Genes and inheritance

Alleles and genes

Each features is controlled by a gene, found on a chromosome

There are two copies of each chromosome and each gene in all body cells except the gametes

The gametes have only one copy of each chromosome and each gene

There are two alleles of each gene

One allele is dominant over the other allele, which is recessive

When two different alleles are in the same cell, only the dominant allele is expressed

An individual can have two dominant (homozygous), two recessive (homozygous), or one of each allele in each cell (heterozygous)

click to edit

Genotype: the alleles each cell has for a certain feature (eg. Tt)

Phenotype: the feature that results from the genotype (eg. tall)

Genetic cross example

Phenotype of parents: tall | dwarf

Genotype of parents: TT | tt

Gametes: (T) | (t)

Genotype of F1: Tt

Phenotype of F1: all tall

Gametes from F1: (T) or (t) | (T) or (t)

Genotypes of F2: cross diagram
1TT : 2Tt : 1tt

Phenotypes of F2: 3 tall : 1 dwarf

Predicted ratio

Ratios in genetic crosses are predicted ratios – in breeding experiments they are unlikely to fit exactly

Factors affecting survival:

Some pollen may not fertilise some ova

Some seedlings may die before they mature

KA chance events

Test cross

Used when you can't tell if something is heterozygous or homozygous (TT or Tt – both appear the same)

Factor under investigation: the unknown genotype of an organism showing the dominant phenotype

Every other possible variable has to be controlled including the genotype of the plant it's bred with
=> the other plant has to be homozygous recessive

  • breed the two plants
  • write out a genetic cross for both possibilities
  • predict the outcome for each
  • compare breeding experiment with predicted outcomes to see which result matches the prediction most closely

Outcomes:
–> homozygous: all offspring are dominant
–> heterozygous: half are dominant, half are recessive

Pedigree

Another genetic diagram

Horizontal lines connecting breeding partners

Vertical - horizontal - vertical lines connecting parents with offspring

Children arranged left–>right = oldest–>youngest

Codominance

If two alleles are expressed in the same phenotype, they are codominant

eg. snapdragon plants have red, white, or pink flowers
–> red flowers + white flowers –––> pink flowers
–> pink flowers + pink flowers –––> 1 red : 2 pink : 1 white

Appearance of a third phenotype => codominance

Sex chromosome

Determined by X and Y chromosomes

Two Xs in every female cell except egg,
one X and one Y in every male cell except sperm

Genetic cross

Parent phenotype: male | female

Parent genotype: XY | XX

Gametes: (X) or (Y) | (X)

Ratio of genotypes: 50% XX : 50% XY

Ratio of phenotypes: 50% female : 50% male

Polygenic inheritance

A characteristic controlled by two or more genes working together

eg. skin colour – several genes where some promote melanin production and some do not
=> produces a wide range of phenotypes

eg. human weight and body mass (weight)