Classical
Genetics

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 phenotypes to 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 for two normal alleles
QuickTime™ and a
Animation decompressor are needed to see this picture.

Homozygous for two non-functional alleles
QuickTime™ and a
Animation decompressor are needed to see this picture.

Heterozygous for a normal and a non-functional allele
Dominance
QuickTime™ and a
Animation decompressor are needed to see this picture.

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).

Mendel figured out how to start a breeding experiment:
A Classical Mendelian Research Program true breeding line “A” true breeding line “a”
A Mendelian Research Program backcross used as a test cross
F
1 generation all Aa monohybrid cross
1/2 Aa , 1/2 aa
F
2 generation
1/4 AA , 2/4 Aa , 1/4 aa backcross used as a test cross

The Three Steps of
Classical Genetic Analysis

Meiosis
Classical genetic analysis involves 3 steps based on the structure of a eukaryotic life cycle gametes
Syngamy
(fertilization) multicellular body
(parents and offspring)

Meiosis
Classical genetic analysis involves 3 steps based on the structure of a eukaryotic life cycle gametes
Syngamy
(fertilization)
1. Parental Genotypes
Offspring

Meiosis
Classical genetic analysis involves 3 steps based on the structure of a eukaryotic life cycle
2. Meiotic products = gametes
Syngamy
(fertilization)
1. Parental Genotypes
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
Classical genetic analysis involves 3 steps based on the structure of a eukaryotic life cycle
2. Meiotic products = gametes
Syngamy
(fertilization)
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 diploid offspring

Predicting products of fertilization:
AA x AA
Step 1 AA x AA
Steps 2-3 predict gametes and combine them randomly
All A gametes
All A gametes
All AA diploid offspring
Genotypic ratio: all AA
Phenotypic ratio: all “A”

Predicting products of fertilization:
AA x Aa
Step 1 AA x Aa
Steps 2-3 predict gametes and combine them randomly
All A
1/2 A 1/2 a
1/2
AA
1/2
Aa
Genotypic ratio: 1/2 AA and 1/2 Aa ; 1:1
Phenotypic ratio: all “A”

Predicting products of fertilization: aa x aa
Step 1 aa x aa
Steps 2-3 predict gametes and combine them randomly
All a gametes
All a gametes
All aa diploid offspring
Genotypic ratio: all aa
Phenotypic ratio: all “a”

Predicting products of fertilization:
AA x aa
Step 1 AA x aa
Steps 2-3 predict gametes and combine them randomly
All a gametes
All A gametes
All Aa diploid offspring
Genotypic ratio: all Aa
Phenotypic ratio: all “A”

Predicting products of fertilization:
Aa x aa
Test
Cross
Step 1 Aa x aa
Steps 2-3 predict gametes and combine them randomly
All a gametes
1/2 A 1/2 a
1/2
Aa
1/2 aa
Genotypic ratio: 1/2 Aa 1/2 aa
Phenotypic ratio: 1/2 “A” 1/2 “a”

Predicting products of fertilization:
Aa x Aa
Monohybrid
Cross
Step 1 Aa x Aa
Steps 2-3 predict gametes and combine them randomly
1/2
1/2
A a
1/2 A 1/2 a
1/4 AA
1/4 Aa
1/4
1/4
Aa aa
Genotypic ratio: 1/4 AA 2/4 Aa 1/4 aa
Phenotypic ratio: 3/4 “A” 1/4 “a”

Summary of the six diallelic crosses
(with dominance)

Mendel’s Experimental Results - Single
Genes

A Classical Mendelian Research Program true breeding line “A” true breeding line “a”
A Mendelian Research Program backcross used as a test cross
F
1 generation all Aa monohybrid cross
1/2 Aa , 1/2 aa
F
2 generation
1/4 AA , 2/4 Aa , 1/4 aa backcross used as a test cross

Only monohybrid and test crosses produce patterns in the progeny red x blue red x red
1/2 red 1/2 blue 3/4 red 1/4 blue

Only monohybrid and test crosses produce patterns in the progeny
Aa red x aa blue
Aa red x
Aa red
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
Two genes will be inherited independently of one another

Meiosis
Classical genetic analysis involves 3 steps based on the structure of a eukaryotic life cycle
2. Meiotic products = gametes
Syngamy
(fertilization)
1. Parental Genotypes = start
3. Fertilization products = Offspring

Mendel’s Second Law
1
1/4 AB 1/4 aB
1/4 Ab 1/4 ab 2
3

Dihybrid Cross - Peas

Dihybrid cross - Eye color

1/4 AB
1/4 Ab
1/4 aB
1/4 ab
Using punnet squares can get cumbersome
BIG and
MESSY

Forking Diagram
27/64

Sex Linkage

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

X

X

Human Y Chromosome

Homogametic and Heterogametic Genotypes
XX XY
In our species
XX = female, XY = male
Other species
XY = female, XX = male

Sex Linkage
Figure 1. Sex Linkage
A ce ntrom ere
X ch romos ome
Y ch romos ome pai ring regi on ce ntrom ere
C differen tial regi ons
B
Y Li nkag e
X Li nkag e pseudo -auto som al

Practice

Hemophelia

Victoria’s Clan

Color Blindness normal color vision:
X C X C , X C X c
X C Y color blindness:
X c X c
X c Y

Inheritance of White Eye wild type (red) eye white eye

Inheritance of White Eyes

Inheritance of White Eyes

White eye revisted

homogametic males
Some species have heterogametic females heterogametic females