Genetics Terms to Know Trait Hybridization Monohybrid cross True-breeding/purebred Allele Dominant, Recessive Homozygous, Heterozygous Phenotype, genotype P generation, F1 generation, F2 generation Testcross Dihybrid cross Law of Segregation Law of Independent Assortment Incomplete dominance Codominance Multiple alleles Sex-Linked Punnett Square

Gregor Mendel – Father of Genetics Born in 1822

Austrian monk

Gardener and teacher

Good observations

Good with numbers

Before Mendel…• The Greeks – Males and females contribute a

fluid and at conception, fluids compete to determine sex.

• Middle Ages – Spontaneous Generation – Living organisms can arise from non-living substances (i.e. flies arise from meat).

• William Harvey – Females had eggs because chickens have eggs.

Gregor Mendel’s Plants• Mendel studied characteristics of PEA plants

• Fortunate choices - lucky

• Peas perfectly suited to genetic research

• Lots of offspring, short generation time

• Number of stable varieties with distinct traits

Pea Plants• Mendel looked at 8 traits• Examples: seed shape, flower color (purple/white), etc.

What happens when…• A green pea is crossed (mates) with a

yellow pea? Mendel’s cross results in only yellow peas.

WHY?

• Mendel crosses those yellow peas with each other? Mendel’s cross results in both yellow

and green peas. WHY?

Mendel Lucks Out! • Every trait selected is a single gene

inheritance• Every trait is a simple dominance

relationship

• Traits were easily distinguishable • Picked for ease – got lucky they all worked• All plants were purebreds – means they

were true-breeding, always gave the same offspring

Mendel Controls Breeding• Had to control self-pollination

• Had to prevent pollination from all other sources

Terms you should know! • Gene – A sequence of DNA that

codes for a trait • Mendel called it a factor, genes

and chromosomes had not yet been discovered

• Gene Locus – Location of a particular gene on a chromosome

• Alleles – Alternate versions of a gene

• Homozygous – having identical alleles (AA or aa)

• Heterozygous – having two different alleles (Aa)

Terms you should know!

Same phenotype (physical appearance), different genotype (genetic make-up)

Phenotype = tall, tallGenotype = AA, Aa

Terms you should know!

• Allele A is DOMINANT to allele a.

• Aa and AA are tall plants.

• Allele a is RECESSIVE to allele A.

• aa is the only combination for short plants.

AA, Aa, and aa are gentoypesTall and short are phenotypes

Hybridization• Crossing of 2

purebred or true breeding parents that are different from each other to produce a hybrid.

• The parents are homozygous.

• The hybrid is heterozygous.

P =Parents

F1 =Offspring, 1st generation

Flower Color Cross

P Generation

(true-breeding parents) Purple

flowersWhiteflowers

F1 Generation (hybrids)

All plants hadpurple flowers

F2 Generation

Purple is dominant to white.True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by the “”).

1.If the parents are true-breeding, are they homozygous or heterozygous? 2.What is the genotype for the purple flower? 3.What is the genotype for the white flower?Both purple-flowered plants and white-flowered plants appeared in the F2 generation. In Mendel’sexperiment, 705 plants had purple flowers, and 224 had white flowers.1.What is the simplified ratio of purple to white?2.What is the genotype of the white flower?3.What are the possible genotypes of the purple flowers?

Punnett Square SolutionTrue-breeding purple genotype = PP (parent

1)True-breeding white genotype = pp (parent

2)Offspring are called F1All offspring are Pp (heterozygous)P P

p Pp Pp

p Pp Pp

Punnett Square SolutionNow cross two of the offspringHeterozygous purple= Pp (F1 #1)Heterozygous purple= Pp (F1 #2)Offspring are called F2

P p

P PP Pp

p Pp pp

Phenotype versus genotypePhenotype: Physical appearance

Purple

Purple

Purple

White

Genotype: Genetic composition

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Mendel’s Law of Segregation (first generation, F1 offspring)

P Generation

F1 Generation

P p

Appearance:Genetic makeup:

Purple flowerPP

White flowerspp

Purple flowersPp

Appearance:Genetic makeup:

Gametes:

Gametes:

Each true-breeding plant of the parental generation has identicalalleles, PP or pp.

Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele.

Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple-flower allele is dominant, allthese hybrids have purple flowers.

When the hybrid plants producegametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.

Law of Segregation, 2nd generationP Generation

F1 Generation

F2 Generation

P p

P p

P p

P

p

PpPP

ppPp

Appearance:Genetic makeup:

Purple flowerPP

White flowerspp

Purple flowersPp

Appearance:Genetic makeup:

Gametes:

Gametes:

F1 sperm

F1 eggs

1/21/2

Each true-breeding plant of the parental generation has identicalalleles, PP or pp.

Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele.

Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple-flower allele is dominant, allthese hybrids have purple flowers.

When the hybrid plants producegametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.

This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F1 F1 (Pp Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottomleft box shows the genetic combinationresulting from a p egg fertilized bya P sperm.

Random combination of the gametesresults in the 3:1 ratio that Mendelobserved in the F2 generation. 3 : 1

Ratios

3

1 1

2

1

Phenotype

Purple

Purple

Purple

White

Genotype

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Phenotypic Ratio 3:1 Genotypic Ratio 1:2:1

HomeworkBook page 199 Practice Problems One-Factor Crosses#1 and 2 Prepare for open-note quiz (these two

problems could definitely be on the quiz!)