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.

Chapter 11 - Mendelian Genetics

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:


1. The Law of Segregation (monohybrid cross)

- 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?


Practice with Punnett Squares

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?


Modern Understanding

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.




The Dihybrid Cross:

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)

1 -  (two recessive traits)

You can solve it with a punnet square

You can also solve it mathematically.






Solve this one mathematically: RrPp x rrpp




2. The Law of Independent Assortment

- states that each trait is separate, the alleles do not influence each other. This is why the 9:3:3:1 ratio is constant.



A note about probability

* 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?