TERMINOLOGY
DNA – Deoxyribonucleic acid, double stranded cellular nuclear material inherited from
parents to offspring. Carrier of genetic information.
Chromosome – Condensed form of DNA arranged around histones.
Gene – Fundamental unit of heredity; the sequence of nucleotides encoding a specific trait.
Allele – Alternative forms of a gene.
o Recessive – Where the allele has no effect on the phenotype in the presence of the
dominant allele. The phenotype is only seen when two of the same alleles co-occur.
o Dominant – The phenotype is determined by the allele irrespective of the second
allele.
Homozygote – When an individual has two identical alleles of a particular trait.
Heterozygote – When an individual has two different alleles of a particular trait; contributes
more to variation.
Hemizygote – When the phenotype is determined by a single allele because of the absence
of the second allele in a diploid organism.
o Absence of the second X-chromosome in males
Mendelian inheritance – When alleles contributing to a trait segregate in a predictable
manner according to expectation from a single locus as postulated by Gregor Mendel.
Haplotype – Set of genetic determinants (alleles) located on a single chromosome; inherited
together from a single parent.
Haplogroup – Aset of genetic markers inherited as a unit from one parent, shared by
descendants of the same ancestor.
o They basically are haplotypes inherited from the same ancestor.
MONOGENIC INHERITANCE
Mendel used pea plants to discover how traits are inherited.
o He used pea plants since they reproduce at a faster rate.
o They give numerous offspring.
o And it was ethical since he could inbreed them.
He found out that: -
o Phenotype is the observable traits of an organism.
o Genotype is the genetic sequence that confers the phenotype.
o Dominant alleles or traits are expressed in the F1 generation.
o Recessive alleles are expressed in the F2 generation but not in the f1 generation
(when crossing opposite homozygote parents, e.g; TT X tt).
o Segregation is the separation of members of a gene pair during gamete formation.
This occurs randomly and contributes to variation.
PEDIGREES AND DIFFERENT MODES/TYPES OF INHERITANCE
AUTOSOMAL DOMINANT TRAITS
,
Homozygous dominant and heterozygous individuals will express the trait.
Every affected individual should have at least one affected parent.
Because it is autosomal, males and females are affected equally.
Homozygous dominant genotype severely affected.
Heterozygous parents have a 25% chance of getting an unaffected child.
AUTOSOMAL RECESSIVE TRAITS
Male and females are affected equally.
Rare traits with most affected individuals having unaffected parents.
Two homozygous affected parents all affected children.
Two heterozygous unaffected parents 25% affected children.
Some examples are Cystic Fibrosis and Sickle Cell Anaemia and Tay Sachs Syndrome. (SCA has
the double effect in the heterozygous form of being resistant to malaria, where TSS also has
the double effect where the heterozygous form are resistant to Tuberculosis).
X-LINKED DOMINANT TRAITS
X-linked is a pattern of inheritance that results from genes located on the X chromosome.
Affected males produce all affected daughters and no affected sons.
A heterozygous affected female will transmit the trait to half of her children, sons and
daughters equally.
DNA – Deoxyribonucleic acid, double stranded cellular nuclear material inherited from
parents to offspring. Carrier of genetic information.
Chromosome – Condensed form of DNA arranged around histones.
Gene – Fundamental unit of heredity; the sequence of nucleotides encoding a specific trait.
Allele – Alternative forms of a gene.
o Recessive – Where the allele has no effect on the phenotype in the presence of the
dominant allele. The phenotype is only seen when two of the same alleles co-occur.
o Dominant – The phenotype is determined by the allele irrespective of the second
allele.
Homozygote – When an individual has two identical alleles of a particular trait.
Heterozygote – When an individual has two different alleles of a particular trait; contributes
more to variation.
Hemizygote – When the phenotype is determined by a single allele because of the absence
of the second allele in a diploid organism.
o Absence of the second X-chromosome in males
Mendelian inheritance – When alleles contributing to a trait segregate in a predictable
manner according to expectation from a single locus as postulated by Gregor Mendel.
Haplotype – Set of genetic determinants (alleles) located on a single chromosome; inherited
together from a single parent.
Haplogroup – Aset of genetic markers inherited as a unit from one parent, shared by
descendants of the same ancestor.
o They basically are haplotypes inherited from the same ancestor.
MONOGENIC INHERITANCE
Mendel used pea plants to discover how traits are inherited.
o He used pea plants since they reproduce at a faster rate.
o They give numerous offspring.
o And it was ethical since he could inbreed them.
He found out that: -
o Phenotype is the observable traits of an organism.
o Genotype is the genetic sequence that confers the phenotype.
o Dominant alleles or traits are expressed in the F1 generation.
o Recessive alleles are expressed in the F2 generation but not in the f1 generation
(when crossing opposite homozygote parents, e.g; TT X tt).
o Segregation is the separation of members of a gene pair during gamete formation.
This occurs randomly and contributes to variation.
PEDIGREES AND DIFFERENT MODES/TYPES OF INHERITANCE
AUTOSOMAL DOMINANT TRAITS
,
Homozygous dominant and heterozygous individuals will express the trait.
Every affected individual should have at least one affected parent.
Because it is autosomal, males and females are affected equally.
Homozygous dominant genotype severely affected.
Heterozygous parents have a 25% chance of getting an unaffected child.
AUTOSOMAL RECESSIVE TRAITS
Male and females are affected equally.
Rare traits with most affected individuals having unaffected parents.
Two homozygous affected parents all affected children.
Two heterozygous unaffected parents 25% affected children.
Some examples are Cystic Fibrosis and Sickle Cell Anaemia and Tay Sachs Syndrome. (SCA has
the double effect in the heterozygous form of being resistant to malaria, where TSS also has
the double effect where the heterozygous form are resistant to Tuberculosis).
X-LINKED DOMINANT TRAITS
X-linked is a pattern of inheritance that results from genes located on the X chromosome.
Affected males produce all affected daughters and no affected sons.
A heterozygous affected female will transmit the trait to half of her children, sons and
daughters equally.
