Term 2 Lecture 12: Basic principles of heredity Flashcards
Gregor Mendel monitored 7 characteristics of his pea plants
- pod colour yellow/green
- pod shape inflated/ constricted
- seed colour yellow/green
- seed shape smooth/ wrinkled
- seed coat white/ gray
- stem length short/tall
- flower position: axial -along the stem terminal -at tip of stem
Key terms
gene- an inherited factor coded in DNA that helps to determine a characteristic
allele - one of two or more alternative forms of a gene
locus - specific position on a chromosome occupied by an allele
genotype - set of alleles possessed by an individual organism
heterozygote - an individual with two different alleles at a locus
homozygote - an individual with two of the same alleles at a locus
phenotype or trait - appearance/ manifestation of a characteristic
characteristic/ character - attribute/ feature of an organism
Aa
Dominant allele capital letter
Recessive allele lower case letter
(both in italics)
A typical Mendel experiment: monohybrid cross
following one trait hence mono
He did this by first deliberately pollinating two homozygous plants together and preventing self fertilisation. He then grew these plants and harvested the seeds. This generation is therefore known as the P or parent generation.
The P plants produce homozygous seeds
One plant makes RR - round and the other rr- wrinkly
These seeds are grown and the plants crossed to produce F1 offspring. All F1 offspring are Rr - round.
When F1 offspring plants are self fertilized 3/4 have round seeds and 1/4 have wrinkled seeds
(Genotypes: RR,Rr,Rr, rr)
Conclusion: the traits of the parent plants do not blend. Although F1 plants display the phenotype of one parent both traits are passed to F2 offspring in a 3:1 ratio.
Mendels conclusions from monohybrid crosses
Monohybrid cross: a cross between 2 parents that differ in a single characteristic e.g. tall/short plant or round/wrinkled seeds
conclusions:
1) one character is encoded by two genetic factors (alleles)
2) the 2 alleles separate when gametes are formed
3) the concept of dominant and recessive traits
4) 2 alleles separate with equal probability into the gametes
Mendels first law: Principle of segregation
Each individual diploid organism possesses 2 alleles for any one trait. These 2 alleles segregate when gametes are formed and one goes into each gamete
The concept of dominance
When 2 different alleles are present in a genotype, only the trait encoded by one of them, the ‘dominant’ allele is observed in the phenotype (denoted by capital italic letter)
Sutton: chromosomal theory of heredity supported Mendels findings
By back crossing an F1 progeny with a parental genotype Mendel established dominance was hereditary
Dihybrid crosses
Examine 2 traits at the same time
expected F2 outcome: 9:3:3:1
Comparison of principle of segregation and independent assortment
Segregation
1) each organism possesses 2 alleles per trait (before meiosis)
2) alleles separate when gametes are formed - anaphase l
3) alleles separate in equal proportions - anaphase l
Independent assortment
Alleles at different loci separate independently- Anaphase l
(^this assumes no crossing over occurs. If crossing over does occur then segregation/ assortment may also occur in anaphase of meiosis ll)