Introduction to Genetics

Introduction to Genetics

Who was Gregor Mendel?• He was known as the

“FATHER OF GENETICS” • He discovered how traits were inherited

•GENETICS – study of heredity

•HEREDITY – the passing of traits from parents to offspring

Mendel’s Peas• Mendel did his study

on pea plants

• Pea plants have many traits

(tall/short, purple flowers/white flowers)

• Pea plants can beself-fertilized or cross-fertilized

Mendel’s PeasHe studied seven traits.

Mendel's Procedures

Types of PlantsTrue-Breeding - these plants always

create plants that look like themselves when crossed. AKA: Purebred

Tall=Tall x Tall or Short =Short x Short

Hybrids – offspring from crosses of two true-breeding plants (with different traits)

Hybrid=Tall x Short

Mendel’s Experiments

• He experimentally crosses different varieties to develop hybrids.

• He then crossed the hybrids and analyzed the results.

P

F1

F2

Genes and Alleles

Mendel discovered that each trait is controlled by two factors (alleles)

Genes* – factors that determines traits

So…. Alleles are two forms of the same gene ex. B vs b

*FYI--Danish botanist Wilhelm Johannsen coined the word "gene“ in 1909.

Explaining the Cross

When a parent makes sperm or eggs, their genes (2 factors) separate        (LAW OF SEGREGATION)

The GAMETES (egg or sperm) contain one allele for each trait. (let T be tall and t be short)

TT tt

T

DominanceSome traits were dominant over others.

PRINCIPLE OF DOMINANCE

Tall x Short = all tall offspring (hybrids)

•Tall is the dominant trait•Short is recessive trait (hidden) or Tall allele is dominant to the Short allele

Mendel's -First Law

In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. 

Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

“One trait will “hide” the other trait.”

The Principle of Dominance

Mendel's Second Law

During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. 

Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

The Law of Segregation

•PHENOTYPE   - what it looks like, the organisms physical description. (tall or short)

•GENOTYPE  -  the genes an organism has, represented by letters, (TT, Tt, tt)

Mendel studied the appearance of his peas but did not know about genes.

Check for understanding

1.  A one-eyed purple people eater is crossed with a two eyed purple people eater.  All of their offspring have two eyes.   Which trait is dominant?

2.  If you use the letter E for this gene.   What is the genotype of the offspring?      Are these offspring the F1 or F2 generation?

4.  If you crossed the offspring with each other?  How many ofthe new offspring would you expect to have two eyes?

Mendel's Third Law

Ma-Ma-Ma-Ma-Mendel's Third Law The Law of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another.

The Law of Independent Assortment

Vocabulary for this section

• Mendel, dominant, recessive, trait, gene, genetics, heredity, allele, heterozygous, homozygous, hybrid, true breeding/pure breeding, test cross, P1, G1, F1, (remember the 1’s are subscripts), Punnett square, phenotype, genotype (know ratios for phenotype/genotype), classic Mendelian ratio, gamete, zygote, law of independent assortment, law of segregation

• This material is covered in ch. 6 – specifically 6.3, 6.4 and 6.5