Sex Chromosomes
Sex Chromosomes
X-linked Traits
Possible genotypes
X+Y  Hemizygous wild type male
XmY Hemizygous mutant male
X+X+  Homozyogus wild female
X+Xm  Heterozygous female carrier
XmXm  Homozygous mutant female
X-linked Recessive Inheritance
 Always
expressed in hemizygous males
 Female homozygotes show the trait but
female heterozygotes do not
 Affected males: Inherited from affected or
heterozygous mother
 Affected females: affected fathers and
affected or heterozygous mothers
X-linked Dominant Inheritance
Expressed with one copy
 Males are often more severely affected
 Typically associated with miscarriage or
lethality in males
 Passed from father to all his daughters
but none of his sons

X-linked Dominant Inheritance:
Congenital Generalized
Hypertrichosis
Figure 6.10
Sex-limited traits
A trait that affects a structure or function
of the body that is present in only one of
the sexes.
 May be X-linked or autosomal
 Example: A gene affecting milk
production will not have an effect in
males. However, males can carry and
pass on the trait.
 Why do men have nipples anyway???

Sex-influenced traits
An allele is dominant in one sex but
recessive in the other sex.
 May be X-linked or autosomal
 Due to hormonal interactions

Men have testosterone
 Women have estrogen

X-inactivation

Females “turn off” one of their X
chromosomes in each cell


In order to be more equal to males who only
have one X chromosome
The X chromosome turned off in each
cell is random
X Chromosome Inactivation
Fur Color in Tortoiseshell Cats
Orange fur
Black fur
Manifesting Heterozygote

A carrier of an X-linked trait who expresses the
phenotype
 If a female is heterozygous for a recessive
trait, the dominant allele will usually mask the
recessive allele…


Unless the dominant allele is on the X chromosome
that was inactivated
Some cells will express the trait and others will
not, depending on which X chromosome is
inactivated
Multifactorial Traits
Genes and the Environment
Polygenic Traits
A trait is influenced by more than one
gene
 May be multifactorial (influenced by
environment)

Polygenic Traits are Continuously
Varying

Each gene in the polygenic trait contributes
to the phenotype to a varying degree

Example: Height


Polygenic (influenced by multiple genes)
Continuous
Pure Polygenic Trait -Eye Color
Figure 7.3
• The number of human eye color genes is unknown
• Analysis will probably reveal many genes
• Mice have more than 60 eye color genes
Analyzing Multifactorial Traits

Difficult, requires multiple techniques

Use human genome sequences, population,
and family studies

The frequency in a specific population =
Empiric risk

The amount of inheritance due to genes =
Heritability
Separating Genes and
Environment

Dizygotic twins:

Monozygotic twins:

Twins raised apart:

Adopted individuals:
Shared environment
and 50% of genes
Identical genotype, and
shared environment
Shared genotype but
not environment
Shared environment
but not genes
Concordance
•The percentage of pairs in which both
twins express the trait
•Used to determine heritability
•Assumes both types of twins share
similar environments
•MZ twins often share more similar
environments
Review