Mendelian Patterns of Inheritance

Chapter 14

Gregor Mendel

• Developed the fundamental laws of heredity 

• He studied science and mathematics 

• Mendel chose garden peas (Pisum sativum)as his subjects as they are easily grown and their pollination is easily controlled. He controlled pollination by manually moving pollen between plants 

• Developed True-breeding plants by self-pollination 

• He could also allow the plants to self-pollinate. 

• Mendel examined varieties of peas for heritable characters and traits for his study. (stem length, pod shape, seed shape, seed color..etc)

Funfact: Mendel originally wanted to breed mice, but wasn't allowed to because it was considered scandalous

Mendel's Law of Segregation (MONOHYBRID CROSS)

• Mendel crossed true-breeding plants that differed for a given character

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

• Pollen from true-breeding pea plants with purple flowers (one trait) was placed on stigmas of true-breeding plants with white flowers (another trait).

• 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.

• The F1 plants were allowed to self-pollinate.

• This step was the monohybrid cross. (or the F1 cross).

• The progeny, called F2, were examined: roughly 1/4 were white, and 3/4 were purple.

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

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)

• When fertilization occurs, the zygote gets one from each parent, restoring the pair.

• Mendel's units of inheritance are now called genes. • Different forms of a gene are called alleles. • Each allele is given a symbol: In the case of purple flowers,

P might represent purple and p white. By convention, uppercase P represents the dominant; lowercase p represents the recessive. Never mix your letters! Choose one letter and use a capital to denote dominate and a lowercase to denote recessive.

                                                                                                            

Parental Cross PP  x  pp

purple x white

• True-breeding individuals would have two copies of the same allele: Purple would be PP.

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

• White true-breeding would be pp.• Two copies of same allele = homozygous.   Therefore both PP and pp are

considered homozygous, just one is purple and the other is white.

• Some purple-flowered plants could be Pp, although they would not be true-breeding. 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 = blackb = 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.

GENOTYPE PHENOTYPE

          Pp        purple flowers

 rr wrinkled seeds

TT tall

tt short

BY CONVENTION:

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                                    HHShort hair = Ss                                       HhLong 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.

4.  Buttons are dominant over snaps.

5.  Pockets 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?Help, help!  I don't know what my genotype is!! Am I Dd or DD?

I can help you!  Let's have offspring!

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: Parents: RrYy x RrYy

complete the punnett Square….