Topic E Flashcards
Gregor Mendel
- Gregor Mendel (1822 – 1884)
- Pisum sativum
- “Experiments in Plant Hybridization“ (1865)
- Re-discovered in 1900
Why was Mendel so successful in
discovering the mechanism of heredity?
- True-breeding lines
- Single gene traits
- Easily scored traits
- Good model organism
- Large number of crosses
Monohybrid Cross
P1 (parental gen) true breeding purple x true breeding white
F1 (first filial gen) all purple (only one of 2 traits observed)
F1 x F1
F2 (2nd filial gen) 705 purple, 224 white (2nd trait reappeared)
Phenotypic ratio of 3: 1
Monohybrid cross
What did Mendel observe in the F1 and F2 generations?
Outcome(s) the same for all traits/characteristics – Phenotypic ratios
Defining dominant and recessive
Dom appear in 1st gen
Rec masked/doesn’t appear in first gen
reciprocal crosses
A cross in which the phenotype(s) associated with each parent has/have been switched
P1 Purple (male) x white (female) becomes white (male) x purple (female)
Always come in pairs
Mendel observed this in his F1 and F2 gens
Monohybrid Cross
Insights provided by mendals work
- Concept of the gene
- genetic characters like flower colour are controlled by what mendal called unit factors, we now call them genes. - Concept of alleles
- diff forms of gene exist
- we call these diff forms of genes alleles
- unit factors exist in pairs
— an individual could have 2 purple alleles (homo)
— 2 white alleles (homo)
— one of each allele (hetero) - Concept of dominance
- when an individual posses 2 diff alleles of a gene (a hetero) one allele may mask the effect of the other.
- allele that masks the phenotypic effect of the other allele is said to be dominant, while the other is recessive.
A punnett square can be used to represent the outcome of crosses
- Male gamete genotypes
- Female gamete genotypes
- Progeny genotypes
Mendel’s 1st Law of Equal Segregation
- There are two alleles of each gene and these segregate equally from each other in the formation of gametes
- This can be seen nicely when you look at our heterozygous F1 individual. The T allele and t allele segregate from one another and do NOT occur in the same gamete.
How this first law is reflected by the movement of chromosomes in meiosis?
the two members of a homologous pair of chromosomes segregate from each other during meiosis and as a result, the alleles present on those homologs, here P and p also segregate from one another.
Review of Concept
- Mendel’s monohybrid cross revealed the first law of inheritance
— Alleles of a gene segregate from one another during meiosis and will be present in different gametes - The monohybrid cross supports a clear definition of dominant vs. recessive alleles which is based upon the phenotype revealed in the F1 heterozygote.
Mendel’s dihybrid cross supports the second
law of inheritance
- The 9:3:3:1 phenotypic ratio observed in the F2 generation of the dihybrid cross, or the 1:1:1:1 phenotypic ratio observed in the testcross progeny, are the result of the alleles of the two genes assorting independently during gamete formation
- “Assorting independently” can be restated as the probability of inheriting a particular allele of one gene is independent of the probability of inheriting a particular allele of another gene
Testcross
A testcross is a cross in which an individual with an unknown genotype is crossed to an individual that we call the TESTER.
A tester is an individual that is homozygous for the recessive allele at all loci under investigation.
So in a monohybrid scenario, there is only one locus under investigation- for example the flower colour locus.
The tester is capable of producing only a single type of gamete – all the tester gametes will carry only recessive alleles for the genes under investigation.
Because the tester is homozygous recessive, the phenotype of the testcross progeny will be determined by the genotype of the gametes of the unknown individual
Gametes have 0.5 proportion
Dihybrid Crosses
P R/R ; y/y (round,green) x r/r ; Y/Y (yellow, wrinkled)
Gametes R;y and r;Y
F1 R/r ; Y/y (round yellow) is hetero (if didn’t know genotype, can look at F1 to find out)
F1 x F1
F2. Ratio
315 round yellow 9 (both dom)
108 round green 3 (res than dom)
101 Wrinkled yellow 3 (dom than rec)
32 wrinkled green 1(both rec)
556 seeds. 16
Each gamete has 0.25 proportion
F1 dihybrids produce two categories of gametes
- Parental Type Gametes (same as parent)
- Recombinant Type Gametes (diff than parent, not in F1)
Mendel’s second law of inheritance –
The Law of Independent Assortment
Dihybrid Crosses reveal this
- The alleles of different genes assort independently from one another during gamete formation
- the inheritance of one chromosome of a homologous pair is independent of the inheritance of another chromosome of a different homologous pair
