4.3 inheritance
Genes and alleles
● Gene = a sequence of DNA occupying a specific locus on a chromosome, normally
coding for a specific polypeptide
○ Usually exist as 2+ alleles = a different form of the same gene, coding for a
specific polypeptide
■ E.g. Rhesus blood group (positive or negative)
○ When both alleles are the same, the organism is homozygous for that gene e.g.
RR or rr
○ When both are different, the organism is heterozygous for that gene e.g. Rr
○ Locus = a specific position on a chromosome where a gene is located
● Genotype = genetic makeup of an organism i.e. the alleles it possesses
● Phenotype = characteristics of an organism, resulting from its genotype & environment
Genetic crosses
● Rules;
○ Chose a single letter to represent each allele
○ Upper case (R) represents dominant features, lowercase (r) represents recessive
features - state what each represents
■ Dominant = alleles are always expressed in the phenotype (in both
homozygote, RR and heterozygote, Rr)
■ Recessive = alleles are only expressed in the homozygote (e.g. rr)
■ Codominant = alleles that are equally expressed in a heterozygote
■ Homozygous = both alleles for a gene are identical
■ Heterozygous = both alleles for a gene are different
○ Genotypes are shown as a pair of alleles, gametes are a single allele
○ Label parents & circle gametes
○ Use a punnett square to calculate crossing
○ State the phenotype and ratios of offspring
■ First generation - F1, second generation - F2
Monohybrid inheritance
● The inheritance of a single gene
● Gregor Mendel
○ Conducted experiments with pea plants because;
■ Easy to grow, showed clear differences in phenotypes, produced large
numbers of seeds for reliability
○ Crossed peas with purple flowers with peas with white
flowers
■ F1 all had purple flowers
■ Self-pollinated F1
● F2 ratio of 3 purple : 1 white
, ● F2 probability of 75% purple, 25% white
● Mendel's first law of inheritance, the law of segregation - ‘the characteristics of an
organism are determined by factors (alleles) which occur in pairs. Only one of a pair of
factors can be present in a single gamete’
Test cross / back cross
● Performed to determine the genotype of an organism with a dominant phenotype i.e. is a
dominant characteristic determined by one or two dominant alleles e.g. PP or Pp
● Involves crossing the unknown organism with a homozygous recessive
○ E.g. crossing a purple flower pea plant (PP or Pp) with a white plant (pp)
■ If F1 all show the dominant phenotype (purple), the parent plant was
homozygous / pure-bred
■ If there were purple and white plants, with a ratio of 1:1 (P:p), the parent
plant was heterozygous / not pure-bred
Co-dominance
● In co-dominance, both alleles involved are dominant ⇾ both expressed equally
○ E.g. ABO blood group - A & B are co-dominant
● Use a letter to represent the gene and subscripts to show the alleles
○ E.g. IAIA blood group is A
IAIB blood group is AB
IBIB blood group is B
● Incomplete dominance - when the phenotype is an intermediate of both alleles, rather
than the expression of them both
○ E.g. Snapdragon flowers
■ Two colours exist: purple & white
■ When both alleles are present flowers appear pink
Dihybrid inheritance
● Dihybrid inheritance = the simultaneous inheritance of two unlinked genes (genes on
different chromosomes)
● Mendel carried out experiments with pea plants involving two different characteristics at
the same time e.g. yellow or green seeds and wrinkled or round seeds
Genes and alleles
● Gene = a sequence of DNA occupying a specific locus on a chromosome, normally
coding for a specific polypeptide
○ Usually exist as 2+ alleles = a different form of the same gene, coding for a
specific polypeptide
■ E.g. Rhesus blood group (positive or negative)
○ When both alleles are the same, the organism is homozygous for that gene e.g.
RR or rr
○ When both are different, the organism is heterozygous for that gene e.g. Rr
○ Locus = a specific position on a chromosome where a gene is located
● Genotype = genetic makeup of an organism i.e. the alleles it possesses
● Phenotype = characteristics of an organism, resulting from its genotype & environment
Genetic crosses
● Rules;
○ Chose a single letter to represent each allele
○ Upper case (R) represents dominant features, lowercase (r) represents recessive
features - state what each represents
■ Dominant = alleles are always expressed in the phenotype (in both
homozygote, RR and heterozygote, Rr)
■ Recessive = alleles are only expressed in the homozygote (e.g. rr)
■ Codominant = alleles that are equally expressed in a heterozygote
■ Homozygous = both alleles for a gene are identical
■ Heterozygous = both alleles for a gene are different
○ Genotypes are shown as a pair of alleles, gametes are a single allele
○ Label parents & circle gametes
○ Use a punnett square to calculate crossing
○ State the phenotype and ratios of offspring
■ First generation - F1, second generation - F2
Monohybrid inheritance
● The inheritance of a single gene
● Gregor Mendel
○ Conducted experiments with pea plants because;
■ Easy to grow, showed clear differences in phenotypes, produced large
numbers of seeds for reliability
○ Crossed peas with purple flowers with peas with white
flowers
■ F1 all had purple flowers
■ Self-pollinated F1
● F2 ratio of 3 purple : 1 white
, ● F2 probability of 75% purple, 25% white
● Mendel's first law of inheritance, the law of segregation - ‘the characteristics of an
organism are determined by factors (alleles) which occur in pairs. Only one of a pair of
factors can be present in a single gamete’
Test cross / back cross
● Performed to determine the genotype of an organism with a dominant phenotype i.e. is a
dominant characteristic determined by one or two dominant alleles e.g. PP or Pp
● Involves crossing the unknown organism with a homozygous recessive
○ E.g. crossing a purple flower pea plant (PP or Pp) with a white plant (pp)
■ If F1 all show the dominant phenotype (purple), the parent plant was
homozygous / pure-bred
■ If there were purple and white plants, with a ratio of 1:1 (P:p), the parent
plant was heterozygous / not pure-bred
Co-dominance
● In co-dominance, both alleles involved are dominant ⇾ both expressed equally
○ E.g. ABO blood group - A & B are co-dominant
● Use a letter to represent the gene and subscripts to show the alleles
○ E.g. IAIA blood group is A
IAIB blood group is AB
IBIB blood group is B
● Incomplete dominance - when the phenotype is an intermediate of both alleles, rather
than the expression of them both
○ E.g. Snapdragon flowers
■ Two colours exist: purple & white
■ When both alleles are present flowers appear pink
Dihybrid inheritance
● Dihybrid inheritance = the simultaneous inheritance of two unlinked genes (genes on
different chromosomes)
● Mendel carried out experiments with pea plants involving two different characteristics at
the same time e.g. yellow or green seeds and wrinkled or round seeds
