Biological Basis of Inheritance
Inheritance Patterns
● Females have two homogolous X chromosomes
● Males have an X chromosome and a Y chromosome
○ Y chromosomes carries the SRY (sex-determining region of Y)gene
needed for creating the testes.
○ Absence of SRY creates the ovaries
Sex-linked genes
● Sex-linked genes - Genes located on either sex chromosome
○ Y-linked genes - Genes located on the Y chromosome
○ X-linked genes - Genes located on the X chromosome
● Sex-linked diseases often more in males
○ Any male that receiving a recessive allele from its mother will express the
trait meanwhile females will need to obtain both recessive traits for the
trait to be apparent. Thus males often have more x-linked traits.
Real examples, Patterns
Colour blindness
● Affected father and a carrier mother can pass along the trait to their
daughter.
Duchenne muscular dystrophy
● Progressively weakens muscles and the loss of coordination
● Absence of muscle protein called dystrophy
● Located on X chromosome
Hemophilia
● Absence of proteins required for blood clotting
● Larger cuts and bleeding in joints can be problematic
● X-linked recessive disorder
Linked vs. Sex-linked genes
● Linked genes
○ Genes located near another on same chromosome are normally inherited
together
○ Two or more genes on the same chromosome
● Sex-linked genes
○ Single gene on X chromosome
, Independent assortment in Meiosis I
● Two genes located on different chromosomes, thus are unlinked.
● Random orientation of homologous chromosomes at metaphase I which leads to
independent assortment of unlinked genes.
Linked genes
● Link genes are within the same chromosome thus independent
assortments do not affect these effects
● Crossing over accounts for gene recombination, as part of one
chromosome exchanges a region of the other chromosome causing
alleles that were once linked to be seperated
Parental Phenotypes
● Offspring expected to inherit a phenotype that matches either parental
phenotypes
Recombinant Phenotypes
● Offspring that form new phenotype combinations
Abnormal chromosome number
● Cause by nondisjunction
○ Members of a pair of homologous chromosome that fail to move apart
properly during meiosis I or sister chromatids during meiosis II
Trisomy: chromosome that is present in the triplicate in the zygote (2n+) chromosomes.
Mitosis will transmit the abnormality to all the embryonic cells, producing a set of traits
caused from abnormal dose of genes. (missing or extra chromosome)
● Polyploidy
○ Organisms which have more than two complete chromosome sets in all
somatic cells
○ Triploidy (3n)
■ Three chromosome sets
■ Occurs from fertilization of an abnormal diploid egg produced by
nondisjunction of all its chromosomes
○ Tetraploidy (4n)
■ Four chromosome sets
■ Occurs from the failure of a 2n zygote to divide after replicating its
chromosomes
Inheritance Patterns
● Females have two homogolous X chromosomes
● Males have an X chromosome and a Y chromosome
○ Y chromosomes carries the SRY (sex-determining region of Y)gene
needed for creating the testes.
○ Absence of SRY creates the ovaries
Sex-linked genes
● Sex-linked genes - Genes located on either sex chromosome
○ Y-linked genes - Genes located on the Y chromosome
○ X-linked genes - Genes located on the X chromosome
● Sex-linked diseases often more in males
○ Any male that receiving a recessive allele from its mother will express the
trait meanwhile females will need to obtain both recessive traits for the
trait to be apparent. Thus males often have more x-linked traits.
Real examples, Patterns
Colour blindness
● Affected father and a carrier mother can pass along the trait to their
daughter.
Duchenne muscular dystrophy
● Progressively weakens muscles and the loss of coordination
● Absence of muscle protein called dystrophy
● Located on X chromosome
Hemophilia
● Absence of proteins required for blood clotting
● Larger cuts and bleeding in joints can be problematic
● X-linked recessive disorder
Linked vs. Sex-linked genes
● Linked genes
○ Genes located near another on same chromosome are normally inherited
together
○ Two or more genes on the same chromosome
● Sex-linked genes
○ Single gene on X chromosome
, Independent assortment in Meiosis I
● Two genes located on different chromosomes, thus are unlinked.
● Random orientation of homologous chromosomes at metaphase I which leads to
independent assortment of unlinked genes.
Linked genes
● Link genes are within the same chromosome thus independent
assortments do not affect these effects
● Crossing over accounts for gene recombination, as part of one
chromosome exchanges a region of the other chromosome causing
alleles that were once linked to be seperated
Parental Phenotypes
● Offspring expected to inherit a phenotype that matches either parental
phenotypes
Recombinant Phenotypes
● Offspring that form new phenotype combinations
Abnormal chromosome number
● Cause by nondisjunction
○ Members of a pair of homologous chromosome that fail to move apart
properly during meiosis I or sister chromatids during meiosis II
Trisomy: chromosome that is present in the triplicate in the zygote (2n+) chromosomes.
Mitosis will transmit the abnormality to all the embryonic cells, producing a set of traits
caused from abnormal dose of genes. (missing or extra chromosome)
● Polyploidy
○ Organisms which have more than two complete chromosome sets in all
somatic cells
○ Triploidy (3n)
■ Three chromosome sets
■ Occurs from fertilization of an abnormal diploid egg produced by
nondisjunction of all its chromosomes
○ Tetraploidy (4n)
■ Four chromosome sets
■ Occurs from the failure of a 2n zygote to divide after replicating its
chromosomes
