BIOLOGY FORM 5

5.1 : MENDEL’S EXPERIMENT

PREPARED BY : NORSHAFIKA BINTI DAOD

CLASS : 5 UTM

• There are similarities and differences between parents and offspring

• Some characteristics are passed on from parents to offspring while others appear to be lost.

• Meaning:– Inheritance: transmission of traits from one

generation to to another generation by means of genetic codes

– Character/characteristic: a distinctive structural or functional feature determined by a gene or group of genes.

– Trait: specific characteristic that varies from one individual to another

Character / Characteristic TraitEye colour in humans Black

BlueFruit shape in tomatoes Round

LongEye colour in fruit flies Red

WhitePod colour in garden peas Green

Yellow

CHARACTERS AND TRAITS IN SOME ORGANISMS

The first person to work out the basic laws that govern the inheritance of genes.

Starting from about 1856, for a period of 10 years, he conducted his famous breeding experiments with the garden pea (Pisium sativum).

GREGOR MENDEL

MENDEL’S EXPERIMENT

Mendel worked with the peas, Pisum sativum because:

The pea flowers have both female & male parts : stamens & carpels.

Many varieties of the pea plants which carry out self-pollination.

Can be easily grown Have short cycle Pollination can easily controlled

The seeds were planted and the character of the first generation offspring were noted

Mendel choose pure-breeding pea plant to study inheritance because:

Plant that self fertilises All its offspring resemble the parent

plant (has the same characters as the parents plant)

Show the same traits as their parents Obtained after many generations of

self-pollination

MENDEL’S EXPERIMENT WITH TALL AND SHORT PEA PLANT

MONOHYBRID INHERITANCE

MEANING

Parental generation/ P generation-original parents

1st filial generation/ F1 generation-results of the parental cross appeared in the first generation

2nd filial generation/ F2 generation-The plants of the F1 generation were allowed to the self-pollinate produced F2 generation

GENES AND ALLELESGENES AND ALLELES

Hereditary factors described by Mendel are known as genes

Genes: basic units of inheritance which occupy specific positions of chromosomes.

Position of gene is called its locus Alleles: different forms of the same

gene for a trait and occupy the same relative position on a pair of homologous chromosomes.

DOMINANT AND RECESSIVE DOMINANT AND RECESSIVE ALLELESALLELES

In F1 generation, plant received one allele from tall plant and one allele from short plant.

However, they are tall. Hence, the allele for the tall trait is called dominant allele. The allele for the short trail is recessive allele. When two different alleles of the same gene in which one is

dominant,the dominant allele will cover the effect of recessive allele.

A recessive allele will expressed when there is no dominant allele.

Usually, letters are used to represent genes or alleles. Capital letter for dominant allele, for example T for tall. Small letter for recessive allele, for example t for tall If the organism is pure-breeding tall, TT is used to represent

it If the organism is pure-breeding short,tt is used to

represent the allele.

EXAMPLES OF DOMINANT & RECESSIVE TRAITS

PHENOTYPE AND GENOTYPEPHENOTYPE AND GENOTYPE

Phenotype: observable characteristic of the some organism like colour, size, form and structure

Eg: tall, short Genotype: genetic composition of

an organism and cannot be seen Eg: TT, tt, Tt

MONOZYGOTE AND MONOZYGOTE AND HETEROZYGOTEHETEROZYGOTE

In Mendel’s experiments,both pea plant in parental generation were pure-breeding.

Therefore, the tall plant had two alleles for tallness(TT) The short plant had two alleles for shortness(tt) This called homozygote. The tall plant produced gametes which carried the

allele T and and short plant which carried the allele t will produced all tall plant.

They had one allele for tallness and one allele for shortness(Tt)

This called heterozygote. Three quarters of the offspring in the F2 generation

were tall and one quarter was short. Hence, the phenotypic ratio is 3:1 The genotype of tall pea plant in F2 generation is TT

and Tt while short pea plant is tt. Hence, the genotypic ratio is 1 TT : 2 Tt : 1 tt

MENDEL’S 1MENDEL’S 1STST LAW OF LAW OF INHERITANCEINHERITANCE

Monohybrid Cross: -cross carried out by Mendel involves only one character

Involves only one pair of alleles Mendel formulated his 1st Law of inheritance This called the Law of Segregation. It states:

-The members of each pair of alleles separate or segregate during the formation of gametes only one allele can be carried in a single gamate.

Monohybrid inheritance:-inheritance involving a single characteristic determined by one genes

Dihybrid inheritance:-a cross which involves two pairs of alleles determining two characteristics

MENDEL’S 2MENDEL’S 2NDND LAW OF LAW OF INHERITANCEINHERITANCE

It is called Law of Indipendent Assortment

It states:- two or more pairs of alleles segregate independently of one another during the formation of gametes.

MONOHYBRID CROSSMONOHYBRID CROSS In the initial set of experiments, Mendel

concentrated only on the pattern of inheritance of a single pair of contrasting characters. This pattern of inheritance involving only one pair of contrasting characters is known as monohybrid inheritance.

In the first set of experiments, Mendel conducted cross-pollination between a pure-breeding tall plant and a pure-breeding dwarf plant. He collected the seeds from this cross pollination and allowed them to germinate. All the resulting plants were found to be tall.

Based on these results, Mendel came to the conclusion that in a cross-involving two contrasting characters, only one character expresses itself in the next generation. Mendel called the character, which expressed as dominant character and the character, which failed to express, as recessive character. This idea came to be known as the principle of dominance (first law).

DIHYBRID DIHYBRID CROSSCROSS

DIHYBRID CROSSDIHYBRID CROSS

Figure 10-7For each dihybrid cross, Mendel cross-fertilized true-breeding plants that were different in two characters. Then

he allowed the F1 hybrids to self-fertilize. In this case, the

two characters, seed color and shape, are displayed by the first stage of each new generation, the seed (pea).

ANOTHER ANOTHER EXAMPLEEXAMPLE

FF1 1 GENERATIONGENERATION

Choose Symbol

Seed Color: Yellow = G; Green = g</I< FONT>

Seed Shape: Round = W; Wrinkled = w

FF2 2 GENERATIONGENERATION   

Female Gametes

   GW Gw gW gw

  GWGGWW (Yellow

,round)

GGWw (Yellow,round)

GgWW (Yellow,round)

GgWw (Yellow,round)

MaleGw

GGWw (Yellow

,round)

GGww (Yellow,wrinkled

)

GgWw (Yellow,round)

Ggww (Yellow,

wrinkled)

Gametes gWGgWW (Yellow

,round)

GgWw (Yellow,round)

ggWW (Green,round)

ggWw (Green,<BR)ROUND)< TD>

  gwGgWw (Yellow

,round)

Ggww (Yellow,wrinkled

)

ggWw (Green,round)

ggww (Green,

wrinkled)

PUNNET SQUAREPUNNET SQUARE

The genotypes and phenotypes resulting from various combination of gametes can be easily determined by Punnet squares, devised by Reginald C. Punnet (1875 1967). Hence each of the possible gametes is placed in an individual column or a row, with vertical column representing the female and horizontal row the male parent. The gametes are then arranged in all possible combinations and the resulting genotypes are entered in the boxes along with the phenotypes.

DIHYBRID CROSS USING SCHEMATIC DIHYBRID CROSS USING SCHEMATIC DIAGRAM AND PUNNET SQUAREDIAGRAM AND PUNNET SQUARE