Often Mendelian genetics refers to basic genetics that use Punnett Squares and statistics to predict the phenotypes of offspring. Modern Genetics usually refers to our modern-day understanding of how DNA and genes control the expression of traits. During Mendel's time, biologists did not know about DNA and genes.
Known as "The Father of Genetics" for his experiments on pea plants which established the basic rules of heredity
Pea Plants had a variety of traits: Examples:
- illustrated by crossing parents of opposing traits (purple x white)
- on trait was dominant, one was recessive
The second generation displayed only dominant traits.
What happened in the F2 generation?
1. A purple plant (RR) is crossed with a wrinkle seeded plant (rr). What are the phenotypes of the offspring?
2. Two heterozygous round-seeded plants (Rr) are crossed. What are the phenotypes of their offspring and in what proportion?
Each letter (R or r ) represents an ALLELE
Two letters represent a GENE
Generally, when setting up problems, use a capital letter to represent the dominant trait and a lowercase letter for the recessive.
RR & rr = homozygous | Rr = heterozygous
Phenotype = the way it looks, the form | Genotype = the genetic composition, represented by letters
A TEST CROSS is performed when the genotype is unknown.
Why does the punnett square work?
*it represents all the possible gametes that each parent can contribute
If a parent has this genotype RrYy, what combinations are possible in the gametes?
What about AaBbRr?
This is a very common cross seen in biology: RrYy x RrYy
The result of the cross is 9:3:3:1 *always*
AaBb x AaBb
9 - (two dominant traits)
3 - (one dominant, one recessive)
3 - (one recessive, one dominant)
You can solve it with a punnet square
You can also solve it mathematically.
Solve this one mathematically: RrPp x rrpp
- states that each trait is separate, the alleles do not influence each other. This is why the 9:3:3:1 ratio is constant.
* It doesn't matter how often you flipped a coin or how many times it's already shown heads, the probability is ALWAYS 50% of heads/tails.
The gambler's fallacy is a logical fallacy where people gambling believe that a losing streak will turn around.
Example: I've tossed a coin 4 times, all four times it came up heads. What is the probability that my next toss will be heads?