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 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 blueyellowwhite

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

Homozygous for two non-functional alleles

Heterozygous for a 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 Program AA x aa all Aa 1/4 AA, 2/4 Aa, 1/4 aa P generation F 1 generation F 2 generation Mendel figured out how to start a breeding experiment: A Classical Mendelian Research Program 1/2 Aa, 1/2 aa monohybrid cross backcross 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 steps based on the structure of a eukaryotic life cycle Syngamy (fertilization) Meiosis multicellular body (parents and offspring) gametes

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

Classical genetic analysis involves 3 steps based on the structure of a eukaryotic life cycle Syngamy (fertilization) 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

Classical genetic analysis involves 3 steps based on the structure of a eukaryotic life cycle Syngamy (fertilization) Meiosis 1. Parental Genotypes = start 2. Meiotic products = gametes 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 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 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 1/2 A 1/2 a All A 1/2 AA Genotypic ratio: 1/2 AA and 1/2 Aa; 1:1 Phenotypic ratio: all “A” 1/2 Aa

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 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 Step 1 Aa x aa Steps 2-3 predict gametes and combine them randomly 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

Predicting products of fertilization: Aa x Aa Step 1 Aa x Aa Steps 2-3 predict gametes and combine them randomly 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 AA1/4 Aa1/2 A 1/2 a 1/4 Aa1/4 aa Monohybrid Cross

Summary of the six diallelic crosses (with dominance)

Mendel’s Experimental Results - Single Genes

A Mendelian Research Program AA x aa all Aa 1/4 AA, 2/4 Aa, 1/4 aa P generation F 1 generation F 2 generation A Classical Mendelian Research Program 1/2 Aa, 1/2 aa monohybrid cross backcross 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 redblue x red x 1/2 red1/2 blue3/4 red1/4 blue

Only monohybrid and test crosses produce patterns in the progeny Aa red aa blue x Aa red Aa red x 1/2 Aa red1/2 aa blue3/4 A_ red1/4 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

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

Mendel’s Second Law /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

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

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 Y

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