Transcript Genetics: Day 5

Genetics: Day 5
Pedigree Charts
• Pedigree charts are diagrams constructed
to show biological relationships and in
genetics are used to show how a trait
passes from one generation to another
• Pedigree Key:
Pedigree Analysis
Determine the pattern of inheritance for the chart?
Is it sex-linked dominant, sex-linked recessive,
autosomal dominant, or autosomal recessive?
Let’s work this one out together on the
board.
Dihybrid Crosses
• Used to determine potential genotypic and
phenotypic ratios from parents using two
characteristics
• Assuming the alleles are located on
different chromosomes and not “linked”
then we can assume the dihybrid cross will
also follow Mendel’s Law of Independent
Assortment
Example
Mrs. Parrott wants to predict whether her
kids with Mr. Parrott will have straight hair,
wavy hair or curly hair and dimples or no
dimples. Below are their alleles.
Mrs.- wavy hair (Hh) and no dimples (dd)
Mr. - straight hair (hh) and dimples (DD)
hD
hD
hD
hD
Hd
HhDd
HhDd
HhDd
HhDd
Hd
HhDd
HhDd
HhDd
HhDd
hd
hhDd
hhDd
hhDd
hhDd
hd
hhDd
hhDd
hhDd
hhDd
Genotypic ratio: 1:1 (HhDd:hhDd)
Phenotypic Ratio 1:1 (wavy hair with
dimples: straight hair with dimples)
Therefore, all offspring with have a 50%
chance to have either wavy hair or straight
hair and 100% of offspring will have
dimples
Complete the cross.
Determine the
resulting phenotypic
ratio for the F1
offspring.
Phenotypic ratio – 9:3:3:1
9 black/short, 3 black/long, 3 brown/short, and 1
brown/long
Objectives for Linked Genes
1. Define linkage group
2. Explain an example of a cross between
two linked genes
3. Identify which of the offspring are
recombinants in a dihybrid cross
involving linked genes.
Linked Genes
• Linkage group – two or more genes
inherited together because they are found
on the same chromosome
• Linked genes do not follow Mendel’s Law
of Independent Assortment which would
give a typical result of 9:3:3:1 (instead they
give a wide variety of ratios)
Common Example of Linked Genes
• For fruit fly Drosophila the gene for body
color and wing length are linked b/c they
are located on same chromosome
• For fruit fly:
G = grey body
g = black body
L = long wings
l = short wings
1. In a lab we have two parent flies with alleles
shown: (linkage shown)
G L
G L
g l
g l
2. These two flies mate and their offspring gets
one trait from each parent
G L
g l
3. Then, the offspring mates with another fly
which is homozygous recessive (grey/long
wings mates with black/short wings)
G L
g l
g l
g l
4. Complete a Punnett
grid to show the
results of the mating
5. Notice the two
offspring in bold are in
combinations unlike
their parents
(grey/short wings and
black/long wings)
therefore they must
have formed through
crossing over and we
call them
recombinants
GL
gl
gl
gl
gl
Gl
gL
gl
GgLl Ggll ggLl ggll
Objectives for Polygenic
Inheritance
1. Define polygenic inheritance
2. Explain that polygenic inheritance can
contribute to continuous variation using
two examples, one of which must be
human skin color
Polygenic Inheritance
• It is when 2 or more genes influence the
expression of one trait
• Since there are 2 or more allelic pairs
found at different loci, the number of
possible genotypes is greatly increased
*This is why interpreting the human genome is
especially difficult since many genes can effect
the same trait
Continuous Variation
• When multiple genes
produce a spectrum of
resulting phenotypes
(this can make it so
the genotype is not
clear)
• Examples include skin
color: multiple genes
effect the intensity of
pigment in the skin (at
least 3 genes affect
skin color)
• Other examples of continuous variation include
eye color, height, body shape and intelligence
• Note: many of these traits are also affected by
the person’s environment (nature vs. nurture)
so they are called multifactorial
• Traits that do not vary along a continuum are
referred to as discontinuous variation; examples
of these traits include your blood group,
widow’s peak, attached vs. unattached earlobes