Transcript Dihybrid crosses and gene linkage

Topic 10.2
10.2.1 Calculate and predict the genotypic and
phenotypic ratio of offspring of dihybrid crosses
involving unlinked autosomal genes.
 10.2.2 Distinguish between autosomes and sex
chromosomes.
 10.2.3 Explain how crossing over between non-sister
chromatids of a homologous pair in prophase I can
result in an exchange of alleles.
 10.2.4 Define linkage group.
 10.2.5 Explain an example of a cross between two
linked genes.
 10.2.6 Identify which of the offspring are
recombinants in a dihybrid cross involving linked
genes.

 Observations:
• Seed shape – some round, others wrinkled
(allele for round is dominant)
• Seed colour – some green, others yellow (allele
for yellow is dominant)
 Mendel
crossed true breeding plants
with each other
• One parent: homozygous dominant for both traits
(round and yellow seeds)
• Other parent: homozygous recessive for both
traits (wrinkled and green)
R
= allele for round
peas
 r = allele for
wrinkled peas
 Y = allele for yellow
peas
 y = allele for green
peas
Parent
Round
phenotypes yellow
Green
wrinkled
Parent
genotypes
RRYY
rryy
Parent
gametes
RY
ry
F1
genotypes
RrYy
F1
Round
phenotypes yellow

Allowing heterozygous
offspring to selfpollinate yields
following proportions:
• Round and yellow 56.6%
• Wrinkled and yellow
18.2%
• Round and green 19.4%
• Wrinkled and green 8%

Ratio
• 9:3:3:1
 Sex chromosomes: X and Y (one pair)
 Autosomes: any chromosome not X or Y
(22
pairs)
 Sex-linked gene or trait is located on a sex
chromosome.
 Autosomal gene or trait is located on one of
the autosomes.
 On which type of chromosome is the gene
for protein production in the testes found?
 So, the gene is known as ___.
 On which type of chromosome is the gene
for protein production in the pancreas
found?
 So, the gene is known as ___.
 Two
non-sister chromatids can swap
segments of their DNA
 A maternal chromosome can end up with
a segment of a paternal chromosome and
vice versa
 Any
two genes which are found on the
same chromosome are said to be linked
to each other.
 Linked genes are usually passed on to
the next generation together.
 Linkage group - groups of genes on the
same chromosome inherited together
 Linked genes are the exception to
Mendel’s law of independent assortment
 Fruit
fly gene for
body color is in the
same linkage group
as the gene for wing
length
 G – grey body
 g – black body
 L – long wings
 l – short wings
 GGLL – gray body,
long wings
 ggll – black body,
short wings





G L
G L
Two horizontal bars
symbolize homologous
chromosomes and that
the locus of G is on the
same chromosome as L
F2 generation genes
located on same pair of
chromosomes, so only
possible gametes are
PL and pl
Phenotypic ratio is 3:1
A
cross between homozygous dominant
true-breeding fruit fly (GGLL) and a
homozygous recessive true-breeding
fruit fly (ggll) would result in flies which
were all heterozygous for both of the
traits (GgLl)
 If heterozygotes for these traits are mixed
with homozygous dominant flies, then
one could not determine the genotype by
sight.
 Cross “mystery” fly
with homozygous
recessive (ggll)
 Assuming the mystery fly is heterozygous
(GgLl) for both traits:
gl
GL
Gl
gL
gl
GgLl
Ggll
ggLl
ggll
A new shuffling of the alleles has created a new combination
which does not match either of the parents’ genotypes
The term recombinant is used to describe both the new
chromosome and the resulting organism.
Recombinants form through the process of crossing over
Topic 10.3
 10.3.1
Define polygenic inheritance.
 10.3.2 Explain that polygenic inheritance
can contribute to continuous variation
using two examples, one of which must
be human skin colour.
 Involves
two or more genes influencing
the expression of one trait
 Increased number of possible genotypes
 Believed that most human traits are too
complex to be determined by one gene
 When
an array of possible phenotypes can
be produced, it is called continuous
variation
• Examples: skin color, height, body shape, and
intelligence
• These traits are also influenced by environmental
conditions
 When
only a number of phenotypes can be
produced, it is called discontinuous
variation
• Examples: earlobe attachment, widow’s peak, blood
type
CONTINUOUS
VARIATION
DISCONTINUOUS
VARIATION
Iris is made up of
zones, rings, streaks or
speckles of different
colored pigments with
varying intensities
 What color are your
eyes, really?
 Since there is so much
variety, eye color must
be influenced by
multiple alleles and
has continuous
variation.

 Many
societies feel the need to label
people with categories
 Oversimplification serves more
administrative purposes than any
biological purpose
 So what is the purpose of skin?!
 PROTECTION FROM THE SUN’S
HARMFUL ULTRAVIOLET RADIATION!!!
 Melanin
protects from serious sunburn
which can lead to melanoma
 Melanin is found in all individuals except
albinos
 Concentration of melanin varies
 How can one increase the melanin level
in their skin?
 EXPOSE ONESELF TO SUNLIGHT
 Tanning is a natural defense against the
negative effects of excess sunlight
 UVB
radiation helps the skin to make
vitamin D
 Vitamin D is necessary for proper growth
and bone formation
 It is good to have a MODERATE amount
of sunlight
 In regions of low Sun exposure people
need light-colored skin
 Why?
 How
do people of varying degrees of skin
color relocated to parts of the world that
receive differing amounts of sunlight get
vitamin D? How do others fight off the sun?
 Should
there be equal esteem for all
humans?
 Why is human diversity so often used to
divide and discriminate, rather than be
appreciated, respected, and celebrated?