Why Colour Blindness Turns Up Far More Often in Boys Than in Girls
Tell autosomes from sex chromosomes, see how an XX and XY cross decides whether a baby is a boy or a girl, follow the inheritance of haemophilia and colour blindness, and understand how mutation and variation differ.
How do chromosomes decide a baby's sex and pass on some disorders?
Every human body cell carries 46 chromosomes in 23 pairs. One of those pairs is different from the rest: it decides whether a person is male or female, and it also carries genes for features such as colour vision and blood clotting.
This part covers autosomes and sex chromosomes, sex determination, X-linked disorders, and the difference between mutation and variation.
This part covers autosomes and sex chromosomes, sex determination, X-linked disorders, and the difference between mutation and variation.
What is the difference between autosomes and allosomes, and how many chromosomes do humans have?
Humans have 46 chromosomes in 23 pairs: 22 pairs of autosomes that are alike in males and females, and one pair of allosomes, or sex chromosomes, which is XX in females and XY in males.
Autosomes
- 22 pairs, the same in both sexes
- Carry genes for body characters such as height and skin colour
Allosomes (sex chromosomes)
- 1 pair that determines sex
- Females: XX — two similar chromosomes
- Males: XY — the X is large and carries many genes; the Y is small and carries few genes, including one that decides maleness
Writing the chromosome set:
Gametes carry half: an ovum has ; a sperm has or .
Worked example. How many autosomes are in a human sperm? .
An everyday example. A genetics laboratory preparing a karyotype photographs a cell's chromosomes and arranges them in pairs, with the sex chromosomes placed last.
The boundary case. Mature red blood cells have no nucleus, so they carry no chromosomes at all.
Autosomes
- 22 pairs, the same in both sexes
- Carry genes for body characters such as height and skin colour
Allosomes (sex chromosomes)
- 1 pair that determines sex
- Females: XX — two similar chromosomes
- Males: XY — the X is large and carries many genes; the Y is small and carries few genes, including one that decides maleness
Writing the chromosome set:
Gametes carry half: an ovum has ; a sperm has or .
Worked example. How many autosomes are in a human sperm? .
An everyday example. A genetics laboratory preparing a karyotype photographs a cell's chromosomes and arranges them in pairs, with the sex chromosomes placed last.
The boundary case. Mature red blood cells have no nucleus, so they carry no chromosomes at all.
How is sex determined in human beings using a cross between XX and XY?
The mother always passes on an X chromosome, while the father passes on either an X or a Y, so the sperm decides the sex: X from the father gives a girl and Y gives a boy, each with an equal chance.
The cross:
- Mother (XX) makes ova that all carry X
- Father (XY) makes sperms, half carrying X and half carrying Y
Combinations:
- X ovum + X sperm gives XX — girl
- X ovum + Y sperm gives XY — boy
An everyday example. Blaming a mother for the birth of a daughter is scientifically wrong, since her ova always carry X; the chromosome from the father's sperm decides the sex.
The misconception. Each birth is independent. A couple with two daughters still has a chance of exactly that the next child is a son — earlier births do not change it.
The cross:
- Mother (XX) makes ova that all carry X
- Father (XY) makes sperms, half carrying X and half carrying Y
Combinations:
- X ovum + X sperm gives XX — girl
- X ovum + Y sperm gives XY — boy
An everyday example. Blaming a mother for the birth of a daughter is scientifically wrong, since her ova always carry X; the chromosome from the father's sperm decides the sex.
The misconception. Each birth is independent. A couple with two daughters still has a chance of exactly that the next child is a son — earlier births do not change it.
How are haemophilia and colour blindness inherited, and why are they commoner in males?
Haemophilia and colour blindness are caused by recessive alleles on the X chromosome; a male has only one X, so a single faulty allele affects him, while a female needs two and is usually only a carrier.
The disorders:
- Haemophilia — blood does not clot properly, so even small wounds bleed for a long time
- Colour blindness — commonly, red and green cannot be told apart
Why males are affected more often:
- Females (XX): need the faulty allele on both X chromosomes; with one, they are carriers with normal vision or clotting
- Males (XY): the Y has no matching allele, so one faulty X is enough
Worked cross — carrier mother × normal father. Write for the colour-blind allele.
- ovum + sperm gives — carrier daughter
- ovum + sperm gives — normal daughter
- ovum + sperm gives — colour-blind son
- ovum + sperm gives — normal son
So half the sons are expected to be colour-blind, and no daughter is affected, though half are carriers.
An everyday example. Some careers, such as those in railways or aviation, require a colour vision test using patterns of coloured dots.
The substance. A son never inherits an X-linked disorder from his father, because the father gives him a Y chromosome.
The disorders:
- Haemophilia — blood does not clot properly, so even small wounds bleed for a long time
- Colour blindness — commonly, red and green cannot be told apart
Why males are affected more often:
- Females (XX): need the faulty allele on both X chromosomes; with one, they are carriers with normal vision or clotting
- Males (XY): the Y has no matching allele, so one faulty X is enough
Worked cross — carrier mother × normal father. Write for the colour-blind allele.
- ovum + sperm gives — carrier daughter
- ovum + sperm gives — normal daughter
- ovum + sperm gives — colour-blind son
- ovum + sperm gives — normal son
So half the sons are expected to be colour-blind, and no daughter is affected, though half are carriers.
An everyday example. Some careers, such as those in railways or aviation, require a colour vision test using patterns of coloured dots.
The substance. A son never inherits an X-linked disorder from his father, because the father gives him a Y chromosome.
What is the difference between mutation and variation, and why does each matter?
Variation is any difference between individuals of the same species, while a mutation is a sudden, inheritable change in a gene or chromosome; mutations create new alleles, and variation provides the raw material for evolution.
Variation
- Differences in height, skin colour, blood group and many other features
- Causes: shuffling of alleles in sexual reproduction, mutations, and the environment
- Importance: helps a species adapt to changing conditions and is the basis of natural selection and plant and animal breeding
Mutation
- A sudden change in the DNA of a gene, or in chromosome structure or number
- Causes: copying errors, radiation such as X-rays and ultraviolet light, and some chemicals
- Examples: sickle-cell anaemia from a gene change; Down syndrome from an extra chromosome 21
- Importance: the only source of completely new alleles; most are harmful or neutral, a few are useful
An everyday example. Insects in a field sprayed with the same pesticide season after season can become resistant, because the rare individuals with a resistance mutation survive and multiply.
The boundary case. Variation acquired during life is not inherited — a wrestler's strong muscles are not passed to his children, since the DNA of his gametes is unchanged.
Variation
- Differences in height, skin colour, blood group and many other features
- Causes: shuffling of alleles in sexual reproduction, mutations, and the environment
- Importance: helps a species adapt to changing conditions and is the basis of natural selection and plant and animal breeding
Mutation
- A sudden change in the DNA of a gene, or in chromosome structure or number
- Causes: copying errors, radiation such as X-rays and ultraviolet light, and some chemicals
- Examples: sickle-cell anaemia from a gene change; Down syndrome from an extra chromosome 21
- Importance: the only source of completely new alleles; most are harmful or neutral, a few are useful
An everyday example. Insects in a field sprayed with the same pesticide season after season can become resistant, because the rare individuals with a resistance mutation survive and multiply.
The boundary case. Variation acquired during life is not inherited — a wrestler's strong muscles are not passed to his children, since the DNA of his gametes is unchanged.
Exam tip
What earns full marks on sex determination and X-linked inheritance?
Always write the full chromosome symbols, show gametes before offspring, and state the result for sons and daughters separately.
- Give the human set as and
- Show the sex-determination cross with both kinds of sperm
- Mark the allele on the X, such as or
- Separate sons from daughters in the result, naming carriers
- Define mutation and variation and give one example each
The trap. Writing a carrier female as affected. A heterozygous female has normal clotting or vision; she only passes the allele on.
- Give the human set as and
- Show the sex-determination cross with both kinds of sperm
- Mark the allele on the X, such as or
- Separate sons from daughters in the result, naming carriers
- Define mutation and variation and give one example each
The trap. Writing a carrier female as affected. A heterozygous female has normal clotting or vision; she only passes the allele on.
Did you know
Why are tortoiseshell cats almost always female?
Cats with patches of both orange and black fur are nearly always female.
In cats, the gene for orange or black fur lies on the X chromosome. A female with two X chromosomes can carry one allele for each colour, and different patches of skin use different X chromosomes, giving the patchwork coat.
A normal male has only one X, so he is orange or black but not both. The rare male tortoiseshell has an extra X chromosome — a chromosome change of the kind described in this lesson.
In cats, the gene for orange or black fur lies on the X chromosome. A female with two X chromosomes can carry one allele for each colour, and different patches of skin use different X chromosomes, giving the patchwork coat.
A normal male has only one X, so he is orange or black but not both. The rare male tortoiseshell has an extra X chromosome — a chromosome change of the kind described in this lesson.
Exam relevance
How do sex determination and X-linked disorders lead into NEET Biology?
This is foundation work for Class 12 Principles of Inheritance and Variation and Evolution in NEET Biology.
What gets built on. Principles of Inheritance and Variation compares sex determination in humans with XO in grasshoppers, ZW in birds and haplodiploidy in honeybees, uses pedigree analysis for disorders, and separates Mendelian disorders such as haemophilia, colour blindness and sickle-cell anaemia from chromosomal disorders such as Down syndrome, Klinefelter syndrome (XXY) and Turner syndrome (XO). Evolution builds on variation and natural selection.
Question types. Pedigree charts, crosses predicting affected sons and carrier daughters, and match-the-column items on disorders.
The trap that costs marks. In birds, the female is ZW, so the egg, not the sperm, decides the sex.
What gets built on. Principles of Inheritance and Variation compares sex determination in humans with XO in grasshoppers, ZW in birds and haplodiploidy in honeybees, uses pedigree analysis for disorders, and separates Mendelian disorders such as haemophilia, colour blindness and sickle-cell anaemia from chromosomal disorders such as Down syndrome, Klinefelter syndrome (XXY) and Turner syndrome (XO). Evolution builds on variation and natural selection.
Question types. Pedigree charts, crosses predicting affected sons and carrier daughters, and match-the-column items on disorders.
The trap that costs marks. In birds, the female is ZW, so the egg, not the sperm, decides the sex.
Key takeaways
What must you be able to do from this part?
- 46 chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes
- Female ; male
- Ova carry X; sperms carry X or Y, so the father's sperm decides sex
- Chance of a boy or girl: each birth, independently
- Haemophilia and colour blindness: recessive alleles on X
- Males affected with one allele; females need two and are usually carriers
- Carrier mother × normal father: half the sons affected, half the daughters carriers
- Variation: differences between individuals; mutation: sudden inheritable change in DNA
Work out on paper what happens when a colour-blind father and a mother with normal genes have children, and check whether any son could be colour-blind.
- Female ; male
- Ova carry X; sperms carry X or Y, so the father's sperm decides sex
- Chance of a boy or girl: each birth, independently
- Haemophilia and colour blindness: recessive alleles on X
- Males affected with one allele; females need two and are usually carriers
- Carrier mother × normal father: half the sons affected, half the daughters carriers
- Variation: differences between individuals; mutation: sudden inheritable change in DNA
Work out on paper what happens when a colour-blind father and a mother with normal genes have children, and check whether any son could be colour-blind.