Mendelian Genetics Flashcards
Who was Gregor Mendel?
Born in 1822 in Czech Republic (formerly Czechia), died 1884
discovered inheritance
What did Mendel study and what did he show?
studied peas (Pisum sativum) in 1850’s
- Have dichotomous traits (traits with only 2 forms)
- Coined the term hereditary units didn’t know they were called genes
Showed inheritance is “particulate”, not blending
Ex a black cat x white cat does not produce a grey cat
Why did Mendel use peas?
Readily available at monestary
Flowers have both male and female structures
Self-fertilize
Can manually fertilize plants by removing male anthers (ie artificial cross-fertilization)
Short life cycles
Self-fertilize
Reproduction by organisms that contain both male and female reproductive structures; the natural reproductive mechanism on many plants, including pea plant
artificial cross-fertilization)
A controlled cross between plants made by an investigator who transfers pollen from one plant to fertilize the other plant.
Pure-breeding (true breeding):
strains that consistently produce the same phenotype
the parents would produce offspring that would carry the same phenotype
he parents are homozygous for every trait.- the parents must be both dominant or both recessive.
Parental (P) generation:
pure-breeding parent plants in a cross
Each parent has alternative forms of the trait
Pink parent plant will only produce pink offspring
First filial (F1 )
progeny(offspring) produced from a parental cross
Self-fertilised or crossed to produce the next generation
Mendel would measure the phenotypes in this generation
Second filial (F2 )
progeny produced from the cross of F1
Mendel would then measure the phenotypes produced here too
Replicate crosses
produced 100’s of F1 plants and thousands of F2 plants
Repeated crosses involving parents with the same genotypes and phenotypes
Reciprocal crosses:
Plants with the same phenotypes crossed but the sexes of the donating parents were swapped
Plant donating the egg in one cross, donated the pollen in the next
Test-crosses
crosses designed to identify the alleles carried by an organism whose genetic make-up (genotype) is not known
Genes
determinants of traits (ie, eye colour)
The physical unit of heredity, composed of a DNA sequence that is transcribed and encodes a polypeptide or another functional molecule.
Alleles
alternative forms of genes (ie gene variants)
Gene for pea seed colour has alleles that code for Yellow (Y) or green (y) seeds
Phenotype
trait that you can measure
Ie appearance, biochemistry (eg amount of enzyme), behaviour
Genotype
Genetic make-up of an organism that results in its phenotype
Determined by combination of alleles
Diploid organisms have 2 alleles for each gene (one from mom and one from dad)
Genotypic ratio
relative proportions between organisms with different genotypes
Mendel’s peas had a 1:2:1 genotypic ratio for GG:Gg:gg \homozygous and heterozygous
Phenotypic ratio:
relative proportions between organisms with different phenotypes
Mendel’s peas had a 3:1 phenotypic ratio for purple: white flowers
What are Mendels genotypic and phenotypic ratios?
Mendel’s peas had a 1:2:1 genotypic ratio for GG:Gg:gg \homozygous and heterozygous
Mendel’s peas had a 3:1 phenotypic ratio for purple: white flowers
Homozygous
both alleles code for the same phenotype (ie both YY or yy)
Heterozygous
Each allele codes for a different phenotype (Yy)
Dominant allele
alleles that mask the effect of the other alleles
Ie Yy results in yellow seeds since Y (yellow) masks y (green)
Recessive allele:
alleles that are masked by other alleles
Phenotype is only seen in the homozygous state (ie yy, green)
Monohybrid cross
cross between two organisms that have the same heterozygous genotype for a gene
What happened when Mendel performed a monohybrid cross?
reveal the segregation of alleles
When Mendel bred two true-breeding parents, the resulting F1 progeny were all the same phenotype as the parent with the dominant trait
When Mendel crossed the F1 , the resulting F2 showed the appearance of the recessive trait (1:2:1 genotype ratio and 3;1 phenotype ratio)
Law of Segregation
proven by the monohybrid cross
Two alleles of the same gene separate from one another during the formation of gametes
explained phenotypes
Meiosis I!- separating out the sister chromatids during anaphase
When an organism makes gametes, each gamete receives just one gene copy, which is selected randomly.
Monohybrid cross ratio
F2- 1:2:1 genotypic ratio and 3:1 phenotypic ratio
What did Mendel predict to be the genotype for the F1 progeny what was done to prove it?
He predicted all the F1 progeny were heterozygotes, but needed to prove it
Performed a test-cross (always with the recessive parent)
F1 (dominant phenotype) x true-breeding recessive parent
If F1 was heterozygous, the resulting F2 would have a phenotypic ratio of 1:1
What did Mendel predict about the F2 generation?
that the F2 plants with the dominant phenotype would produce twice as many heterozygotes as homozygous dominant individuals
Ie 3⁄4 of the offspring would be yellow seeds, 1⁄4 green seeds (phenotypic ratio)
What was the result of self fertilization in the F2?
Homozygous dominant plants would produce offspring that were homozygous dominant (ie all yellow)
Heterozygous plants would produce some plants with yellow seeds and some with green seeds
3:1 phenotypic ratio
Dihybrid crosses
two traits and their simultaneous transmission (inheritance)
Mendel began each dihybrid cross with pure -breeding parentals
F1 will be heterozygous for both traits (ie dihybrids)
Independent assortment
The segregation of alleles of one gene is independent of the segregation of alleles of the other gene
*Law of segregation occurs during
anaphase I of meiosis I since the two alleles will segregate independently of each other during gamete formation and have equal opportunity of inclusion in the gamete
supported by dihybrid cross
What does Crossing the F1 heterozygotes produce
Crossing the F1 heterozygotes produce a 9:3:3:1 phenotypic ratio in the F2
Trihybrid crosses
involve three independently assorting traits
Dihybrid cross features
Parental and nonparental phenotypes in the F 2 differ from one another ie 9:3:3:1
Compare that to the monohybrid cross
Monohybrids in F2 generation will have a 3:1 ratio
hen we look at each trait independently, we still get the 3:1 phenotypic ratio
Conditional probability
Probability prediction that is dependent on another previous event having taken place
i.e. “What is the probability that F2 yellow-seeded progeny plants are heterozygous like their parents”?
We know that F2 progeny will be 3⁄4 yellow and 1⁄4 green, and of the 3 yellow, 2/3 will be Gg and 1/3 will be true-breeding GG
