6.1.4Inheritance, Variation and Evolution
Genetic Inheritance
5 min read 9 parts Lesson 5 of 16 2 quizzes
Section 1 of 9Key vocabulary
Each gene occurs in different forms called alleles. A body cell has two alleles of each gene, one from each parent. A dominant allele (shown with a capital letter) is expressed even if only one copy is present. A recessive allele (lower case) is only expressed when two copies are present, in the absence of the dominant allele.
| Term | Meaning |
|---|---|
| Genotype | The combination of alleles a person has, e.g. Bb |
| Phenotype | The characteristic that is expressed, e.g. brown eyes |
| Homozygous | Two identical alleles (BB or bb) |
| Heterozygous | Two different alleles (Bb) |
Section 2 of 9Monohybrid crosses
A monohybrid cross follows the inheritance of one gene. Suppose B is the dominant allele for brown eyes and b is the recessive allele for blue eyes. A heterozygous parent (Bb) produces gametes B and b. A Punnett square shows how the gametes can combine.
Cross Bb × Bb:
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
The offspring genotypes are 1 BB : 2 Bb : 1 bb.
Section 3 of 9Steps for solving a cross
A cross of a heterozygous and a homozygous recessive parent (Bb × bb) gives 1:1, so 50 percent Bb and 50 percent bb. This kind of cross is used to find the genotype of an organism showing the dominant trait: if any offspring show the recessive trait, the parent was heterozygous.
Section 4 of 9Family trees and ratios
A family tree (pedigree) shows how a characteristic passes through generations.
Most characteristics are not controlled by a single gene. Many, such as height or skin colour, are controlled by several genes working together and are also affected by the environment. Only a few, such as the ability to roll the tongue or cystic fibrosis, follow simple single-gene patterns, which is why monohybrid crosses are used.
Section 5 of 9Links
Genetic crosses depend on the gametes produced by meiosis (6.1.2) and fertilisation being random (6.1.1). They also link to inherited disorders (6.1.7) and sex determination (6.1.8), which is another example of a cross where the probability is 50 percent. In evolution, alleles that give an advantage are passed to more offspring.
Section 6 of 9Using probability correctly
Probability questions are very common in genetics and it is worth being clear about the difference between a prediction and an outcome.
- so a plant breeder with 200 offspring expects about 50 recessive ones,
- but not necessarily exactly 50.
When two events must both happen, multiply probabilities.
- Be careful to read the question for whether it asks about a particular child or about the family as a whole.
Show working, give the answer as a fraction, percentage or ratio as requested, and state what the value represents.
Section 7 of 9Exam-style questions
In pea plants, tall (T) is dominant to dwarf (t). Two heterozygous tall plants are crossed. Draw a Punnett square and give the phenotype ratio. [4]4 marks
Parental genotypes Tt and Tt, gametes T, t [1]. Punnett square: TT, Tt, Tt, tt [2]. Phenotype ratio 3 tall : 1 dwarf [1].
Explain what is meant by the terms homozygous and heterozygous. [2]2 marks
Homozygous: the two alleles of a gene are the same [1]. Heterozygous: the two alleles are different [1].
Two brown-eyed parents have a blue-eyed child. Explain how this is possible. [3]3 marks
Blue eyes is recessive, so the child is bb [1]. Each parent must have passed on a b allele, so both are carriers, Bb [1]. Their brown phenotype is due to the dominant B allele [1].
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Allele | A version of a gene. |
| Dominant | Expressed when at least one copy is present. |
| Recessive | Expressed only when two copies are present. |
| Genotype | The alleles an organism has. |
| Phenotype | The observable characteristic. |
| Homozygous | Having two identical alleles. |
| Heterozygous | Having two different alleles. |
| Punnett square | A diagram used to predict the outcome of a genetic cross. |