GENETICS
Non-Mendelian Genetics
Linked Genes

Genes for 2 traits are located on the same
chromosome and travel together into the same
gamete
P
L
p
l
Crossing Over

New combinations of alleles in the offspring that
occur when neighboring segments of tetrads
break off where they meet and exchange genetic
material
Sex-Linked Genes
X-linked Genes: genes found only on the
X chromosome
 Y-linked Genes: genes found only on the
Y chromosome

X-Linked Genes


Many hereditary diseases are X-linked, meaning
the gene is found on the X chromosome
Ex: red-green color blindness
hemophilia
x
x
x
XX XX
Y XY XY
XX= normal female
XX= carrier female
XY= normal male
XY= affected male
Test for Red-Green Color
Blindness
Sex-Limited Traits




Expressed only in the presence of sex hormones
and are only observed in one sex or the other
Controlled by genes not found on the sex
chromosomes
Although both males and females have these
genes, they are only expressed by one sex since
the appropriate sex hormone must be present
EX: color in birds
beard growth in human males
Sex-Influenced Traits
Expressed in both sexes, but they are
expressed differently
 EX: Baldness

 In
the presence of the male sex hormone, the
allele is dominant
 In the presence of the female sex hormone,
the allele is recessive
Intermediate Inheritance
The heterozygous offspring have traits
that are not exactly like the trait of either
purebred parent
 There are 3 types of intermediate
inheritance:

 Incomplete
Dominance
 Codominance
 Polygenic Inheritance
Incomplete Dominance


In the heterozygous offspring, the dominant
allele does not completely mask the effect of
the recessive gene
The offspring shows a phenotype that is in
between the phenotype of the two
homozygous parents
Example of Incomplete
Dominance
Codominance


Both alleles in the heterozygote express
themselves fully
Ex: Blood Types
 The IA
& IB genes are both dominant, so if an
individual has one of each of these genes, their
blood type is AB
 The iO gene is recessive

EX: Feather Color
 White
(WW) and Black (BB) are both
dominant, so a cross between a white chicken
and a black rooster will produce a speckled
offspring (BW)
Inheritance by Multiple Alleles



When there are more than two alleles in a
population, these genes are said to have
multiple alleles
EX: blood type—there are 3 alleles in the
population: IA, IB & iO
Possible combinations:
Possible
Genotypes
IA IA
IA iO
IB I B
IB iO
Blood
Types
Type A
Type A
Type B
Type B
IA IB
iO iO
Type AB Type O
Polygenic Inheritance


Some traits are controlled by 2 or more genes
EX: eye color
Pleiotrophy
When a single gene affects more than
one trait
 EX: sickle cell anemia

 Changes
the cell shape
 Causes anemia
 Causes brain damage
 Causes spleen damage
 Causes heart damage
Body Temperature and Gene
Expression


In some organisms, body temperature can
affect the expression of genes
EX: Himalayan rabbit’s coat color
 The
colder body parts (i.e. feet, ears, nose) are
black
 The warmer body parts are white