1. Sex-Linked Traits &
Pedigrees

2. Linked Genes
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.

3. Autosomal vs. Sex-Linked
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

4. Human Karyotype Picture of Human Chromosomes 22 Autosomes and 2 Sex Chromosomes

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

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

9. What this means… X-linked traits are passed from mother to son.
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.

10. copyright cmassengale
Mendelian Genetics
8/2/2018
Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
XX chromosome - female
XY chromosome - male
fruit fly
eye color
copyright cmassengale

11. Sex-linked Trait Problem
Mendelian Genetics
Sex-linked Trait Problem
8/2/2018
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female) XRY x XrXr
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 XR Xr Y
copyright cmassengale

12. Sex-linked Trait Solution:
Mendelian Genetics
8/2/2018
Sex-linked Trait Solution: XR Xr Y
50% red eyed female
50% white eyed male
XR Xr
Xr Y
copyright cmassengale

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

14. Heterozygote (has the recessive allele but the trait is NOT expressed)
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.

15. copyright cmassengale
Mendelian Genetics
8/2/2018
Sex-Linked Pedigrees
copyright cmassengale

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

17. 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

18. Recessive Alleles
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)

19. Autosomal Traits
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)

22. Sex-Linked Traits 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

23. Y-Linked Traits No affected females
ALL sons of affected males are also affected

24. X-Linked Recessive Traits
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

25. X-Linked Recessive

27. X-Linked Dominant Traits
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