Lecture Outline for February 28, 2005


Topic: Mendelian Genetics: monohybrid crosses, dihybrid crosses

Chapter 10 sections 10.1, 10.2

Figures used in lecture: 10.1 through10.6, 10.8, 10.11

Additional Resources

Introduction to Mendelian Genetics – a great website

Mendel’s Experiment – a great animation from the Mendel Museum of Genetics

Mendelian Genetics – site includes problem sets, tutorials, web resources

MendelWeb Table of Contents 97.1 - An extensive collection of information about Mendel, including links to his publications.


Mendel’s Work

Up until the time of Mendel, it was thought that crossing two parents caused a blending of their traits in the offspring. It was thought to be like mixing paint – white paint plus red paint produces pink paint and you can’t retrieve white paint from pink.


Mendel’s Experiments

Mendel chose 7 traits of pea plants and started with true-breeding parents

True-breeding tall X true-breeding short ® F1 generation was all tall

If the F1 plants are allowed to self-pollinate, the F2 generation is 75% tall and 25% short

This does not support the idea of blending since the F1 are not intermediate between the appearance of the 2 parents and because the trait hidden in the F1 is seen again in the F2. Mendel saw this pattern for all the traits he examined. Instead of blending, Mendel proposed factors that are donated by each parent to produce the offspring. The factors are not mixed or blended or lost in the F1. They are only masked but reappear in the F2. Today we call Mendel’s factors genes.

Define/explain: self-fertilize (self-pollinate), true-breeding, P generation, F1 generation, F2 generation, gene, allele, homozygous, heterozygous, dominant, recessive, genotype, phenotype


Today’s Lecture (2-28-05)

Mendelian Genetics

            monohybrid crosses

            dihybrid crosses


Mendelian genetics: study of the inheritance of traits from parent to offspring

Gregor Mendel 


Each pea flower can pollinate itself. Mendel had to prevent this.

Prior to Mendel, inheritance was thought to be via blending.

For each trait, Mendel chose strains of peas that “bred true”.

True-breeding organisms show the same trait every generation as long as they self-fertilize.

Mendel saw specific patterns:

P      true-breeding tall        X      true-breeding short

F1     100% tall (dominance)

F2     3 tall : 1 short   (3/4 to 1/4)

            75% tall : 25% short















Mendel’s First Law – The Principle of Segregation

Two members of a gene pair (alleles) segregate (separate) from each other in the formation of gametes.

This occurs in meiosis I – specifically anaphase I – when homologous chromosomes separate.

Define/explain: monohybrid cross, Punnett square, test cross

Monohybrid Cross Problem:

Q.  In guinea pigs, rough coat (R) is dominant over smooth coat (r). A rough-coated guinea pig is bred to a smooth one, producing 8 rough and 7 smooth progeny in the F1.

What are the genotypes of the parents and their offspring?

A.  The first step in any genetics problem is to write down everything the question tells you.

                The gene is for coat texture.  R = rough coat (dominant allele) and r = smmoth coat (recessive allele.

               P generation     rough (R-)  X  smooth (rr)

               F1 consists of    8 rough (R-)   and 7 smooth (rr)

The genotypes of the smooth guinea pigs can only be rr.  The genotypes of the rough guinea pigs could be RR or Rr.  To determine whether the rough parent is RR or Rr consider the fact that smooth offspring are present in the F1 generation.  Those smooth offspring must recieve a recessive allele (r) from EACH of their parents.   Therefore, the rough parent must have a recessive allele and must be heterozygous (Rr).  The rough offspring in the F1 generation must also be Rr but for a different reason.  The smooth parent has 2 recesive (r) alleles.  Every gamete produced by that smooth parent will contain a recessive (r) allele.  Every offspring of that smooth parent will recieve a recessive allele (r).  Therefore, the rough offspring in F1 must be heterozygous (Rr).


What happens when you examine the inheritance of 2 genes at the same time?

Does the inheritance pattern of one gene influence the inheritance of the other?


Mendel’s Second Law – Law of Independent Assortment

Alleles of different genes assort independently of one another during gamete production.

Define/explain: dihybrid cross, 9:3:3:1 ratio



Mendelian Genetics

9:3:3:1 ratio is : (3:1) X (3:1)


Dihybrid ratio is the result of 2 independent events.



Dihybrid Cross Problem

Q.   In summer squash white fruit (W) is dominant over yellow (w) and disk-shaped fruit (D) is dominant over sphere-shaped fruit (d). The following crosses were performed and the outcome of each cross is shown. What are the genotypes of the parents for each cross?

Phenotypes of Parents                                                            Offspring

a. white, disc-shaped X yellow, sphere-shaped                     ½ white, disc; ½ white, sphere


A.  P    white, disc-shaped fruit        X          yellow, sphere-shaped fruit

           The white, disc-shaped squash must have at least one dominant allele for color and at least one dominant allele for shape.  We can start by identifying its genotype as W- D- and all that remains is to determine whether it is WW DD or WwDd.  The yellow, sphere-shaped squash is showing the recessive phenotype for both genes.  It's genotype must be ww dd.  The white, disc-shaped parent's genotype can be determined by considering what kind of offspring it can produce.  Since all of it's offspring are white, the   parent must be WW for the color gene.  Since there are sphere-shaped offspring produced, the white, disc-shaped parent must possess a recessive allele and is therefore heterozygous for the shape gene.  The complete genotype of the white, disc-shaped parent is WW Dd.