4.2 - Genetic inheritance

Phenotypic ratios (monohybrid crosses)

all one type, 1:1, 3:1

Phenotypic ratios (dihybrid crosses)

all one type, 1:1:1:1, 9:3:3:1, 3:3:1:1

Dominant allele

allele that has the same effect on the phenotype when in either the homozygous or heterozygous state

Recessive allele

allele that only has an effect on the phenotype when in homozygous state

Continuous variation

variation that doesn’t fall into categories, but has a range of measures

Discrete (discontinuous) variation

variation with clearly distinguishable categories with no intermediaries
Single nucleotide polymorphism (SNPs)

Codominance

both alleles in the genotype are expressed in the phenotype

Incomplete dominance

the phenotype of heterozygous offspring is a combination of the two homozygous phenotypes (completely new phenotype)

Sex-linked (X-linked) genes

found only on the X chromosome

Examples of x-linked diseases

haemophilia and red-green colourblindness (both recessive)

Pedigree charts

trace the pattern of inheritance of a genetic condition in a family (males squares, females circles)

Example of an autosomal dominant disease

Huntington’s disease

Phenylketonuria (PKU)

autosomal recessive condition, mutates liver enzyme tyrosine hydroxylase (turns phenylalanine into tyrosine) on chromosome 12 so decreased metabolism of amino acid phenylalanine (build up in brain and blood), affects children til puberty

Polygenes

groups of genes which together control a characteristic (often unlinked, create continuous variation like skin colour/height)

Phenotypic plasticity

ability of individual genotypes to produce different phenotypes when exposed to different environmental conditions
Polyphenism

Types of polyphenism

seasonal polyphenism (butterfly wings colourful in wet season, dull in dry season), caste polyphenism (ants can develop into queen or sterile worker ant), sexual polyphenism (sex of crocodiles determined by the temperature of egg incubation - aka ESD)

Environmental sex determination (ESD)

determination of sex based on environmental factors like temperature, nutrient availability, photoperiodism (changes in day length), location (some snails form towers: if next to female become male, if alone become female, once female cannot change back to male)

Dihybrid crosses

genetic crosses for two genes

Likelihood of genes inherited together

increased if genes are linked (loci on same chromosomes) and close together (unlikely to cross over), if unlinked (different chromosomes) then Mendelian genetics applies

Linked genes representation

Chi squared test (𝛘2)

𝛘² = 𝚺(O - E)²/E statistical test to check if the difference between observed and expected results is significant, use a table of values (degrees of freedom or df = n° of phenotypes - 1)

Example of codominance

ABO blood groups (I^AI^A = A, I^BI^B = B, I^Ai = A, ii = O, I^AI^B = AB)