• Some genes are linked because they are on the
SAME chromosome .
• A chromosome is a set of linked genes.
• Remember: Offspring get one copy of each chromosome from each parent.
• Chromosomes assort independently, NOT individual genes.
• Crossing-over during meiosis can “un-link” genes.

Traits can be either:
• Autosomal : traits (genes) are located on the non-sex chromosomes
• Sex-Linked : traits (genes) are located on the sex chromosomes
• Sex chromosomes determine gender ( X & Y )
• XX genotype for females
• XY genotype for males
• Many sex-linked traits are carried on X chromosome

Picture of Human Chromosomes
22 Autosomes and 2 Sex Chromosomes

Examples of
Autosomal Traits
Autosomal Dominant:
Huntington’s Disease
Achondroplasia (Dwarfism)
Osteoporosis
Autosomal Recessive:
Cystic Fibrosis
Sickle Cell Anemia

Examples of
Sex-Linked Traits
X-linked Recessive:
Colorblindness
Hemophilia
Duchenne Muscular
Dystrophy
Y-linked:
Ear Hair

• Males have only one X chromosome, so ALL X-linked alleles are expressed in males, even if the trait is recessive.
• This is why males exhibit some traits more frequently than females.
• X-linked traits are passed from mother to son.
• Y-linked traits are passed from father to son.

Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes fruit fly eye color
XX chromosome - female copyright cmassengale
XY chromosome - male
10

• Example: Eye color in fruit flies
• (red-eyed male) x (white-eyed female)
X R Y x X r X r
• Remember: the Y chromosome in males does not carry an allele for the trait.
• RR = red eyed
• Rr = red eyed
• rr = white eyed
• XY = male
• XX = female
X R
X r X r
Y copyright cmassengale 11

Sex-linked Trait Solution:
X r X r
X R X R X r X R X r
Y X r Y X r Y
50% red eyed female
50% white eyed male copyright cmassengale 12

Pedigrees
A tool that can be used to trace genetic trait or disease throughout several generations.
Show ACTUAL results.

Carriers
• Heterozygote (has the recessive allele but the trait is NOT expressed)
• Shown in a Pedigree by shading half of the circle
(female) or square (male)
• In autosomal traits, carriers can be Male or
Female.
• In Sex-Linked traits, ONLY
Females can be carriers. All
Sex-Linked traits are expressed in Males.

Sex-Linked Pedigrees copyright cmassengale 15

You must figure out 2 things in order to correctly solve a problem involving a Pedigree:
1. Determine if trait is
Dominant or Recessive
2. Determine if trait is
Autosomal or Sex-Linked

Dominant Alleles
• Appears EVERY generation
• Expressed even in heterozygotes
• Will appear to be relatively common in a pedigree
• A dominant trait will not occur in an individual unless it also appears in at least one of the parents.
• Unaffected parents will have only unaffected offspring

• A recessive trait commonly skips one or more generations
• Frequently found in pedigrees that include marriage between close relatives
• If BOTH Parents are affected, ALL of the offspring will be affected
• If both parents are heterozygous
(dominant allele is expressed), each offspring:
– 50% chance of being heterozygous (carrier)
– 25% chance of being homozygous recessive (affected)

• If there is a male Heterozygote
(carrier), the trait MUST be autosomal .
• An autosomal trait can be passed on from father to son OR daughter
• Approximately the same number of males and females will be affected (especially if autosomal recessive)

• Must determine if the trait is
X-linked or Y-linked
• An X-linked trait can NEVER be passed from father to son
– Any example of father to son transmission means that the trait is
NOT X-linked
• Y-linked traits will NEVER affect females
– Any example of a female with the trait means that the trait is
NOT Y-linked

• No affected females
• ALL sons of affected males are also affected

• Both males and females can be affected, but the trait is much more common in males
• Affected males transmit the allele to ALL of their daughters & NONE of their sons
• Daughters of affected males are always be carriers (not affected) unless mother also has the allele
• All sons of an affected female will also be affected

• If the trait is dominant, it will be expected to occur slightly more often in females
• ALL daughters of an affected male will be affected (NO carriers)
• Can not be passed on from father to son