Inheritance, revision notes

Overview

Inheritance is how characteristics are passed from parents to offspring through genes. This chapter covers monohybrid and dihybrid inheritance, genes and alleles, and inheritance in humans such as sex determination and blood groups.

Punnett squares and correct genetic terms are essential, the genetics tool pages give extra practice.

A test cross is used to find out whether an individual showing the dominant phenotype is homozygous or heterozygous: it is crossed with a homozygous recessive individual, and the ratio of phenotypes among the offspring reveals the unknown genotype. If all offspring show the dominant phenotype, the tested individual was homozygous dominant; if about half show the recessive phenotype, it was heterozygous.

Not every characteristic follows a simple dominant-recessive pattern. The ABO blood group system shows codominance and multiple alleles: the alleles I^A and I^B are both fully expressed when present together, giving blood group AB, while the allele i is recessive to both.

Some conditions, such as red-green colour blindness and haemophilia, are sex-linked, carried on the X chromosome, which is why they occur far more often in males than in females.

Incomplete dominance is different again: neither allele is fully dominant, so the heterozygote shows a blended or intermediate phenotype. A cross between red-flowered and white-flowered snapdragons, for example, produces pink-flowered offspring, because neither the red nor the white allele completely masks the other.

A person who carries one copy of a recessive allele for a condition, without showing the condition themselves, is called a carrier. Two carrier parents can each pass on either their dominant or their recessive allele, so predicting the chance that a particular child inherits the condition uses exactly the same Punnett square logic as any other monohybrid cross.

Genes, alleles and the language of genetics (CS 26.3)

A gene is a section of DNA that codes for a particular characteristic and occupies a fixed position, or locus, on a chromosome. An allele is one particular version of that gene; an organism inherits one allele of each gene from each parent, so it carries two alleles for every gene at a given locus, one on each chromosome of a homologous pair.

A dominant allele produces its effect in the phenotype whenever it is present, and is written with a capital letter; a recessive allele produces its effect only when paired with another recessive allele, and is written with the same letter in lower case, for example T for tall and t for dwarf. An organism with two identical alleles for a gene is homozygous (TT or tt); one with two different alleles is heterozygous (Tt).

Genotype refers to the actual alleles an organism carries, written as a letter pair such as Tt, while phenotype refers to the observable characteristic that results, such as being tall. Two organisms can share the same phenotype while having different genotypes, TT and Tt both produce a tall plant, because the dominant allele masks the recessive one in the heterozygote.

Monohybrid inheritance and the law of segregation (CS 26.1)

Monohybrid inheritance follows a single characteristic controlled by one gene. Mendel's first law, the law of segregation, states that the two alleles an organism carries for a gene separate from each other during gamete formation, so each gamete carries only one allele of the pair.

To predict the outcome of a cross, the gametes each parent can produce are identified and combined in a Punnett square: a heterozygote Tt produces two gamete types, T and t, in equal proportion. Crossing two heterozygotes (Tt x Tt) gives offspring in the genotype ratio 1 TT : 2 Tt : 1 tt, and because both TT and Tt show the dominant phenotype, this becomes a phenotype ratio of 3 tall : 1 dwarf.

A common exam task gives the phenotypes of two parents and the offspring ratio observed, and asks for the parents' genotypes to be deduced, or gives the genotypes and asks for the predicted ratio; both directions of this reasoning are tested.

The test cross (CS 26.1)

An organism showing a dominant phenotype could be either homozygous dominant or heterozygous, since both genotypes look identical. A test cross resolves this by crossing the unknown individual with a homozygous recessive individual, whose genotype is already certain because a recessive phenotype can only result from two recessive alleles.

If every offspring shows the dominant phenotype, the unknown parent must be homozygous dominant, since a homozygous recessive partner can only contribute a recessive allele, and a dominant allele from the other parent would mask it every time. If roughly half the offspring show the recessive phenotype instead, the unknown parent must be heterozygous, since half its gametes carry the recessive allele.

Exam note: a test cross always pairs the unknown individual with a homozygous recessive partner, never with another individual of unknown or dominant phenotype, since only a homozygous recessive genotype gives an unambiguous result.

Dihybrid inheritance and the law of independent assortment (CS 26.2)

Dihybrid inheritance follows two characteristics, controlled by two different genes, at the same time. Mendel's second law, the law of independent assortment, states that the alleles of one gene separate into gametes independently of the alleles of a different gene, provided the two genes are on different chromosomes.

A double heterozygote such as PpIi produces four gamete types in equal proportion, PI, Pi, pI and pi, because each of the two alleles for flower colour can combine with either of the two alleles for pod shape. Crossing two such double heterozygotes (PpIi x PpIi) requires a 4 x 4 Punnett square with 16 boxes, which group into four phenotype classes in the ratio 9:3:3:1.

Exam note: state clearly which two phenotypes correspond to the 9 and which correspond to each 3 and the 1, since simply reciting the numbers '9:3:3:1' without linking them to specific phenotype combinations does not answer most dihybrid questions.

Comparing monohybrid and dihybrid crosses (CS 26.1, CS 26.2)

Monohybrid and dihybrid crosses use exactly the same underlying logic, identifying gametes and combining them in a Punnett square, but differ in scale because a dihybrid cross tracks twice as many genes.

Codominance and multiple alleles: the ABO blood group system (CS 26.3, CS 26.4)

In codominance, two different alleles are both fully and separately expressed in the heterozygote, rather than one masking the other. The ABO blood group gene also shows multiple alleles: three different alleles, I^A, I^B and i, exist in the human population for this single gene, although any one person still carries only two of them, one from each parent.

I^A and I^B are codominant with each other, so a person with genotype I^A I^B expresses both A and B antigens on their red blood cells, giving blood group AB. Both I^A and I^B are dominant to i, so genotype I^A I^A or I^A i gives blood group A, and I^B I^B or I^B i gives blood group B; genotype ii gives blood group O.

Exam note: when working an ABO problem, first identify which two of the three possible alleles each parent actually carries, then apply the law of segregation to each parent exactly as in any other cross, remembering that a parent with genotype I^A I^B passes on either I^A OR I^B to a given child, never both together.

Incomplete dominance (CS 26.3)

In incomplete dominance, neither allele fully masks the other, so the heterozygote shows a blended, intermediate phenotype rather than the phenotype of either homozygous parent. A cross between red-flowered (C^R C^R) and white-flowered (C^W C^W) snapdragons produces pink-flowered (C^R C^W) offspring, because neither pigment fully dominates the other in a single flower.

Sex determination and sex-linked inheritance (CS 26.4)

Human sex is determined by the sex chromosomes: females carry two X chromosomes (XX) and males carry one X and one Y (XY). Every egg cell carries an X chromosome, while sperm cells carry either an X or a Y chromosome in equal proportion, so the father's contribution determines the offspring's sex, with an equal chance of each outcome.

A characteristic controlled by a gene located on the X chromosome, such as red-green colour blindness or haemophilia, is described as sex-linked. Because a male has only one X chromosome, a single recessive allele on that X chromosome is enough to produce the recessive phenotype, since there is no second X chromosome carrying a possible dominant allele to mask it.

A female, with two X chromosomes, needs a recessive allele on both of her X chromosomes to show the same condition; a female with only one recessive allele is an unaffected carrier. This is why sex-linked recessive conditions such as colour blindness occur far more often in males than in females.

Pedigree analysis and carriers (CS 26.4)

A pedigree diagram is a family tree that records which relatives across generations show a particular characteristic or condition, using shaded symbols for affected individuals and unshaded symbols for unaffected ones. It is used to trace how a trait has actually been inherited, in contrast to a Punnett square, which predicts probabilities for a hypothetical cross.

Two unaffected parents having an affected child shows that the condition is recessive, since a dominant allele carried by either parent would have produced the condition in that parent as well. Whether the condition is autosomal or sex-linked can often be deduced from whether it affects sons and daughters equally or appears mainly in sons, combined with whether an affected father's daughters are carriers.

A carrier holds one recessive allele for a condition without showing it, because the single dominant allele in the heterozygote is enough to produce a normal phenotype. When two carrier parents (heterozygotes) have children, a Punnett square shows that one quarter of the possible offspring genotypes are homozygous recessive, so on average one in four of their children is expected to show the condition, with each pregnancy an independent event carrying that same one-in-four chance.

Key concepts to master

• Genes and alleles, A gene is a section of DNA that codes for a characteristic; alleles are its different versions, one from each parent.
• Dominant and recessive, A dominant allele shows in the phenotype whenever present; a recessive allele shows only when both alleles are recessive.
• Genotype and phenotype, The genotype is the alleles an organism has; the phenotype is the characteristic that shows.
• Monohybrid inheritance, The inheritance of a single characteristic, worked out with a Punnett square to predict ratios.
• Dihybrid inheritance, The inheritance of two characteristics at once, giving a 9:3:3:1 ratio in a typical cross.
• Inheritance in humans, Sex is determined by the X and Y chromosomes; blood groups and some conditions follow specific inheritance patterns.
• Test cross, 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.
• Codominance and multiple alleles, 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.
• Sex-linked inheritance, 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.
• Pedigree diagram, 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.
• Incomplete dominance, 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.
• Carrier, 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.

Quick recall checklist

1. Can you define and explain Genes and alleles?
2. Can you define and explain Dominant and recessive?
3. Can you define and explain Genotype and phenotype?
4. Can you define and explain Monohybrid inheritance?
5. Can you define and explain Dihybrid inheritance?
6. Can you define and explain Inheritance in humans?
7. Can you define and explain Test cross?
8. Can you define and explain Codominance and multiple alleles?
9. Can you define and explain Sex-linked inheritance?
10. Can you define and explain Pedigree diagram?
11. Can you define and explain Incomplete dominance?
12. Can you define and explain Carrier?

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Source:SRC-DSKP-EN