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3.3.3Topic 3 · Voice of the Genome

Genes and Linkage — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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3.3.3 Genes and Linkage — Edexcel A Level Biology A (SNAB) 9BI0 notes: Define gene, locus and allele, explain autosomal and sex linkage and use crosses and ratios to identify linked genes.A gene is a length of DNA that codes for a polypeptide (or functional RNA).Each gene occupie…

  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Recombinant
  • Hemizygous
  • Chi-squared test

Section 1 of 9Genes, loci and alleles

A gene is a length of DNA that codes for a polypeptide (or functional RNA).

Genes, loci and alleles
Locus
Each gene occupies a fixed position, or locus, on a chromosome.
Alleles
Different forms of the same gene are called alleles.
In a diploid organism chromosomes come in homologous pairs, one inherited from each parent,
  • and a homologous pair carries the same genes at the same loci, though perhaps different alleles.
A person with two identical alleles is homozygous; one with two different alleles is heterozygous.
Genotype
The genotype is the alleles present and the phenotype is the observable characteristic, resulting from the genotype and the environment.
A dominant allele is expressed in the heterozygote; a recessive allele is only expressed when homozygous.

Section 2 of 9Monohybrid and dihybrid crosses

For genes on different chromosomes, alleles assort independently at meiosis (independent assortment).

Monohybrid and dihybrid crosses
A cross between two individuals heterozygous at two genes (AaBb x AaBb) therefore gives a phenotypic ratio of 9 : 3 : 3 : 1 in the offspring when each gene shows complete dominance.
A test cross with a double recessive (AaBb x aabb) gives 1 : 1 : 1 : 1.
  • A monohybrid cross between two heterozygotes (Aa x Aa) gives 3 dominant : 1 recessive, and a test cross of a heterozygote gives 1 : 1.

Section 3 of 9Autosomal linkage

Genes located on the same chromosome are said to be linked, and they tend to be inherited together because they travel on the same chromosome through meiosis.

Autosomal linkage
For linked genes the 9 : 3 : 3 : 1 ratio is not obtained.
If the parent is AaBb with A and B on one chromosome and a and b on the other,
  • and there is no crossing over, only two types of gamete (AB and ab) are produced,
  • so the offspring show a 3 : 1 ratio in a cross to another heterozygote, or 1 : 1 in a test cross.

Crossing over between non-sister chromatids in prophase I may separate the alleles, producing a small number of recombinant gametes (Ab and aB).

Autosomal linkage
Offspring with the recombinant phenotypes appear in low numbers.
The closer two loci are on a chromosome, the less likely that crossing over will occur between them, so the fewer recombinants.
Recombination frequency can therefore be used to map genes: recombinants / total offspring x 100 gives the map distance in map units (centimorgans).

Section 4 of 9Sex linkage

In humans, females are XX and males XY.

The X chromosome is larger and carries many genes that have no equivalent on the short Y chromosome.
Sex linkage
A gene on the X chromosome shows sex linkage.
Males have only one X so a single recessive allele is expressed (they are hemizygous),
  • meaning that X-linked recessive conditions, such as red-green colour blindness and haemophilia A, are much more common in males.
Sex linkage
An affected male inherits the allele from his mother, who is a carrier or affected.
He passes the allele to all daughters (who become carriers) but to none of his sons.
A carrier mother (XHXh) with an unaffected father has a 50% chance of an affected son, and a 50% chance of a carrier daughter.

Section 5 of 9Working with crosses and the chi-squared test

To find out whether the observed offspring match an expected ratio, use the chi-squared test: chi-squared = sum of (O - E)2 / E, where O is the observed and E the expected number.

1
The degrees of freedom are the number of categories minus one.
2
If the calculated value is greater than the critical value at p = 0.05, the difference is statistically significant,
  • so the null hypothesis is rejected, and linkage is a likely explanation.
    • (that the genes assort independently)
3
If it is lower, any difference is due to chance.
4
The test needs numbers rather than percentages or ratios and works best with large samples.

Always write out the key (for example N = normal, n = recessive), the parental genotypes, the gametes, the Punnett grid, and the offspring ratio.

Section 6 of 9Exam technique

For breeding questions, always use a genetic diagram with standard layout.

1
Show the parental phenotypes, genotypes, gametes (circled), the offspring genotypes and the ratio.
2
If the question says that two genes are linked, write the alleles on the same chromosome, for example AB / ab, to show clearly which alleles travel together.
3
In multiple-choice items, test each candidate ratio against the numbers given and choose the closest match,
  • then use chi-squared if the question requires a statistical decision.
4
State the null hypothesis, calculate, compare with the critical value at the stated degrees of freedom and write a conclusion in the context of the genes.

Section 7 of 9Exam-style questions

  1. A fruit fly heterozygous for body colour and wing length was crossed with a double recessive. Of 400 offspring, 185 were grey long, 180 black short, 17 grey short and 18 black long. Explain these results. [4]4 marks

  2. A man with normal colour vision and a woman who is a carrier for red-green colour blindness have children. Give the probability that a child is a colour-blind son. [3]3 marks

  3. Explain why X-linked recessive disorders are more common in males than in females. [2]2 marks

Section 8 of 9Key terms

TermMeaning
LocusThe fixed position of a gene on a chromosome.
AlleleOne of the different forms of a gene.
LinkageInheritance of genes together because they lie on the same chromosome.
RecombinantAn offspring (or gamete) with a combination of alleles different from that of either parent.
HemizygousHaving only one copy of a gene, as for X-linked genes in males.
CarrierA heterozygous individual who has a recessive allele but does not show the phenotype.
Chi-squared testA statistical test comparing observed with expected frequencies.

Section 9 of 9Exam technique

1
Using 'gene' and 'allele' as if they were the same. An allele is one version of a gene.
2
Expecting 9 : 3 : 3 : 1 whenever two genes are involved, forgetting that linkage changes the ratio.
3
Thinking a father can pass an X-linked allele to his son. Sons receive the Y from their father.
4
Using percentages in the chi-squared test instead of observed numbers.
5
Saying crossing over creates new alleles. It creates new combinations of existing alleles.

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