SPM Biology genetics problems

Genotype vs Phenotype

The difference between genotype and phenotype, homozygous vs heterozygous, and dominant vs recessive alleles, with a worked example.

Mendel's First Law (Law of Segregation)

Mendel's law of segregation explained, what it states, why it happens during meiosis, and how it produces gametes with a single allele.

Punnett Square Method

The step-by-step method to set up and read a Punnett square for predicting the genotypes and phenotypes of offspring.

Monohybrid Cross

How a monohybrid cross works, from parental generation to F1 and F2, with a fully worked Tt x Tt example giving a 3:1 ratio.

Monohybrid Ratios: 3:1 vs 1:1

When a monohybrid cross gives a 3:1 phenotype ratio and when it gives a 1:1 ratio, and how to tell which cross you are dealing with.

Test Cross

What a test cross is, why it is used to find an unknown genotype, and a worked example distinguishing TT from Tt.

Incomplete Dominance

How incomplete dominance produces an intermediate phenotype in heterozygotes, with a worked red x white four o'clock flower cross giving a 1:2:1 ratio.

Codominance

How codominance differs from incomplete dominance, with roan cattle coat colour and the AB blood group as worked examples.

Multiple Alleles

What multiple alleles means, introduced through the ABO blood group system, where three alleles exist for one gene.

ABO Blood Groups

The ABO blood group alleles I^A, I^B and i, the genotype-phenotype table, and a worked cross predicting children's blood groups.

Mendel's Second Law (Law of Independent Assortment)

Mendel's law of independent assortment explained, what it states, and how it produces the four gamete types behind a dihybrid cross.

Dihybrid Cross

How a dihybrid cross tracks two genes at once, with a fully worked TtYy x TtYy example giving the classic 9:3:3:1 ratio.

How the 9:3:3:1 Dihybrid Ratio Is Derived

A step-by-step breakdown of where the 9:3:3:1 phenotype ratio in a dihybrid cross comes from, using probability and the 4 x 4 grid.

Sex Determination

How the X and Y chromosomes determine sex in humans, with a Punnett square showing the 1:1 male-to-female ratio.

Sex-Linked Inheritance

How genes carried on the X chromosome are inherited, and why X-linked recessive conditions affect males more often than females.

Colour Blindness

Colour blindness as an X-linked recessive condition, with a worked cross between a carrier mother and a normal-vision father.

Haemophilia

Haemophilia as an X-linked recessive blood-clotting disorder, with a worked cross and the resulting offspring ratios.

Pedigree Analysis

How to read a pedigree (family tree) diagram, the standard symbols used, and how to deduce genotypes and the mode of inheritance.

Genetics Answering Technique

How to lay out a full genetics answer for SPM, defining the key, writing parental genotypes, gametes in circles, the Punnett square, offspring, and ratio.

Continuous vs Discontinuous Variation

The difference between continuous and discontinuous variation, their causes, examples, and the shape of graph each one produces.