Monohybrid InheritanceMonohybrid Inheritance

Section 11Section 11

Monohybrid InheritanceMonohybrid Inheritance

Gregor Mendel (1822-1884):Gregor Mendel (1822-1884): - Used varieties of pea plant- Used varieties of pea plant - Ensured large numbers were used- Ensured large numbers were used - Studied only 1 characteristic- Studied only 1 characteristic - Observed 3:1 phenotypic ratio- Observed 3:1 phenotypic ratio Principle of segregation:Principle of segregation: ‘ ‘ the alleles of a the alleles of a

gene exist in pairs but when gametes are gene exist in pairs but when gametes are formed, the numbers of each pair pass formed, the numbers of each pair pass into different gametes. Thus each gamete into different gametes. Thus each gamete contains only one allele of each gene.contains only one allele of each gene.

Homozygous (TT) or (tt) – 2 of the Homozygous (TT) or (tt) – 2 of the same allelesame allele

Heterozygous (Tt) – 2 different allelesHeterozygous (Tt) – 2 different alleles

Law of probabilityLaw of probability Most monohybrid crosses Most monohybrid crosses

give roughly 3:1 give roughly 3:1 phenotypic ratiophenotypic ratio

1 in 4 chance of a 1 in 4 chance of a recessive allele being recessive allele being expressedexpressed

200 animals – roughly 200 animals – roughly 150 have 1 phenotype, 150 have 1 phenotype, 50 will have another50 will have another

Since fertilisation is Since fertilisation is random – rarely 3:1 random – rarely 3:1 exactlyexactly

Monohybrid Inheritance in Monohybrid Inheritance in humanshumans

Family trees used to predict Family trees used to predict genetic trendsgenetic trends

E.g. tongue rollingE.g. tongue rolling Rhesus D antigen:Rhesus D antigen: D-antigen – found on some D-antigen – found on some

blood cellsblood cells Present = Rhesus positive blood Present = Rhesus positive blood

(Rh+)(Rh+) Absent = Rhesus negative (Rh-)Absent = Rhesus negative (Rh-) Rh- people produce anti-D Rh- people produce anti-D

antibodies –’sensitised’antibodies –’sensitised’ Antigen-D present (DD/Dd)Antigen-D present (DD/Dd) Antigen-D absent (dd)Antigen-D absent (dd)

Monohybrid Inheritance in Monohybrid Inheritance in humanshumans

Albinism:Albinism: Inability to make melanin Inability to make melanin

– pigment for skin, eyes, – pigment for skin, eyes, hair etc.hair etc.

Is a recessive traitIs a recessive trait AA or Aa = normalAA or Aa = normal aa = albinoaa = albino Other examplesOther examples - Cystic Fibrosis (mucus - Cystic Fibrosis (mucus

secretion)secretion) - Phenylketonuria (PKU)- Phenylketonuria (PKU) - both are recessive traits- both are recessive traits

Monohybrid Inheritance in Monohybrid Inheritance in humanshumans

Huntingdon’s Chorea:Huntingdon’s Chorea: Determined by a Determined by a

dominant alleledominant allele Only expressed after Only expressed after

average of 38 years oldaverage of 38 years old 50% chance of children 50% chance of children

of Huntingdon’s sufferer of Huntingdon’s sufferer getting the disordergetting the disorder

1 in 20,000 cases1 in 20,000 cases

Incomplete dominance – sickle Incomplete dominance – sickle cell anaemiacell anaemia

Mutation of haemoglobin gene – Mutation of haemoglobin gene – haemoglobin S synthesisedhaemoglobin S synthesised

Homozygous (SS) – Homozygous (SS) – Sickle cell Sickle cell anaemiaanaemia

- reduced oxygen carrying - reduced oxygen carrying capacitycapacity

- blood cells malformed (sickle-- blood cells malformed (sickle-shaped)shaped)

Heterozygous – (HS) – Heterozygous – (HS) – Sickle cell Sickle cell traittrait

- both types of haemoglobin - both types of haemoglobin presentpresent

- neither H or S are dominant - neither H or S are dominant (incomplete dominance)(incomplete dominance)

- sickle cell = malaria resistance- sickle cell = malaria resistance

Incomplete / Co-DominanceIncomplete / Co-Dominance

Incomplete – both alleles expressed in the Incomplete – both alleles expressed in the phenotype, but unevenlyphenotype, but unevenly

- sickle-cell: more H than S expressed- sickle-cell: more H than S expressed Co-Dominance - both alleles Co-Dominance - both alleles equallyequally

expressed in the phenotype expressed in the phenotype - AB blood – exactly 50:50 split - AB blood – exactly 50:50 split - MN blood - another blood type antigen- MN blood - another blood type antigen - no antibodies produced - no antibodies produced - equal expression (equal - equal expression (equal

dominancedominance

Multiple AllelesMultiple Alleles 3 or more alleles for a 3 or more alleles for a

characteristiccharacteristic 3 alleles = 6 possible genotypes3 alleles = 6 possible genotypes No. of phenotypes depend on the No. of phenotypes depend on the

type of dominance type of dominance – – complete, incomplete or co-complete, incomplete or co-

dominancedominance E.g ABO blood – 6 genotypes, 4 E.g ABO blood – 6 genotypes, 4

phenotypesphenotypes