Mendel’s Principle of Dominance or Recessiveness

Mendel & The Gene Idea

The Father of Genetics

It All Began with Mendel (FYI) Gregor Mendel was born in 1822. Called the “Father of Genetics” Late 1800 chromosomes and the process of

meiosis were unknown. Mendel’s work was considered obscure and

unimportant until 1900 Walter Sutton proposed the Chromosome

Theory and people began to listen to his ideas. Chromosome Theory – specific genes are

located on specific chromosomes

1. Mendel’s Findingsa. Principle of Dominance and

Recessivenessa. One allele in a pair may mask the

effect of the other

b. Principle of Segregationa. The two alleles for a characteristic

separate during the formation of eggs and sperm

c. Principle of Independent Assortmenta. The alleles for different characteristics

are distributed to reproductive cells independently.

2. Terminologya. Character

(heritable feature, i.e., fur color)

b. Trait (variant for a character, i.e., brown)

c. True-bred (all offspring of same variety)

d. Hybridization

e. (crossing of 2 different true-breds)

f. P generation (parents)

g. F1 generation (first filial generation)

h. F2 generation (second filial generation)

2. Terminologye. Punnett square: f. Gene: point on a chromosome

that controls the traitg. Allele: an alternate form of a gene

A or ah. Homozygous: identical alleles

for a characteri. Heterozygous: different alleles

for a genej. Phenotype: physical traitsk. Genotype: genetic makeupl. Testcross: breeding of a

recessive homozygote X dominate phenotype (but unknown genotype)

What about 2 Traits? BbLl x BbLl The Gametes contain one

of each of the alleles. (BL).

Each of the offspring contain four alleles exactly like the parents.(BbLl).

Notice the number of possible offspring has increased.

The phenotypic ratio is 9:3:3:1

BbLl x BbLlBL Bl bL bl

BL BBLL BBLl BbLL BbLl

Bl BBLl BBll BbLl Bbll

bL BbLL BbLl bbLL bbLl

bl BbLl Bbll bbLl bbll

Now on your own… Work the Dihybrid Crosses on your own.

Exceptions to Mendel’s Rule

Non Mendelian Genetics

Incomplete Dominance The phenotype of the

heterozygote is intermediate between those of the two homozygotes.

Ex) Snap Dragon Color

Red, Pink, White

Co-dominance Phenotype of both

homozygotes are produced in heterozygotes individuals.

Both alleles are expressed equally.

Ex)Roan Cattle White-feathered birds are both homozygotes for both B and W alleles

Multiple Alleles

Ex )Blood type Blood type A and B are co-dominant,

while O is recessive. Forms possible blood types of A, B, AB,

and O.

Blood Also Shows Codominance

Sex-Determination Chromosome pairs 1-22 are autosomes Chromosome pair 23 are sex chromosomes They determine the sex of an individual XX = female XY = male

Sex-Linked Inheritance Traits that are only found on the X

chromosome Colorblindness and Hemophilia are

examples of sex-linked traits. These genes are recessive and found only

on the X chromosome.

Polygenic Inheritance Inheritance pattern of a

trait that is controlled by two or more genes.

Gene may be on the same chromosome or on different chromosomes.

Ex) Skin color and Height