D3.2 Inheritance

Question 1

D3.2.1—Production of haploid gametes in parents and their fusion to form a diploid zygote as the means
of inheritance

Answer 1

Students should understand that this pattern of inheritance is common to all eukaryotes with a sexual life
cycle. They should also understand that a diploid cell has two copies of each autosomal gene.

Question 2

D3.2.2—Methods for conducting genetic crosses in flowering plants

Answer 2

Use the terms “P generation”, “F1 generation”, “F2 generation” and “Punnett grid”. Students should
understand that pollen contains male gametes and that female gametes are located in the ovary, so
pollination is needed to carry out a cross. They should also understand that plants such as peas produce
both male and female gametes on the same plant, allowing self-pollination and therefore self-fertilization.
Mention that genetic crosses are widely used to breed new varieties of crop or ornamental plants.

Question 3

D3.2.3—Genotype as the combination of alleles inherited by an organism.

Answer 3

Students should use and understand the terms “homozygous” and “heterozygous”, and appreciate the
distinction between genes and alleles

Question 4

D3.2.4—Phenotype as the observable traits of an organism resulting from genotype and environmental
factors

Answer 4

Students should be able to suggest examples of traits in humans due to genotype only and due to
environment only, and also traits due to interaction between genotype and environment.

Question 5

D3.2.5—Effects of dominant and recessive alleles on phenotype

Answer 5

Students should understand the reasons that both a homozygous-dominant genotype and a
heterozygous genotype for a particular trait will produce the same phenotype.

Question 6

D3.2.6—Phenotypic plasticity as the capacity to develop traits suited to the environment experienced by
an organism, by varying patterns of gene expression

Answer 6

Phenotypic plasticity is not due to changes in genotype, and the changes in traits may be reversible during
the lifetime of an individual.

Question 7

D3.2.7—Phenylketonuria as an example of a human disease due to a recessive allele

Answer 7

Phenylketonuria (PKU) is a recessive genetic condition caused by mutation in an autosomal gene that
codes for the enzyme needed to convert phenylalanine to tyrosine.

Question 8

D3.2.8—Single-nucleotide polymorphisms and multiple alleles in gene pools

Answer 8

Students should understand that any number of alleles of a gene can exist in the gene pool but an
individual only inherits two

Question 9

D3.2.9—ABO blood groups as an example of multiple alleles

Answer 9

Use IA
, IB
and i to denote the alleles. (a and b is small)

Question 10

D3.2.10—Incomplete dominance and codominance

Answer 10

Students should understand the differences between these patterns of inheritance at the phenotypic
level. In codominance, heterozygotes have a dual phenotype. Include the AB blood type (IA
I
B
) as an
example. In incomplete dominance, heterozygotes have an intermediate phenotype. Include four o’clock
flower or marvel of Peru (Mirabilis jalapa) as an example.

Question 11

D3.2.11—Sex determination in humans and inheritance of genes on sex chromosomes

Answer 11

Students should understand that the sex chromosome in sperm determines whether a zygote develops
certain male-typical or female-typical physical characteristics and that far more genes are carried by the X
chromosome than the Y chromosome.

Question 12

D3.2.12—Haemophilia as an example of a sex-linked genetic disorder

Answer 12

Show alleles carried on X chromosomes as superscript letters on an uppercase X.

Question 13

D3.2.13—Pedigree charts to deduce patterns of inheritance of genetic disorders

Answer 13

Students should understand the genetic basis for the prohibition of marriage between close relatives in
many societies.

Question 14

D3.2.14—Continuous variation due to polygenic inheritance and/or environmental factors

Answer 14

Use skin colour in humans as an example.
Application of skills: Students should understand the distinction between continuous variables such as
skin colour and discrete variables such as ABO blood group. They should also be able to apply measures of
central tendency such as mean, median and mode.

Question 15

D3.2.15—Box-and-whisker plots to represent data for a continuous variable such as student height

Answer 15

Application of skills: Students should use a box-and-whisker plot to display six aspects of data: outliers,
minimum, first quartile, median, third quartile and maximum. A data point is categorized as an outlier if it
is more than 1.5 × IQR (interquartile range) above the third quartile or below the first quartile.

Question 16

D3.2.16—Segregation and independent assortment of unlinked genes in meiosis

Answer 16

Students should understand the link between the movements of chromosomes in meiosis and the
outcome of dihybrid crosses involving pairs of unlinked genes

Question 17

D3.2.17—Punnett grids for predicting genotypic and phenotypic ratios in dihybrid crosses involving pairs
of unlinked autosomal genes

Answer 17

Students should understand how the 9:3:3:1 and 1:1:1:1 ratios are derived.
NOS: 9:3:3:1 and 1:1:1:1 ratios for dihybrid crosses are based on what has been called Mendel’s second law.
This law only applies if genes are on different chromosomes or are far apart enough on one chromosome
for recombination rates to reach 50%. Students should recognize that there are exceptions to all biological
“laws” under certain conditions

Question 18

D3.2.18—Loci of human genes and their polypeptide products

Answer 18

Application of skills: Students should explore genes and their polypeptide products in databases. They
should find pairs of genes with loci on different chromosomes and also in close proximity on the same
chromosome

Question 19

D3.2.19—Autosomal gene linkage

Answer 19

In crosses involving linkage, the symbols used to denote alleles should be shown alongside vertical lines
representing homologous chromosomes.
Students should understand the reason that alleles of linked
genes can fail to assort independently

Question 20

D3.2.20—Recombinants in crosses involving two linked or unlinked genes

Answer 20

Students should understand how to determine the outcomes of crosses between an individual
heterozygous for both genes and an individual homozygous recessive for both genes. Identify
recombinants in gametes, in genotypes of offspring and in phenotypes of offspring.

Question 21

D3.2.21—Use of a chi-squared test on data from dihybrid crosses

Answer 21

Students should understand the concept of statistical significance, the p = 0.05 level, null/alternative
hypothesis and the idea of observed versus expected results.
NOS: Students should recognize that statistical testing often involves using a sample to represent a
population. In this case the sample is the F2 generation. In many experiments the sample is the replicated
or repeated measurements.