7A - Linkage and epistasis Flashcards

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1
Q

When is a characteristic said to be sex-linked?

A

When the allele that codes for it is located on a sex-chromosome.

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2
Q

What are examples of genetic disorders caused by faulty alleles on sex chromosomes?

A

Colour blindness and haemophilia.

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3
Q

Why are males more likely than females to show recessive phenotypes for genes that are sex-linked?

A

Because males only have one X chromosome so they often only have one allele for sex-linked genes. Therefore, because they only have one copy, they express the characteristic of this allele even if it’s recessive.

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4
Q

What is it called when the faulty alleles are carried on the X chromosome?

A

X-linked disorders.

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5
Q

What is an autosome?

A

Any chromosomes that isn’t a sex chromosome.

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6
Q

What are autosomal genes?

A

Genes located on the autosomes.

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7
Q

Why are genes on the same autosome said to be linked?

A

Because they’re on the same autosome they’ll stay together during the independent segregation of chromosomes in meiosis I, and their alleles will be passed on to the offspring together. The only reason this won’t happen is if crossing over splits them up first.

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8
Q

The closer together two genes are on the autosome…

A

…the more closely they are said to be linked.

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9
Q

Why is it that the closer together two genes are on the autosome, the more closely they are said to be linked?

A

Because crossing over is less likely to split them up.

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10
Q

What would you expect the phenotypic ratio to be like if two genes are autosomally linked?

A

You won’t get the ratio you expect in the offspring of a cross.

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11
Q

How are the X and Y chromosomes different (structurally)?

A

Y chromosomes are smaller and carry fewer genes.

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12
Q

What is an X-linked gene?

A

Most genes on the sex chromosomes are only carried on the X chromosome and this means they are X-linked genes.

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13
Q

What are genetic disorders caused by?

A

Faulty alleles on sex chromosomes.

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14
Q

Are males or females more likely to show recessive phenotypes for genes that are sex-linked?

A

Males

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15
Q

Explain how colour blindness (a sex-linked disorder) becomes expressed in the phenotype.

A

1) Colour blindness is a sex-linked disorder caused by a faulty allele carried on the X-chromosome.
2) As it is sex-linked both the chromosome and the allele are represented in the genetic diagram, e.g. Xn, where X represents the X chromosome and n the faulty allele for colour vision.
3) The Y chromosomes doesn’t have an allele for colour vision so is just represented by Y.
4) Females would need two copies of the recessive allele to be colour blind, while males only need one copy. This means colour blindness is much rarer in women than men.

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16
Q

What would the ratio be of not colour-blind to colour-blind if you crossed a carrier female and an unaffected male?

A

3:1 (normal to colour-blind).

17
Q

What would to ratio be of female offspring without colour-blindness:male offspring without colour-blindness:male offspring with colour-blindness if you crossed a carrier female and an unaffected male?

A

2:1:1 (female offspring without colour-blindness:male offspring without colour-blindness:male offspring with colour-blindness).

18
Q

What would the ratio be of offspring with colour-blindness:offspring without colour-blindness if you did a monohybrid cross between a carrier female and a colorblind male?

A

1:1 (with:without).

19
Q

What are the genes located on the autosomes called?

A

Autosomal genes.

20
Q

Why is it that if two genes are autosomally linked, you won’t get the phenotypic ratio you expect in the offspring of a cross?

A

Because the two autosomally-linked alleles are inherited together meaning that a higher proportion of the offspring will have their parents’ genotype and phenotype.

21
Q

Why is it that if two genes are autosomally linked, you won’t get the phenotypic ratio you expect in the offspring of a cross?

Use an example this time…

A

For example, in a dihybrid cross between two heterozygous parents you’d expect a 9:3:3:1 ratio in the offspring. Instead the phenotypic ratio is more likely to be that expected for a monohybrid cross between two heterozygous parents (3:1) (because the two autosomally-linked alleles are inherited together meaning that a higher proportion of the offspring will have their parents’ genotype and phenotype).

22
Q

What does the fact that if two genes are autosomally linked, you won’t get the phenotypic ratio you expect in the offspring of a cross because the two autosomally-linked alleles are inherited together meaning that a higher proportion of the offspring will have their parents’ genotype and phenotype allow us to do?

A

Allows us to use the predicted phenotypic ratio to identify autosomal linkage.

23
Q

What is a back cross?

A

Crossing the offspring with one of the parents.

24
Q

What is it called when the offspring is crossed with one of the parents?

A

A back cross.

25
Q

What does an epistatic gene do?

A

Masks the expression of another gene.

26
Q

What is epistasis?

A

When the allele of one gene masks (blocks) the expression of the alleles of the genes.

27
Q

What do crosses involving epistatic genes not result in?

A

The expected phenotypic ratios.

28
Q

Predict the ratio of a dihybrid cross involving a recessive epistatic allele

A

9:3:4 (dominant both:dominant epistatic recessive other:recessive epistatic).

29
Q

Explain why the ratio of a dihybrid cross involving a recessive epistatic allele is 9:3:4

A

Having two copies of the recessive epistatic allele masks (blocks) the expression of the other gene. If you cross a homozygous recessive parent with a homozygous dominant parent you will get a 9:3:4 phenotypic ratio of dominant both:dominant epistatic recessive other:recessive epistatic in the F2 generation.

30
Q

Predict the ratio of a dihybrid cross involving a dominant epistatic allele

A

12:3:1 (dominant epistatic:recessive epistatic dominant other:recessive both).

31
Q

Explain why the ratio of a dihybrid cross involving a dominant epistatic allele is 12:3:1

A

Having at least one copy of the dominant epistatic allele masks (blocks) the expression of the other gene. Crossing a homozygous recessive parent with a homozygous dominant parent will produce a 12:3:1 phenotypic ratio of dominant epistatic:recessive epistatic dominant other:recessive both in the F2 generation.