Topic 10.2: InHERITAnCE
InHERITAnCE!!
Unlinked Genes Linked Genes
The inheritance of two different genes / traits will occur A linkage group describes a group of genes whose loci are
independently provided the genes are on separate chromosomes on the same chromosome (i.e. linked genes)
¥ This is because unlinked genes segregate independently ¥ Linked genes will function as a single inheritable unit
during meiosis (random assortment of homologous pairs) and will not follow the law of independent assortment
MendelÕs Law of Independent Assortment Linked genes are represented as pairs:
¥ Separation of alleles for one gene occurs independently
AB one chromosome
of the separation of alleles for another gene
one chromosome ab
Dihybrid Crosses
Linked genes can be separated (unlinked) by crossing over
Dihybrid crosses determine allele combinations of offspring ¥ The novel allele combinations are called recombinants
for two genes that are unlinked (on different chromosomes)
¥ 2 genes × 2 alleles = 4 potential gamete combinations Recombinant phenotypes will only be evident if crossing
over has occurred and thus occur at lower frequencies
Gamete combinations are calculated via the FOIL method: ¥ Frequency increases the further apart the two genes are
¥ First (AaBb = AB)
¥ Outside (AaBb = Ab)
Linkage Studies
¥ Inside (AaBb = aB) A a B b
¥ Last (AaBb = ab) Thomas Morgan determined the concept of gene linkage
via breeding experiments involving fruit flies (Drosophila)
Once gamete combinations are identified, a punnett square is
Morgan identified a number of different traits that did not
then used to calculate genotype and phenotype frequencies
conform to Mendelian ratios and surmised the following:
¥ These traits represented linked genes (on same chromosome)
YR Yr yR yr ¥ Linked genes can be uncoupled via recombination, but
Example:
such events are uncommon (hence occur at low frequencies)
Pea seed cross
YR
(heterozygous) YYRR YYRr YyRR YyRr
YyRr × YyRr Yr Grey Grey
YYRr YYrr YyRr Yyrr Black Black
normal vestigial vestigial normal
Y = yellow
y = green yR
YyRR YyRr yyRR yyRr
R = round
r = wrinkled yr
YyRr Yyrr yyRr yyrr Normal offspring Recombinants
Polygenic Inheritance
Variation can be discrete (finite patterns) or continuous (normal distribution)
Monogenic traits are characteristics that are controlled by a single gene locus
¥ They therefore have a finite pattern of expression (discrete variation)
Polygenic traits are characteristics controlled by more than two gene loci
¥ They exhibit a bell-shaped distribution pattern (continuous variation)
1 gene 4 genes
Polygenic traits (e.g. human height) may also be influenced by environment (monogenic) (polygenic)
InHERITAnCE!!
Unlinked Genes Linked Genes
The inheritance of two different genes / traits will occur A linkage group describes a group of genes whose loci are
independently provided the genes are on separate chromosomes on the same chromosome (i.e. linked genes)
¥ This is because unlinked genes segregate independently ¥ Linked genes will function as a single inheritable unit
during meiosis (random assortment of homologous pairs) and will not follow the law of independent assortment
MendelÕs Law of Independent Assortment Linked genes are represented as pairs:
¥ Separation of alleles for one gene occurs independently
AB one chromosome
of the separation of alleles for another gene
one chromosome ab
Dihybrid Crosses
Linked genes can be separated (unlinked) by crossing over
Dihybrid crosses determine allele combinations of offspring ¥ The novel allele combinations are called recombinants
for two genes that are unlinked (on different chromosomes)
¥ 2 genes × 2 alleles = 4 potential gamete combinations Recombinant phenotypes will only be evident if crossing
over has occurred and thus occur at lower frequencies
Gamete combinations are calculated via the FOIL method: ¥ Frequency increases the further apart the two genes are
¥ First (AaBb = AB)
¥ Outside (AaBb = Ab)
Linkage Studies
¥ Inside (AaBb = aB) A a B b
¥ Last (AaBb = ab) Thomas Morgan determined the concept of gene linkage
via breeding experiments involving fruit flies (Drosophila)
Once gamete combinations are identified, a punnett square is
Morgan identified a number of different traits that did not
then used to calculate genotype and phenotype frequencies
conform to Mendelian ratios and surmised the following:
¥ These traits represented linked genes (on same chromosome)
YR Yr yR yr ¥ Linked genes can be uncoupled via recombination, but
Example:
such events are uncommon (hence occur at low frequencies)
Pea seed cross
YR
(heterozygous) YYRR YYRr YyRR YyRr
YyRr × YyRr Yr Grey Grey
YYRr YYrr YyRr Yyrr Black Black
normal vestigial vestigial normal
Y = yellow
y = green yR
YyRR YyRr yyRR yyRr
R = round
r = wrinkled yr
YyRr Yyrr yyRr yyrr Normal offspring Recombinants
Polygenic Inheritance
Variation can be discrete (finite patterns) or continuous (normal distribution)
Monogenic traits are characteristics that are controlled by a single gene locus
¥ They therefore have a finite pattern of expression (discrete variation)
Polygenic traits are characteristics controlled by more than two gene loci
¥ They exhibit a bell-shaped distribution pattern (continuous variation)
1 gene 4 genes
Polygenic traits (e.g. human height) may also be influenced by environment (monogenic) (polygenic)
