Sex linkage
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of inheritance terms and sex chromosomes. You can test your knowledge on these below.
What do the terms dominant and recessive mean in genetics?
Dominant alleles are always expressed if present; recessive alleles are only expressed when there is no dominant allele present.
What is meant by genotype and phenotype?
Genotype is the genetic makeup (alleles present), while phenotype is the observable characteristics determined by genotype and the environment.
What sex chromosomes do males and females have in humans?
Males have XY, females have XX.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains sex linkage or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What Is Sex Linkage?
Sex linkage refers to when a gene is located on one of the sex chromosomes (usually the X chromosome in humans).
Since males and females have different sex chromosomes (XX for females, XY for males), genes on these chromosomes inherit differently between the sexes.
Differences Between X and Y Chromosomes
|
Feature |
X Chromosome |
Y Chromosome |
|
Size |
Large |
Small |
|
Number of Genes |
Many (over 1,000 genes) |
Very few (~100 genes) |
|
Presence in Sexes |
Two copies in females (XX), one in males (XY) |
Only in males |
As the sex chromosomes are different shapes and sizes, most of the X and Y chromosomes are non-homologous (they carry different genes).
Why Are Males More Likely to Inherit Sex-Linked Disorders?
-
Males only have one X chromosome.
-
If a male inherits a faulty allele on the X chromosome, he has no second copy to mask it.
-
Therefore, recessive alleles on the X chromosome are always expressed in males, even though they only have 1 copy.
In contrast, females have two X chromosomes and would therefore need two copies of the faulty allele (they would need to be homozygous recessive) for the disorder to be expressed - if they only have one copy, they are carriers but usually healthy.
Detailed Example
The allele for colour vision is carried on the part of the X chromosome that is missing from the Y chromosome.
There is a recessive allele that causes colour blindness.
Females inherit 2 alleles for colour vision, one on each X chromosome, if they inherit one faulty allele, they are unaffected.
Males only inherit one allele from their mother, if they inherit the faulty allele they are colour blind. As a result, colour blindness is more prevalent in males.
Examples of X-linked Disorders
|
Condition |
Effect |
|---|---|
|
Haemophilia |
Blood doesn't clot properly |
|
Red-Green Colour Blindness |
Difficulty distinguishing red from green |
|
Duchenne Muscular Dystrophy |
Progressive muscle weakness |
No answer provided.
Sex Linkage Inheritance Patterns
-
Use X with a superscript for the allele, e.g.
-
Xᴴ = normal clotting
-
Xʰ = haemophilia
-
Males are written as XᴴY or XʰY.
-
Females are written as XᴴXᴴ, XᴴXʰ, or XʰXʰ.
Example genetic cross with sex linkage
If a female cat inherits a black allele and a ginger allele she will be tortoiseshell. XBXG
A male cat can only inherit one of these alleles so he will be either black or ginger. XBY or XGY
Cross a black female cat with a ginger male cat. Remember the allele is sex linked (carried on the X chromosome.)
|
Parental phenotype |
Black female cat |
Ginger male cat |
|
Parental genotype |
XBXB |
XGY |
|
Gametes produced |
XB or XB |
XG or Y |
|
XB |
XB |
|
|
XG |
XBXG |
XBXG |
|
Y |
XBY |
XBY |
XBXG = Tortoiseshell female
XBY = Black male
1:1 ratio
Note - a male cat could never be tortoiseshell as they would not be able to inherit both the ginger and black alleles (as they only have one X chromosome)
Key Terms
- Sex Linkage: When a gene is located on a sex chromosome (usually X)
- Carrier: An individual with one normal and one faulty allele; does not show symptoms
- X Chromosome: The larger sex chromosome carrying many genes not related to sex
- Y Chromosome: The smaller sex chromosome with few genes, mainly involved in male development
No answer provided.
Exam Tips
In Punnett square questions about sex linkage:
Always include the sex chromosomes (X and Y), not just alleles.
Be careful: males cannot be carriers for X-linked disorders — they either have the disorder or they don't.
State clearly why males are more likely affected (only one X chromosome = only need to inherit one recessive allele to have a recessive, sex-linked disorder).
No answer provided.
Colour blindness is controlled by a gene on the X chromosome. The allele for colour blindness, Xb, is recessive to the allele for normal colour vision, XB . The gene controlling the presence of a white streak in the hair is not sex linked, with the allele for the presence of a white streak, H, being dominant to the allele for the absence of a white streak, h.
a) Explain why colour blindness is more common in men than in women. (2 marks)
b) The diagram shows a family tree in which some of the individuals have colour blindness or have a white streak present in the hair.
What is the probability that the first child of individuals 5 and 6 will be a colour blind boy with a white streak in his hair? Show your working. (4 marks)
a)
-
Males have only one X chromosome (and one Y chromosome).
-
A single recessive allele on the X chromosome will be expressed because there is no dominant allele to mask it.
b)
-
1 mark for correct parental genotypes
-
1 mark for parental gametes
-
1 mark for offspring genotypes correct in genetic cross
-
1 mark for correct probability
5 = no white streak and colourblind male = hhXbY
6 = white streak and normal vision female = HhXBXb
|
hXb |
hY |
|
|
HXB |
HhXBXb |
HhXBY |
|
HXb |
HhXbXb |
HhXbY |
|
hXB |
hhXBXb |
hhXBY |
|
hXb |
hhXbXb |
hhXbY |
HhXbY = colour blind boy with a white streak in his hair
1/8 or 0.125 or 12.5%
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!