An in-depth informative booklet on Inheritance and Evolution, covering monohybrid and dihybrid crosses, epistasis, linkage, genetic drift, speciation etc. Includes useful summary questions and the specification for this topic.
term definition example
Gene A base sequence of DNA that codes for the The gene for tongue rolling.
amino acid sequence of a polypeptide which
results in a characteristic.
Allele One or more alternative versions of the same The gene for tongue rolling has
gene. two alleles T and t.
Genotype The genetic constitution of an organism (the One person has TT and one has
different alleles it has). tt.
Phenotype The expression of the genetic constitution and One person can roll his tongue
its interaction with the environment. (What and the other cannot.
characteristics an organism has as a result of
its genes and the effects of the environment on
them).
Gene locus The position on a chromosome where a
particular gene is found.
Dominant alleles An allele whose characteristic appears in the A person with one T allele will be
of a gene phenotype even when there is only one copy. able to roll his/her tongue.
Recessive alleles An allele whose characteristic only appears in A person will only be unable to
of a gene the phenotype if two copies are present. roll the tongue if he/she has two
copies of the t allele
Co-dominant An allele whose characteristics appear together A plant with an allele for red
alleles of a gene with another allele in the phenotype because flowers and an allele for white
neither allele is recessive. flowers will have pink flowers.
Multiple alleles More than two alleles of a gene. There are three alleles of the
blood group type gene
Diploid A cell with two of each type of chromosome All human cells are diploid
(homologous pairs of chromosomes).
Haploid A cell with only one of each type of Human gametes are haploid
chromosome
Heterozygous Two different alleles at the same gene locus on Tt
genotype each of a pair of chromosomes
Homozygous Two of the same alleles at the same gene TT or tt
genotype locus on each of a pair of chromosomes
Monohybrid The inheritance of a single characteristic The inheritance of tongue rolling
inheritance controlled by one gene with two or more in humans.
alleles.
Dihybrid The inheritance of two characteristics which The inheritance of tongue rolling
inheritance are controlled by different genes. and blood type in humans.
Sex-linked Alleles that code for the characteristic are Recessive alleles for
characteristic located on the sex chromosomes (X or Y). haemophilia, red-green colour-
blindness.
Autosomal A chromosome that is not a sex chromosome. The 22 pairs of chromosomes
chromosomes other than the X and Y in
(autosome) humans.
Autosomal When two genes coding for different Genes for hair colour and eye
linkage characteristics are located on the same colour are both on chromosome
chromosome and consequently are inherited 19.
together.
Epistasis When the alleles of one gene mask the
expression of alleles on another gene.
Dominant Just one copy of the epistatic allele will block
epistasis expression of the second gene
Recessive Two copies of the epistatic allele are required
epistasis to block expression of the second gene
2
,Recap of causes of genetic variation
1. Mutation: random changes in base sequences of genes to give new allele.
2. Meiosis: crossing over and independent segregation of homologous pairs.
Homologous pairs form bivalents at the
start of meiosis I.
Crossing over occurs: sections of
non-sister chromatids are exchanged to
give new combinations of alleles in
the gametes. Chiasma are the points at
which the crossing over occurs.
(singular chiasmata)
During meiosis I homologous pairs are
separated. The homologous pairs
segregate independently giving many
combinations of maternal and
paternal alleles in the gametes.
3. Random fusion of gametes: any gamete from one parent is equally likely to fuse with any gamete
from the other parent regardless of the allele combinations they contain.
3
, Monohybrid inheritance – inheritance of one gene
Genetic diagrams are used to give the probability of two parents passing on a particular phenotype to their
offspring. The diagrams have to be written in a conventional manner.
Example:
The gene for tongue rolling has two alleles T for tongue rolling t for non-rolling. Find the probability of two
parents, one a heterozygote for rolling and one unable to tongue roll, of having a child unable to tongue roll.
Parental phenotype
Parental genotype
Gametes
Offspring genotype
Offspring phenotype
Ratio
4
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