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Dihybrid crosses and gene linkage

Topic 10.2

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10.2.1 Dihybrid crosses

Dihybrid cross: A cross that tracks the inheritance of two characters, controlled by 2 genes on two different chromosomes

(Y) = yellow seeds (y)= green seeds

(S) = smooth seeds (s) = rough seeds

SSYY

ssyy

SsYy

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What is the predicted phenotype ratio for a cross between two pea plants which are heterozygous for both genes?

Make sure each new genotype has two alleles for each trait. Alphabetical order, then dominant alleles first

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Dihybrid test cross

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10.2.2 Distinguish between autosomes

and sex chromosomes

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10.2.3 Explain how crossing-over occurs…

During prophase I of meiosis, homologous chromosomes come together in pairs.

Each chromosome is divided into 2 chromatids. The homologous chromosomes twist around each other. This creates a tension, which may cause breaks to occur along the length of the chromatids.

During crossing over, corresponding fragments of chromatids may get swapped over.

A chiasma is an X-shaped structure formed between non-sister chromatids during prophase I of meiosis. This chiasma is a physical manifestation of crossing over. Usually between one and three chiasmata form per homologous pair. The chiasmata persist through metaphase I and play a role in the prevention of non-disjunction.

This cutting and sticking of chromatids means that the genetic material is exchanged. In this way, new genetic combinations are produced and variation in the gametes is increased.

So, together with random segregation of chromosomes, crossing over can produce an enormous amount of variation, both in the gametes and the resulting offspring.

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10.2.3 Explain how crossing-over occurs…

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10.2.4 Define linkage group

Gen linkage occurs when two different genes are located on the same chromosome.

Linkage Group, group of genes whose loci are on the same chromosome. For this reason, they are usually inherited together, because they move together during meiosis and end up in the same gamete.

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10.2.5 Explain an example of a cross

between two linked genes

Linked genes may become separated through crossing over. Crossing over could produce weird combinations.

Some crossing over will occur

here and B and b are exchanged

Can you see that,

because of crossing over,

Four types of offspring

are possible?

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10.2.5 Explain an example of a cross

between two linked genes

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Homozygous

Recessive

Ebony bodied

Curled Wings

MITOSIS

PHENOTYPE

GENOTYPE

DIPLOID

SOMATIC CELL

HAPLOID CELLS

GAMETES

	GS	gs	Gs	gS
gs	GgSs	ggss	Ggss	ggSs

25% = GgSs = Normal body, straight wings

25% = ggss = Ebony body, curled wings

25% = Ggss = Normal body, curled wings

25% = ggSs = Ebony body, straight wings

GgSs

ggss

Heterozygous

Normal Body

Straight Winged

Female Fly

45% = GgSs = Normal body, straight wings

45% = ggss = Ebony body, curled wings

6% = Ggss = Normal body, curled wings

4% = ggSs = Ebony body, straight wings

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Heterozygous

Normal Body

Straight Winged

Female Fly

GgSs x

Homozygous

Recessive

Ebony bodied

Curled Wings

ggss

MEIOSIS

PHENOTYPE

GENOTYPE

DIPLOID

SOMATIC CELL

HAPLOID CELLS

GAMETES

	GS	gs	Gs	gS
gs	GgSs	ggss	Ggss	ggSs

Expected Results Assuming Independent Assortment

25% = GgSs = Normal body, straight wings

25% = ggss = Ebony body, curled wings

25% = Ggss = Normal body, curled wings

25% = ggSs = Ebony body, straight wings

RECOMBINANTS

CREATED BY CROSSING OVER

Actual Results

45% = GgSs = Normal body, straight wings

45% = ggss = Ebony body, curled wings

6% = Ggss = Normal body, curled wings

4% = ggSs = Ebony body, straight wings

Because the genes are linked the only way to produce the recombinants is by crossing over during Meiosis

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10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes

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Recombinants formed by crossing over during meiosis

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Videos to review Gene Linkage in dihybrid cross.