Transcript Genes and Heredity - Nova Scotia Department of Education

Genes and Heredity
Test Cross
Multiple Alleles
Incomplete Dominance
Test Cross
Sheep- white wool is desirable because
black wool is too brittle and difficult to dye,
 W=dominant allele - white wool
 w=recessive allele - black wool
 “How would you know that a ram is
homozygous for the white phenotype?”

W
w
W
W
w
Ww
ww
w
Ww
Ww
w
Ww
ww
w
Ww
Ww
The test cross is always performed between an unknown
genotype and a homozygous recessive genotype . (what is it
here?) WHAT ARE THE OFFSPRING IN EACH
CASE?(%)
Multiple Alleles
It is possible to have more than two
different alleles for one trait - multiple
alleles, examples: blood types, eye colour in
Drosophila (fruit flies)
 Phenotypes Wild type, Apricot, Honey,
White. Red or wild type is dominant to
apricot is dominant to honey is dominant to
white. Capital letters and superscripts are
used.

Crosses That Involve Two Traits
When two pairs of contrasting traits are
considered in a cross, it is described as a
DIHYBRID CROSS.
 The same principles apply to such a cross as
apply to a monohybrid cross. It is more
complex because there are more possible
combinations.

Law of Independent Assortment

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RR = pure breeding round
rr = pure breeding wrinkled
YY = pure breeding yellow
yy = pure breeding green
Phenotype = pure yellow, round parent, the
genotype is RRYY
Phenotype = pure green, wrinkled parent, the
genotype is rryy
The offspring produced when these cross (F1) are
heterozygous for both the yellow and round genotypes.
YYRR Yellow/round
cross with
yyrr green/wrinkled
Gametes formed by meiosis (the alleles for these genes are
on separate chromosomes) are
YR
YR
yr
yr
When these are crossed, the F1 generation looks like this:
YR
YR
yr
YyRr YyRr
yr
YyRr YyRr
If a cross is considered between a pure green, round pea plant
and a pure yellow wrinkled pea plant, the resulting off spring
are yellow, round (all YyRr). See Figure 24.18.
Inheritance of the gene for colour is not affected by either the
wrinkled or round alleles.
This is a demonstration of the LAW OF INDEPENDENT
ASSORTMENT. Shape and colour are inherited independently.
Progressing to the F2 generation, (let the yellow, round plants
self-fertilize), each heterozygous yellow, round plant can
produce four different phenotypes,
YR, yR, Yr, yr
A modified Punnet square, dihybrid cross diagram is used to
show the phenotypes of the F2 generation.
Gametes
YR
yR
Yr
YR
YYRR
YyRR
YYRr
YyRr
yyRR
YyRr
yyRr
yR
YyRR
yr
Yr
YYRr
YyRr
YYrr
Yyrr
yr
YyRr
yyRr
Yyrr
yyrr