To study genetics and biotechnology, we must begin by looking at Gregor Mendel’s work and try to understand how traits are passed from one generation to the next. We will follow this by learning about the actual shape and design of chromosomes and genes along with how technology and the world continue to drive this topic.

BI30-GB1 Investigate the mechanisms and patterns of inheritance.

a. Discuss the importance of Gregor Mendel as the “father of genetics”. (STSE, K)

b. Discuss the historical development of scientific understanding of Mendelian genetics, including the importance of statistical analysis, probability, and significance. (STSE, K)

c. Distinguish among the mechanisms of inheritance (i.e., dominant and recessive alleles, sex-linked traits, codominance, incomplete dominance, and multiple alleles). (K)

d. Determine an organism’s phenotype from its genotype, and where possible, its genotype from its phenotype. (K)

e. Construct Punnett squares using P1 genotypes (i.e., homozygous and heterozygous) to determine genotypic and phenotypic frequencies for F1 and F2 generations. (S)

A discussion on Genetics must begin with by looking at the work of Gregor Mendel and a discussion of independent events and probability.

The term independent events means that events are independent of each other if the probability of one occurring does not affect the probability of the others. The probability of something is the measure of how likely an event is. For instance, when my son was born there was a 1 in 2, or 0.50 probability that Eli would be a boy. There was also a 1 in 2, or 0.50 probability that he would have been a girl and hopefully would have a different name. So, when my second son Ti was born, was the probability still 1 in 2, or 0.5 for having another boy? Yes it was because the sex of one child has no impact on the second child. So, these would be independent events.

Gregor Mendel:"father of genetics"

Blending Theory of Inheritance- offspring of two parents "blend" the traits of both parents
Particulate Theory of Inheritance- traits are inherited as "particles", offspring receive a "particle" from each parent.

Evidence for Particulate Theory of Inheritance: A plant with purple flowers is crossed with another plant that has purple flowers. Some of the offspring have white flowers (wow!). Mendel set out to discover how this could happen.

Some stuff on Mendel

  • parents were farmers
  • he became ordained as a priest
  • studied science and mathemathics at the University of Vienna

Mendel's Experiments

Mendel chose pea plants as his experimental subjects, mainly because they were easy to cross and showed a variety of contrasting traits (purple vs white flowers, tall vs short stems, round vs wrinkled seeds)

1. Mendel chosetrue-breeding linesof each plant/trait he studied (true breeding lines always produced offspring of the same type)

2. He crossed a true breeding plant with a plant of the opposite trait (purple x white). He called this theParental (P) generation. (In this case, he cross-pollinated the plants)

3. He recorded data on the offspring of this cross (First Filial, F1)

4. He self pollinated the F1 offspring

5. He recorded data on the offspring of the second generation, calling it theSecond Filial generation (F2)


  • The F1 generation always displayed one trait (he later called this the dominant trait)
  • The F1 generation must have within it the trait from the original parents - the white trait
  • The F2 generation displayed the hidden trait, 1/4 of the F2 generation had it (he later called this hidden trait the recessive trait)
  • Each individual has two "factors" that determine what external appearance the offspring will have. (We now call these factors genes or alleles)

Mendel established three principles (or Laws) from his research

1.The Principle of Dominance and Recessiveness- one trait is masked or covered up by another trait

2.Principle of Segregation- the two factors (alleles) for a trait separate during gamete formation

3.Principle of Independent Assortment- factors of a trait separate independently of one another during gamete formation; another way to look at this is, whether a flower is purple has nothing to do with the length of the plants stems - each trait is independently inherited

Modern Genetics

Mendel's factors are now calledALLELES. For every trait a person have, two alleles determine how that trait is expressed.

We use letters to denote alleles, since every gene has two alleles, all genes can be represented by a pair of letters.

PP = purple, Pp = purple, pp = white

Homozogyous: when the alleles are the same, the individual is said to be homozygous, or true breeding. Letters designating a homozgyous individual could be capital or lowercase, as long as they are the same. Ex. AA, bb, EE, dd

Heterozygous: when the alleles are different, in this case the DOMINANT allele is expressed. Ex. Pp, Aa

Monohybrid cross= a cross involving one pair of contrasting traits. Ex. Pp x Pp

Punnet Square: used to determine the PROBABILITY of having a certain type of offspring given the alleles of the parents

Genotype: letters used to denote alleles (BB, Pp..etc)
Phenotype: what an organism looks like (brown, purple..)

  • a simple box-like device that helps us to consider all genetic combinations and show the expected frequencies of genotypes.
  • The P and p symbols represent the single allele each gamete receives.
  • Fertilization provides the two alleles for the new individual, one from the male (sperm) and one from the female (egg).
  • The Punnett square shows that the genotypes and associated ratios for a monohybrid cross are 1 PP :2 Pp : 1 pp.
  • Any progeny with a P would have the dominant (purple) phenotype, so the phenotypic ratio is 3 purple to 1 white.
  • Now it is known that a gene is a portion of the chromosomal DNA that resides at a particular site, called a locus (plural is loci). The gene codes for a particular function or trait.
  • Mendel arrived at the law of segregation with no knowledge of meiosis or chromosomes. The mechanism of chromosome separation in meiosis I today explains his law of segregation.

How to Solve a Punnet Square

1. Determine the genotypes (letters) of the parents. Bb x Bb
2. Set up the punnet square with one parent on each side.
3. Fill out the Punnet square middle
4. Analyze the number of offspring of each type.

In pea plants, round seeds are dominant to wrinkled. The genotypes and phenotypes are:

RR = round
Rr = round
rr = wrinkled

If you get stuck make a "key". Sometimes the problems won't give you obvious information.

Example: In radishes, a bent root is a dominant trait, though some roots are straight (which is recessive). If a straight rooted plant is crossed with a heterozyous bent root plant, how many of the offspring will have straight roots?

Say what?First of all, this problem doesn't make it easy. Start by assigning genotypes and phenotypes. It doesn't matter what letter you pick, but it may be easiest to pick a letter that represents the dominant trait. In this case, use the letter B for bent.

BB = bent
Bb = bent
bb = straight

Now use the key to figure out your parents. In this case you have a straight root plant (bb) crossed with a heterozyous bent plant (Bb). Once you've figured that out, the cross is simple!

If a heteroyzous round seed is crossed with itself (Rr x Rr) a punnett square can help you figure out the ratios of the offspring.


3/4 round, 1/4 wrinkled

One Trait Test Cross

  • A test cross can determine the genotype (heterozygous or homozygous) of an individual with a dominant trait.
  • It involves crossing the individual to a true-breeding recessive (homozygous recessive).
  • If the unknown is heterozygous, approximately half the progeny will have the dominant trait and half the recessive trait.
  • If the unknown is homozygous dominant, all the progeny will have the dominant trait

style / WILD TYPE = refers to the "normal" genotype
Often designated with a + symbol
  1. Please go to the following link and complete this assignment in your book:

Dihybrid Crosses: Crosses that involve 2 traits.

These type of crosses can be challenging to set up, and the square you create will be 4x4. This simple guide will walk you through the steps of solving a typical dihybrid cross common in genetics. The method can also work for any cross that involves two traits.

Consider this cross

A pea plant that is heterozygous for round, yellow seeds is self fertilized, what are the phenotypic ratios of the resulting offspring?

Step 1: Determine the parental genotypes from the text above, the word "heteroyzous" is the most important clue, and you would also need to understand that self fertilized means you just cross it with itself.


Step 2: Determine the gametes. This might feel a little like the FOIL method you learned in math class. Combine the R's and Ys of each parent to represent sperm and egg. Do this for both parents

Gametes after "FOIL"

RY, Ry, rY, ry(parent 1) andRY, Ry, rY, ry(parent 2)

Step 3: Set up a large 4x4 Punnet square, place one gamete set from the parent on the top, and the other on the side


Step 4: Write the genotypes of the offspring in each box and determine how many of each phenotype you have. In this case, you will have 9 round, yellow; 3 round, green; 3 wrinkled, yellow; and 1 wrinkled green

Some Shortcuts

In any case where the parents are heterozygous for both traits (AaBb x AaBb) you will always get a 9:3:3:1 ratio.

9 is the number for the two dominant traits, 3 is the number for a dominant/recessive combination, and only 1 individual will display both recessive traits.

Another way to determine the ratios is to do it mathematically

3/4 of all the offspring will have round seeds
3/4 of all the offspring will have yellow seeds

3/4 x 3/4 = 9/16 will have round, yellow seeds.

Crosses that Involve 2 Traits


The square is set up as shown

You might notice that all four rows have the same genotype. In this case, you really only need to fill out the top row, because 1/4 is the same thing as 4/16

A Mathematical Alternative (LAWS OF PROBABILITY)

A punnet square is not needed to determine the ratios of genotypes and phenotypes. Simple statistics and math can save you the trouble of filling out a square.

In a monohybrid cross Pp x Pp, each parent produced P gametes and p gametes

If you wanted to determine how many of the offspring are pp: x =

Example 2: H is dominate for long hair (h = short) and B is dominate for black eyes (b = red eyes)

If the parents are: HhBb x hhBb

How many off the offspring will be short haired and red eyed?


Task: Use mathematical analysis to determine the number of short haired, black eyed offspring from the cross above.


Used to determine the genotype of an "unknown" by crossing it with an individual that is homozygous recessive for both traits.

In flies (Long wings is dominant to short wings, Gray body is dominant to black)

A L __ G ___ is test crossed.

The offspring are 1:1:1:1 --> What is the genotype of the unknown parent?
If the offspring are half long winged & gray, and half long winged and black --> What is the genotype of the unknown parent?

We will know complete some assignments with the use of punnett squares and our knowledge of genetics.

  1. We will try to complete the following assignment as a group:

Incomplete Dominance

In some cases, an intermediate phenotype is shown (meaning three phenotypes)

Neither allele is dominant

In snapdragons, flower color can be red, pink, or white. The heterozygous condition results in pink flowers (or an intermediate trait)

A white snapdragon crossed with a red snapdragon produces all pink offspring

Two pinks crossed together produce 1/4 white, 2/4 pink, and 1/4 red

When dealing with incomplete dominance and codominance it does not matter what letter you use, as long as the heterozygous condition always denotes the intermediate trait. In the diagram R is used, but you could also use W or even P. Ww = pink, Pp = pink if these letters are used.

Description style

Sickle cell disease is incompletely dominant in humans. AA x aa = Aa (sickle cell trait), where some blood cells will have abnormal shapes


Both alleles can be expressed

For example, red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches.

This phenotype might seem to support the blending theory. (The blending theory predicts pink F1 progeny.)

The F2 progeny, however, demonstrate Mendelian genetics. When the F1 roan individuals self-fertilize, the F2 progeny have a phenotypic ratio of 1 red:2 roan:1 white.

This mode of inheritance is called incomplete dominance.

The phenotypic outcomes for cow color and incomplete dominance in general can be explained biochemically.

One allele of the gene codes for an enzyme that functions in the production of the red color. The other allele codes for the gene to make white color. If both alleles are present, both are expressed, resulting in a cow that has some red and some white.

Mendel's laws are not compromised here, he just happened to find in peas examples of complete dominance only.

Polygenic Traits

Traits controlled by many genes, resulting phenotypes are in a range with a central average

Each allele intensifies or diminishes the phenotype

Examples: height, skin color, seed color in wheat

(P) AABBCC x aabbcc

(F1) AaBbCc x AaBbCc

(F2) Seven possible phenotypes

A bell curve indicates polygenic inheritance, with a center average and a small number of extremes

Epistatic Alleles

Indicate the genotypes and phenotypes found in labrador retrievers

Labrador Retriever Genetics

Black is dominant to chocolate B or b
Yellow is recessive epistatic (when present, it blocks the expression of the black and chocolate alleles) E or e

Phenotype / Possible Genotypes
/ bbEE
/ BBee

Show the crosses

1. A black lab (BBEe) x yellow lab (bbee)

2. A chocolate lab (bbEe) x black lab (BbEe)

3. Two black labs (BBEE x BbEe)

Sex-Linked Genes

Genes located on the X chromosomes (some cause diseases)

The Y is much smaller, contains few genes

Sex Linked Disorders
Color Blindness (red-green)
Hemophilia ("bleeders disease)
Duchenne Muscular Dystrophy (weakening/loss of skeletal muscles)

Crosses Involving Sex - Linked Genes

**remember the Y is wimpy, no genes go there **

Consider the Hemophilia - this disease is caused by an allele on the X chromosome

punnett square

Muliple Allele Traits

Traits that are controlled by more than two alleles. Blood type in humans is controlled by three alleles: A, B, and O

Phenotype / Genotype
A / AA or AO
B / BB or BO
AB / AB only
O / OO only

Examples of Blood type crosses

Blood Transfusions

Blood can only be transferred to a body of a person who's immune system will "recognize" the blood. A and B are antigens on the blood that will be recognized. If the antigen is unfamiliar to the body, your body will attack and destroy the transfused blood as if it were a hostile invader (which can cause death).

O is like a blank, it has no antigens. O is called the universal donor because a person can receive a transfusion from O blood without having an immune response

AB is the universal acceptor, because a person with AB blood has both the A and B antigens already in the body, A and B blood can be transfused to the person (as well as O) and the body will recognize it and not attack.

Polygenic Traits

Traits controlled by many genes: hair color, height weight, intelligence

Sex Influenced Traits

Traits are influenced by the environment. Pattern baldness affects men because testosterone activates the genes.

Environmentally Influenced Traits

Siamese cats have dark ears and feet due to the temperature. Height in humans is influenced by the environment (diet)

Please complete the advanced genetics worksheets and From Parents to Child Lab.

f. Explore patterns of inheritance by interpreting pedigrees. (K, S)

Complete pedigree worksheets.

g. Explain how allelic frequencies change over time within a population’s gene pool, with reference to gene flow. (K)

Hardy Weinberg Principle

This video is very mathematical – we may not get into the math of Hardy Weinberg depending on the time we have left at this stage.

All the following information was taken from Wikipedia at the following link:

The Hardy–Weinberg principle (also known by a variety of names: HWP, Hardy–Weinberg equilibrium, Hardy–Weinberg Theorem, HWE, or Hardy–Weinberg law) states that both allele and genotype frequencies in a population remain constant—that is, they are in equilibrium—from generation to generation unless specific disturbing influences are introduced.

So, essentially it means … (please write it out in your words)…

The factors that can affect the HWP:

Violations of the Hardy–Weinberg assumptions can cause deviations from expectation. How this affects the population depends on the assumptions that are violated. Generally, deviation from the Hardy–Weinberg equilibrium denotes the evolution of a species.

  • Random mating. The HWP states the population will have the given genotypic frequencies (called Hardy–Weinberg proportions) after a single generation of random mating within the population. When violations of this provision occur, the population will not have Hardy–Weinberg proportions. Three such violations are:
  • Inbreeding, which causes an increase in homozygosity for all genes.
  • Assortative mating, which causes an increase in homozygosity only for those genes involved in the trait that is assortatively mated (and genes in linkage disequilibrium with them).
  • Small population size, which causes a random change in genotypic frequencies, particularly if the population is very small. This is due to a sampling effect, and is called genetic drift.

The remaining assumptions affect the allele frequencies, but do not, in themselves, affect random mating. If a population violates one of these, the population will continue to have Hardy–Weinberg proportions each generation, but the allele frequencies will change with that force.