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Inheritance

Biology: Cambridge International A Level, topic 16.2.2

Monohybrid Crosses

When two parents that differ in only one characteristic breed, the process is called a monohybrid cross. Monohybrid crosses allow the genotype of offspring to be predicted.

Parental genotype

Parental genotype

  • The first step in constructing a monohybrid cross involves identifying the parental genotypes.
  • E.g. Two true-breeding pea plants have yellow or green peas.
    • The dominant seed colour is green so the parental genotype is GG for green pea plants and gg for yellow pea plants.
Gamete alleles

Gamete alleles

  • Gametes are haploid, so only one allele from each parent is found in the gametes.
  • All possible combinations of the parental alleles should be identified. This represents the meiotic segregation into haploid gametes. In our pea plant example:
  • 100% of the gametes of green pea plants will have G alleles.
  • 100% of the gametes of yellow pea plants will have g alleles.
F1 offspring

F1 offspring

  • F1 offspring are the first generation of offspring.
  • A monohybrid cross produces four different combinations of possible offspring.
  • For the pea plants, both parents are homozygous. This means all the F1 offspring produced have a Gg genotype.
Gamete alleles

Gamete alleles

  • The F1 pea plants have two different alleles. They are heterozygous.
  • The gametes for an individual F1 offspring may contain either the G allele or the g allele.
  • 50% of an organism's gametes will contain the G allele.
  • 50% of an organism's gametes will contain the g allele.
F2 offspring

F2 offspring

  • F2 offspring are the second generation of offspring.
  • When the F1 pea plants breed, there are three possible genotypic combinations:
  • GG
  • Gg
  • gg
Predicting genotypic ratios

Predicting genotypic ratios

  • Monohybrid crosses allow predictions to be made about the genotypic and phenotypic ratios of offspring.
  • In the pea plant example, the ratio of yellow peas to green peas is 3:1. A monohybrid cross between two heterozygotes will always produce this ratio.
  • Monohybrid crosses can be drawn in two ways:
  • Genetic diagrams.
  • Punnett squares.

Dihybrid Crosses

When two parents that differ in two characteristics breed, the process is called a dihybrid cross.

Independent assortment

Independent assortment

  • Mendel proved that genes do not influence each other with regard to the sorting of alleles into gametes. This is called the law of independent assortment.
  • The law of independent assortment means that genes separate independently of each other when gametes are made.
  • The combination of alleles can be shown in a dihybrid cross.
Dihybrid gamete alleles

Dihybrid gamete alleles

  • In a dihybrid cross between two homozygotes, there is one possible gamete allele combination for each homozygote.
  • E.g. two pea plants differ in two characteristics: seed colour and seed texture. One plant has green, wrinkled seeds (yyrr) and one plant has yellow, round seeds (YYRR).
  • 100% of the gametes of the green/wrinkled plant are yr.
  • 100% of the gametes of the yellow/round plant are YR.
F1 offspring

F1 offspring

  • When two homozygotes breed, all the F1 offspring have the same genotype.
  • E.g. The offspring of the pea plants all have a YyRr genotype.
F1 gamete alleles

F1 gamete alleles

  • The law of segregation predicts that each gamete in F1 generation has an equal probability of receiving any allele (e.g. R, r, Y or y).
  • This means there are four possible combinations of gametes produced by the F1 offspring. For example:
  • YR.
  • Yr.
  • yR.
  • yr.
F2 offspring

F2 offspring

  • When the F1 offspring breed, the four possible gametes from one individual can combine with any of the four possible gametes from the other individual.
  • The total possible combinations in the F2 generation is 16.
Predicting phenotypic ratios

Predicting phenotypic ratios

  • Dihybrid crosses can be used in this way to predict genotypic ratios of the F2 offspring.
  • In the pea plant example, the ratio of offspring is:
  • Nine round/yellow.
  • Three round/green.
  • Three wrinkled/yellow.
  • One wrinkled/green.
  • When two dihybrid heterozygotes breed, the ratio is always expected to be 9:3:3:1.

Codominance

Codominance is where both alleles for the same characteristic are simultaneously expressed in the heterozygote. This can influence the outcome of monohybrid and dihybrid crosses.

Codominance

Codominance

  • Codominant alleles are both expressed in a heterozygote.
  • Neither of the alleles are recessive.
  • Codominance influences the phenotypic ratios of monohybrid and dihybrid crosses.
E.g. sickle-cell anaemia

E.g. sickle-cell anaemia

  • An example of codominance is sickle-cell anaemia.
  • There are two alleles for sickle-cell anaemia:
  • HN - normal haemoglobin.
  • HS - sickle haemoglobin.
Sickle-cell phenotypes

Sickle-cell phenotypes

  • People who have two copies of the HN allele (homozygotes) do not have sickle-cell anaemia.
  • People who have two copies of the HS alleles (homozygotes) do have sickle-cell anaemia.
  • People who have one HN allele and HS allele (heterozygotes) have both normal haemoglobin and sickled haemoglobin.
  • HN and HS are codominant.
Phenotypic ratios

Phenotypic ratios

  • Codominance affects the phenotypic ratios of monohybrid and dihybrid crosses.
  • E.g:
  • If two heterozygous (HNHS) breed, the ratio becomes 1:2:1 instead of the normal 3:1 that is expected in a monohybrid cross.

Test yourself on Inheritance

  • If a true-breeding yellow pea plant (YY) breeds with a true-breeding green pea plant (yy), what is the genotype of the offspring?

  • Which of the following statements about monohybrid crosses are true?

  • What is the predicted ratio of a monohybrid cross between two heterozygotes?

  • Which of the following statements about dihybrid crosses are true?

  • When heterozygote peas with yellow, round seeds (YyRr) interbreed, what are the resulting offspring?

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