Unit 1.2 Laws of MendelVyshali V M

Introduction

• The biological science which deals with the mechanism of heredity and cause of variation in living being is known as Genetics.

• The word genetics- greek word ; gen –to become or to grow into

• Coined by Batson, 1906

• Heredity is the cause of similarities between the individual

• Variation is the cause of difference between the individuals. The heredity and variation play an important role in the formation of new species.

History

• Mendel-19th century

1. Vapour and fluid theory: Pythogorus, Aristotle

2. Preformation theories: Malphigi, 1673; Homunculus; Ovists; spermatists; K.F. Wolff (1794)

3. Particulate theories: French biologist; Maupertuis-minute particles

4. Pangenic theories: 1868, Charles Darwin-Pangenesis; Gemules; disproved Galton, weissmann

5. Germplasm theories: August Weismann,1904; somatic cells and germ cells; determinant-chromosome of nucleus

Mendelism

• Gregor Johan Mendel; July 22, 1822 in Austria

• 1843, augustrain monastery at Brunn- given name ‘Gregor’

• 1851, sent to Vienna- natural science-school master; 3years- studied mathematics and natural history.

• Returned to brunn –teacher of science in high school; same time, began experiments with garden peas.

• After 7years of experiment on hybridization- presented in natural history society of brunn, 1865; printed in the annual proceedings of the society

• Nobody appreciated his work until 1900

• Mendel died in 1884.

Rediscovery of mendel`s principles

• Dervis: from Holland

• Correns: Germany

• Tscherma: Austria

• Bateson confirmed mendels work- hybridization experiments

Mendel began his work –collecting several types of garden pea and studied the difference among them

• Its flowers are normal- self fertilized

• Shows a number of clear cut contrasting characters

• Hybrids are perfectly fertile

• Cross pollination and fertilization is not difficult and successful

• Factors for seven pairs of characters are located on different homologous pairs of chromosomes

• Many pure breeding varieties are available for the experiments

• It is very to cultivate the pea plant in open ground

• They have short growth period and life cycle.

Terminologies

• Genes: It is a unit of inheritance

• Alleles/ allelomorphs: A pair of genes representing the two alternatives of the same character and located at the same locus in the homologous chromosomes

• E.g., Gene content of F1 plant---Tt: T and t

Homozygous and heterozygous

• Homozygous: In a diploid organism the members of a pair of contrasting characters or alleles situated at identical loci on the homologous chromosomes are similar then the condition is termed homozygous

• E.g., TT

• Heterozygous: In a diploid organism the members of a pair of contrasting characters or alleles situated at identical loci on the homologous chromosomes are dissimilar then the condition is termed heterozygous

• E.g., Tt

Genotype • The genetic make up or the gene content of a plant or animal

• Genotype for tall plant – TT• Genotype for dwarf plant – tt• Genotype for F1 plant – Tt

• F2 generation– 3types• 25% TT (Homozygous)• 50% Tt (heterozygous)• 25% tt (Homozygous)

• 1:2:1 ratio--- genotypic ratio

Phenotype

• The external visible appearance of a plant or animal is called Phenotype

• F2– phenotypically two types: tall and dwarf

• 3:1 ratio– phenotypic ratio

Back cross and test cross

• To identify the heterozygous plant, mendel crossed the F1 with one of the parent--- back cross

• Two types

• Dominant back cross: If the F1 hybrid is crossed with dominant parent

• Recessive back cross: If the F1 hybrid is crossed with recessive parent, this helps to identify heterozygosity--- Test cross; 1:1 ratio

Monohybrid experiment

Punnett Square:• It is a graphical representation to calculate the probability of all

possible genotypes of off springs in a genetic cross . The production of gametes by the parents, the formation of zygote, the F1 and F2 –germinations can be explained by Punnett square. It was developed by British geneticist RC Punnett.

T t

T TTtall

Tttall

t TtTall

TtDwarf

Dihybrid Experiment• In a di-hybrid cross the parents differ through two characters.

• Mendel conducted a cross between a true breeding Round Yellow plants (RRYY) with true breeding Wrinkled Green plant (rryy).

• Round and Wrinkled are the shapes of seed coat whereas Yellow and Green are the colours of the seed coat.

• Mendel observed the seeds of the Fj plants were all Round and Yellow.

• This showed Round and Yellow were dominant over Wrinkled, Green.

• In the Fv generation four types of combinations were observed such as Round Yellow, Wrinkled Yellow, Round Green and Wrinkled Green.

• Thus the above types of offspring’s of F2 generation were produced in the ratio of 9:3:3:1. This ratio is called Di-hybrid ratio

•

Dihybrid Experiment

Genotypic and phenotypic ratio of dihybrid cross

Explanation:

• In his dihybrid cross experiment Mendel observed round and yellow characters are dominant over wrinkled and green so that all the F1 offspring’s are round and yellow.

• In the F2 generation, he noticed an allele (dominant or recessive) of a given character freely combines with either one (dominant or recessive) of the alleles of another character.

• Hence, a dominant allele of a character combines, not only with the dominant, but also with the recessive allele of another character.

Mendels Laws

• Based up on mendel’s experimental results certain principles are formed. These principles are called mendels laws. They are as fallows

• 1. Law of Dominance (First Law):• The law of dominance states that when two alternative forms of a trait or

character (genes) are present in an organism, only one factor expresses itself in F1-progeny and is called dominant, while the other that remains masked is called recessive.

• This law is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 -generation and the expression of both in the F2-generation. It also explains the proportion of 3:1 obtained in theF2-generation.

• Example –Monohybrid experiment

• 2. Law of Segregation (Second Law):

• This law states that the alleles do not show any blending and both the characters are recovered as such in the F2-generation, though one of these is not seen in the F1 -generation.

• Due to this, the gametes are pure for a character. 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.

• Example –incomplete dominance

• 3. Law of Independent Assortment (Third Law):

• This law states that when two pairs of traits are combined in a hybrid, segregation of one pair of character is independent of the other pair of characters at the time of gamete formation.

• Example –Dihybrid experiment

Incomplete dominance• Incomplete Dominance:

• The law of dominance is not always found to be correct as there are many examples where the complete dominance is absent. In such cases some traits of F1 phenotype is intermediate between those of parental traits.

• A good example of incomplete dominance is seen in Mirabilis jalapa(Four o’clock plant)

• Here, when a cross is made between red flowered (RR) and white flowered (rr) varieties, F1 (Rr) progeny produced is an all pink flowered. When these F1 pink flowers are self pollinated or crossed among themselves to raise F2 generation, they produce red (RR), pink (Rr) and white (rr) flowers giving 1:2:1 ratio. This phenotypic ratio is identical with genotypic ratio because heterozygotes are phenotypically intermediate between two homozygous types.

Incomplete dominance

1. The round shape of pea seeds is dominant over the

wrinkled shape of seeds. If two homozygous parents

belonging to round and wrinkled character are crossed,

then describe the F2 generation including test cross ratio.

• In fruit fly, Drosophila melanogaster, genes for normal

developed wings are dominant over the genes for vestigial

wings. Describe the genotypes of the parents and

F2 genotypic and phenotypic ratio of the flies.

• In fruit fly the normal fly will contain V and vestigial wings will be represented by v genotype.

After self fertilization Vv x Vv, the F2 will be:

Phenotypic ratio = 3 Normal: 1 Vestigial wingedGenotypic ratio = 1:2:1.

2. When a red fruited tomato plant is

selfed, red and yellow fruited plants are

produced in the ratio of 32:10. What is the

genotype of the parent?

• Note. In tomato red fruit colour is dominant over yellow.

• Given ratio-Red: Yellow = 32: 10

• Approximate phenotypic ratio = 3: 1

• This 3:1 is the monohybrid ratio. The red fruited plant must be a hybrid one. The genotype is Rr. While crossing a pure bred red with pure yellow, this gives a hybrid but with red fruit.

• Phenotype: 3 red: 1 Yellow

• Thus, the genotype of parent is Rr.

• In fowls the gene for black feathers B is incompletely dominant over its

allele for white feather b, the heterozygous being slate blue.

Determine the F2 phenotypic and genotypic ratio of a cross between a

black and a white bird. What will be the result when F1 bird produced

in above cross is mated to a black bird and to a white bird ?

• Bateson & Punnett demonstrated in sweet pea that when

two white flowered varieties were crossed, F1 hybrids

were all purple coloured but in F2 generation it

segregated in to purple and white colour. Give the

genotype of parents and grand parents.

• In maize purple colour (P) is dominant over the green colour (p) and

crinkled or cut character (c) is recessive to normal character (C) of

leaves. What will be the phenotype and genotype of F1 and

F2 generation if purple normal maize is crossed with green crinkled.

Genetic problems

1. A heterozygous round seeded plant (Rr) is crossed with a homozygous round seeded plant (RR).What percentage of the offspring will be homozygous (RR)

2. A homozygous round seeded plant is crossed with a homozygous wrinkled seeded plant.

What are the genotypes of the parents? __________ x __________

What percentage of the offspring will also be homozygous? _______ What is the genotype of all of the offspring? ______

• A variety of pepper having brown fruits was crossed with a variety

having yellow fruits. All the F1 plants had red fruits. When the red

fruited F1 plants were crossed together, the F2 consisted of 180

plants with red fruits, 60 with brown fruits, 58 with yellow fruits

and 18 with green fruits. What appears to be the genetic basis of

these fruit colours in pepper?

• The fruit colour is governed by two different allelic pair of genes. Individually one dominant gene shows brown (B) and the other yellow (Y) colour. But when both the dominant genes meet together, after interaction produce red (BY) colour and the double recessive condition gives green colour (bbyy).

• According to this hypothesis the segregation in F2 is in the ratio of 9:3:3:1 (interaction of genes).