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17.7Inheritance

Sex linkage and colour blindness — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

5 min read 10 parts Lesson 8 of 9 2 quizzes

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Lesson summary

17.7 Sex linkage and colour blindness — IGCSE BIOLOGY notes: Describe sex determination and sex-linked inheritance, using red-green colour blindness as an example.Sex chromosomes — females are XX and males are XY.Eggs — every egg carries an X.Sperm — half carry X and half carry Y…

  • X egg + X sperm
  • X egg + Y sperm
  • Each pregnancy
  • Sex-linked characteristic
  • Linked alleles
  • Y chromosome

Part 1 of 8Sex determination

Sex chromosomes
  • Sex chromosomes females are XX and males are XY.
Eggs
  • Eggs every egg carries an X.
Sperm
  • Sperm half carry X and half carry Y.
X egg + X sperm
  • X egg + X sperm gives a girl.
X egg + Y sperm
  • X egg + Y sperm gives a boy.
Each pregnancy
  • Each pregnancy has a 50% chance of being a boy.

Part 2 of 8Sex-linked characteristics

Sex-linked characteristics
Sex-linked characteristic
Sex-linked characteristic a feature controlled by a gene located on a sex chromosome.
  • It is more common in one sex than the other.
Linked alleles
  • Linked alleles alleles on the same chromosome are said to be linked.
X chromosome
  • X chromosome almost all sex-linked alleles are on the X because the Y is much smaller.

Part 3 of 8Writing sex-linked alleles

Notation
  • Notation write the allele as a superscript on the X, XB for normal colour vision and Xb for colour blindness.
Y chromosome
  • Y chromosome carries no allele for this gene, so it is written as Y alone.
GenotypePhenotype
XBXBNormal female
XBXbCarrier female with normal vision
XbXbColour-blind female
XBYNormal male
XbYColour-blind male

Part 4 of 8Why more males are affected

Males
  • Males need only one recessive allele (XbY).
Females
  • Females need two recessive alleles, which is much less likely.
Y chromosome
  • Y chromosome cannot carry a dominant allele to mask the recessive one.
Other examples
  • Other examples haemophilia, a blood-clotting disorder, follows the same pattern.

Part 5 of 8Rules of inheritance

Son
  • Son always receives his X from his mother.
Father
  • Father passes his X to all his daughters and his Y to all his sons.
Colour-blind father
  • Colour-blind father cannot pass the condition to his son, but all his daughters are at least carriers.
Carrier female
  • Carrier female has a 50% chance of passing the recessive allele to each child.

Part 6 of 8Carrier mother with a normal father

ChildChance
Normal girl25%
Carrier girl25%
Normal boy25%
Colour-blind boy25%
1Probability a son is colour blind
  • 50%.
2
No daughter is colour blind because the father passes XB to all daughters.

Part 7 of 8Method for sex-linked crosses

1
Write the parental phenotypes, including the sex.
2
Write the genotypes with alleles on the X.
3
List the gametes, including Y.
4
Complete the Punnett square.
5
State each child's sex and phenotype, and the ratio or probability.

Part 8 of 8More from the specification

1
Sex-linked genes
  • Carried on the X chromosome; males (XY) express a recessive allele because they have no second copy.
2
Colour blindness
  • A recessive X-linked allele; a carrier mother and normal father give 50% of sons affected.
3
Codominance
  • Both alleles are expressed in the heterozygous individual, as in blood group IA IB.
4
Meiosis and offspring
  • Meiosis separates the sex chromosomes into gametes, so each offspring inherits an X from its mother and X or Y from father.
5
Genetic crosses
  • Use Punnett squares with symbols such as XH and Xh to show homozygous and heterozygous genotypes.
6
Specific
  • Able to act on only one type of substrate.
7
Heterozygous
  • Having two different alleles.
8
Blood group
  • A phenotype set by alleles I-A, I-B and I-O.
9
Protein synthesis
  • Making proteins from amino acids using the DNA code.
10
Monohybrid inheritance
  • Inheritance of one gene.
11
Mitosis
  • Cell division producing genetically identical cells.
12
Protein
  • A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
13
Meiosis
  • Reduction division giving four genetically different haploid cells.
14
Homozygous
  • Having two identical alleles.
15
Sequence
  • The order of bases in DNA or amino acids in a protein.

Key terms

TermMeaning
Sex chromosomeA chromosome that determines sex: X or Y.
Sex-linkedControlled by a gene on a sex chromosome.
CarrierA heterozygous female who has the recessive allele but is unaffected.
Red-green colour blindnessA sex-linked recessive condition affecting colour vision.
HaemophiliaA sex-linked recessive blood-clotting disorder.

Avoid these mistakes

1
Saying the mother decides the child's sex — the sperm carries X or Y.
2
Writing an allele on the Y — the Y has no allele for these genes.
3
Saying a colour-blind father passes it to his son — the son gets the father's Y.
4
Saying carrier females are affected — they have normal vision.

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