A free printable sex-linked inheritance worksheet for Grade 11 biology.
Students explore sex determination, write sex-linked genotypes, and complete X-linked recessive Punnett squares for colour blindness, haemophilia, Duchenne muscular dystrophy and Morgan’s fruit flies. They also compare X-linked dominant and Y-linked traits, solve a combined autosomal and X-linked cross, and read a colour-blindness pedigree. Includes 24 questions (27 marks) and a full answer key.
This free sex-linked traits worksheet explains why some traits show up far more often in boys than in girls. Students start with how X and Y chromosomes decide sex, learn to write sex-linked genotypes such as XBXb, and then complete Punnett squares for colour blindness, haemophilia, Duchenne muscular dystrophy and Morgan's famous white-eyed fruit flies.
It goes further than most sheets, with X-linked dominant and Y-linked traits, a cross that combines an autosomal gene with an X-linked gene, and a colour-blindness pedigree. Every cross in the answer key has been computed and checked.
X-linked recessive crosses (colour blindness, haemophilia, DMD, fruit flies)
8
D
X-linked dominant and Y-linked traits
4
E
One autosomal gene + one X-linked gene
3
F
Reading a colour-blindness pedigree
3
G
Spot the mistake
2
This sex-linked traits worksheet supports the Ontario Grade 11 biology curriculum expectations in the Genetic Processes unit.
How Sex Is Determined
Females have two X chromosomes and males have one X and one Y. Every egg carries an X, but sperm carry either an X or a Y, so the father's sperm decides the baby's sex:
X
Y
X
XX
XY
X
XX
XY
Each child has a 1 in 2 chance of being a girl. On the Y chromosome, the SRY gene switches on male development. When meiosis goes wrong, unusual combinations like XXY can occur; the meiosis worksheet covers nondisjunction.
X-Linked Recessive Traits
Genes on the X chromosome are written as superscripts. For colour blindness, XB is normal vision and Xb is colour blindness. A male needs only one Xb to be colour-blind, but a female needs two. A female with one is a carrier. Here is a carrier woman crossed with a colour-blind man:
Xb
Y
XB
XBXb
XBY
Xb
XbXb
XbY
That gives 1 carrier daughter : 1 colour-blind daughter : 1 normal son : 1 colour-blind son. Half the daughters are colour-blind, which only happens when the father is affected. Part C of the sex-linked traits worksheet also covers haemophilia, Duchenne muscular dystrophy and Thomas Morgan's fruit fly experiment, which first linked a gene to the X chromosome. Read more about sex linkage on Wikipedia.
X-Linked Dominant and Y-Linked Traits
X-linked dominant traits, such as hypophosphatemic rickets, need only one allele in either sex. An affected father passes the trait to every daughter and no sons:
XA
Y
Xa
XAXa
XaY
Xa
XAXa
XaY
Y-linked traits are simpler still: they pass from a father to all of his sons and never to daughters. Comparing all three patterns side by side is the best way to stop students from mixing them up on a sex-linked traits worksheet.
Sex-Linked Traits in Pedigrees
Part F of the sex-linked traits worksheet uses this colour-blindness pedigree. The colour-blind grandfather's daughter (II-2) must be a carrier, and her son inherits the trait. It skips a generation through the female line, the classic X-linked recessive pattern.
For more practice, the pedigree worksheet compares X-linked pedigrees with autosomal ones.
Sex-Linked Traits Worksheet Practice Questions
These questions match the sex-linked traits worksheet PDF, which includes blank Punnett squares, a genotype table and the full-size pedigree.
Part A – Sex Determination
Which parent determines the sex of a child? Explain.
What is the probability that a child will be a girl? Show it with the Punnett square.
Which gene on the Y chromosome switches on male development?
Part B – Writing Sex-Linked Genotypes
Questions 4–9: write the genotype for each phenotype.
Male with normal vision
Colour-blind male
Female with normal vision (not a carrier)
Carrier female
Colour-blind female
Carrier male (write “not possible” if it cannot exist)
Why can a female be a carrier of colour blindness but a male cannot?
Part C – X-Linked Recessive Crosses
Colour blindness is X-linked recessive. A carrier woman has children with a colour-blind man. What fraction of their daughters will be colour-blind?
Haemophilia is X-linked recessive. A woman who is not a carrier has children with a man who has haemophilia. Describe the sons and daughters.
Duchenne muscular dystrophy (DMD) is X-linked recessive. A carrier mother and an unaffected father have a child. What is the probability the child is a son with DMD?
Thomas Morgan crossed a white-eyed female fruit fly with a red-eyed male. Red (XR) is dominant to white (Xr). Describe the offspring.
Part D – X-Linked Dominant and Y-Linked Traits
Hypophosphatemic rickets is X-linked dominant. An affected man has children with an unaffected woman. Describe the offspring.
Why does a father with an X-linked dominant trait never pass it to his sons?
A trait is Y-linked. Which children of an affected father will have it?
Part E – Autosomal + X-Linked Cross
In fruit flies, red eyes (XR) are dominant to white eyes (Xr), and normal wings (V) are dominant to vestigial wings (v) on an autosome. A V v XRXr female is crossed with a v v XrY male. What is the probability of a white-eyed male with vestigial wings? (2 marks)
What is the probability of a red-eyed fly with normal wings?
Part F – Reading a Pedigree
II-2 has normal vision. Explain why she must be a carrier.
I-1 is colour-blind, but his son II-3 is not. Explain.
What is the probability that III-2 is a carrier?
Part G – Spot the Mistake
A student says: “A colour-blind man will pass colour blindness on to his sons.” Explain the error and correct it.
A student says: “Females can’t be colour-blind.” Explain the error and correct it.
Sex-Linked Traits Worksheet Answer Key
Open each part of the sex-linked traits worksheet answers to check your work. The printable key with completed Punnett squares is at the end of the PDF.
Part A answers (Q1–3)
The father. Every egg carries an X, but a sperm carries either an X (girl) or a Y (boy).
1/2 (XX × XY gives 1 girl : 1 boy)
The SRY gene (sex-determining region Y)
Part B answers (Q4–10)
Male with normal vision: XBY
Colour-blind male: XbY
Female with normal vision (not a carrier): XBXB
Carrier female: XBXb
Colour-blind female: XbXb
Carrier male (write “not possible” if it cannot exist): Not possible: a male has only one X
Females have two X chromosomes, so a normal allele can hide a recessive one. Males have only one X, so any recessive allele on it is expressed.
Yes. The worksheet and its answer key are free to download and print as a PDF, with no account needed.
What is a sex-linked trait?
A sex-linked trait is controlled by a gene on a sex chromosome, usually the X. Examples include red-green colour blindness, haemophilia and Duchenne muscular dystrophy.
Why are males more likely to have X-linked recessive traits?
Males have only one X chromosome, so a single recessive allele is expressed. Females have two X chromosomes and need two recessive alleles to show the trait; with one, they are carriers.
How do you write genotypes for sex-linked traits?
Write the allele as a superscript on the X: XB for normal vision and Xb for colour blindness. A carrier female is XBXb and a colour-blind male is XbY.
Can a father pass an X-linked trait to his son?
No. A son inherits his father's Y chromosome, not his X. A father passes his X to every daughter instead.
Ready to start? Download the free sex-linked traits worksheet PDF (use the Download button above), print it double-sided, and keep the answer key pages for marking.