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Lecture # 6
Date _________
 Chapter 14~
Mendel &
The Gene Idea
Mendelian genetics
 Character
(heritable feature, i.e., fur color)
 Trait




(variant for a character, i.e., brown)
True-bred
(all offspring of same variety)
Hybridization
(crossing of 2 different truebreds)
P generation (parents)
F1 generation (first filial generation)
Leading to the Law of Segregation
 Alternative versions of genes




(alleles) account for variations in
inherited characteristics
For each character, an organism
inherits 2 alleles, one from each
parent
If the two alleles differ, then one,
the dominant allele, is fully
expressed in the organism’s
appearance; the other, the recessive
allele, has no noticeable effect on
the organism’s appearance
The alleles for each character
segregate (separate) during gamete
production (meiosis).
Mendel’s Law of Segregation
Genetic vocabulary…….
 Punnett square: predicts the results





of a genetic cross between
individuals of known genotype
Homozygous: pair of identical
alleles for a character
Heterozygous: two different alleles
for a gene
Phenotype: an organism’s traits
Genotype: an organism’s genetic
makeup
Testcross: breeding of a recessive
homozygote X dominate phenotype
(but unknown genotype)
The Law of Independent Assortment
 Law of Segregation
involves 1 character. What
about 2 (or more)
characters?
 Monohybrid cross vs.
dihybrid cross
 The two pairs of alleles
segregate independently of
each other.
 Mendel’s Law of
Independent Assortment
Non-single gene genetics, I
 Incomplete dominance: appearance
between the phenotypes of the 2
parents. Ex: snapdragons
 Codominance: two alleles affect
the phenotype in separate,
distinguishable ways.
Ex: Tay-Sachs disease
 Multiple alleles: more than 2
possible alleles for a gene.
Ex: human blood types
What is the difference between
incomplete and codominance?
 Incomplete-heterozygous phenotype that
“appears” to be a blend of both
homozygotes. (pink flowers)
 Codominance-heterozygote that equally
expresses the traits from both alleles.
(flowers with some red and white)
P Generation
Red
CRCR
Fig. 14-10-3
White
CWCW
CR
Gametes
CW
Pink
CRCW
F1 Generation
Gametes 1/2 CR
1/
CW
2
Sperm
1/
2
CR
1/
2
CW
F2 Generation
1/
2
CR
Eggs
1/
2
CRCR
CRCW
CRCW
CWCW
CW
Human Blood Types









Human blood types come from 3 different alleles.
Codominance and multiple alleles
Ia, Ib, I
Ia and Ib are codominant and produce an antigen (protein)
i-no antigen
Genotype-Ia,i=A blood type
Genotype Ib, i=B blood type
Genotype ii=O blood type
Book pg 205
Fig. 14-11
Allele
IA
IB
Carbohydrate
A
B
i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Genotype
Red blood cell
appearance
Phenotype
(blood group)
IAIA or IA i
A
IBIB or IB i
B
IAIB
AB
ii
O
(b) Blood group genotypes and phenotypes
Human Blood Types
 Genotype-Ia,i=A blood type
 Genotype Ib, i=B blood type
 Genotype ii=O blood type
 Genotype Ia,Ib=AB blood type
 Book pg 205
 Donors-
Non-single gene genetics, II
 Pleiotropy: genes with multiple
phenotypic effect. Ex: sickle-cell
anemia
 Epistasis: a gene at one locus
(chromosomal location) affects the
phenotypic expression of a gene at
a second locus. Ex: mice coat
color
 Polygenic Inheritance: an additive
effect of two or more genes on a
single phenotypic character Ex:
human skin pigmentation and
height
Pleiotropy
 Genes have multiple phenotypic effects.
 Pea plants-the gene that effects flower color
also affects the color of the seed coating.
Polygenic Traits
 Human skin color result of four genes that
interact to produce a color range.
 Eye color-blue, green, brown-still do not
know all genes used in eye color.
Epistasis
 Expression of one gene can interfere with
the expression of other genes.
 Fur color in mammals-2 genes give general
color, 2 genes give shading
 5th gene-interfers with the expression of
other genes (albinism)
Human disorders
 The family pedigree
 Recessive disorders:
•Cystic fibrosis
•Tay-Sachs
•Sickle-cell
 Dominant disorders:
•Huntington’s

Achondroplasia
 Testing:
•amniocentesis
•chorionic villus
sampling (CVS)
Fig. 14-15c
1st generation
(grandparents)
Ff
2nd generation
(parents, aunts,
and uncles)
FF or Ff ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd generation
(two sisters)
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?
Fig. 14-16
Parents
Normal
Aa

Normal
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
Fig. 14-17
Parents
Dwarf
Dd

Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
d
Dd
Dwarf
Eggs
Normal
dd
Normal
Fetal Screening