Potential command term assessment statements
Topic 3: Genetics
3.4 Inheritance
Essential Idea: The inheritance of genes follows patterns.
Nature of science: Making quantitative measurements with replicates to ensure reliability. Mendel’s
genetic crosses with pea plants generated numerical data.
State that Mendel discovered the principles of inheritance with experiments in which large numbers of pea
plants were crossed.
State that gametes are haploid so contain only one allele of each gene.
Outline the separation of the two alleles of each gene into different haploid daughter nuclei during meiosis.
Explain how the fusion of gametes results in diploid zygotes with two alleles of each gene that may be the
same allele or different alleles.
Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, homozygous, heterozygous,
carrier and test cross.
Genotype: the (symbolic representations of the) alleles of an organism (AA, Aa, aa).
Phenotype: the characteristics of an organism.
Homozygous: having two identical alleles of a gene (AA or aa).
Heterozygous: having two different alleles of a gene (Aa).
Dominant allele: an allele (A) that has the same effect on the phenotype whether it is present the
homozygous or heterozygous state (AA of Aa).
Recessive allele: an allele (a) that only has an effect on the phenotype when present in the
homozygous state (aa).
Codominant alleles: pairs of alleles (IA and IB) that both affect the phenotype (AB blood group) when
present in a heterozygote (IAIB).
Carrier: an individual that has one copy of a recessive allele (heterozygous Aa) that causes genetic
disease (albinism) in individuals that are homozygous (aa) for this allele.
Test cross: testing a suspected heterozygote by crossing it with a known homozygous recessive (Aa x
aa give mixed offspring showing both the dominant and recessive phenotypes. AA x aa gives only
offspring showing the dominant phenotype).
Construct Punnett grids to predict the outcomes of monohybrid genetic crosses. Clearly identify or label:
 parental genotypes
 gametes
 offspring genotypes
 offspring phenotypes
 genotypic and phenotypic ratios.
Compare predicted and actual outcomes of genetics crosses using real data.
Potential command term assessment statements
Describe ABO Blood groups as an example of codominance and multiple alleles. Dominant alleles mask the
effects of recessive alleles but co-dominant alleles have joint effects.
Phenotype
Genotype
O
ii
A
IAIA or IAIa
B
IBIB or IBi
AB
IAIB
State that many genetic diseases have been identified in humans but most are very rare.
Describe how many genetic diseases in humans are due to recessive alleles of autosomal genes, although
some genetic diseases are due to dominant or co-dominant alleles.
 Cystic fibrosis: autosomal recessive, FF=no disease, Ff=carrier of disease, ff=cystic fibrosis.
 Huntington’s chorea: autosomal dominant, hh= no disease, HH and Hh=Huntington’s disease.
 Sickle Cell Anemia: autosomal codominant, HbAHbA=normal, HbSHbS=sickle cell disease, HbAHbS=sickle
cell trait
State that some genes are present on the X chromosome and absent from the shorter Y chromosome in
humans.
Explain how some genetic diseases are sex-linked. The pattern of inheritance is different with sex-linked genes
due to their location on sex chromosomes.
 Red-Green colour blindness: XBY and XBXB=normal, XBXb=carrier, XbY and XbXb=color blind.
 Hemophilia: XHY and XHXH=normal, XHXh=carrier, XhY and XhXh=hemophilia.
Analyze pedigree charts to deduce the genotypes and phenotypes of individuals and pattern of inheritance of
genetic disease.
Discuss how radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and
cancer including the consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl.
Theory of knowledge: Mendel’s theories were not accepted by the scientific community for a long
time. What factors would encourage the acceptance of new ideas by the scientific community?