Lesson 8 of 11 / Inheritance
The usual XX and XY model
Which gamete contribution distinguishes XX from XY?
I can use a genetic diagram to explain the usual chromosomal determination of sex in humans.
I can explain usual XX/XY sex determination.Syllabus
Syllabus K325, Topic 14(g). Explain usual XX/XY sex determination.
About 4 min
Make a guess. You are not marked.
Which parent's gamete decides whether a baby is XX or XY?
Show the answer
The father's sperm.
Every egg carries an X. A sperm carries X or Y, so the sperm decides the outcome.
The key idea
In the usual XX/XY model, eggs carry X while sperm carry X or Y. Fertilisation gives an expected 1:1 ratio of XX to XY.
Use the usual XX parent and XY parent. Choose a possible offspring combination below.
| Gametes | X | Y |
|---|---|---|
| X |
Selected: XX -> XX (usual female). Each displayed combination has probability 1/2. The egg contributes X; the sperm contributes X or Y. The expected XX:XY ratio is 1:1, with each fertilisation a separate chance event in this usual model.
Expected outcomes: XX (usual female): 1/2; XY (usual male): 1/2
Rows: egg sex chromosome. Columns: possible sperm sex chromosomes. Boxes are equally likely combinations for the stated crosses, not a required sequence of offspring. Repeated genotypes can occur in different boxes. This shows the usual XX/XY model; real sex development can have biological variations.
Explanation
The usual human chromosomal model describes females as XX and males as XY. The other chromosomes are autosomes. Sex-chromosome inheritance can be followed using the same gamete-combination method as other genetic diagrams.
An XX parent produces eggs carrying one X chromosome. An XY parent produces sperm carrying either X or Y, with roughly equal probability in this simple model. Every gamete also carries a set of autosomes.
An X-bearing sperm combining with the egg gives XX, while a Y-bearing sperm gives XY. The expected ratio is 1 XX : 1 XY. The sperm contribution distinguishes the two outcomes, but it is a chance process rather than a decision or a reason to blame either parent.
An expected 1:1 ratio does not guarantee equal numbers in a small family or an alternating sequence. This is the usual syllabus model of chromosomal sex determination; actual sex development can involve biological variations beyond this simplified diagram.
Step by step
- 1
State parental chromosomes
Use XX and XY for the usual model.
- 2
List gamete contributions
The egg carries one X; a sperm carries either X or Y.
- 3
Combine and interpret
Give XX or XY and an expected equal probability under the model.
Worked example
Another child in the model
A family already has two XY children. Under the simple model, what is the probability of XX in another fertilisation?
One way to explain it
The probability remains 1/2. An X-bearing or Y-bearing sperm can fertilise the X-bearing egg independently of the previous outcomes.
Why this answer works
- Previous children do not change the egg contribution.
- The next gamete combination is a separate chance event.
Watch out for this
A student says: "The egg must switch between X and Y to balance a family." What is wrong with this?
Show the answer
In the usual model eggs carry X. Sperm carry X or Y, and earlier outcomes do not force the next one.
Check your understanding
Which combination produces XY in the usual human model?
Show the answer
An X-bearing egg and a Y-bearing sperm
Their chromosome contributions combine to give XY.
Exam-style questions
Check your understanding
Two parents are both carriers (Ff) of a recessive allele for cystic fibrosis. What is the probability that their next child is a boy with cystic fibrosis?
Show the answer
1/8
P(ff) = 1/4 and P(boy) = 1/2, so P(boy with ff) = 1/4 x 1/2 = 1/8.
Quick recall
Which gamete contribution distinguishes XX from XY?