6.1.6a Genetic inheritance

Alleles

1. Most genes come in different versions called alleles.
2. You inherit two alleles of each gene, one from each parent, carried on a pair of chromosomes.
3. The two alleles you have for a gene may be the same or different.

Dominant and recessive alleles

1. A dominant allele shows up in the phenotype whenever it is inherited, so a single copy is enough for its effect to appear.
2. A recessive allele only affects the phenotype when both copies are recessive, because any dominant allele present will override it.
3. Dominant alleles are written with a capital letter and recessive alleles with the matching lower-case letter, such as $B$ and $b$.
4. So an organism with the alleles $Bb$ shows the dominant feature, because the dominant $B$ is expressed.

Homozygous and heterozygous

1. When both copies of a gene carry the matching allele, the organism is homozygous for that gene, for example $BB$ or $bb$.
2. When the two copies carry different alleles, the organism is heterozygous, for example $Bb$.
3. A homozygous recessive organism ($bb$) is the only genotype that shows a recessive feature.
4. A dominant feature can be shown by either a homozygous dominant ($BB$) or a heterozygous ($Bb$) organism.

Genotype and phenotype

1. The genotype is the combination of alleles an organism has, such as $Bb$.
2. The phenotype is the characteristic you can actually observe, such as brown eyes.
3. The phenotype is produced by the genotype together with the environment.

One gene or many genes

1. A few characteristics are controlled by a single gene, such as fur colour in mice or red-green colour blindness.
2. Most characteristics are controlled by several genes working together, not by one gene alone.
3. Features such as height and body mass are affected by many genes and by the environment.

Genetic diagrams and family trees

1. A monohybrid cross follows the inheritance of a single gene from the parents to their offspring.
2. Each parent has two alleles, but each gamete carries only one of them, because meiosis halves the number.
3. A Punnett square is a grid that combines the parents' gametes to show the possible offspring genotypes.
4. Write one parent's alleles along the top and the other parent's down the side, then fill each box by combining them.
5. Crossing two heterozygotes ($Bb\times Bb$) gives offspring $BB$, $Bb$, $Bb$ and $bb$, a phenotype ratio of $3:1$ dominant to recessive.
6. A family tree shows how a characteristic has been inherited through the generations of a real family.
7. You can read a family tree to work out whether an allele is dominant or recessive and to predict genotypes.

6.1.6b Genetic inheritance (Higher tier)

Constructing a genetic cross

1. To construct a cross, first write the two parents' genotypes, for example $Bb\times Bb$.
2. Work out the gametes: each $Bb$ parent makes gametes carrying either $B$ or $b$.
3. Put one parent's gametes along the top of the Punnett square and the other's down the side.
4. Fill each box by combining the alleles, giving $BB$, $Bb$, $Bb$ and $bb$.

Ratios, fractions and percentages

1. From $Bb\times Bb$, three of the four boxes show the dominant feature, a ratio of $3:1$ dominant to recessive.
2. As a fraction, that is $\frac{3}{4}$ dominant and $\frac{1}{4}$ recessive.
3. As percentages, that is $75\%$ dominant and $25\%$ recessive.
4. The same outcome can be written as a ratio, a fraction or a percentage.

Using probability to predict offspring

1. From $Bb\times Bb$, the probability that one offspring shows the recessive feature is $\frac{1}{4}$.
2. A probability can be written as a fraction, a decimal ($0.25$) or a percentage ($25\%$).
3. These are expected proportions, not exact counts.
4. Real offspring numbers vary by chance, and the more offspring there are, the closer the results usually get to the expected ratio.