Lecture 20 (X-linked traits and recombination)

Question 1

Sex-linked traits

Answer 1

Involving genes on the sex chromosome, X or Y rather than autosomes
As a result their inheritance patterns differ between males (XY) and females (XX)
Traits that are linked to the sex chromosomes

Question 2

Examples of sex-linked traits

Answer 2

Haemophilia

Red-green colour blindness

Question 3

Sex determination

Answer 3

Sex chromosomes (X and Y) determine the sex of an individual. It is a 50/50 whether X or Y is chosen on top of the other X

XX - female
XY - male
XXY - klinefelter
XO - turner syndrome

Question 4

Autosomes

Answer 4

Chromosomes that aren’t sex chromosomes

Question 5

Sex-linked trait ratios

Answer 5

Homozygous dominant trait female x hemizygous recessive trait male (hemizygous means that if you are a male you only have one X)

• All daughters are dominant heterozygotes
• All sons are dominant hemizygotes

Homozygous recessive trait female x hemizygous dominant male

• All daughters are dominant heterozygotes
• All sons are recessive hemizygotes

Question 6

Why are sex-linked trait diseases/disorders more common in males? (X linked recessive)

Answer 6

You are more likely to have a sex-linked disorder if you are male because you only have on X chromosome. This means that if you have the allele for the disorder it will be expressed.

Having 2 X chromosome is like having a safety net, if you get one allele for the disorder you won’t express it as it is recessive and you will just be a carrier. Females have to get 2 copies of these recessive alleles that cause the disorder.

Question 7

Why are sex-linked trait diseases/disorders more common in males? (X linked dominant)

Answer 7

More female than males get the disorder

Question 8

Sex-linked trait (X linked recessive) - Male affected and female homozygote unaffected (dominant homozygote)

Answer 8

A father with the trait will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation

Question 9

Sex-linked trait (X linked recessive) - A carrier mates with a normal male (homozygous dominant)

Answer 9

A carrier who mates with a normal male will pass the mutation to half her sons and half her daughters. The sons with the mutation will have the disorder. The daughters who have the mutation in a single dose will be carriers just like the mother

Question 10

Sex-linked trait (X linked recessive) - A carrier mates with a male with the trait

Answer 10

A carrier mates with a male with the trait, there is a 50% chance that each child will have the trait. Daughters who do not have the trait will be carriers, whereas males without the trait will be completely free of the harmful recessive allele =

Question 11

Linked genes

Answer 11

Linked genes means that both genes are found on the same chromosome therefore the alleles for these genes found on the same chromosome will be inherited together

Sometimes when we do a dihybrid cross we find that the ratio is not 9:3:3:1, this is because the genes that control this trait are linked

They do not sort independently (like genes on different chromosomes) therefore the gametes are likely to resemble parental or sibling allele combinations. This results in fewer genetic combinations of alleles in offspring.

Question 12

How many genes are there in humans?

Answer 12

Around 22,000

Question 13

How many chromosomes are there in humans?

Answer 13

23 pairs (thats about 1000 linked genes per chromosome)

Question 14

How are linked genes inherited?

Answer 14

Linked genes show biases towards parental genotypes

Question 15

How can we determine the order and location of genes within a chromosome?

Answer 15

Using recombination frequencies

Question 16

If two genes do not sort independently …

Answer 16

They are linked on the same chromosome

Mendel’s second law only applies to genes on different chromosomes. Once genes are on the same chromosome it isn’t random where genotypes of that gene goes as they are linked.

This is shown by a bias towards the parental phenotype. Fewer recombinants means the genes must be linked (on the same chromosome) (violates Mendel’s 2nd law (independent assortment))

Theoretically, meiosis should cause all gametes to have parental genotypes however we do get some recombinants due to crossing over.

Question 17

Crossing over

Answer 17

Crossing over during meiosis explains recombination in linked genes

Two chromatids of a tetrad (one from each pair) cross over at random points and swap genetic material. (chromosomes effectively break and repair themselves)

Allows for new combinations of genotypes to occur

Question 18

Recombination frequency

Answer 18

The proportion of recombinant gametes

The amount of recombination that occurs differs depending on how close or how far apart genes are on a chromosome

Total recombinants / total = recombination frequency (times by 100 to get the percentage)
(the percentage found will tell you that only ____% of the gametes contain a chromosome that has formed a chiasma between the two genes)

Question 19

Closeness of genes and how that affects crossing over

Answer 19

The smaller the distance between two genes - the less likely a chiasma will be formed between them

Distant (unlinked) genes have recombination frequencies of around 50% (half recombinant and half parental types). Close genes have recombination frequencies of 0-50%. There is a near linear relationship between distance and recombination frequency

When you have a random process and a small target (close genes), the chance of crossing over is small

Crossing over will only sometimes seperate alleles. Smaller the distance between the genes, the less likely they are to seperate and create a recombination