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One gene at a time: genetic diagrams that actually score

A monohybrid cross is four lines of working and a ratio. Most of the marks sit in the layout rather than the biology, and most of the marks lost are lost by skipping a line — usually the gametes, which is the line the examiner is looking for hardest.

Before this Meiosis and the origin of variation · The genetic code and mutation

Before you start

A 3 : 1 ratio means three out of every four offspring show the dominant characteristic. Read literally that would mean a family of four children with two heterozygous parents contains exactly one child with the recessive phenotype, which is plainly not how families work. The ratio is a statement about probability: each fertilisation independently has a 3 in 4 chance of producing the dominant phenotype, and only over large numbers of offspring does the count settle near 3 : 1. A pea plant produces hundreds of seeds and so obliges; a human couple has three children and need not.

What you should be able to do

The words, used precisely

Genetics questions are unusually strict about vocabulary, because the words do genuinely different jobs and an answer that swaps two of them stops making sense. A gene is a length of DNA coding for a polypeptide. An allele is one of the alternative versions of that gene. The locus is the fixed position on the chromosome where the gene sits — the same position on both members of a homologous pair, which is why you carry two alleles of every autosomal gene.

Genotype
The alleles an organism carries, for the gene or genes under discussion.
Phenotype
The observable characteristics of an organism, produced by its genotype interacting with its environment.
Homozygous
Carrying two identical alleles of a gene.
Heterozygous
Carrying two different alleles of a gene.
Dominant
An allele whose effect appears in the phenotype even when only one copy is present.
Recessive
An allele whose effect appears only when two copies are present.
Carrier
An individual heterozygous for a recessive allele, so not showing the characteristic but able to pass the allele on.

Two habits are worth forming now. Choose a letter whose capital and lower-case forms look different — T and t are safe, S and s and C and c are not — and use the same letter for both alleles of a gene, never one letter for the dominant and a different letter for the recessive. And say what your symbols mean before you use them. A line reading 'let T be the allele for tall and t the allele for dwarf' costs you eight words and secures a mark that is genuinely available on many papers.

The environment belongs in the definition of phenotype for a reason. Two genetically identical plants grown in different light give different heights; identical twins are not identical in mass. Where a question asks why two organisms of the same genotype look different, the environment is the answer it wants, and it is a mark most candidates walk past.

The layout that earns the marks

A genetic diagram is not a sketch. It is a piece of working with a fixed order, and each line is credited separately, which means a correct final ratio arrived at without showing gametes can score less than a wrong ratio with the working laid out.

The gametes are the line candidates skip and the line examiners look for. Circling them is the convention: it separates single alleles from the two-allele genotypes above and below, so nothing on the page is ambiguous.

The order is: parental phenotypes, parental genotypes, gametes, offspring genotypes, offspring phenotypes and ratio. Set the offspring out in a Punnett square unless you are confident with crossed lines; the square is harder to get wrong and easier for a marker to follow. Then read the ratio off the square rather than reciting it from memory, because the ratio you remember is the ratio for a cross you are not necessarily being asked about.

Three crosses cover almost everything at this level. Two heterozygotes give 1 TT : 2 Tt : 1 tt, and so 3 tall : 1 dwarf. A heterozygote crossed with a homozygous recessive gives 1 Tt : 1 tt, and so 1 : 1. A homozygous dominant crossed with anything gives all dominant offspring, because every offspring receives at least one dominant allele. Notice that the first of those is the only one where the genotype ratio and the phenotype ratio differ.

The second cross has a name and a job. A test cross pairs an organism showing the dominant phenotype with a homozygous recessive, to find out whether the first one is homozygous or heterozygous. If any offspring shows the recessive phenotype, the unknown parent must have carried a recessive allele and is heterozygous. If a large number of offspring all show the dominant phenotype, the unknown parent is very probably homozygous — and 'very probably' is the honest word, because a heterozygote could produce eight dominant offspring in a row by chance about once in every 256 attempts.

Reading a pedigree backwards

Two pea plants, both tall, are crossed. Of 160 offspring, 118 are tall and 42 are dwarf. Deduce the genotypes of the parents and explain your reasoning.

Dwarf offspring appear from two tall parents, so dwarf must be recessive and each parent must carry a dwarf allele. Both parents are tall, so neither can be homozygous dwarf. Both are therefore heterozygous, Tt.

Check against the numbers. Tt × Tt predicts 3 tall : 1 dwarf, which for 160 offspring is 120 tall and 40 dwarf. The observed 118 and 42 are close to that, and the small difference is what chance produces.

The reasoning that scores here is the appearance of a phenotype that neither parent shows. That, on its own, tells you the phenotype is recessive and both parents are heterozygous, and it is worth saying in those words.

When neither allele gives way

Complete dominance is a special case that happens to be common, not a law. Where two alleles are codominant, both are expressed in the heterozygote and the phenotype shows both. Codominant alleles cannot be written as a capital and a lower-case letter, because neither is subordinate to the other; the convention is a capital letter for the gene with a raised letter for each allele, such as CR and CW.

Snapdragon flower colour is the standard plant example. CRCR plants are red, CWCW are white, and CRCW are pink. Cross two pink plants and the offspring come out 1 red : 2 pink : 1 white — a 1 : 2 : 1 phenotype ratio, because with codominance the phenotype ratio is the genotype ratio. Any monohybrid cross producing three phenotypes in a 1 : 2 : 1 ratio is telling you the alleles are codominant.

Human sickle-cell anaemia works the same way at the molecular level. Someone with genotype HbAHbS makes both normal haemoglobin and sickle haemoglobin, so both alleles are expressed and the alleles are codominant. Whether the person is described as having the disease is a different question from which alleles are expressed, and that gap is where questions on this example are usually set.

A gene can also have more than two alleles in the population, which is called having multiple alleles. Any one individual still carries only two, because there are only two loci to carry them at. The human ABO blood group system is the example every board uses, and it combines multiple alleles with codominance in one gene.

Six genotypes, four phenotypes. Group A and group B each arise two ways, which is why a group A parent can have a group O child and why paternity questions on this topic are answerable at all.

The three alleles are IA, IB and IO. IA codes for antigen A on the red cell surface and IB for antigen B; IO codes for neither. IA and IB are codominant with each other, and both are dominant to IO. Some textbooks and some boards write the third allele as a lower-case i rather than IO; the biology is identical, so use whichever notation your specification uses and define it at the top of your answer.

TRY IT — A question of parentage

A woman of blood group O has a child of blood group B. She names a man of blood group AB as the father. A second man, of blood group A, is also considered. Determine whether each man could be the father, showing your reasoning.

Check your answer

The mother is group O, so her genotype is IOIO and every egg she produces carries IO. The child is group B, so the child's genotype must be IBIO, and the IB must have come from the father.

The group AB man is IAIB. Half his sperm carry IB, so he could be the father.

The group A man is either IAIA or IAIO. Neither genotype contains IB, so he could only give the child IA or IO, producing a group A or group O child. He cannot be the father.

Blood groups can exclude a man but never confirm one: millions of other men are also group AB. Saying so is often the final mark.

Genes on the X chromosome

The sex chromosomes break the symmetry the rest of genetics relies on. In humans a female is XX and a male XY, and the Y is much smaller than the X and carries far fewer genes. A gene present on the X with no equivalent on the Y is sex-linked. A female carries two alleles of it and can be homozygous or heterozygous; a male carries one, and whichever allele he has is expressed, because there is no second copy to mask it.

That asymmetry is the whole of the topic. Recessive sex-linked conditions — haemophilia A, red-green colour blindness, Duchenne muscular dystrophy — are far more common in males, because a male needs only one recessive allele to show the condition while a female needs two. A female heterozygote is a carrier and shows nothing.

The notation matters as much as the biology. Write the allele as a raised letter on the X — XH and Xh — and always write the Y where it belongs, so a male genotype reads XhY rather than a bare h. Dropping the X and Y makes it impossible for a marker to see that you understood the linkage, and it is the commonest way marks disappear from an otherwise correct diagram.

Follow the alleles through a family and two rules fall out that are worth having. A father passes his X to every daughter and never to a son, so an affected father cannot pass a sex-linked condition to his sons at all, and every one of his daughters is at least a carrier. A son gets his single X from his mother, so an affected son inherited the allele from his mother, whatever the father's phenotype. Those two sentences answer most pedigree questions on the topic before you draw anything.

Working a carrier cross

Haemophilia is caused by a recessive allele on the X chromosome. A woman whose father had haemophilia marries a man who does not have it. Calculate the probability that their first child is a boy with haemophilia, and the probability that a daughter is a carrier.

The woman's father was XhY, and he gave his only X to every daughter, so she must have received Xh. She does not have haemophilia, so her genotype is XHXh. Her husband is unaffected, so he is XHY.

The four equally likely offspring are XHXH, XHXh, XHY and XhY. The probability of a boy with haemophilia is 1 in 4, or 0.25.

Of the two possible daughters, one is a carrier, so the probability that a daughter is a carrier is 1 in 2. The two answers differ because they are taken over different populations of children: all children in the first case, daughters only in the second.

The step worth practising is the first one. The woman's genotype was not given; it was deduced from her father's phenotype and the rule that a father gives his X to every daughter.

In the exam

Check yourself

In cattle, coat colour is controlled by a single gene with two codominant alleles: CRCR cattle are red, CWCW are white, and CRCW are roan, a mixture of red and white hairs. A farmer crosses a roan bull with a roan cow and gets 24 calves. Set out the cross and state how many calves of each colour you would expect. The farmer then crosses the same bull with a white cow: state the expected ratio for that cross.

Answer

Roan × roan is CRCW × CRCW. Each parent produces gametes carrying CR or CW in equal numbers, so the offspring are 1 CRCR : 2 CRCW : 1 CWCW.

That is 1 red : 2 roan : 1 white, so of 24 calves you would expect 6 red, 12 roan and 6 white.

Roan × white is CRCW × CWCW. The white cow can only give CW, so the offspring are half CRCW and half CWCW: 1 roan : 1 white, with no red calves at all.

The give-away that these alleles are codominant rather than showing incomplete blending is the roan coat itself: it is red hairs and white hairs side by side, both alleles expressed, rather than a pink blend.

Questions

Written to the command words the boards use. Try them on paper before opening a scheme: the marks go to points made, not to length.

Question 15 marks

In tomatoes, red fruit is dominant to yellow fruit. Complete a full genetic diagram for a cross between a heterozygous red-fruited plant and a yellow-fruited plant, showing the parental genotypes, the gametes, the offspring genotypes and the expected phenotype ratio.

Mark scheme
  1. B1 the symbols are defined, for example R for the allele for red fruit and r for the allele for yellow fruit, using one letter for both alleles
  2. B1 parental genotypes given as Rr for the red-fruited plant and rr for the yellow-fruited plant
  3. B1 gametes shown as circled single alleles rather than pairs: R and r from the red-fruited parent, r and r from the yellow-fruited parent
  4. B1 offspring genotypes obtained from the gametes as Rr and rr, in equal numbers
  5. A1 offspring phenotypes and ratio given as 1 red-fruited : 1 yellow-fruited

Question 24 marks

A breeder has a tall pea plant and needs to know whether it is homozygous or heterozygous. Explain how a test cross would answer the question, and explain why a result suggesting the plant is homozygous can never be certain.

Mark scheme
  1. B1 cross the tall plant with a homozygous recessive dwarf plant, which can only contribute a recessive allele to every offspring
  2. B1 if any offspring is dwarf, the tall parent must have supplied a recessive allele, so it is heterozygous
  3. B1 if a large number of offspring are all tall, the tall parent is very probably homozygous, because a heterozygote would be expected to give a 1 : 1 ratio
  4. A1 each fertilisation is independent, so a heterozygote can give a run of dominant offspring by chance: eight in a row happens about once in every 256 attempts

Question 34 marks

Cystic fibrosis is caused by a recessive allele. Two parents are both carriers and neither has the condition. Calculate the probability that their first child has cystic fibrosis, and calculate the probability that their first two children both have it.

Mark scheme
  1. M1 both parents are heterozygous, so the cross is Ff × Ff and the offspring genotypes are 1 FF : 2 Ff : 1 ff
  2. A1 one of the four equally likely outcomes is homozygous recessive, so the probability is 1 in 4, or 0.25
  3. M1 the two fertilisations are independent events, so multiply the separate probabilities: 0.25 × 0.25
  4. A1 1 in 16, or 0.0625

Question 44 marks

Haemophilia is caused by a recessive allele carried on the X chromosome. Explain why the condition is far more common in males than in females, and explain why a man with haemophilia can never pass it to his sons.

Mark scheme
  1. B1 a male is XY, and the Y chromosome is much smaller and carries no equivalent of the gene
  2. B1 a male therefore carries only one allele of the gene and expresses it whichever it is, because there is no second copy to mask it, so one recessive allele is enough
  3. B1 a female is XX and needs two copies of the recessive allele to show the condition; with one she is an unaffected carrier, and inheriting two is much less likely
  4. A1 a father passes his X to every daughter and his Y to every son, so his sons receive no allele of the gene from him at all and every daughter is at least a carrier

Question 54 marks

A woman of blood group AB has children with a man of blood group O. Explain why none of their children can be blood group AB, and explain why none of them can be blood group O.

Mark scheme
  1. B1 the woman's genotype is IAIB, so every egg carries either IA or IB and never both
  2. B1 the man's genotype is IOIO, so every sperm carries IO
  3. B1 every child is therefore IAIO or IBIO, giving blood group A or blood group B, because IA and IB are each dominant to IO
  4. A1 group AB would need IA and IB together, which the father cannot supply, and group O would need two copies of IO, which the mother cannot supply

Question 62 marks

State what is meant by the term allele, and state what is meant by describing someone as a carrier of a recessive condition.

Mark scheme
  1. B1 an allele is one of the alternative versions of a gene, found at the same locus on a chromosome
  2. B1 a carrier is an individual heterozygous for a recessive allele, so they do not show the condition but can pass the allele on

Worth remembering

← Meiosis: two divisions, and why no two gametes come out the same · Two genes at once: 9 : 3 : 3 : 1, the ways it breaks, and the chi-squared test →