International A Level Biology / Gene expression and genetics
Genetic terms and monohybrid crosses revision notes
Edexcel International A Level Biology (YBI11), chapter 6: Gene expression and genetics, specification point 2.15. Free lesson.
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A monohybrid cross follows the inheritance of one gene. Knowing the genetic terms and setting out each stage clearly helps you predict offspring and give the correct phenotype ratio.
How a gene differs from an allele
A gene is a sequence of DNA bases that codes for a polypeptide. It has a fixed position, called a locus, on a chromosome. A homologous pair consists of one chromosome from each parent. The chromosomes carry the same genes in the same order, but their versions of a gene can differ. For pea seed shape, R is an allele for round seeds and r is an allele for wrinkled seeds. Both alleles are versions of the same gene, found at the same locus. So there is one gene for seed shape, with two possible alleles.
What the genetic terms mean
A genotype is the alleles an organism has. A phenotype is an observable characteristic, which results from the genotype and the environment together. For seed shape, the phenotype is round or wrinkled. A dominant allele is expressed when it is present, even as one copy. So Rr plants have round seeds. A recessive allele is expressed only when there is no dominant allele. So only rr plants have wrinkled seeds. A homozygote has two identical alleles: RR or rr. A heterozygote has two different alleles: Rr. A round-seeded plant could be RR or Rr.…
How to set out a monohybrid cross
A monohybrid cross follows one gene at a time. Set out the stages in order so the examiner can follow how you reach the answer.
Predicting offspring from Rr × Rr
The gametes from one parent go across the top of the Punnett square, and those from the other parent go down the side. Combining one allele from each gamete gives four possible offspring genotypes. RR is homozygous dominant and has round seeds. Rr has round seeds because R is dominant. rr has wrinkled seeds because it has no dominant allele.
Giving the phenotype ratio
For Rr × Rr, three of the four possible offspring have round seeds and one has wrinkled seeds. The genotype ratio is different from the phenotype ratio, so check which one the question asks for. These are expected ratios because which gametes meet is due to chance. Genotype ratio: 1 RR : 2 Rr : 1 rr. Phenotype ratio: 3 round : 1 wrinkled. Mendel counted 5474 round and 1850 wrinkled seeds, which is close to the expected 3 : 1 ratio.
Using offspring to find a parent's genotype
A wrinkled-seeded parent must be rr, so it can only pass on r. If any offspring are wrinkled, they must get another r from the round-seeded parent. This can help you work out that parent's genotype. If half the offspring from a round × wrinkled cross are wrinkled, the round parent is Rr. The cross Rr × rr gives Rr round and rr wrinkled offspring, in a 1 round : 1 wrinkled ratio.