INHERITANCE IN LIFE

HEREDITY

• Heredity is the transmission of particular

characteristics from parent to offspring.

Gregor Johann Mendel

1822-1884

• Mendel presented completely new theory of

inheritance in the journal ‘Transactions of the

Natural History society of Brunn’.

• His work was rediscovered in 1900,

simultaneously by Hugo de Vries in Holland,

Carl Correns in Germany and Erich

Tschermak in Austria.

• Mendel is often referred to as Father of

Genetics,

• His experiments and principles collectively

form “Mendelian genetics”.

Gregor Mendel (1822 – 1884)

• Studied garden peas

• 1st to use mathematics to examine outcomes of crosses

• Number of pea varieties with at least 7 easily distinguished traits

• Peas are small, easy to grow, short generation time

• Peas can self-fertilize; bisexual

Dominance and recessive traits

• Principal of dominance is only one of the two

contrasting character will appear or expressed

• F1 generation showed only purple flowers (dominant)

• White is a recessive trait in F1 generation

Terminologies

1. Mendelian factors are now called genes

2. Alleles are different versions of the same gene

3. An individual with two identical alleles is termed homozygous

4. An individual with two different alleles, is termed heterozygous

5. Genotype refers to the specific allelic composition of an individual

6. Phenotype refers to the outward appearance of an individual

Genotype and Phenotype

• Hereditary traits are under the control of genes (Mendel called them factors).

• Genotype is the genetic makeup of an organism, a description of the genes it

contains.

• Phenotype is the characteristics that can be observed in an organism.

• Phenotype is determined by interaction of genes and environment

• Genes provide potential, but environment determines whether that potential

is realized

Terminologies

• Dominant - stronger of two genes expressed in the hybrid; represented by a

capital letter (R)

• Recessive - gene that shows up less often in a cross; represented by a

lowercase letter (r)

• Self-fertilization can occur in the same flower

• Cross-fertilization can occur between flowers

• P/F1/F2 generations

Keys to Mendel’s success

• The garden pea was an ideal organism for study because:

–Vigorous growth

–Self fertilization

–Easy to cross fertilize

– Produces large number of offspring each generation

Keys to Mendel’s success

• Mendel analysed traits with discrete alternative forms (one of two options)

– purple vs. white flowers

– yellow vs. green peas

– round vs. wrinkled seeds

– long vs. short stem length

• Mendel established pure/true breeding lines to conduct Controlled

fertilization experiments

– (traits remain constant from generation to generation)

Monohybrid cross

• In monohybrid cross Mendel selected one character for his experiment.

• Crosses were made between white flowered and purple flowered plants.

• Pollens from the purple flowers were placed onto the stigma of white

flower.

• Allowed it to cross fertilization

• All the seeds in the pod resulted from this pollination were hybrids.

LAW OF SEGREGATION

• Whenever a pair of ‘factors’ for character brought together in a hybrid, they

segregate during the formation of gametes. Hence each gamete is pure with

reference to this character.

Test cross and Back cross

• A cross involving the F1 individuals with either of the two parents is called

back cross.

• The cross between F1 and recessive parent is called test cross (Pp X pp).

• All the test crosses are back cross but all the back crosses are not a test

cross

• Test cross helps to test whether the individual is homozygous or

heterozygous.

Back cross - Testcross

Dihybrid cross

• Mendel crossed a pea plant producing Round yellow seeds with one

producing green and wrinkled seeds of pure breed variety.

• In F1 generation plants obtained producing only round yellow seeds

• F1 were allowed for self-pollination to get F2 generation.

• In F2 generation, 4 different types of plants were produced that is a) Round

yellow b) Round green c) wrinkled yellow d) wrinkled green.

• Phenotype ratio of 4 types of plants were 9:3:3:1

Principle of Independent Assortment (Mendel’s 2nd

Law)

• The factors for two or more pairs of contrasting characters are distributed

independently of one another at the time of gamete formation.

• In a dihybrid cross, the alleles of each gene assort independently

Dihybrid test cross

RY Ry rY ry

ry RrYy

Round

Yellow

Rryy

Round

Green

rrYy

Wrinkled

Yellow

rryy

Wrinkled

Green

A dihybrid test cross involves crossing of the F1 dihybrid with a double

recessive parental type.

RrYy (F1) X rryy (P)

Genotype and Phenotype ratio is 1:1:1:1

Chromosomal theory of inheritance

• The chromosome theory of inheritance allows us to see the relationship

between Mendel’s laws and chromosome transmission

• Mendel’s law of segregation and independent assortment can be explained

by the homologous pairing and segregation of chromosomes during meiosis

Cell division

Mitosis

Meiosis

Meiosis

Meiosis

Chromosomal theory of inheritance

• The chromosome theory of inheritance describes how the transmission of

chromosomes account for the Mendelian patterns of inheritance

• This theory was independently proposed in 1902-03 by

• Theodore Boveri, a German Scientist

• Walter Sutton, an American Scientist

Chromosomal theory

• The hereditary material must reside within the nuclei of the gametes.

• Chromosomes segregate during meiosis.

• Gametes have a copy of one member of each pair of homologous

chromosomes

• diploid individuals have a copy of both members of the pair.

• During meiosis, each pair of homologous chromosomes orients on the

metaphase plate independently of any other pair.

• It is the chromosomes that undergo segregation and independent

assortment.

• Humans have 46 chromosomes

44 autosomes

2 sex chromosomes

• Males contain one X and one Y chromosome

They are termed as heterogametic

• Females have two X chromosomes

They are homogametic

• The Y chromosome determines the gender

Morgan’s experiment

• In 1910 the American geneticist Thomas Hunt Morgan, studying the fly

Drosophila melanogaster,

• He detected a mutant fly, a male fly that differed strikingly from normal

flies of the same species.

• The eyes were white instead of the normal red.

• Morgan’s discovery of a white eyed trait that correlated with the sex of flies

was a key episode in the development of the chromosome theory of

inheritance.

Morgan’s Experimental evidence

Recombination of linked genes crossing over

Alterations in Chromosomal number

Down syndrome

Genetic disorder

Alterations in chromosomal structure

Alterations in chromosomal structure

Chromosomal translocations

Pedigree analysis

• Collecting the information about a family’s history for a particular trait and

assembling this information into a family tree describing the traits of

parents and children across the generations is the family pedigree.

Pedigree of dominant trait

Widow’s peak is a dominant or recessive trait?

Pedigree of recessive trait

Is attached earlobe a dominant or recessive trait?