Mendelian Genetics Flashcards
Gregor Mendel
“The father of Modern Genetics”
–> Started out as a monk growing peas
–> In 1865, he published a paper that reported on his pea plant experiments that suggested a model for how traits are inherited
Mendel was the first to…
Deduce clear laws that explained the process and patterns of inheritance (through experimental observations and mathematics)
2 major hypotheses existed for inheritance of traits:
1) The “Blending” Hypothesis
2) Inheritance of Acquired Characters Hypothesis
Blending Hypothesis
Genetic material contributed by BOTH parents MIX together (blend)
Ex: Black + White Sheep = Grey Sheep
Inheritance of Acquired Characters Hypothesis
Experience Dependent Inheritance
–> An organism passes on traits that the parent acquired through use/disuse in its lifetime
EX: The passing down of long necks in giraffes due to parents stretching to reach high hanging fruit
Benefits of the pea plant model (5)
1) Easy + Inexpensive to cultivate
2) Reproduce Quickly (short generation time)
3) Lots of variety in pea plants that are easily distinguishable
4) Pea physiology lends itself to genetic analyses
5) Each plant could be easily mated and this mating could be well controlled (allowing for certainty of parentage)
Character
A heritable feature that varies among individuals
Ex: Flower Color
Trait
Each variant of a character
Ex: Red or White (flower color)
7 Characters Observed by Mendel
1) Seed Color
2) Seed Shape
3) Pod Color
4) Flower Color
5) Flower Position
6) Stem Length
7) ?? Didn’t write it down
Each character Mendel studied had…
2 distinct forms/variants
How did Mendel begin his experiments?
True Breeding Lines
True Breeding Lines
Plants whose offspring resembles the parent over many generations
How are true breeding lines obtained?
Self-Pollination
Pea plants make both…
Male and female gametes
Anther
The plant part where pollen (male gamete) is kept
Pollen
Male plant gamete
Self-Fertilization
Pollen from within the same plant enters its own stigma
What was a commonality between the true breeding plants Mendel created?
All the true breeding plants he made were HOMOZYGOUS for the give trait that he was isolating/studying
Cross Pollination Process
1) Took one pea plant and cut its anthers (removed the male gamete parts –> Preventing self-pollination)
2) Collected some pollen from the other plant in the cross onto a brush
3) Brushed this pollen onto the plant that had its anthers cut to initiate fertilization
4) Waited for the seeds in the plant to mature and then planted the seeds to grow the crossed plants and assess their phenotypes
Hybridization
The mating or crossing of 2 true-breeding varieties
–> Producing a hybrid (mixed variety) AKA heterozygote
Mendel’s First Experiment (cross)
Cross pollinated true breeding lines with different traits for the same character
–> Produced the F1 generation of plants
P Generation
Parental Generation (True breeding parents)
F1 Generation
First Generation derived from P generation
–> The hybrid offspring of the true breeding P-generation
Results of Mendel’s First Experiment (flower color trait)
Crossed true breeding pea plants with purple flower to plants with white flowers
Result –> All offspring had purple flowers (white trait “disappeared”)
Mendel’s Second Experiment (cross)
Cross pollinated and allowed for self pollinated of plants in the F1 generation
–> Produced the F2 generation
F2 Generation
2nd generation derived from P-generation
–> Made through crossing F1 plants
Results of Mendel’s Second Experiment (flower color trait)
The offspring had a ratio of 3 (purple) : 1 (white)
–> The white trait had “reappeared”
Mendel coined the terms…
And their initial definitions were…
Dominant = Trait observed in F1
Recessive = Trait that “disappeared” in F1
Mendel’s Model
–> The idea of heritable factors
For each character, an organism inherits 2 versions (alleles) of a heritable factor (gene), one from each parent
Mendel’s Model
–> The idea of versions of characters
There exists alternative versions of each heritable factor
–> Alleles
Alleles account for _____ of inherited characters
variation
Mendel’s Model
–> The idea of alleles differing at a locus
If the 2 alleles at a locus differ, then ONE (dominant allele) determines the organism’s appearance and the OTHER (recessive allele) has no noticeable effect on appearance
Dominant
Trait that is always expressed when present
Recessive
Trait that gets masked by the presence of the dominant form
Mendel’s Law of Segregation
During gamete formation, the 2 alleles of each gene segregate and end up in different gametes
–> Thus, each gamete has ONE allele (of the two present in a diploid) of each gene
What does Mendel’s Law of Segregation account for?
Meiosis: Specifically, Anaphase I (in which homolog separation occurs)
Mendel’s Model
–> The role of fertilization
When fertilization occurs, the zygote receives ONE unit from each parent, restoring the pair
Homozygous
Having a pair of identical alleles
(Homozygote)
Heterozygous
Having 2 different alleles for a given gene
(Heterozygote)
Monohybrid Cross
A cross between 2 heterozygotes (both heterozygous for the same character)
Phenotype
Outward appearance (looks)
Genotype
Genetic Makeup (the genes)
Phenotype ration does NOT always equal the
Genotype ratio
An organism’s phenotype doesn’t always reveal its…
genotype
–> Especially in determining between homozygous dominant and heterozygous (usually have the same phenotype)
Test Cross
A cross of an organism with UNKNOWN genotype with a HOMOZYGOUS RECESSIVE organism
Purpose of Test Crosses
Used to determine if an individual exhibiting a dominant trait is homozygous or heterozygous for that trait
Test Cross:
Meaning of all offspring being dominant phenotype
Means that the unknown genotype is HOMOZYGOUS DOMINANT
Test Cross:
Meaning of 1/2 offspring = dominant phenotype and 1/2 = recessive phenotype
Means that the unknown genotype is HETEROZYGOUS
Dihybrid Cross
A cross between two individuals that are heterozygous for TWO characters
–>EX: YyRr x YyRr
What did Mendel use the dihybrid crosses for?
To test whether traits segregated independently or dependently from one another
Mendel’s Law of Independent Assortment
2 or more genes assort independently
–> Each pair of alleles segregates independently of any other pair of alleles during gamete formation
Exceptions to Mendel’s Law of Independent Assortment
Only applies to genes on DIFFERENT, NON HOMOLOGOUS chromosomes or those that are far apart on the same chromosome
–> Genes Located near each other on the same chromosome tend to be inherited TOGETHER*
What does Mendel’s Law of Independent Assortment account for?
Meiosis –> Metaphase I
–> There is random assignment of chromosomes (homologous pairs have 2 potential orientations and the orientation they end up is random)
Punnet Square
A diagnostic device for predicting the allele composition of offspring from a cross between individuals of known genetic makeup
Each square of a punnet square represents…
An EQUALLY PROBABLE fertilization product (assuming random fertilization)
Purpose of Punnet Squares
Predicts the outcome of a genetic cross
Probability Rules:
0 –> 1 Rule
Probability scales range from ZERO to ONE
Event that is CERTAIN to occur…
Probability = 1
Event that is certain NOT to occur (impossible)…
Probability = 0
The probabilities of all possible outcomes for an event must…
add up to ONE
Probability Rules:
event/Total
The probability of an event occurring is the # of time that event could occur over ALL possible events (total)
Probability Rules:
What is the probability of pulling the ace of spades out of a deck of cards?
1/52
Ace of spades has one occurrence in a deck
A deck has 52 cards total
Probability Rules:
Previous Trials Rule
The outcome of any Independent event is NOT affected by previous trials
–> Doesn’t matter what happened in the past
Product/Multiplication Rule
To determine the probability of one event AND another occurring in combination
–>***we multiple the probability of one event by the probability of the other event
(Prob Event #1) x (Prob Event #2) = Prob. of Independent Events Occurring Together
Sum/Addition Rule
To determine the probability that any ONE of two or more mutually exclusive events will occur
–> The probability that one event OR the other will occur
–>Add the probability of each individual event together
(Prob Event #1) + (Prob Event #2) = Prob. that one or the other will occur
