Inheritance, practice questions

B, A monohybrid cross between two heterozygotes gives a 3:1 phenotype ratio of dominant to recessive.

B, A Tt x Tt cross gives offspring genotypes in the ratio 1 TT : 2 Tt : 1 tt.

C, Two independently assorting genes crossed as double heterozygotes give the classic 9:3:3:1 phenotype ratio.

B, A test cross reveals whether an individual with a dominant phenotype is homozygous dominant or heterozygous.

B, I^A and I^B are codominant, so both are fully expressed together in the heterozygote, producing blood group AB.

C, Incomplete dominance produces a single blended phenotype, such as pink flowers from red and white parents.

C, A recessive allele on the X chromosome is enough to produce colour blindness in a son, who has only one X chromosome.

C, A shaded symbol in a pedigree diagram represents an individual who shows the trait or condition being traced.

B, An Aa x Aa cross gives offspring genotypes in the ratio 1 AA : 2 Aa : 1 aa, so one quarter are expected to be homozygous recessive.

B, Genotype refers to the alleles an organism carries; phenotype refers to the observable characteristic that results.

• (a) The black parent's most likely genotype is Bb, heterozygous, since the litter contains offspring of both phenotypes in roughly similar numbers rather than only black offspring.
• (b) This is a test cross, since the black parent of unknown genotype was crossed with a white parent, whose genotype (bb) is already certain as homozygous recessive.
• If the black parent were homozygous dominant (BB), every offspring would receive a B allele from it, and all offspring would be black, with no white offspring appearing at all.
• (c) A larger litter would make the observed ratio of black to white offspring closer to the expected 1:1 ratio for a Bb x bb cross.
• With a small litter, chance alone can produce an uneven split, such as 4:3 instead of exactly half and half, even when the underlying genotype is correctly Bb.
• A larger sample size reduces the effect of this chance variation, giving a more reliable basis for concluding the parent's genotype.

• (a) All F1 offspring are roan, since a cross between C^R C^R and C^W C^W produces only C^R C^W offspring.
• Both the C^R and C^W alleles are expressed together in the heterozygote, each allele producing its own hair colour rather than blending into a single intermediate colour.
• (b) Crossing two roan (C^R C^W) individuals gives offspring in the ratio 1 red (C^R C^R) : 2 roan (C^R C^W) : 1 white (C^W C^W).
• (c) This is codominance, not incomplete dominance, because the roan coat consists of patches of pure red hair and pure white hair appearing separately side by side.
• In incomplete dominance, every individual hair would instead show a single blended intermediate colour, rather than distinct patches of each parental colour.
• Since both C^R and C^W are fully and separately visible in the roan coat, both alleles are considered codominant rather than blending.

• (a) The condition is more likely recessive, since both parents are unaffected yet their son shows the condition, meaning the responsible allele can be present but hidden in an unaffected parent's genotype.
• (b) The mother is most likely a carrier with genotype X^N Xⁿ, since she is herself unaffected but has clearly passed the condition allele on to her son.
• (c) A son has only one X chromosome, inherited from the mother, so inheriting a single Xⁿ allele from a carrier mother is enough on its own to produce the condition.
• A daughter, however, would need to inherit an Xⁿ allele from both her mother and her father to show the condition, since she has two X chromosomes.
• The father in this pedigree is unaffected, with genotype X^N Y, so he cannot supply an Xⁿ allele to any daughter.
• Because of this, none of this couple's daughters can show the condition, even though their son does, which explains why the condition is far less common in daughters than in sons.

A gene is a section of DNA that codes for a characteristic; alleles are its different versions, one from each parent.

A dominant allele shows in the phenotype whenever present; a recessive allele shows only when both alleles are recessive.

The genotype is the alleles an organism has; the phenotype is the characteristic that shows.

The inheritance of a single characteristic, worked out with a Punnett square to predict ratios.

The inheritance of two characteristics at once, giving a 9:3:3:1 ratio in a typical cross.

Sex is determined by the X and Y chromosomes; blood groups and some conditions follow specific inheritance patterns.

Crossing an organism showing a dominant phenotype with a homozygous recessive individual reveals whether the unknown parent is homozygous or heterozygous, based on whether any offspring show the recessive phenotype.

In codominance, two different alleles are both fully expressed in the heterozygote, as seen in the ABO blood group system, where a gene has three possible alleles, I^A, I^B and i, rather than just two.

A characteristic controlled by a gene on the X chromosome, such as red-green colour blindness or haemophilia, is called sex-linked; because males have only one X chromosome, a single recessive allele is enough to produce the recessive phenotype in a male.

A pedigree is a family tree showing which relatives have a particular characteristic or condition, used to trace how a trait, especially a sex-linked or recessive one, has been inherited across generations.

Unlike simple dominance, neither allele is fully expressed over the other, so a heterozygote shows an intermediate phenotype, such as pink flowers from a cross between red and white parents.

A carrier has one dominant and one recessive allele for a condition and does not show the condition, but can pass the recessive allele on to offspring, who may show the condition if they inherit a recessive allele from both parents.