
More than one gene responsible for a character. Shows cumulative phenotypic effect
Eg. Polygenic inheritance- skin colour in man
Controlled by three genes A, B &C
AABBCC X aabbcc
(Very dark) (Very light)
ABC X abc
F1 AaBbCc
On Selling F1
F2 generation shows 64 combinations
Pleiotropy
Phenomenon when single gene may produce more than one effect
i.e., the multiple effect of a gene
Eg. In Drosophila, white eye mutation leads to depigmentation In many other parts of the body-this Is pleiotropic effect
Sex determination and sex chromosome
Organism Male Female
Human beings XY XX
Birds ZZ ZW
Insects XO XX
Sex Determination In Honey Bee
Male Female
16 32
16 16 16
32 16
Female Male
Female would lay infertile eggs, but it hatches into normal fertile haploid male.
Pedigree Analysis
Pedigreeis a chart of graphic representation of record of inheritance of a trait through several generations in a family.
Genetic Disorders- Two types
1. Mendelian
a) Determined by alteration or mutation in single gene
b) Transmitted to offspring on the same line of principle of inheritance
c) May be dominant or recessive
d) Some trait linked to sex, such X-linked recessive traits transmitted from carrier female to male.
Eg.Haemophilia, Sickle Cell Anaemia, Phenyl Ketonuria
2. Chromosomal
a) Is due to aneuploidy or polyploidy conditions
b) It may be due to trisomy or monosomy conditions
Eg. Downs Syndrome, Klinefelter’s Syndrome, Turner’s Syndrome
Colour Blindness
i) It is a recessive sex linked trail
ii) Here eye fails to distinguish red and green colours
iii) Gene for normal vision dominant
iv) Male has one X-chromosome. So if diseased X-chromosome is present, they directly express it.
v) Female in heterozygous condition have normal vision
XCXc X XCY
Carrier female Normal male
F1 XCXC XCY XCXc XcY
N N CarrierAffected
Thalassemia
i) Genetic defect occurs due to mutation or deletion
ii) Affected person produce very few globins
iii) Group of disorders-Caused in synthesis of globin. Absence or reduced synthesis of one globin chain results’ In excess of another. This leaves free globin chains which are insoluble and accumulate inside the RBC, forms precipitate which damages the cell. This cause Lysis and Anaemia.
Two types:
i) Alpha Thalassemia
ii) Beta Thalassemia
There is third type-Delta Thalassemia where about 3% of adult haemoglobin is made of alpha and chains.
HOTS
1. In a monohybrid cross the monotypic and phenotypic ratio is 1:2:1. What type of inheritance is it example of? Give one example.
2. If a human zygote has XXY sex chromosomes along with 22 pairs of autosomes. What sex will the individual be? Name the syndrome.
3. A mother with blood group ‘B’ has a fetus with a blood group ‘A’ father is ‘A’. Explain the situations.
4. The genes for hemophilia are located on sex chromosome of humans. It Is normally impossible for a hemophilic father to pass the gene to his son. Why?
5. Justify the situation that in human beings sex of the child is determined by father and not by mother?
6. Distinguish between Complete and Incomplete linkage.
7. A man with AB blood group marries a woman with AB blood group.
i) Work out all possible genotypes& phenotypes of the progeny.
ii) Discuss the kind of domination in the parents & progeny.
8. A dihybrid heterozygous round, yellow seeded garden pea was crossed with arecessive plant.
i) What type of cross is this?
ii) Work out the genotype & phenotype of the progeny.
iii) What principle of Mendel is illustrated by it?
Diagram-Based Questions
1. Identify the traits shown in pedigree chart. Give one example each.
BIOLOGY 