Trace an expressing father’s X contribution to daughters in the usual chromosome model
Follow the chromosome route: paternal X to daughters, paternal Y to sons in this model.
Section B16.3 · Monohybrid inheritance
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall chromosomes, genes and alleles, meiosis, haploid gametes and fertilisation restoring two chromosome sets.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What can the supplied XNXN mother contribute?
What you will learn
- Construct XNXN crossed with XnY using one contribution per parent
- Explain daughter carrier status and the absence of direct father-to-son X transmission
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Parents | XNXN × XnY |
| Daughters | XNXn; unaffected carriers under the rule |
| Sons | XNY; unaffected |
| Scope | The non-carrier maternal genotype is supplied, not inferred from appearance |
Construct XNXN crossed with XnY using one contribution per parent
This lesson supplies an unaffected non-carrier mother XNXN and an expressing father XnY in the usual XX/XY X-linked recessive model. The mother supplies XN at the tracked locus in every normal gamete. The father can supply Xn or Y. Combining those contributions gives XNXn daughters or XNY sons; repeated XN rows represent the same available allele type.
Every model daughter receives the father’s Xn and the mother’s XN, making her an unaffected carrier under the supplied rule. Every model son receives the father’s Y and the mother’s XN, so he does not express the feature. All possible offspring are unaffected in this specific cross, but daughters and sons have different allele records and carrier interpretations.
Explain daughter carrier status and the absence of direct father-to-son X transmission
The father’s X-linked n contribution goes to daughters rather than directly to sons in this usual model. A father and a son can both express an X-linked recessive feature in another family model if the mother supplies n to that son; shared expression would not mean the father passed his X chromosome to him. Follow the parental chromosome contributions instead of inferring a route from visual similarity.
The result depends on the supplied non-carrier maternal genotype XNXN. An unaffected mother could instead be XNXn under the classroom rule, giving different possibilities. State the genotype and expression assumptions before predicting. The diagram is an original model of transmission and does not guarantee observed sex counts or establish a real family’s genotype or clinical status.
Sources for this lesson
Pause and explain
What is the predicted daughter genotype here?
Worked example
For XNXN × XnY, predict daughter and son genotypes in the usual supplied model.
Show the worked solution
- The mother supplies XN in each gamete; the father supplies Xn to daughter positions and Y to son positions.
- Daughters are XNXn and unaffected carriers; sons are XNY and unaffected. The father does not pass his Xn directly to sons in this model.
Answer XNXN × XnY predicts XNXn daughters and XNY sons. The father’s Xn goes to daughters; sons receive his Y in the usual supplied model.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
List the possible offspring genotypes and expression categories for XNXN × XnY.
Give me a hint
Combine maternal XN with paternal Xn or Y.
Compare my reasoning
- Form XNXn daughters.
- Form XNY sons.
- Apply the supplied expression key.
Daughters XNXn are unaffected carriers; sons XNY are unaffected under the rule. No expressing offspring is predicted in this particular normal model.
Look for these in your work
- Correct daughter and son genotypes
- Correct expression and carrier distinction
For 40 expected offspring with equal model sex probabilities, give daughter-carrier and unaffected-son counts.
Give me a hint
Each of the two grouped categories has probability one half.
Compare my reasoning
- Calculate 40 × 1/2 for daughters.
- Calculate 40 × 1/2 for sons.
Expected 20 carrier daughters and 20 unaffected sons; both groups are unaffected in this cross under the rule.
Look for these in your work
- Correct twenty and twenty expected counts
- Carrier status not counted as trait expression
A student sees an expressing father and son in another unspecified family and concludes the father passed his Xn directly to the son. Explain the usual-model contribution rule and the missing maternal information.
Give me a hint
A model son receives paternal Y and maternal X.
Compare my reasoning
- Trace Y from father to son.
- Trace the son’s X to the mother.
- Request the maternal allele record.
In the usual XX/XY model a son receives his father’s Y and his mother’s X. A son’s n could come from the mother; shared father-son expression does not demonstrate direct father-to-son X transmission, and the maternal genotype is needed.
Look for these in your work
- Correct parental chromosome route
- Explains why maternal evidence matters
Common mistakes
- Calling all offspring genotype-identical because all lack the feature
- Passing the father’s X directly to a model son
- Assuming every unaffected mother is a non-carrier
What can you explain now?
What is the predicted daughter genotype here?
Compare with the explanation
XNXn, an unaffected carrier under the rule
She receives XN from the mother and Xn from the father.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, construct xnxn crossed with xny using one contribution per parent and explain daughter carrier status and the absence of direct father-to-son x transmission. Use the supplied allele key and state what the evidence can establish.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.