1). Introduction 2). Gregor Johann Mendel 3). Mendel’ Mendel’ss Experiments Experiments 4). Terminology 5). Monohybrid Cross 6). Di-hybrid Cross . en e s aws 8). Exceptions to Mendel’ Mendel’ss Law 9). Sex Chromosomes Chromosomes in Human 10). Determination of Sex 11). Sex Linkage 12). Sex Linked inheritance of diseases
Introduction •
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It is commonly observed that children resemble their parents. An individual inherits the char characteris acteristic tic features features of the species from its paren parents. ts. This res This esem emb bla lanc nce e is bec because th the e off ffsp spri rin ng in inhe herrit itss cer a n ac ors rom e paren s. All the characters that offspring can inherit from the paren entts are called He Herreditary Characters and the process of their trans transfer fer is called Heredity. Heredity. Thus height, types of leaves, flower color, shape, seed structure etc…..are hereditary characters in plants.
- In the same manner curly hair, eye color, color blindness etc….in man are the hereditary characters. - These factors are now know as genes that are located at specific points on chromosomes. - These genes control a particular ‘Character’ or ‘Trait’.
mean anss tr tran ansm smis issi sion on of gen enet etic ical ally ly ba base sed d - Heredity: me char ch arac actter eris isti tics cs fr from om ar aren ents ts to of offs fs ri rin n . - Each gamete has a complete haploid set of these genes. - Thus, the zygote inherits one half of its genetic material from either parent. This is the one reason why children children resemble both the paren parents. ts.
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Althou Alth ough gh of offfsp spri ring ng res esem embl ble e th thei eirr pa parren ents ts,, they are rarely identical to them and they show some dissimilarities from their parents. Such dis isssim imil ila arit itiies in a speci cie es are calle led d . These differences are cause The sed d by gen ene etic / inherited variations and environmental / non inherited differences. Only inherited variations can be passed to offspring and have an influence in changing the genetic genetic constitution of the species.
Gregor Mendel Know
as Father of GENETICS Born in 1822 in Czechoslovakia. Czechoslovakia. He carried out expts on garden pea for 8 yrs. On the basis of his results, he derived a few fundamental fundamental principles. He presented his results in a paper “Expts in Plant Hybridization” before Brunn Natural History Society in 1865. Mendel repeated his work with some other plants also but failed to repeat his results. Hugo de Vries, Carl Correns & Erich von Tschermak workin rking g ind indepen epende den ntly tly redis edisccovered ered his his work and and brought Mendel’s expts to the limelight.
Reasons for Mendel’s Success
He chose the garden pea for his expts.
Pea plants were easy to cultivate, had a short lifespan and showed self-pollination as well as easily distinguishable characters. This helped him.
His success was also due to his meticulous planning and laborious observa rvations and record that enable him to have enough data to be analyzed statistically.
Mendel’s Mendel’s Experiments Experiments & Techniqu echniques es Hybridization (cross breed) involves crossing of two individuals with erent es re char charac actters ers to prod produc uce e an offspring that has desi esired characters ers of both parents.
New terms •
Homozygous: refers to alleles for a single trait.
having
identical
Eg. The gene for seed shape in pea plants , round seed shape (R) and the other for wrinkled seed shape (r). A homozygous plant would contain the following alleles for seed shape: (RR) or (rr).
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Heterozygous: refers to having two different alleles for a single trait. E.g. The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). A heterozygous plant would contain the following alleles for seed shape: (Rr). Organis ism ms hav ave e two all llel ele es for ea eac ch trait it.. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. Using the previous example, round seed shape (R) is dominant and wrinkled seed shape (r) is recessive. Round: (RR) or (Rr), Wrinkled: (rr).
Picture showing Homozygous and Heterozygous
Mendel’s experiment •
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He cultivated the pea plants with different characters and studied till he obtained TRUE BREEDING / HOMOZYGOUS plants (TT). E.g. Homozygosity for tall plants was tested by grow ng see s o a p an s game es o pure breed tall plants are TT and for dwarf plants are tt ]. Bef Before ore exper xperim imen enti ting ng he cons consid ider ered ed some some characters for his experiments.
Characters Characters Mendel considered are 1. Stem length: Tall / Dwarf 2. Flower position: Axial / Terminal 3. Flower color: Red / White 4. Pod color: Green / Yellow . Constricted 6. Seed shape: Round / Wrinkled 7. Cotyledon color: Yellow / White
The next step was HYBRIDIZATION •
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He chose 2 parents with alternate forms of a character. Eg. Red / White flower color, Tall / Dwarf stem length etc….. Then he removed the anthers of the plant, he pollen of desired male and bagged it.
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Such a cross was called because it involved cross between alternate forms of one character character only. only. The offspring was F1 generation .
Further…… # Then, Mendel made a reciprocal cross where the parents in the earlier cross were reserved i.e. male as fe female male and vice ver versa. sa. # In all these, the offspring demonstrated Red # The F2 generation was obtained by bagging complete flowers of F1 generation enabling them to self pollinate and produce seeds.
TERMINOLOGY Term
Explanation
Example
GENE
The basic unit of Height of the inheritance for a given plant character
ALLELE
Alternate forms of the Tall (T) or same gene which Dwarf (t) determine contrasting characters
iploid condition whe where TT or tt Homozygous Dipl both the identical
alleles
are
Heterozygous
Diploid condition where Tt both the alleles are different
Phenotype
The physical or external and Tall, observable expression of a Dwarf character
Genot
The genetic expression of a TT tt Tt character in terms of alleles written in symbols
e
Dominant
An allele whi which expresses TT or Tt itself externally when (both represent pres presen entt in homo homozzygous ous or tallness: T is heterozygous conditions dominant )
Recessive
An allele whi which expresses t or dwarf itself externally when present in homozygous condition but remains suppressed in heterozygous condition.
Monohybrid When only one pair of Tall x Dwarf Dwarf alleles is used during (TT) x (tt) hybridization.
F1 Generation
The generation generation produced by (P) TT x tt crossing two parental parental stocks stocks ↓ is called first filial (F1) Tt generation.
The generation (P) TT x tt F2 tt produc uced ed by cros crossi sing ng Generation prod ↓ two individuals of F1 (F1) Tt x Tt generation is called ↓ second filial (F2) TT Tt Tt tt generation.
Test Cross
A cross between the (P) tt x Tt (F1) recessive parent and an individual of F1 generation.
Definition •
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Variation: Small differences individuals due to inheritance i nheritance
between
Mutation: Sudden changes in one or more , not have existed in the parents, grand parents or even great grand parents. E.g. Albinism (total loss of skin pigment)
