GENETICS - s3-ap-southeast-2.amazonaws.com · 8.4 Cytological basis for Crossing over 8.5 ......

Dr.Sujana Papani, PVKN Govt College, Chittoor

GENETICS

By

Dr. P. SUJANA,

Assistant Professor of Botany,

P.V.K.N Govt College,

CHITTOOR.


TABLE OF CONTENTS

Chapter CONTENTS Page No

6 CLASSICAL GENETICS

6.1. Introduction

6.2. Mendal Experiments

6.3 . Mono Hybrid cross

6.4. Dihybrid cross

6.5. Mendal’s law of Inheritance.

6.6 Back cross and Test cross

6.6.1 Back cross

6.6.2 Test cross

7 INTERACTION OF GENES 7.1 Modification of Monohybrid ratio

7.2. Modification of dihybrid ratio

7.2.1. Epistasis

7.2.2.. Non- Epistatic interaction

8 LINKAGE AND CROSSINGOVER 8.1 Linkage

8.1.1 Theories of Linkage

8.1.2 Types of Linkage

8.1.3 Linkage groups

8.2. Crossing over

8.2.1 Types of Crossing over

8.2.2 Theories of Crossing over

8.3. Recombination during Meiosis

8.4 Cytological basis for Crossing over 8.5 Crossing over and Linkage maps

9 MUTATI ONS

9.1 Classification

9.2 Chromosomal Aberrations

9.3 Gene Mutations and Point Mutation

9.4 Transposable Genetic elements (Transpons)

10 THE CONCEPT OF GENE

10.1 The Gene

10.2 The Genetic Code

10.3 Transcription

10.3.1 Mechanism of Transcription in prokaryotes

10.3.2 Transcription in Eukaryotes

10.4 Translation

10.5 Regulation of Gene expression

10.6 The Extra Nuclear Genome

SUGGESTED READINGS


CHAPTER - 6

CLASSICAL GENETICS – MENDELIAN GENETICS – MENDELISM

6.1. Introduction

Genetics is a branch of Biology that deals with the study of heredity and variations.

In general we can say that, Genetics is the study to understand how genes bring about

characteristics, or traits, in living things and how those characteristics are inherited. Genes

are specific sequences of nucleotides that code for particular proteins. Through the

processes of meiosis and sexual reproduction, genes are transmitted from one generation

to the next.

Gregor Johnn Mendel is the father of Genetics. He conducted hybridization

experiments on garden pea plant ( Pisum sativum ) and formulated Laws of inheritance.

These are commonly known as Mendel’s Laws of inheritance. Mendel performed his

experiments in the 1860s and 1870s, but the scientific community did not accept his work

until early in the twentieth century. Because the principles established by Mendel form the

basis for genetics, the science is often referred to as Mendelian genetics. It is also called

classical genetics to distinguish it from another branch of biology known as molecular

genetics. All these inventions were recognized after the rediscovery by Carl Correns, Erich

Von Tschermak and Hugo de Vries. These are all together known as Classical Genetics or

Mendelism or Mendelian Genetics.

Some Important Technical Terms:

i. Trait : Any characteristic that can be passed from parent to offspring. ii. Heredity - passing of traits from parent to offspring iii. Genetics - study of heredity iv. Phenotype: External appearance of an individual with respect to any character. v. Genotype: Genetic constitution of an individual. vi. Allelomorphs (Alleles): One of two or more forms that can exist at a single

gene locus distinguished by their differing effects on the phenotype. vii. Dominant allele: Expressing allele. Stronger of two genes expressed in the

hybrid; represented by a capital letter. Dominant alleles always express themselves.

viii. Recessive allele: The “overshadowed” allele. Recessive alleles express themselves only when two recessive alleles exist together in an individual. Represented by a lowercase letter.

ix. Genome : The set of all genes that specify an organism’s traits. x. Homozygote (Homozygous): The organism having two similar alleles for a

particular character in a homologous pair of chromosomes is known as


homozygous or genetically pure that particular character. Gene combination involving 2 dominant or 2 recessive genes.

xi. Heterozygote (Heterozygous): An individual containing both dominant and recessive alleles for a particular character. This is generally a hybrid.

xii. Monohybrid cross : Cross involving a single trait. xiii. Dihybrid cross : Cross involving two traits. xiv. F1 hybrid: The first generation of the given cross. xv. F2 hybrid: The second generation which is resulted by interbreeding or selfing

of F1 offspring. xvi. Back cross: The cross made between the F1 hybrid and any of the parents. xvii. Test cross: The cross made between the F1 hybrid and the recessive parent. xviii. Reciprocal cross: If in the cross, individual X is used as male and Y as female

and in the next cross Y is used as male and X as female. It is called reciprocal cross.

Gregor Mendel:

Austrian monk Studied science & math at the University of Vienna Formulated the laws of heredity in the early 1860's Did a statistical study of traits in garden

peas over an eight year period Why peas, Pisum sativum?

Can be grown in a small area Produce lots of offspring Produce pure plants when allowed to self-pollinate

several generations GARDEN PEA

Can be artificially cross-pollinate

6.2. Mendel's Experiments:

Mendel studied simple traits from 22 varieties of pea plants (seed color & shape, pod color & shape, etc.)

Mendel traced the inheritance of individual traits & kept careful records of numbers of offspring

He used his math principles of probability to interpret results Mendel studied pea traits, each of which had a dominant & a recessive form (alleles) The dominant (shows up most often) gene or allele is represented with a capital

letter, & the recessive gene with a lower case of that same letter (e.g. T, t)


Mendel's traits included:

a. Seed shape --- Round (R) or Wrinkled (r) b. Seed Color --- Yellow (Y) or Green (y) c. Pod Shape --- Smooth (S) or wrinkled (s) d. Pod Color --- Green (G) or Yellow (g) e. Seed Coat Color --- Gray (G) or White (g) f. Flower position --- Axial (A) or Terminal (a) g. Plant Height --- Tall (T) or Short (t) h. Flower color --- Purple (P) or white (p)

Mendel selected Pisum sativum for his experiment because – It has a number of well

defined contrasting characters with bisexual flowers. It shows predominantly self

fertilization. Hybridization or crossing is easy in Pea plant and this plant shows a short life

span.

Mendel chose true-breeding lines of each plant/trait he studied (true

breeding lines always produced offspring of the same type)

He crossed a true breeding plant with a plant of the opposite trait (purple x

white). He called this the Parental (P) generation. (In this case, he cross-

pollinated the plants)

He recorded data on the offspring of this cross (First Filial, F1)

He self pollinated the F1 offspring


He recorded data on the offspring of the second generation, calling it the

Second Filial generation (F2)

Analysis:

The F1 generation always displayed one trait

(he later called this the dominant trait)

The F1 generation must have within it the

trait from the original parents - the white trait

The F2 generation displayed the hidden trait,

1/4 of the F2 generation had it (he later called

this hidden trait the recessive trait)

Each individual has two "factors" that

determine what external appearance the

offspring will have. (We now call these factors

genes or alleles)

Mendel experiment involved four steps - selection, hybridization, selfing and

calculations. First he selected a true breeding variety. Then he cross pollinated two

contrasting true breeding varieties to get F1generation. Then he self pollinated F1 offspring

to get F2 and self pollinated F2 to get F3 generation.

6.3. MONOHYBRID CROSS:

A cross between two parents differing in a single pair of contrasting Characters is

known as monohybrid cross. Mendal self pollinated pea plants for several generations to

get pure homozygous plants.

Mendel crossed a tall (T) plant with dwarf plant (t ) and obtained all tall offspring in

the F1generation (Law of Dominance). On selfing of F1 the F2generation plants appeared in

the phenotypic ratio of 3 tall : 1 dwarf. The character of dwarfness disappeared in the first

generation (hybrid), but again reappeared in the second generation (F2). When plant

produces gametes, the alleles segregate or separate in separate in such a way that each

gamete receives only one of the two alternative alleles.( Law of Seregation).


Genetically F2 generation consisted of three types of plants ( instead of apparent

two types):

(i) Tall homozygous (Pure) – 25% (TT)

(ii) Tall heterozygous (hybrid) – 50% (Tt)

(iii) Dwarf homozygous (pure) – 25% (tt)

Monohybrid cross:

Genotypic ratio: 1:2:1 ; Phenotypic ratio: 3:1

F1 gamates

T

t

T

TT

Tall

Tt

Tall

t

Tt

Tall

tt

Dwarf


6.4. DIHYBRID CROSS:

A cross between two parents differing in two parents in two pairs of contracting

characters is known as dihybrid cross. The progeny of such crosses called dihybrids.

Mendel crossed a dominant homozygous yellow (YY) and round seeded(RR) plant

with a recessive homozygous green (yy) and wrinkled seeded (rr) plant. The F1 hybrids

were found to have yellow and round seed. This indicates that yellow colour and round

shape seeds are dominant and green and wrinkled are recessive. The F1 hybrids were

selfed and the F2 offspring appeared in the phenotypic ratio of 9 Yellow, Round : 3 Yellow,

wrinkled : 3 green, Round : 1 green, wrinkled. These characters lie independent of each

other. This experiment gives evidence for principle of Law of independent assortment

Fig: Dihybrid Cross

Genotypic ratio: 1:2:2:4:1:2:1:2:1

Phenotypic ratio: 9:3:3:1

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