INHERITANCE
INHERITANCE TERMS
❏ Genotype: The genetic make-up of a characteristic.
❏ Phenotype: An observable characteristic of an organism.
ZYGOSITY
❏ Homozygous - When an organism contains 2 identical alleles for a trait
❏ Heterozygous - When an organism contains 2 different alleles for a trait.
DOMINANT AND RECESSIVE ALLELES
❏ Dominant Trait - A trait that is expressed in homozygous and
heterozygous conditions.
❏ Recessive Trait - A trait that is masked in heterozygous conditions and only
expressed in homozygous conditions.
MONOHYBRID INHERITANCE
A single characteristic passed down from parent to offspring
ALBINISM
People with albinism produce very little or no melanin in their skin, eyes,
and hair.
❏ N stimulates melanin production (dominant)
❏ n fails to stimulate melanin production (recessive)
GENETIC DIAGRAMS
A genetic diagram shows the cross between two genotypes and the possible
genotypes and phenotypes of the offspring.
GENETIC
CROSS
● B is the dominant allele for black hair.
● b is the recessive allele for red hair.
❏ Phenotypic Ratio - 3 black hair : 1 red hair
❏ Genotypic Ratio - 1 BB : 2 Bb : 1 bb
❏ 25% probability of having an offspring with red hair.
PUNNETT SQUARES
❏ Phenotypic Ratio - 1 normal pigmentation : 1 albino
❏ Genotypic Ratio - 1 Nn : 1 nn
❏ 50% of the offspring have albinism.
INCOMPLETE DOMINANCE
In incomplete dominance, both alleles are partially expressed in the
heterozygous genotype, resulting in an intermediate or different phenotype.
In some plants, such as Impatiens, R produces red flowers, r produces white
flowers, and Rr produces pink flowers (a blending of red and white flowers).
CO-DOMINANCE
In co-dominance, both alleles are equally expressed in the heterozygous
genotype. There is no blending in the phenotypes
SICKLE CELL
ANAEMIA
The normal allele HbA stimulates the
production of normal haemoglobin A,
the abnormal allele HbS stimulates the
production of abnormal haemoglobin S.
These alleles show co-dominance.
SICKLE CELL ANAEMIA
MULTIPLE ALLELES IN BLOOD GROUPS
ABO blood groups are controlled by a single gene with three alleles, but
only 2 are present in a cell. IA and IB are co-dominant to each other, and
both are dominant to IO.
❏ The A and B alleles produce A or B antigens.
❏ The O allele does not produce an antigen.
ABO BLOOD GROUPS
PEDIGREE CHART
A pedigree chart shows the occurrence of a particular characteristic in a family
tree. This information is used by genetic counsellors to identify potential risks for
future offspring developing a genetic disorder.
Phenylthiocarbamide
PTC tastes bitter to some people and is tasteless to others.
● T is the dominant allele for tasting PTC
● t is the recessive allele for not tasting PTC.
Phenylthiocarbamide
● All the unshaded individuals must be tt since they have the recessive trait.
● Individual 1, 4, and 10 must be Tt since they all produced at least one
offspring with the recessive trait.
● Individuals 15, 16 and 18 could be TT or Tt since both parents were
heterozygous.
SEX DETERMINATION
The X and Y chromosomes determine
someone’s sex.
❏ XX is female
❏ XY is male.
❏ Phenotypic ratio - 1 female : 1 male
❏ Genotypic ratio - 1 XX : 1 XY
❏ 50% of the offspring will be male.
SEX-LINKED CHARACTERISTICS
Sex-linked characteristics are characteristics determined by genes carried on
the sex chromosomes, particularly the X chromosome. Hemophilia and color
blindness are recessive sex-linked diseases. For sex-linked conditions:
❏ Only females can be carriers (males
cannot be carriers).
❏ Males will always inherit a sex-linked
trait from their mother as they inherit
a Y chromosome from their father.
❏ Females cannot inherit a sex-linked
recessive condition from an
unaffected father
❏ Sex-linked recessive traits are more
common in males
SEX-LINKED
INHERITANCE
HEMOPHILIA
Haemophilia is a sex-linked condition where the blood fails to clot at a cut, which
takes a long time to heal.
❏ The dominant allele, H, causes blood to clot normally.
❏ The recessive allele, h, causes haemophilia.
HEMOPHILIA
❏ All females will have normal blood clotting.
❏ There’s a 25% chance of having a hemophilic son
COLOR BLINDNESS
Colour blindness is a sex-linked condition where the person is unable to
distinguish differences between certain colours.
The dominant allele, N, allows normal vision and the recessive allele, n, causes
colour blindness. It is inherited in the same way as haemophilia.
COLOR BLINDNESS
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