Inheritance
Variation is when individual organisms of the same species differ in their characteristics. Variation can
be caused by environmental factors; for example, the height of a plant is affected by sunlight exposure.
However, there is also genetic variation which occurs because of inheritance; biological inheritance
refers to which phenotypes a living organism inherits from the parent organism genomes.
The genome is an organism’s complete set of genetic material (DNA) which is located within the nucleus.
DNA consists of many sections called genes; each gene codes for distinct characteristics, and the
inherited characteristic is known as the phenotype. DNA is coiled into structures known as
chromosomes which exist as pairs; within each pair is one chromosome inherited from the mother and
one from the father. The pair typically contains a different version of the same characteristic called
alleles. A specific pair of alleles are known as genotypes; for example, the eye colour genotype. The
characteristic inherited from the genotype is known as the phenotype; for example, green eye color.
So, the inherited phenotype is determined by Mendel’s laws of inheritance:
Law of segregation: Each gene has an allele, and the alleles determine the chromosome pairs.
During meiosis, the diploid parent cell segregates the chromosome pairs so that each daughter
gamete cell receives one of each allele. So, the law of segregation states that each gamete
(sperm or egg) only receives one of the two gene copies (alleles) from their parent cell by being
randomly allocated. For example, if the parent cell has the genotype Aa, half of its daughter cells
will contain the “A allele” and the other half will contain the “a allele”.
Law of independent assortment: During meiosis 1, the chromosome pairs assort themselves
independently along the equator and during meiosis 2, the chromosomes again assort
themselves independently. Independant assortment refers to how the allele pair separates and
is not inherited as a unit, instead they can form several combinations. In other words, the allele
a gamete receives for one gene does not affect the allele received for another gene.
Law of dominance: If a genotype codes for the same characteristic, it is homozygous and if a
genotype codes for a different characteristic, it is heterozygous. Recessive alleles need to be
homozygous to be inherited while dominant alleles are inherited even when heterozygous.
A monohybrid cross is when one characteristic is being interpreted and a dihybrid cross is when two
characteristics are being interpreted. The organisms being crossed are known as the parental generation
(P) and the generation they produce as the F1 (Filial) generation. If the F1 generation are crossed, the
generation they produce is called the F2 (Filial) generation.
The punnet square is a method that utilizes Mendel’s laws of inheritance to predict the inherited
phenotypes. Here is an example of a monohybrid cross punnet square:
The organism: Plant
The characteristic: Stem length
Parent phenotype:
Mother plant: long-stemmed (T)
Father plant: short-stemmed (t)
, Parent genotype:
Mother genotype: Tt
Father genotype: tt
You represent each allele of the genotype from both parent gametes with a letter of your choosing and
distinguish which allele is dominant or recessive by representing dominant alleles with capitals and the
recessive alleles with lower case. Then, the letters representing the mother gamete’s genotype are
placed as column titles and the letters representing the father gamete’s genotype are placed as row
titles. Then the letters are combined to show the possible phenotypes from which the most likely
phenotype can be observed.
Gametes T t
t Tt tt
t Tt tt
❑
The ratio of long-stemmed to short-stemmed phenotype ration is 2:2. So, the F 1❑ generation has a
50 % chance of inheriting the long-stemmed/short-stemmed phenotype.
The punnet square predicts which phenotype the next generation will inherit by displaying the parent
genotypes and crossing their alleles. The possibilities produced are based on the law of segregation and
the law of dominance:
Referring to the law of segregation, there are different phenotype possibilities because there are
different allele combinations (genotypes) that can be produced due to each gamete only receiving one
allele from each of the parent genotypes.
Then, according to the law of dominance, the phenotype that each genotype codes for depends on
which allele is recessive/dominant. The long-stemmed gene is dominant, which means it masks the
other allele even when heterozygous. This allows us to produce the phenotype ratio.
Here is an example of dihybrid cross punnet square:
The organism: Plant
The characteristics: seed shape (R/r) and seed colour (Y/y)
Parent phenotypes:
Mother plant: round (R) and yellow (Y)
Father plant: wrinkled (r) and green (y)
Parent genotypes:
Mother genotypes: RRYY
Father genotypes: rryy
You represent the allele pair of each genotype of both parent gametes by assigning a letter of your
choosing to each genotype and distinguish which allele is dominant or recessive by representing
dominant alleles with capitals and the recessive alleles with lower case. Then, the letters representing
the mother gamete’s genotype are placed as column titles and the letters representing the father’s
Variation is when individual organisms of the same species differ in their characteristics. Variation can
be caused by environmental factors; for example, the height of a plant is affected by sunlight exposure.
However, there is also genetic variation which occurs because of inheritance; biological inheritance
refers to which phenotypes a living organism inherits from the parent organism genomes.
The genome is an organism’s complete set of genetic material (DNA) which is located within the nucleus.
DNA consists of many sections called genes; each gene codes for distinct characteristics, and the
inherited characteristic is known as the phenotype. DNA is coiled into structures known as
chromosomes which exist as pairs; within each pair is one chromosome inherited from the mother and
one from the father. The pair typically contains a different version of the same characteristic called
alleles. A specific pair of alleles are known as genotypes; for example, the eye colour genotype. The
characteristic inherited from the genotype is known as the phenotype; for example, green eye color.
So, the inherited phenotype is determined by Mendel’s laws of inheritance:
Law of segregation: Each gene has an allele, and the alleles determine the chromosome pairs.
During meiosis, the diploid parent cell segregates the chromosome pairs so that each daughter
gamete cell receives one of each allele. So, the law of segregation states that each gamete
(sperm or egg) only receives one of the two gene copies (alleles) from their parent cell by being
randomly allocated. For example, if the parent cell has the genotype Aa, half of its daughter cells
will contain the “A allele” and the other half will contain the “a allele”.
Law of independent assortment: During meiosis 1, the chromosome pairs assort themselves
independently along the equator and during meiosis 2, the chromosomes again assort
themselves independently. Independant assortment refers to how the allele pair separates and
is not inherited as a unit, instead they can form several combinations. In other words, the allele
a gamete receives for one gene does not affect the allele received for another gene.
Law of dominance: If a genotype codes for the same characteristic, it is homozygous and if a
genotype codes for a different characteristic, it is heterozygous. Recessive alleles need to be
homozygous to be inherited while dominant alleles are inherited even when heterozygous.
A monohybrid cross is when one characteristic is being interpreted and a dihybrid cross is when two
characteristics are being interpreted. The organisms being crossed are known as the parental generation
(P) and the generation they produce as the F1 (Filial) generation. If the F1 generation are crossed, the
generation they produce is called the F2 (Filial) generation.
The punnet square is a method that utilizes Mendel’s laws of inheritance to predict the inherited
phenotypes. Here is an example of a monohybrid cross punnet square:
The organism: Plant
The characteristic: Stem length
Parent phenotype:
Mother plant: long-stemmed (T)
Father plant: short-stemmed (t)
, Parent genotype:
Mother genotype: Tt
Father genotype: tt
You represent each allele of the genotype from both parent gametes with a letter of your choosing and
distinguish which allele is dominant or recessive by representing dominant alleles with capitals and the
recessive alleles with lower case. Then, the letters representing the mother gamete’s genotype are
placed as column titles and the letters representing the father gamete’s genotype are placed as row
titles. Then the letters are combined to show the possible phenotypes from which the most likely
phenotype can be observed.
Gametes T t
t Tt tt
t Tt tt
❑
The ratio of long-stemmed to short-stemmed phenotype ration is 2:2. So, the F 1❑ generation has a
50 % chance of inheriting the long-stemmed/short-stemmed phenotype.
The punnet square predicts which phenotype the next generation will inherit by displaying the parent
genotypes and crossing their alleles. The possibilities produced are based on the law of segregation and
the law of dominance:
Referring to the law of segregation, there are different phenotype possibilities because there are
different allele combinations (genotypes) that can be produced due to each gamete only receiving one
allele from each of the parent genotypes.
Then, according to the law of dominance, the phenotype that each genotype codes for depends on
which allele is recessive/dominant. The long-stemmed gene is dominant, which means it masks the
other allele even when heterozygous. This allows us to produce the phenotype ratio.
Here is an example of dihybrid cross punnet square:
The organism: Plant
The characteristics: seed shape (R/r) and seed colour (Y/y)
Parent phenotypes:
Mother plant: round (R) and yellow (Y)
Father plant: wrinkled (r) and green (y)
Parent genotypes:
Mother genotypes: RRYY
Father genotypes: rryy
You represent the allele pair of each genotype of both parent gametes by assigning a letter of your
choosing to each genotype and distinguish which allele is dominant or recessive by representing
dominant alleles with capitals and the recessive alleles with lower case. Then, the letters representing
the mother gamete’s genotype are placed as column titles and the letters representing the father’s
