Dihybrid crosses and gene linkage

HL Genetics Topic 10.2

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

Polygenic InheritanceTopic 10.3

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.