Unit 3: Sex Determination, Sex Linkage, and Pedigree Analysis
How chromosomes set an organism's sex, how genes on those chromosomes inherit differently as a result, and how family trees expose both patterns.
8 topics · 29 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Chromosomal Sex Determination Systems
- How it worksHow does the SRY gene on the Y chromosome trigger male development in mammals?
- Going furtherWhat changes about sex determination in birds, where females are ZW and males are ZZ instead?
- Common mistakeWhy isn't having two X chromosomes what actively makes a mammalian embryo develop as female?
- Worked exampleTrace the sex chromosomes present in a person with X0 Turner syndrome compared to a typical XX or XY karyotype.
Your turn1 of 2
A mammalian embryo with two X chromosomes (XX) develops as female. A student concludes that having two X chromosomes is what actively triggers female development, the same way the SRY gene actively triggers male development. What's wrong with that idea?X-Linked Inheritance and the Criss-Cross Pattern
- How it worksHow does an X-linked recessive allele pass from an affected father to all his daughters but none of his sons?
- Worked exampleFind the offspring ratio from crossing a white-eyed male fruit fly with a homozygous red-eyed female.
- Common mistakeWhy can't a father pass an X-linked trait directly to his son?
- Going furtherWhat happens to the cross ratios when the red-eyed female fly is heterozygous instead of homozygous?
Your turn1 of 2
A heterozygous red-eyed female fruit fly (carrier) is crossed with a red-eyed male. Out of 247 offspring, how many are expected to be white-eyed? (Only sons can show the recessive X-linked trait, and half of sons inherit the recessive X, so 1/4 of all offspring are affected.)X-Inactivation and Mosaic Phenotypes
- How it worksHow does one X chromosome get randomly silenced as a Barr body in every cell of a female embryo?
- Going furtherWhat happens to X-inactivation patterns in a female who inherits one X chromosome carrying a harmful mutation?
- Common mistakeWhy doesn't X-inactivation mean female mammals end up with less genetic material active than males?
Your turn1 of 2
In a heterozygous calico cat embryo, X-inactivation occurs randomly and independently in each of 76 early progenitor cell lineages, with a 50 percent chance the orange-allele X stays active in any given lineage. How many of the 76 lineages are expected to display the orange coat patch?Y-Linked Inheritance
- How it worksHow does the non-recombining region of the Y chromosome guarantee strict father-to-son transmission?
- Worked exampleTrace a Y-linked trait through four generations of a pedigree where every male descendant is affected.
- Common mistakeWhy can't a Y-linked trait ever appear in a daughter no matter what her mother carries?
Your turn1 of 2
A Y-linked trait is traced through a pedigree spanning 6 generations, with one son per generation continuing the direct paternal line. How many men in that direct paternal line are affected?Sex-Limited and Sex-Influenced Traits
- How it worksHow does a hormone environment let the same genotype produce different phenotypes in males and females?
- Common mistakeWhy isn't a sex-influenced trait like pattern baldness the same thing as an X-linked trait?
- Going furtherWhat happens to the dominance relationship of a sex-influenced allele when it moves from a male body to a female body?
- Worked exampleFind the expected phenotypes of male and female offspring who each inherit one copy of a sex-influenced pattern-baldness allele.
Your turn1 of 2
A rooster can carry the gene for a hen's egg-laying trait, pass it on to his daughters, and yet never show any hint of it himself, no matter his genotype. How is this different from a sex-influenced trait like pattern baldness, where a genotype's dominance simply flips between males and females?Pedigree Analysis of Autosomal Traits
- How it worksHow does tracing filled and unfilled symbols across a pedigree reveal whether a trait is dominant or recessive?
- Worked exampleFind whether a trait is autosomal dominant or autosomal recessive given two unaffected parents have an affected child.
- Common mistakeWhy doesn't an autosomal dominant trait have to appear in every single generation of a pedigree?
- Worked exampleFind the probability that two unaffected carrier parents have three unaffected children in a row for an autosomal recessive disorder.
- Going furtherWhat happens to a pedigree's pattern when an autosomal recessive disorder appears in a family with no history of consanguinity?
Your turn1 of 2
Two unaffected parents, both known carriers for an autosomal recessive disorder, have 5 children. What is the probability that all 5 children are unaffected?Pedigree Analysis of X-Linked Traits
- How it worksHow does an X-linked recessive pattern in a pedigree differ from an autosomal recessive one?
- Worked exampleFind the genotypes of every individual in a three-generation pedigree segregating X-linked color blindness.
- Going furtherWhat pattern would the same pedigree show if the trait were X-linked dominant instead of recessive?
Your turn1 of 2
A carrier mother for X-linked color blindness (heterozygous) and an unaffected father have 124 children. How many of the 124 children are expected to be color blind?Calculating Recurrence Risk from a Pedigree
- How it worksHow does combining a pedigree's genotype logic with the product rule produce a recurrence risk?
- Worked exampleFind the probability that a couple's next child is affected by an autosomal recessive disorder given both parents are known carriers.
- Worked exampleFind the probability that a woman with an affected brother but unaffected parents is a carrier for an X-linked recessive disease.
Your turn1 of 2
A couple are both known carriers for 4 different, independently inherited autosomal recessive disorders. What is the probability that their next child is affected by at least one of the 4 disorders?