classical genetics. humans have a long history of animal and plant breeding… but without an...

Classical Genetics

Humans have a long history of animal and plant breeding…

but without an understanding of the underlying process

Humans have a long history of animal and plant breeding…

but without an understanding of the underlying process

Gregor Mendel

Mendel conducted experimental crosses

Classical Mendelian Genetics has a limitation: The requirement for observable phenotypic differences in different genotypes

Mendel chose single gene mutants with extreme phenotypesto study.

This made different genotypes recognizable and countable.

Terminology

• Genes and alleles

• Genotype and Phenotype

• Homozygote, Heterozygote, Hemizygote

• Dominance

• Meiosis and Syngamy (Fertilization)

• Parents, Gametes, Offspring

Genes and Alleles

• A gene is a nucleotide sequence of a DNA molecule that codes for the primary structure of a protein or RNA molecule

• Alleles are gene variants. They differ in their nucleotide sequences.

Genotype and Phenotype

• Genotype: An individual’s genetic constitution

AA, Aa, aa are diploid genotypes

• Phenotype: An organism’s appearance, reflecting genotypic and environmental influences

blue yellow white

Dominance

• Many alleles are mutations whose gene products (proteins) work poorly or not at all (e.g., allele a). These alleles are recessive to normal alleles in the sense that they affect the phenotype only when there are no functional alleles present, i.e., in the homozygous recessive genotype aa.

• Both homozygotes (e.g., AA) for the normal allele and heterozygotes (e.g., Aa) share the functional allele (A) and exhibit the normal phenotype. However, aa individuals are unable to perform the function that this gene is responsible for and they will have a different phenotype.

• Operationally, one allele is said to be dominant over another if the heterozygote has the same phenotype as a homozygote (e.g., Aa and AA look alike).

Homozygous fortwo normal alleles

QuickTime™ and aAnimation decompressor

are needed to see this picture.

Homozygous fortwo non-functional alleles



Heterozygous fora normal and a non-functional allele

Dominance



Siamese Cats:

An enzyme that catalyzes pigment synthesis is denatured under warmer physiological conditions, like warmer parts of the cat’s body. Only cooler extremities reveal intense pigmentation.

Similarly, the enzyme can be deactivated not only under conditions that are too warm (below), but also under conditions that are cooler (above).

A Mendelian Research ProgramAA x aa

all Aa

1/4 AA, 2/4 Aa, 1/4 aa

P generation

F1 generation

F2 generation

Mendel figured out how to start a breeding experiment:

A Classical Mendelian Research Program

1/2 Aa, 1/2 aa

monohybrid crossbackcross used as a test cross

backcross used as a test cross

true breeding line “A”

true breeding line “a”

The Three Steps of Classical Genetic Analysis

Classical genetic analysis involves 3 stepsbased on the structure of a eukaryotic life cycle

Syngamy(fertilization)

Meiosis

multicellular body(parents and offspring)

gametes



Meiosis

1. Parental Genotypes

gametes

Offspring



Meiosis

1. Parental Genotypes

2. Meiotic products = gametes

Offspring

Rules for step 2:Diploid parents making haploid gamete genotypes

AA parents produce all A gametes

aa parents produce all a gametes

but

Aa parents produce 1/2 A and 1/2 a gametes

MENDEL’S FIRST LAW

Genetic Segregation is Based on Chromosomal Segregation



Meiosis

1. Parental Genotypes = start


3. Fertilization products = Offspring

Fertilization: Sperm, Egg, and Zygote

Predicting products of fertilization

Step 1 parental genotypes

Steps 2-3 predict gametes and combine them randomly

haploid gametes

haploid gametes

diploidoffspring

Predicting products of fertilization:

AA x AA

Step 1 AA x AA


All A gametes

All A gametes

All AAdiploid

offspring

Genotypic ratio: all AA

Phenotypic ratio: all “A”


AA x Aa

Step 1 AA x Aa


1/2 A 1/2 a

All A1/2 AA

Genotypic ratio: 1/2 AA and 1/2 Aa; 1:1


1/2 Aa


aa x aa

Step 1 aa x aa


All a gametes

All a gametes

All aadiploid

offspring

Genotypic ratio: all aa

Phenotypic ratio: all “a”


AA x aa

Step 1 AA x aa


All A gametes

All a gametes

All Aadiploid

offspring

Genotypic ratio: all Aa



Aa x aa

Step 1 Aa x aa


1/2 A 1/2 a

All a gametes

Genotypic ratio: 1/2 Aa 1/2 aa

Phenotypic ratio: 1/2 “A” 1/2 “a”

1/2 Aa

1/2 aa

Test

Cross


Aa x Aa

Step 1 Aa x Aa


1/2 A 1/2 a

Genotypic ratio: 1/4 AA 2/4 Aa 1/4 aa

Phenotypic ratio: 3/4 “A” 1/4 “a”

1/4 AA 1/4 Aa1/2 A

1/2 a 1/4 Aa 1/4 aa

Monohybrid

Cross

Summary of the six diallelic crosses (with dominance)

Mendel’s Experimental Results - Single Genes

A Mendelian Research ProgramAA x aa

all Aa

1/4 AA, 2/4 Aa, 1/4 aa

P generation

F1 generation

F2 generation

A Classical Mendelian Research Program

1/2 Aa, 1/2 aa

monohybrid crossbackcross used as a test cross

backcross used as a test cross

true breeding line “A”

true breeding line “a”

Only monohybrid and test crosses produce patterns in the progeny

red bluex red redx

1/2 red 1/2 blue 3/4 red 1/4 blue

Only monohybrid and test crosses produce patterns in the progeny

Aared

aablue

xAared

Aared

x

1/2 Aa red 1/2 aa blue 3/4 A_ red 1/4 aa blue

Brain Teasers

• Mother and father both find the taste of phenylthiourea very bitter, but three of their four children find it tasteless. Assuming that this difference is caused by a single gene with two alleles, is the non-taster phenotype dominant or recessive (circle the correct answer)? What kind of cross is this? Be prepared to explain with a diagram of the cross that identifies phenotypes and their genotypes.

• Mother finds the taste of phenylthiourea very bitter, but father and three of their four children find it tasteless. Assuming that this difference is caused by a single gene with two alleles, is the non-taster phenotype dominant or recessive (circle the correct answer) )? What kind of cross is this? Be prepared to explain with a diagram of the cross that identifies phenotypes and their genotypes.

Remember

Monohybrid crosses provide the most information:

Informing about both dominance and the number of genes

...and the parents in monohybrid crosses look alike

Test crosses also produce different progeny phenotypes, but

...whereas the parents in test crosses look different

Hints:

• Each family produced both phenotypes in their children, so the matings must be either test crosses or monohybrid crosses.

• Parents look alike in monohybrid crosses, but not in test crosses.

Brain Teasers

• Mother and father both find the taste of phenylthiourea very bitter, but three of their four children find it tasteless. Assuming that this difference is caused by a single gene with two alleles, is the non-taster phenotype dominant or recessive (circle the correct answer)? What kind of cross is this? Be prepared to explain with a diagram of the cross that identifies phenotypes and their genotypes.

Two progeny phenotypes, parents alike:

Therefore a monohybrid cross, taster dominant:

Taster(Aa) x Taster(Aa)

3 Non-taster (aa) and Taster (AA, Aa)

Brain Teasers

• Mother finds the taste of phenylthiourea very bitter, but father and three of their four children find it tasteless. Assuming that this difference is caused by a single gene with two alleles, is the non-taster phenotype dominant or recessive (circle the correct answer) )? What kind of cross is this? Be prepared to explain with a diagram of the cross that identifies phenotypes and their genotypes.

Two progeny phenotypes, parents not alike:

Therefore a test cross, but can’t resolve dominance relationships:

Aa x aa

1/2 Aa and 1/2 aa

No Dominance

• Some heterozygotes have phenotypes unlike either homozygote. The alleles of these heterozygotes are said not to exhibit dominance.

• In this case, each genotype has a unique phenotype.

Incomplete Dominance

white

Summary of the six diallelic crossses (no dominance)

Mendel’s Second Law

Independent Assortment

Two genes will be inherited independently of one another



Meiosis

1. Parental Genotypes = start


3. Fertilization products = Offspring

Mendel’s Second Law

1 2

3

1/4 AB 1/4 aB

1/4 Ab 1/4 ab

Dihybrid Cross - Peas

Dihybrid cross - Eye color

Using punnet squares can get

cumbersome

BIG

and

MESSY

1/4 AB 1/4 aB

1/4 Ab 1/4 ab

Forking Diagram

27/64

Sex Linkage

The Human Chromosome Complement:22 autosomes and a heteromorphic pair of sex chromosomes

Human Y Chromosome

Homogametic and Heterogametic Genotypes XX XY

In our speciesXX = female, XY = male

Other speciesXY = female, XX = male

Sex Linkage

centromere

pairing region

differential regions

A B

Ccentromere

X chromosome

Y chromosome

Figure 1. Sex Linkage

Y Linkage

X Linkage

pseudo-autosomal

Practice

Hemophelia

Victoria’s Clan

Color Blindness

normal color vision:

XCXC, XCXc

XCY

color blindness:

XcXc

XcY

Inheritance of White Eye

white eyewild type (red) eye

Inheritance of White Eyes

White eye revisted

Some species have heterogametic females

heterogametic females

homogametic males

classical genetics. humans have a long history of animal and plant breeding… but without an...

Documents

gregor mendel slide

underlying process slide

aa individuals

aa share

aa look

normal alleles

alleles genotype

normal phenotype