Mendelian patterns of inheritance
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Mendelian Patterns of Inheritance. Chapter 11. Mendel's Law of Segregation (MONOHYBRID CROSS). A monohybrid cross involves one (mono) character and different (hybrid) traits. The F1 seeds were all purple; the white flower trait failed to appear at all.

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Mendelian Patterns of Inheritance

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Mendelian Patterns of Inheritance

Chapter 11

Mendel's Law of Segregation (MONOHYBRID CROSS)

  • A monohybrid cross involves one (mono) character and different (hybrid) traits.

  • The F1 seeds were all purple; the white flower trait failed to appear at all.

Because the purple flower trait completely masks the white flower trait when true-breeding plants are crossed, the purple flower trait is called dominant, and the white flower trait is called recessive.

Creating the F2 generation

*Cross the F1 generation together to create F2

*Ratio is always 3:1

Mendel proposed that the units responsible for inheritance were discrete particles - particulate theory of inheritance

The same pattern occurred over and over, for seven different traits. 

As Viewed by Modern Genetics

  • During production of gametes, only one of the pair members for a given character passes to the gamete. (LAW OF SEGREGATION)

  • Mendel's units of inheritance are now called genes.

  • Different forms of a gene are called alleles.

  • Each allele is given a symbol:

Parental Cross 

PP  x  pp

purple x white

  •  Two copies of same allele = homozygous. Homo means "the same" 

  • Therefore both PP and pp are considered homozygous, just one is purple and the other is white.

  • Some purple-flowered plants could be Pp.  Individuals that are purple, but had a white parent, are heterozygous: Pp. Hetero means "different".

The F1 cross

Pp  x Pp   

purple x purple





When an organism is studied for three different genes and has the alleles AABbCC, it is homozygous for A and C genes but heterozygous for the B gene.

A = antennaa = no antenna

B = black

b = purple

C = chompersc = no chompers



The physical appearance of an organism is its phenotype. Purple-flowered would be a phenotype.

The actual composition of the organism's alleles for a gene is its genotype: Pp is a genotype.

Organisms have many different genes some have thousands, and complex organisms have 10 times that number.


The dominant trait is given a capitol letter, the lowercase of that same letter is the recessive trait.  DO NOT MIX LETTERS.  Pick one and stick to it.

Also, some letters are better than others.  Capital S looks a lot like a lowercase (s).  Pick a different letter...

   Okay                                                   Better (use H for hair)

Short  hair  = SS                                    HH

Short hair = Ss                                       Hh

Long hair = ss                                        hh 

Practice picking letters.... the following traits are found in the common Shirtus americanus.

1.  Polka dots are dominant to stripes.

2.  Long sleeves are dominant to short sleeves.

3.  Collared shirts are recessive.

Practice with Punnett Squares

1.  A  round seeded plant (RR) is crossed with a wrinkle seeded plant (rr).  What are the phenotypes of the offspring?

2.  Two heterozygous purple flowered pea plants are crossed.  What are the phenotypes of their offspring and in what proportion?

3.  A plant with green seeds (yy) is crossed with a heterozygous plant.  What percentage of their offspring have yellow seeds?

In dragons...

Wings are a dominant trait, but some dragons are born wingless.

What are the chances that two heterozygous dragons have a whelp that is wingless?

If a wingless dragon is crossed with one that is heterozygous, how many of its offspring will also be wingless?

What is a test cross?

I can help you!  Let's have offspring!

Help, help!  I don't know what my genotype is!!

Am I Dd or DD?

Why does the punnett square work?

It all goes back to meiosis.. each side represents a sperm or egg.  The boxes filled out simply give you the statistical chance that a certain sperm will fertilize a certain egg. 

Consider a pea plant that is RrTt  (round seed, tall)

When this plant's cells go through MEIOSIS, the alleles segregate - each sperm receives a random combination...

Possible Gametes:

R T            R t

r T              r t

What are the gametes possible for an organism that is  AaBbRr?

Mendel’s Three Laws 1. Dominance & Recessiveness 2. Segregation

 3. Independent Assortment

Mendel's Law of Independent Assortment – Illustrated by the DIHYBRID cross

The second law describes the outcome of dihybrid (two character) crosses, or hybrid crosses involving additional characters.

A dihybrid is an individual that is a double heterozygote (e.g., with the genotype RrYy - round seed, yellow seed).

What are the gametes that can be produced by this individual?

Dihybrid Cross: RrYy x RrYy

All of these type of crosses will follow the same ratio

AaBb x AaBb

9 - (two dominant traits)

3 - (one dominant, one recessive)

3 - (one recessive, one dominant)

1 - (two recessive traits)

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?

Try another mathematical model..

Winged, Fire breathing dragon DdFf


Wingless, Fire breathing dragon ddFf

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