Sex-Linked Traits & Pedigrees

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.

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

Human Karyotype 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

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.

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

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

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

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

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.

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

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

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

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)

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)

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

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

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

X-Linked Recessive

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