Inheritance

Why it matters

Inheritance patterns show up across lab careers. Clinical labs test for inherited conditions such as sickle cell disease and cystic fibrosis. Blood banks rely on the inheritance of ABO and Rh blood groups. Research labs breed model organisms (mice, fruit flies, plants) and must predict which offspring will carry a gene. The Punnett square is the simple tool behind all of these predictions.

The core idea: alleles, genotypes, phenotypes

• A gene can come in different versions called alleles. For a gene with alleles A and a, a body cell has two copies: one from each parent.
• The pair of alleles is the genotype: AA and aa are homozygous (two of the same), Aa is heterozygous.
• The observable trait is the phenotype.
• A dominant allele (capital letter) shows its effect with one copy. A recessive allele (lower case) shows only when there are two copies.

Gregor Mendel's pea experiments gave two key rules:

1. Segregation: each parent passes on one of its two alleles, chosen at random, in each egg or sperm (gamete).
2. Independent assortment: alleles of different genes (on different chromosomes) are passed on independently of each other.

Worked example 1: a monohybrid cross

Cystic fibrosis is recessive. Two parents are both carriers (Aa, where a is the CF allele). What are the chances for each child?

Write one parent's gametes across the top and the other's down the side, then fill each box with one allele from each:

• Genotypes: 1 AA : 2 Aa : 1 aa
• Phenotypes: 3 unaffected : 1 affected
• Each child has a 1 in 4 (25%) chance of having CF, a 1 in 2 chance of being a carrier, and a 1 in 4 chance of inheriting neither CF allele.

Note that this is a probability for each child. A family with four children could have zero, one or several affected children.

Worked example 2: the test cross

A breeder has a mouse with the dominant phenotype. Is it AA or Aa? Cross it with a homozygous recessive (aa) mouse.

• If it is AA: every offspring is Aa and shows the dominant phenotype.
• If it is Aa: half the offspring are Aa and half are aa (recessive phenotype).

Seeing even one recessive offspring shows the parent is heterozygous. This is a standard tool in research animal facilities.

Beyond simple dominance

Many traits do not follow the simple pattern:

• Codominance: both alleles show. ABO blood groups have alleles I^A, I^B and i. A person with I^A I^B is blood group AB, with both A and B antigens on their red cells.
• Incomplete dominance: the heterozygote is in between, like pink flowers from red × white snapdragons.
• Sex-linked traits: genes on the X chromosome, such as those for haemophilia A and red-green colour blindness, show up more often in males, who have one X.
• Polygenic traits: height and skin colour depend on many genes plus environment.

Thinking like a programmer

A Punnett square is just "every allele from parent 1 combined with every allele from parent 2". That is two nested loops. In the Do activity, you finish a short Python function that does exactly that and counts the results. One detail matters: Aa and aA are the same genotype, so the code must write the dominant allele first before counting.

Common mistakes

• Putting both of a parent's alleles in one gamete. Each gamete gets one allele.
• Reading a ratio as a guarantee. 3:1 is a probability per offspring, not an exact count in a small family.
• Assuming dominant means common. Some dominant alleles, such as the one for Huntington's disease, are rare.
• Counting "aA" separately from "Aa". They are the same heterozygous genotype.

Quick recap

• Each parent passes one allele per gene to each offspring.
• A Punnett square combines every gamete from one parent with every gamete from the other.
• Aa × Aa gives 1 AA : 2 Aa : 1 aa, or 3 dominant : 1 recessive.
• A test cross with aa reveals whether a dominant-looking parent is AA or Aa.
• Codominance, incomplete dominance, sex linkage and polygenic traits extend the simple rules.

Try it

Open the Do tab and complete a Python Punnett square counter.