Mendel’s Principle of Dominance or Recessiveness
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Transcript of Mendel’s Principle of Dominance or Recessiveness
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