Lecture 3: Mendelian Genetics I Flashcards
SINGLE - GENE INHERITANCE; first analysed? History? Conclusion from it?
- first analysed by Gregor Mendel - an Augustinian monk
- garden peas
- interested in the inheritance of traits (properties, characteristics), not the properties of the peas
- 10 years of crosses (controlled matings), counting the resulting progeny, and testing hypotheses in subsequent experiments
(kept records, repeated experiments and user controls) - results published in 1866, but were not known or ignored until 1900
CONCLUSION:
- traits controlled by discrete units of inheritance (still knew nothing about genes or even chromosomes at that time!)
- units existed in pairs, with the two members separating when gametes formed
Single-Gene Inheritance: WHO? What happened during that time?
- Born in 1822- son of farmers, Brno. Scientific education entered the monastery Founder of Genetics!!!
The thinking at the time of Mendel’s experiments…
- Darwin’s On the Origin of Species had been
published in 1859 - Theory of the time:
The blending of inheritance - progeny have traits midway between those of mother and father - Mendel showed - traits were inherited randomly and independently as discrete units (no blending)
Explain Mendel Experiments:
Mendel examined - 7 traits of peas
(Seed colour Seed shape Seed coat
Pod colour Pod shape Flower position Stem length)
- contrasting phenotypes (easy to count and categorise
HE
- Documented over many years/generations
- Replicated
- Recorded numbers/ratios
- Simple, elegant and reproducible
Mendel did cross-pollination and self-crosses; what did he find out about the pollen and stigma?
Mendel did - cross pollination crosses - self crosses
pollen - contains a male gamete
- made in the anthers (male part of a flower)
stigma - receptive surface for pollen grain on the female part of a flower
What did Mendel’s Experiments ENCOMPASS?
- how, is parental generation? F1 generation?
early experiment, found?
Mendel’s Experiments
Mendel used - pure lines for particular traits (all progeny resulting from crosses within a line are identical for a particular trait, e.g. purple flowers)
- parental (P) generation
Progeny resulting from crosses of P generation
are called first filial (F1) generation
In an early experiment:
pollinated purple-flowered plant with pollen from white-flowered plant
found - progeny plants (F1 generation) all had purple flowers
Mendel’s Experiments: Did the reciprocal cross
- found? hypothesis?
- Did the reciprocal cross (pollen from purple- flowered plant used to pollinate white-flowered plant):
- found - all F1 individuals had purple flowers
For both crosses - saw no blending of inheritance - Hypothesis that “discrete units of inheritance existed” was supported
- Reciprocal crosses are crosses between different genetic stocks, strains or species where the sexes of the parents are reversed.
strains A and B, (A×B) and (B×A) are reciprocal crosses.
What did Mendel find out about the F1 generation?
Experiments with the F1 generation:
- Grew F1 yellow seeds (resulting from a cross between a yellow-seeded plant and a green-seeded plant) into plants and self-crossed (monohybrid cross)
- The resulting progeny represent the second filial (F2) generation
- Found 3/4 of the F2 generation seed were yellow, 1/4 were green → 3:1 phenotypic ratio
Mendel’s Monohybrid cross vs Dihybrid?
- monohybrid cross is a cross between first-generation offspring of parents who differ in one trait
- dihybrid cross is a cross between first-generation offspring of parents who differ in two traits.
Mendel’s experiment: monohybrid crosses results;
Ressesive vs dominate?
3:1 phenotypic ratio held in the F2 generation for each trait Mendel studied
For each cross
- one parental trait disappeared in the F1 generation but reappeared in the F2 generation
*unit of inheritance present in F1 but not expressed
*recessive allele (form) of the gene responsible for the trait (recessive phenotype)
- parental trait expressed in the F1 represents the dominant phenotype (dominant allele of the gene)
Mendel’s Experiments - The Explanation
Meiosis, fertilisation?
At meiosis, the two alleles of a gene separate (segregate) equally into the male and female gametes.
At fertilisation, the gametes combine at random.
Phenotypic ratio of 3:1 (3 yellow: 1 green)
Genotypic ratio of 1:2:1 (1Y/Y : 2Y/y : 1y/y)
Mendel’s Experiments - The Explanation; overall
- Discrete units of inheritance (genes), no blending
- Each plant has two units (alleles)
- Each gamete carries only one allele
- Each unit segregates equally and randomly into the gametes
- Gametes combine randomly to form a zygote
A/A and a/a - homozygotes (pure lines)
A/a - heterozygotes (also called hybrids)
Mendel’s Experiments - The Test
Crossed F1 yellow-seeded plant with any green-seeded plant
*homozygous recessive (yy)
*also called a tester
F2 generation were 1/2 yellow seed, 1/2 green seed → 1:1 ratio
Found the same ratio with the reciprocal cross
TESTCROSS
- test crosses determine the genotype of an individual with a dominant phenotype
- the tester is homozygous recessive
Mendel’s Experiments - The Test; explaining the test cross
TESTCROSS
Tester contributes only recessive alleles - allows gametes produced by individuals of unknown genotype to be determined from the progeny resulting from the cross
- test crosses determine the genotype of an individual with a dominant phenotype
- the tester is homozygous recessive
Phenotypic ratio of 1:1 (1 yellow: 1 green)
Genotypic ratio of 1:1 (1Y/y: 1y/y)
Explain Mendel’s First Law - The Law of Equal Segregation
“The two members of a gene pair segregate from each other into the gametes; so half the gametes carry one member of the gene pair and the other half of the gametes carry the other member of the gene pair.”
- Mendel explained the reappearance of hidden traits
- Parents make Equal contribution
- Disproved blended inheritance
All before we knew anything about DNA structure/chromosomes
What would Mendel think today?
10 years work to follow the segregation of genes
Now we can sequence the whole genome in 4 hrs @ $500
First genome took 13 years and cost $3 billion
