PATTERNS OF INHERITANCE
(a) (i) The contribution of both environmental and genetic factors to phenotypic variation.
● To include examples of both genetic and environmental contributions – environmental
examples could include diet in animals and etiolation or chlorosis in plants.
The phenotype of an organism is influenced by both its genotype and its environment. Variation
can also be caused by the environment in plants. Mutations have contributed to the process of
evolution by increasing the number of different alleles there are for each gene. This may involve
gene mutations that if they occur during gamete formation, will be persistent through
generations, and chromosome mutations, polyploidy, which can cause speciation by creating a
fertile hybrid.
A lack of light can change gene expression to induce etiolation which increases the likelihood a
plant will reach a light source. It results in long, weak stems, smaller leaves due to longer
internodes, and chlorosis. Chlorosis occurs when plants grow in insufficient light for
photosynthesis to provide the energy to produce chlorophyll, with a lack of iron to provide
enzyme cofactors or magnesium to synthesise the chlorophyll molecule, or as a result of virus
infections that interfere with the metabolism of the cell. As a result chlorotic plants have the
genotype for making chlorophyll, but environmental factors prevent the expression of these
genes. In animals, the diet affects many characteristics, such as body mass and height.
(ii) How sexual reproduction can lead to genetic variation within a species.
● Meiosis and the random fusion of gametes at fertilisation.
Meiosis produces genetically different gametes. During meiosis, genetic variation results from
the swapping of alleles between non-sister chromatids during prophase 1, and independent
assortment during metaphase/anaphase I and II. This produces genetically unique, haploid
gametes, which fuse randomly during fertilisation with another genetically unique gamete
increases genetic diversity further.
(b) (i) The use of phenotypic ratios to identify linkage (autosomal and sex linkage) and epistasis.
● To include monogenic inheritance, dihybrid inheritance, multiple alleles, sex linkage and
co-dominance.
● To include explanations of linkage and epistasis.
Monogenic inheritance is the inheritance of a phenotype coded for by just one gene loci. If the
phenotypes are coded for by two alleles, the phenotypic ratio ratio will b 3 dominant: 1
recessive.
Dihybrid inheritance is the inheritance of two non-linked gene loci, coding for distinct
phenotypes. An F1 cross between two individuals heterozygous for both gene loci will produce
an F2 generation of four distinct phenotypes, in a ratio of 9:3:3:1.
Some genes have multiple alleles for a specific gene locus, only two of which can be expressed
on in a pair of homologous chromosomes. In addition to being determined by multiple alleles,
there are three alleles controlling the inheritance of human ABO blood groups. However,
dominance of A, and B produces four phenotypes: A, B, AB, O. A and B are dominant over O.
Codominance occurs where both alleles present in the genotype of a heterozygous individual
contribute to the individual’s phenotype as the two alleles are responsible for two distinct gene
products. If homozygous individuals of two different phenotypes are crossed, the offspring will
(a) (i) The contribution of both environmental and genetic factors to phenotypic variation.
● To include examples of both genetic and environmental contributions – environmental
examples could include diet in animals and etiolation or chlorosis in plants.
The phenotype of an organism is influenced by both its genotype and its environment. Variation
can also be caused by the environment in plants. Mutations have contributed to the process of
evolution by increasing the number of different alleles there are for each gene. This may involve
gene mutations that if they occur during gamete formation, will be persistent through
generations, and chromosome mutations, polyploidy, which can cause speciation by creating a
fertile hybrid.
A lack of light can change gene expression to induce etiolation which increases the likelihood a
plant will reach a light source. It results in long, weak stems, smaller leaves due to longer
internodes, and chlorosis. Chlorosis occurs when plants grow in insufficient light for
photosynthesis to provide the energy to produce chlorophyll, with a lack of iron to provide
enzyme cofactors or magnesium to synthesise the chlorophyll molecule, or as a result of virus
infections that interfere with the metabolism of the cell. As a result chlorotic plants have the
genotype for making chlorophyll, but environmental factors prevent the expression of these
genes. In animals, the diet affects many characteristics, such as body mass and height.
(ii) How sexual reproduction can lead to genetic variation within a species.
● Meiosis and the random fusion of gametes at fertilisation.
Meiosis produces genetically different gametes. During meiosis, genetic variation results from
the swapping of alleles between non-sister chromatids during prophase 1, and independent
assortment during metaphase/anaphase I and II. This produces genetically unique, haploid
gametes, which fuse randomly during fertilisation with another genetically unique gamete
increases genetic diversity further.
(b) (i) The use of phenotypic ratios to identify linkage (autosomal and sex linkage) and epistasis.
● To include monogenic inheritance, dihybrid inheritance, multiple alleles, sex linkage and
co-dominance.
● To include explanations of linkage and epistasis.
Monogenic inheritance is the inheritance of a phenotype coded for by just one gene loci. If the
phenotypes are coded for by two alleles, the phenotypic ratio ratio will b 3 dominant: 1
recessive.
Dihybrid inheritance is the inheritance of two non-linked gene loci, coding for distinct
phenotypes. An F1 cross between two individuals heterozygous for both gene loci will produce
an F2 generation of four distinct phenotypes, in a ratio of 9:3:3:1.
Some genes have multiple alleles for a specific gene locus, only two of which can be expressed
on in a pair of homologous chromosomes. In addition to being determined by multiple alleles,
there are three alleles controlling the inheritance of human ABO blood groups. However,
dominance of A, and B produces four phenotypes: A, B, AB, O. A and B are dominant over O.
Codominance occurs where both alleles present in the genotype of a heterozygous individual
contribute to the individual’s phenotype as the two alleles are responsible for two distinct gene
products. If homozygous individuals of two different phenotypes are crossed, the offspring will
