Download Complex Inheritance

Page 3 left Column
Types of Complex
Inheritance
1.
2.
3.
4.
Incomplete Dominance
Codominance
Multiple Alleles
Polygenic Traits
Incomplete dominance
• Both alleles partially show by blending
together
– Analogy: Mixing Together Paint
Yellow
Blue
Green
codominance
Both alleles are completely
expressed at the same time.
-Analogy: Mixing Together Pebbles
Yellow
Blue
Yellow AND Blue
Multiple alleles
• Having more than two alleles for a
specific trait
– Ex. An example is blood groups in humans.
Blood type, for example, is determined by
three alleles.
• A is codominant IA
• B is codominant IB
• O is recessive i
Blood Types
Alleles
(Genotype)
AA or Ao
IAIA or IAi
Blood Type
(Phenotype)
Type A
BBor Bo
IBIB or IBi
Type B
AB only
IAI B
Type AB
oo only
ii
Type O
Polygenic Traits
Traits that are
determined by the
interaction of
multiple pairs of
genes
Ex: skin color,
height
Page 4 left Column
Multiple Allele Traits:
Blood Type
Blood type exhibits codominance and multiple
alleles. The chart below shows the phenotype
and genotype of blood types.
Phenotype
Genotype
Type A
AA or Ao
IAIA or IAi
Type B
BB or Bo
IBIB or IBi
Type AB
AB
IA IB
Type O
oo
ii
Blood Type is controlled by 3 alleles: A, B, O. A &
B are codominant, O is recessive.
1. a) What are the two genotypes possible
for a person who as A blood? IAIA or IAi
b) What genotype does a person with AB
blood have? IAIB
c ) What genotype does a person with O
blood have? ii
d) What are the two genotypes possible
for a person who as B blood? IBIB or IBi
Page 5 left Column
Sex-Linked
Which Chromosome Determines
our Sex?
• Introduction: A human has 23 pairs of
homologous chromosomes
– Pairs 1-22 are called Autosomes –
chromosomes not involved in determining sex
– Pair 23 – Sex Chromosomes
How is Sex Determined?
Sex chromosomes
are X and Y
Females are XX
Males are XY
How is Sex Determined?
• Females can only
make eggs with the X
chromosome
• Males make sperm
with the X or Y
chromosome, so it is
the male that
determines the sex of
a baby
How is Sex Determined?
50% chance male
50% chance female
Discovery of Sex-Linked Genes
• Discovered in fruit flies (drosophila
melanogaster) by Thomas Hunt Morgan
Discovery of Sex-Linked Genes
• Eye color is sex-linked in fruit flies. He
discovered that alleles for eye color is
carried on the X chromosome ONLY!
• NO ALLELES ON THE Y CHROMOSOME
Why Fruit Flies?
• Easy to maintain
• 10 – 15 day life cycle
• Many offspring produce in a short time
Sex-Linked Traits
Traits that are controlled by the sex
chromosomes
Colorblindness – can’t see the difference
between certain colors
Hemophilia – blood does not clot properly
Example
H = Normal
h = hemophilia
Important Rule: Only write the gene over
the X chromosome
Female Genotypes
XHXH : Normal Female (Homozygous Dominant)
XHXh :
Normal Female Carrier (Heterozygous)
XhXh :
Female with hemophilia (Homozygous
Recessive)
Male Genotypes
• XHY :
normal male without hemophilia
• XhY :
male with hemophilia
• Sex-linked traits occur more in males than
females because
– Males only need one “bad” gene on the X
chromosome
– But females need two “bad” genes on both X
chromosomes