1. Classical (Mendelian) Genetics

2. • Gregor Mendel was born by July 1822 in Heizendorf (today Hynice in the Czech Republic)
• From 1851 to 1853, studied zoology, botany, chemistry, and physics at the University of Vienna
• He became a member of the Zoologico-Botanical Society of Austria and has published two scientific treatises in the "Verhandlungen" of this scientific organization (1853 and 1854)
• Probably due to health reasons (epilepsia ?), Mendel has returned to Brno without formally finishing the University in Vienna.
Mendelian genetics

3. • Experimentation in the monastery
garden results in laws of inheritance
• Mendel began his experiments with
the hybrid cultivation of pea plants in
1856
• He reported on the results of his
observations at the meetings of the
Association for Natural Research in
Brno in 1865.
• The Association published the written
accounts of these observations in 1866,
under the title Versuche über Pflanzen-
Hybride.
Mendelian genetics

4. There was little reaction from the scientific community
– Nature of his experimentation was unconventional for his age, nobody before him had use mathematical and statistical analysis as a means of interpreting the results of biological inquiry
– Mendel was known as a relatively shy person and might not have presented his results with the necessary emphasis and Stress
– Scientific fraternity of the day will have been the limited number of people who read the Brno Association’s records
– Although it in fact dealt with no fewer than 355 cross-bred strains and 12,980 resultant hybrids, his work was described as "incomplete“.
– Mendel received a "fatal" advice: to continue his investigations using the hawkweed (Hieracium), later botanists discovered that these plants have asexual reproduction.
Mendelian genetics

5. • The rediscovery of Mendel’s Laws
– It was to take thirty-four years before Mendel’s prediction came true.
– The year 1900, Carl Correns in Germany, Hugo de Vries in the Netherlands and Erich von Tschermak-Seysenegg in Austria.
– Their achievement was to realize that Mendel had not merely conducted experiments in successful hybridization
– But had in fact studied the heredity of specific characteristics as they were passed on from parent plants to their offspring.
Mendelian genetics

6. Terms to Know and Use
Gene – basic unit of genetic information. Genes determine the inherited characters.
Genome – the collection of genetic information.
Chromosomes – storage units of genes.
Mendelian genetics

7. Chromosome Logical Structure
Locus – location of a gene/marker on the chromosome.
Allele – one variant form of a gene/marker at a particular locus.
Locus1
Possible Alleles: A1,A2
Locus2
Possible Alleles: B1,B2,B3
Mendelian genetics

8. Genotypes Phenotypes
At each locus (except for sex chromosomes) there are 2 genes. These constitute the individual’s genotype at the locus.
The expression of a genotype is termed a phenotype. For example, hair color, weight, or the presence or absence of a disease.
Mendelian genetics

9. Mendel choose garden pea plant for his experiment ,
because it had the following
Advantages:
Well defined characters
Bisexual flowers
Self fertilization
Easy hybridization
Mendelian genetics

10. Mendelian genetics

11. Mendel’s First Experiment
Mendel crossed purebred plants with opposite forms of a trait. He called these plants the parental generation , or P generation. For instance, purebred tall plants were crossed with purebred short plants. X
Parent Short P generation
Parent Tall P generation
Offspring Tall F1 generation
Mendel observed that all of the offspring grew to be tall plants. None resembled the short short parent. He called this generation of offspring the first filial , or F1 generation, (The word filial means “son” in Latin.)
Mendelian genetics

12. Mendel’s Second Experiment
Mendel then crossed two of the offspring tall plants produced from his first experiment.
Parent Plants
Offspring X
Tall F1 generation
Mendel called this second generation of plants the second filial, F2, generation. To his surprise, Mendel observed that this generation had a mix of tall and short plants. This occurred even though none of the F1 parents were short.
Mendelian genetics

13. Mendel’s Law of Segregation
Mendel’s first law, the Law of Segregation, has three parts. From his experiments, Mendel concluded that:
1. Plant traits are handed down through “hereditary factors” in the sperm and egg.
2. Because offspring obtain hereditary factors from both parents, each plant must contain two factors for every trait.
3. The factors in a pair segregate (separate) during the formation of sex cells, and each sperm or egg receives only one member of the pair.
Mendelian genetics

14. Dominant and Recessive Genes
Mendel went on to reason that one factor (gene) in a pair may mask, or hide, the other factor. For instance, in his first experiment, when he crossed a purebred tall plant with a purebred short plant, all offspring were tall.
Although the F1 offspring all had both tall and short factors, they only displayed the tall factor. He concluded that the tallness factor masked the shortness factor.
Today, scientists refer to the “factors” that control traits as genes. The different forms of a gene are called alleles.
Alleles that mask or hide other alleles, such as the “tall” allele, are said to be dominant.
A recessive allele, such as the short allele, is masked, or covered up, whenever the dominant allele is present.
Mendelian genetics

15. Homozygous Genes
What Mendel refered to as a “purebred” plant we now know this to mean that the plant has two identical genes for a particular trait. For instance, a purebred tall plant has two tall genes and a purebred short plant has two short genes. The modern scientific term for “purebred” is homozygous.
short-short
short-short
short-short X
Short Offspring
Short Parents
Mendelian genetics

16. Hybrid Alleles
In Mendel’s first experiment, F1 offspring plants received one tall gene and one short gene from the parent plants. Therefore, all offspring contained both alleles, a short allele and a tall allele. When both alleles for a trait are present, the plant is said to be a hybrid for that trait.
tall-tall
short-tall
short-tall
short-short X
Parent Short P generation
Parent Tall P generation
Offspring Tall F1 generation
Although the offspring have both a tall and a short allele, only the tall allele is expressed and is therefore dominant over short.
Mendelian genetics

17. Dominant Alleles
Mendel observed a variety of dominant alleles in pea plants other than the tall allele. For instance, hybrid plants for seed color always have yellow seeds.
Green & Yellow Allele
Yellow Seed
However, a plant that is a hybrid for pod color always displays the green allele.
Green Pod
Green & Yellow Allele
In addition, round seeds are dominant over wrinkled seeds, and smooth pods are dominant over wrinkled pods.
Mendelian genetics

18. Punnett Squares
Genetic problems can be easily solved using a tool called a punnett square.
Tool for calculating genetic probabilities
A punnett square
Mendelian genetics

19. Monohybrid cross (cross with only 1 trait)
Tallness (T) is dominant over shortness (t) in pea plants. A Homozygous tall plant (TT) is crossed with a short plant (tt). What is the genotypic makeup of the offspring? The phenotypic makeup ?
Determine alleles of each parent, these are given as TT, and tt respectively.
Take each possible allele of each parent, separate them, and place each allele either along the top, or along the side of the punnett square.
Mendelian genetics

20. Here we have some more interesting results: First we now have 3 genotypes (TT, Tt, & tt) in a 1:2:1 genotypic ratio.
We now have 2 different phenotypes (Tall & short) in a 3:1 Phenotypic ratio. This is the common outcome from such crosses.
Mendelian genetics

21. Mendel’s 2nd Law, Independent Assortment
Mendel’s second law, the Law of Independent Assortment, states that each pair of genes separate independently of each other in the production of sex cells.
This “law” is true only in some cases.
Gene pairs on SEPARATE CHROMOSOMES
assort independently at meiosis.
Mendelian genetics

22. Dihybrid crosses Example:
Dihybrid crosses are made when phenotypes and genotypes composed of 2 independent alleles are analyzed.
Process is very similar to monohybrid crosses.
Example:
2 traits are being analyzed
Plant height (Tt) with tall being dominant to short,
Flower color (Ww) with Purple flowers being dominant to white.
Mendelian genetics

23. Dihybrid cross example
The cross with a pure-breeding (homozygous) Tall, Purple plant with a pure-breeding Short, white plant should look like this.
F1 generation
Mendelian genetics

24. Dihybrid cross example continued
Note that there is a 9:3:3:1 phenotypic ratio. 9/16 showing both dominant traits, 3/16 & 3/16 showing one of the recessive traits, and 1/16 showing both recessive traits.
Also note that this also indicates that these alleles are separating independently of each other. This is evidence of Mendel's Law of independent assortment
Mendelian genetics

25. Dihybrid Cross
Mendelian genetics

26. Mother contributes: Father contributes: Dihybrid Cross SB Sb sB sb SB
SSBB
SsBb
SSBb
SsBB
SSbB
SSbb
SsbB
Ssbb Sb
Father contributes:
sSBB
sSBb
ssBB
ssBb sB
sSbB
sSbb
ssbB sb
ssbb
Mendelian genetics

27. Mendel Genetics in Human
Mendelian genetics

28. Sex Linkage All chromosomes are homologous except on sex chromosomes.
Sex chromosomes are either X or Y.
If an organism is XX, it is a female, if XY it is male.
If a recessive allele exists on the X chromosome. It will not have a corresponding allele on the Y chromosome, and will therefore always be expressed
Mendelian genetics

29. Human Sex Linkage Hemophilia:
Disorder of the blood where clotting does not occur properly due to a faulty protein.
Occurs on the X chromosome, and is recessive.
Thus a vast majority of those affected are males.
First known person known to carry the disorder was Queen Victoria of England. Thus all those affected are related to European royalty.
Mendelian genetics

30. Medical Genetics X-linked dominant
Affected males pass the disorder to all daughters but to none of their sons.
Affected heterozygous females married to unaffected males pass the condition to half their sons and daughters
e.g. fragile X syndrome
Mendelian genetics

31. Medical Genetics Codominant inheritance
Two different versions (alleles) of a gene can be expressed, and each version makes a slightly different protein
Both alleles influence the genetic trait or determine the characteristics of the genetic condition.
E.g. ABO locus
Mendelian genetics

32. Next topic:
Mendelian genetics_ two
Mendelian genetics