D3.2 InheritanceIB Biology SL: Revision notes

Inheritance in all eukaryotes with a sexual life cycle depends on parents producing haploid gametes that fuse to form a diploid zygote. A diploid cell therefore has two copies of each autosomal gene, one from each parent.

In flowering plants, pollen carries the male gametes and the female gametes are in the ovary, so a cross needs pollination. Peas produce both on the same plant and can self-pollinate, so to cross two varieties the anthers are removed from one parent and pollen from the other is transferred to its stigma. The parents are the P generation, their offspring the F1, and offspring of the F1 the F2. A Punnett grid shows all possible fusions of gametes. Crosses like these are used to breed new crop and ornamental varieties.

A gene is a heritable factor; alleles are its alternative forms. The genotype is the combination of alleles an organism inherits: homozygous (two identical alleles) or heterozygous (two different). The phenotype is the set of observable traits, produced by genotype and environment together.

• Genotype only: ABO blood group, PKU.
• Environment only: a scar, a language spoken.
• Interaction: height, skin colour.

A dominant allele determines the phenotype whenever it is present, so homozygous-dominant and heterozygous individuals look the same: one functional allele produces enough of the protein. A recessive allele only affects the phenotype when homozygous.

Phenotypic plasticity is the capacity to develop traits suited to the environment by varying gene expression, for example tanning of skin in sunlight. The genotype does not change and the change may be reversible.

Phenylketonuria (PKU) is caused by a recessive allele of an autosomal gene coding for the enzyme that converts phenylalanine to tyrosine. Homozygous recessive children accumulate phenylalanine, which damages the developing brain unless they follow a low-phenylalanine diet. Two heterozygous carriers have a 1 in 4 chance of an affected child.

New alleles usually arise as single-nucleotide polymorphisms (SNPs), where one base differs. Any number of alleles can exist in the gene pool, but an individual inherits only two. The ABO gene has three alleles, $I^A$, $I^B$ and $i$: groups are A ($I^AI^A$ or $I^Ai$), B ($I^BI^B$ or $I^Bi$), AB ($I^AI^B$) and O ($ii$).

In codominance both alleles are fully expressed in the heterozygote, giving a dual phenotype: $I^AI^B$ gives both A and B antigens (blood group AB).

In incomplete dominance the heterozygote has an intermediate phenotype: crossing red and white four o'clock flowers (Mirabilis jalapa, marvel of Peru) gives pink F1 plants, and the F2 is 1 red : 2 pink : 1 white.

Females are XX and males XY. Eggs always carry X; sperm carry X or Y, so the sperm determines sex, and a gene on the Y triggers male-typical development. The X carries far more genes than the Y, so males have only one copy of most X-linked genes.

Haemophilia is caused by a recessive allele on the X chromosome, written $X^h$. Males ($X^hY$) are affected with a single copy; females are usually carriers ($X^HX^h$). Sons inherit their X from their mother.

Pedigree charts are used to deduce patterns: two unaffected parents with an affected child show the allele is recessive. Close relatives are more likely to carry the same recessive allele, which is the genetic basis for prohibiting marriage between them.

NOS: deducing a general pattern from some cases is inductive reasoning; using that pattern to work out a particular person's genotype is deductive reasoning.

Discrete variables fall into distinct classes (ABO blood group). Continuous variables such as height or skin colour take any value in a range, because of polygenic inheritance (many genes with small additive effects) and/or environmental factors such as sunlight or diet. Summarise them with the mean, median or mode.

A box-and-whisker plot shows minimum, first quartile, median, third quartile, maximum and outliers. A point is an outlier if it is more than 1.5 × IQR above Q3 or below Q1.