P generation: purebred organisms that are used as the parent generation

F1 generation: offspring of the P generation

F2 generation:

◦ offspring that arise from a F1 x F1 cross

Gene: part of chromosome that governs expression of a particular trait

eg. hair colour , plant height

Allele: alternate form of a gene

eg. brown hair (B), red hair (b); tall (T), short (t)

Phenotype: the observable trait or characteristic ( what you can see )

Genotype: the genetic make-up of the trait ( which 2 alleles are present? ) Types of Genotypes

◦ Homozygous: 2 identical alleles are present eg. BB, tt

◦ Heterozygous: 2 different alleles are present eg. Bb, Tt

Types of Alleles (traits):

Dominant: trait that is fully expressed (visible)

Recessive: trait that is usually masked (hidden)

Punnett Square:

◦ method of predicting offspring if the genotypes of parents are known

Cross-fertilized two purebred pea plants that showed short and tall plant heights, respectively Parent (P) generation

First Filial (F1) Generation offspring obtained were all tall

100 % tall (phenotype)

After repeating this experiment while examining other pairs of traits

he observed that one trait was always DOMINANT than the other.

Principle of Dominance:

When an organism is a hybrid (crossbred) for a pair of contrasting traits, it will show only the dominant trait.

Hybrid tall plants from F1 generation in 1st experiment self-pollinated

Second Filial (F2) Generation obtained were mostly tall but some were short

75% tall and 25% short ( ratio – 3 tall : 1 short )

After repeating this experiment while examining other pairs of traits

he observed the same ratio of 3 : 1 (called the Mendelian ratio)

Law (Principle) of Segregation:

Inherited traits are determined by a pair of “factors” that segregate during the formation of gametes.

Let the tall trait = T (dominant)

Let the short trait = t (recessive)

P T T x t t male female T T t t F1 100% T t only combination possible all plants were tall tall is dominant; short is masked

F2 T t x T t

self-pollinated

Possible gametes: T, t T, t

Observed:

ratio 3:1 (tall : short)

3 tall : 1 short

Alternate forms of a gene exist called ALLELES

For each inherited trait, an organism has two genes (alleles)

one allele is inherited from each parent.

A sperm or egg carries only one allele for each inherited trait because allele pairs segregate (separate) from each other during the production of gametes by meiosis

When two alleles of a pair are different,

◦ one is fully expressed (DOMINANT allele) and

◦ the other is completely masked (RECESSIVE allele)

NOTE: Today, we know this is not true in all circumstances !

From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic ◦ One characteristic

comes from each parent!!!

P GENERATION (true-breeding parents)

F1 generation

F2

generation

Purple flowers White flowers

All plants have purple flowers

Fertilization among F1 plants (F1 x F1)

3/4 of plants have purple flowers

1/4 of plants have white flowers

Figure 9.3A

A sperm or egg carries only one allele of each pair

–The 2 alleles for a gene separate during gamete formation, and each gamete gets a different one

–This is the law of segregation

GENETIC MAKEUP (ALLELES)

P PLANTS

F1 PLANTS (hybrids)

F2 PLANTS

PP pp

All P All p

All Pp

1/2 P 1/2 p

Eggs P

p

P

PP p

Sperm

Pp Pp

pp

Gametes

Gametes

Phenotypic ratio 3 purple : 1 white

Genotypic ratio 1 PP : 2 Pp : 1 pp

Figure 9.3B

(1) Define symbols:

(2) State the cross

(3) Diagram the gametes

(4) Complete the squares

(5) Summarize the results:

Genotype

Phenotype

Monohybrid Cross: Punnett Square Method

Genotypic ratio:

1EE : 2Ee : 1ee

Phenotypic ratio: 3:1

In mice, gray coat colour is dominant over white coat colour.

What phenotypic and genotypic ratios would you

expect if: Heterozygous gray mouse crossed with homozygous

white mouse. Heterozygous gray mouse crossed with heterozygous

gray mouse. Work with the people at your table to solve this

question.

HOMEWORK: complete worksheet