Transcript Gene Linkage and Polygenic Traits

Syllabus Points 10.2 and 10.3
GENE LINKAGE AND POLYGENIC TRAITS
10.2.1 DIHYBRID CROSSES
Calculate and predict the genotypic and
phenotypic ratio of offspring of dihybrid crosses
involving unlinked autosomal genes
Mendel’s law of Independent assortment states
that inheritance of one trait does not influence
the inheritance of other traits – only true if
genes are on different chromosomes or far
apart on the same chromosome

DIHYBRID CROSS OF HETEROZYGOUS PARENTS
http://users.rcn.com/jkimball.ma.ult
ranet/BiologyPages/L/Linkage.html
10.2.2 AUTOSOMAL VS SEX CHROMOSOMES

Autosomal are any chromosomes except the
sex chromosomes
10.2.3 CROSSING OVER
Crossing over exchanges alleles between nonsister chromatids during prophase 1 of meiosis
 Chiasmata are the places where the crossing
over takes place
 3 steps – enzyme cuts, swap segments
(strands find complementary sequences to join
to), ligase enzyme hooks them back together
 Must be exact switch or frameshift mutation
can result

CHIASMATA

Humans average just over two chiasmata per
chromosome
In this photomicrograph (courtesy of Prof. Bernard John), a tetrad of the
grasshopper Chorthippus parallelus shows 5 chiasmata.
10.2.4 GENE LINKAGE GROUP

Gene linkage groups are groups of alleles that
tend to be inherited together – this usually
means that they are close together on the
same chromosome
10.2.5 EXAMPLE LINKED GENES
Represented as vertical pairs:
For parents TtBb and ttbb
TB
tb

tb
tb
If no crossing over offspring are either TB or tb
If crossing over Tb or tB
LINKAGE OF KERNEL COLOR AND TEXTURE IN
CORN
10.2.6 IDENTIFY RECOMBINANTS IN DIHYBRID
CROSS OF LINKED GENES
Recombinants are the offspring that have
genotypes not found in the parents – the result
of crossing over
 The percentage of recombinant offspring is
used to calculate the distance between the two
genes on the chromosome
 Expressed in cenitmorgans (cM) so 3%
recombinants = distance of 3 cM

10.3.1 POLYGENIC INHERITANCE
Polygenic inheritance is a trait (continuous
trait) that is controlled by multiple alleles at
multiple loci
 Effect of these genes are combined to produce
phenotype
 Environment can contribute to actual
expresssion

10.3.2 CONTINUOUS VARIATION
Polygenic traits can result in continuous
variation in phenotype
 Continuous variation traits exhibit a bell curve
distribution


Height, skin color, hair and eye color
BELL CURVE HEIGHT
http://www.hf.faa.gov/webtraining/HFModel/Variance/anthropometrics2.htm
10.3.2 GENETICS OF SKIN COLOR
Primary determinant of variability is pigment
density and distribution of melanin
 Varies in populations with latitude and diet
 Balance between protection from UV and ability
to synthesize vitamin D
 Selective advantage now overcome with
sunblock and vitamin D supplements
