Date post: | 14-Jan-2016 |
Category: |
Documents |
Upload: | jamie-liddell |
View: | 223 times |
Download: | 0 times |
Assessment Statements10.2.1 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.
Dihybrid crossesMendel’s Peas
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 otherOne parent: homozygous dominant for both
traits (round and yellow seeds) RRYYOther parent: homozygous recessive for both
traits (wrinkled and green) rryy
When both parents are homozgous – all the F1 offspring are the same genotype and phenotype
R = allele for round peas
r = allele for wrinkled peas
Y = allele for yellow peas
y = allele for green peas
Parent phenotypes
Round yellow
Green wrinkled
Parent genotypes
RRYY rryy
Parent gametes
RY ry
F1 genotypes
RrYy
F1 phenotypes
Round yellow
Cross the F1 double heterozygous Allowing heterozygous offspring to self-pollinate
Phenotype Ratio
is:9331
Dihybrid Phenotype RatiosHomozygous parentsAABB x aabb
All F1 offspring the same Heterozygous parents
AaBb x AaBb9:3:3:1 phenotype
ratio 9AB 3Ab 3aB 1ab
A new shuffling of the alleles has created a new combination which does not match either of the parents’ genotypes. Recombinants
Autosomes and sex chromosomesSex chromosomes: X and Y (one pair)Autosomes: any chromosome not X or Y (22
pairs)Sex-linked gene is located on a sex
chromosome.Autosomal gene is located on one of the
autosomes.On which type of chromosome is the gene for
haemophilia 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 ___.
Exchange of alleles by crossing overTwo non-sister chromatids can swap
segments of their DNA during prophase I of meiosis.
This increases genetic variety of chromosomes in gametes
There are now thought to be 20,500 human genes on 23 chromosomes
Source: Broad Institute of MIT and Harvard (2008, January 15). Human Gene Count Tumbles
There must be approx 1000 genes on each chromosome.
Linkage groupAny 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
Linked genesFruit fly gene for
body color is in the same linkage group as the gene for wing length
Alleles are G – grey bodyg – black bodyL – long wingsl – short wings
Notation of linked genes is
G LG L
The two horizontal bars symbolize homologous chromosomes and that the locus of G is on the same chromosome as L
G L
G L
Offspring of a dihybrid cross
A cross between homozygous fruit flies
GGLL x ggll Grey body & long wings x Black body, short wings
F1 flies will be all heterozygous for both of the traits
GgLlPhenotype : Grey body & long wings
If these F1 heterozygotes were allowed to reproduce together the ratio of phenotypes produced would be
expected to be
9:3:3:1
RecombinantsAfter the F2 flies were identified the
phenotypes were;Grey long wing
Grey short wing
Black long wing
Black short wing
1600 1020 210 200 170
This is far from the expected 9:3:3:1 ratio which should give 900 300 300 100
This is a sign that the genes are on the same chromosome (genes are linked)
Offspring of a dihybrid crossusing linkage notation
A cross between homozygous fruit flies
GL glGL x gl
Grey body & long wings x Black body, short wings F1 flies will be all heterozygous for both of the traits
GLgl
Phenotype : Grey body & long wings
If these F1 heterozygotes were allowed to reproduce together the ratio of phenotypes produced would be expected to be
9:3:3:1
Offspring of a dihybrid crossusing linkage notation
If these F1 heterozygotes make gametesThe gametes will be
GL or gl unless crossing over happens – because the
genes are on the same chromosome The F1 will contain more than expected of
these genotypes
G L
G L
GL GL gl gl GL gl GL gl
Offspring of a dihybrid crossusing linkage notation
If The F1 will contain more than expected of these genotypes
The Grey short wing and black long wing phenotypes
Are Recombinants -
G L
G L
GL GL gl gl GL gl GL gl
Grey long wing
Grey short wing
Black long wing
Black short wing
1600 1020 210 200 170
Assessment Statements10.3.1 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.
Polygenic Inheritancewhen two or more genes influence the expression of one trait
Eg Skin Colour, Height
Continuous and discontinuous variationWhen an array of possible phenotypes can be
produced, it is called continuous variationExamples: skin color, height, body shape, and
intelligenceThese traits are also influenced by
environmental conditionsWhen only a number of phenotypes can be
produced, it is called discontinuous variationExamples: earlobe attachment, blood group
Graphical representationContinuousvariation
Discontinuous variation
4 5 6 70
10
20
30
40
50
60
Height in humans
Frequency
A B AB O05
1015202530354045
Blood Type
% of popula-tion
Eye ColorIris 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.
Skin colorHow does the existence of multiple alleles
controlling skin colour result in the appearance of many different shades of skin colour in humans?
ThoughtsHow 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?
Polygenic inheritance of color in wheat.Kernal color in wheat is determined by two genes. A range of colors occur, from white to dark red, depending on the combinations of alleles.
Dark red plants are homozygous AABB and white plants are homozygous aabb.
Crossing individuals with the phenotype extremes yield offspring that are a 'blend' of the two parents.
When these homozygous phenotypes are crossed AABB x aabb
Dark x white
the F1 offspring are all double heterozygous
AaBb.
But what happens when the two double heterozygous genotypes are crossed?
Parent Phenotypes: all brownGenotypes: AaBb x AbBb
Punnet square:AB Ab aB ab
AB AABB AABb AaBB AaBb
Ab AABb AAbb AaBb Aabb
aB AaBB AaBb aaBB aaBb
ab AaBb Aabb aaBb aabb
There is no blending in the offspring. Offspring can be more extreme than either parentThere are grades of colour – evidence of polygenic inheritance.
Skin colorHow does the existence of multiple alleles
controlling skin colour result in the appearance of many different shades of skin colour in humans?
Human skin colour is controlled by multiple alleles (and the environment)It is known that at least three genes control
skin color, let’s call them genes A, B, and C. Someone who is AABBCC would have very
dark skin color and someone who is aabbcc would have very light skin color.
If they got married and had children, their children would all be AaBbCc and have mid-brown skin.
If two of those people would get married and have children, the Punnett square would look like the one above.