What you'll learn
- What inheritance means: how characteristics are passed from parents to offspring.
- How chromosomes, genes and alleles control inherited characteristics.
- How to use genetic diagrams, Punnett squares and family pedigrees to predict outcomes.
- How biological sex, ABO blood group, and Higher Tier sex-linked disorders are inherited.
Why inheritance was hard to understand
Inheritance is the passing of genetic information from parents to offspring. Before scientists understood DNA, chromosomes and cell division, inheritance was mysterious. People could see that offspring often resembled their parents, but they did not know what was being passed on.
Gregor Mendel studied pea plants in the 1800s. He carefully crossed plants with different characteristics, such as tall or short stems, and counted the offspring. From his results, he suggested that characteristics were controlled by “factors” passed from parents to offspring. We now call these factors genes.
Mendel’s work was difficult for people to accept at the time because chromosomes, genes, DNA and meiosis had not yet been discovered or properly understood.
Mendel’s big idea
Characteristics are controlled by inherited factors, now called genes. Offspring receive one copy of each gene from each parent.
Chromosomes, genes and alleles
DNA is the chemical that stores genetic information. In the nucleus of a body cell, DNA is arranged into long coiled structures called chromosomes. A gene is a section of DNA that codes for a specific protein, which can affect a characteristic.
An allele is an alternative version of the same gene. For example, a gene for flower colour might have one allele for purple flowers and another allele for white flowers.
In body cells, chromosomes usually come in pairs. The two chromosomes in a pair carry the same genes in the same positions, but they may carry different alleles.
Allele
An allele is a different version of a gene. You inherit one allele for each gene from your mother and one allele from your father.
From alleles to characteristics
A genotype is the combination of alleles an organism has for a gene, such as BB, Bb or bb.
A phenotype is the observable characteristic, such as having the dominant trait or the recessive trait. Phenotype is usually affected by genotype, although the environment can sometimes affect it too.
A dominant allele is expressed in the phenotype even if only one copy is present. A recessive allele is expressed only if two copies are present.
A person or organism is homozygous for a gene if both alleles are the same, such as BB or bb. It is heterozygous if the two alleles are different, such as Bb.
Genotype vs phenotype
Genotype means the alleles written as letters. Phenotype means the characteristic that is actually shown.
Gametes and zygotes
A gamete is a sex cell: sperm in males and eggs in females. Gametes contain only one allele for each gene, because they contain one chromosome from each chromosome pair.
At fertilisation, a sperm cell joins with an egg cell. The new cell formed is called a zygote. The zygote has two alleles for each gene again: one from each parent.
Monohybrid inheritance
Monohybrid inheritance is the inheritance of one characteristic controlled by one gene. GCSE genetic crosses usually use letters to represent alleles. The dominant allele is written as a capital letter, such as B, and the recessive allele is written as the same letter in lowercase, such as b.
A genetic diagram shows how alleles can be passed from parents to offspring. A Punnett square is a grid used to show all the possible allele combinations in offspring.
Predicting offspring from heterozygous parents
A dominant allele B gives the dominant phenotype. A recessive allele b gives the recessive phenotype only when two copies are present. Two heterozygous parents have genotypes Bb and Bb.
- Work out the gametes each parent can make. A Bb parent can pass on either B or b.
- Combine the possible gametes in a Punnett square: BB, Bb, Bb and bb.
- Decide the phenotype for each genotype. BB and Bb show the dominant phenotype; bb shows the recessive phenotype.
- Convert the four boxes into outcomes: genotype ratio 1 BB : 2 Bb : 1 bb, and phenotype ratio 3 dominant : 1 recessive.
- Calculate probabilities: the chance of the recessive phenotype is p=14p = \frac{1}{4}p=41, which is 25%. The chance of the dominant phenotype is 75%.
Ratios, fractions and percentages
In a four-box Punnett square, one box means 25%, two boxes means 50%, three boxes means 75%, and four boxes means 100%.
Probability is not a promise
A Punnett square predicts chances for each offspring. It does not guarantee that exactly three out of four children will show the dominant phenotype.
Family pedigrees
A family pedigree is a family tree diagram used to track how a characteristic is inherited. In exam questions, shaded symbols usually show people who have the phenotype being studied.
A carrier is someone who has one recessive allele for a condition but does not show the condition because they also have a dominant allele.
Inferring carriers in a pedigree
A recessive disorder is caused by allele a. Two unaffected parents have an affected child. Work out the parents’ genotypes.
- Because the disorder is recessive, the affected child must have genotype aa.
- The child received one a allele from each parent, so both parents must carry allele a.
- The parents are unaffected, so they cannot be aa. Each must also have a dominant allele A.
- Therefore both parents are carriers with genotype Aa.
Sex determination
Sex chromosomes are chromosomes involved in determining biological sex. In humans, typical females have two X chromosomes, written XX. Typical males have one X chromosome and one Y chromosome, written XY.
Egg cells always carry an X chromosome. Sperm cells carry either an X chromosome or a Y chromosome. The sperm determines the sex chromosome combination of the zygote.
Calculating the chance of a male child
- The mother is XX, so every egg cell carries an X chromosome.
- The father is XY, so sperm cells can carry either X or Y.
- An X sperm produces XX, which is female. A Y sperm produces XY, which is male.
- There are two equally likely outcomes, so the chance of a male child is p=12p = \frac{1}{2}p=21, which is 50%.
ABO blood groups: codominance and multiple alleles
In Biology, you also need ABO blood groups. Multiple alleles means there are more than two alleles for a gene in the population. For the ABO blood group gene, the alleles are IAI^AIA, IBI^BIB and iii.
Codominance means both alleles are expressed in the phenotype when they are present together. The alleles IAI^AIA and IBI^BIB are codominant, so a person with genotype IAIBI^A I^BIAIB has blood group AB. The allele iii is recessive.
The possible genotypes are:
- Blood group A: IAIAI^A I^AIAIA or IAiI^A iIAi
- Blood group B: IBIBI^B I^BIBIB or IBiI^B iIBi
- Blood group AB: IAIBI^A I^BIAIB
- Blood group O: iiiiii
Predicting ABO blood groups
One parent has genotype IAiI^A iIAi. The other parent has genotype IBiI^B iIBi.
- Work out the gametes. The first parent can pass on IAI^AIA or iii. The second parent can pass on IBI^BIB or iii.
- Combine the alleles: IAIBI^A I^BIAIB, IAiI^A iIAi, IBiI^B iIBi and iiiiii.
- Convert genotypes into phenotypes: AB, A, B and O.
- Each outcome appears once out of four, so each blood group has a 25% chance.
Sex-linked genetic disorders
For Higher Tier Biology, you also need to explain sex-linked genetic disorders. A sex-linked disorder is caused by an allele on a sex chromosome, usually the X chromosome.
Males are more likely to show recessive sex-linked disorders because they have only one X chromosome. If their X chromosome carries the recessive disorder allele, the Y chromosome usually does not carry a matching dominant allele to mask it.
Predicting a sex-linked disorder
A recessive sex-linked disorder is caused by allele h on the X chromosome. A carrier mother has genotype XHXhX^H X^hXHXh. An unaffected father has genotype XHYX^H YXHY.
- Work out the gametes. The mother can pass on XHX^HXH or XhX^hXh. The father can pass on XHX^HXH or YYY.
- Combine the gametes: XHXHX^H X^HXHXH, XHXhX^H X^hXHXh, XHYX^H YXHY and XhYX^h YXhY.
- Identify the phenotypes: unaffected female, carrier female, unaffected male and affected male.
- The chance of an affected child is p=14p = \frac{1}{4}p=41, which is 25%. Among sons only, the chance of being affected is p=12p = \frac{1}{2}p=21, which is 50%.
In the exam
- Always define the alleles first, including which allele is dominant or recessive.
- Show the parents’ genotypes, the gametes, and the offspring genotypes before writing probabilities.
- Convert your final answer into the form asked for: ratio, fraction, percentage or phenotype.
Check yourself
- What is the difference between a gene and an allele?
- In a cross between Bb and Bb, why can bb offspring appear even though both parents show the dominant phenotype?
- Why are males more likely to show recessive sex-linked disorders?
