Topic 3: Linkage and sex linkage

Question 1

Define Locus (1 point)

Answer 1

1. the location of a gene/allele on a chromosome

Question 2

What is the plural of locus?

Answer 2

Loci

Question 3

What are linked genes? (1 point)

Answer 3

1. two genes which are on the same chromosome

Question 4

How are genes which are not linked usually inherited?

2 points

Answer 4

1. They are usually inherited independently of each other
   during independent assortment of chromosomes in meiosis,
2. as each chromosome (and genes on it) ends up in a different gamete

Question 5

Linked genes usually end up being _________ together, so end up in the same _______

Answer 5

inherited

gamete

Question 6

How can linked genes be separated and end up on different chromatids
(which become chromosomes) and in different gametes? (1 point)

Answer 6

1. Sometimes two genes/alleles which are on THE SAME chromosome (so are linked genes) at the beginning of meiosis I do not end up on the same chromosome by the end of meiosis I – due to CROSSING OVER

Question 7

The closer together the ____ of two genes on a chromosome, the more closely _______ they are
and the less likely ________ _____ is to separate them.

This is because the probability of a _______ occurring is fairly low,
and if one does occur the probability of it occurring in the small region
between the two close genes is even lower.

Therefore, two genes located at opposite ends of a chromosome, rather than
near the middle, are more likely to be __________ during crossing over in ________.

Answer 7

loci

linked

crossing over

chiasma

separated

meiosis

Question 8

How can unlinked genes on different chromosomes end up together on the same chromosome? (1 point)

Answer 8

1. Sometimes two genes/alleles which are on DIFFERENT chromosomes
   at the beginning of meiosis I DO end up on the same chromosome (become linked) by the end of meiosis I – due to CROSSING OVER.

Question 9

The further apart the genes on the different chromosomes,
the more likely that _______ _____ will occur between them and
they will end up on the same chromosome (linked) and therefore in the same _______.

The closer together the genes on the two different ___________ chromosomes,
the _____ likely that crossing over will NOT occur between them
and they will stay on different chromosomes.

Answer 9

crossing over

gamete

homologous

more

Question 10

The greater the ________ between the loci of two ______,

the greater the likelihood of ________ _____ occurring between the _____.

Answer 10

distance

genes

crossing over

genes

Question 11

Key Summary: Explain linkage (4 points)

Answer 11

1. Linked genes are on the same chromosome, but are found at different gene loci
2. Linked genes have a high chance of entering the same gamete, as when chromatids separate in meiosis II, all the genes on the chromatid (which becomes a chromosome) usually stay together and move as one unit into a gamete
3. Linked genes can be separated by crossing over in meiosis
4. The greater distance between loci of two linked genes, the greater the likelihood of crossing over happening between them and separating them

Question 12

In addition to the ___ pairs of autosomal chromosomes/ autosomes in the
nucleus of a human somatic/body cell, there are TWO ____ chromosomes (which control gender).

Answer 12

22

sex

Question 13

Females have ___
(two ____ chromosomes)

Male have ___ (one ____ X
and one ____ Y chromosome,
so not homologous)

Answer 13

XX

long

XY

long

short

Question 14

Key Summary: Define sex linkage

2 points

Answer 14

1. Genes found on the sex chromosomes (X and Y)
   (with the genes that determine sex/gender)
2. When an allele/gene is found on the X chromosome, females have two copies of the allele/gene and males have one copy of the allele/gene

Question 15

All genes on the sex chromosomes are passed on to future generations
with the genes that determine sex/gender - they are said to be ‘___ _____’ genes

Humans have around ____ sex linked genes on the X chromosome

If there are mutations in sex linked genes on the X chromosome,
they can lead to sex linked conditions which affect a higher percentage of _____ than _____.

Answer 15

sex linked

1098

males than females

Question 16

(sex linked conditions)
Give examples of X linked recessive disorders
(3 examples)

Answer 16

1. haemophilia
2. red-green colour blindness
3. Duchenne muscular dystrophy

Question 17

(sex linked conditions)
Give examples of X linked dominant disorders (very rare)
(2 examples)

Answer 17

1. Rett syndrome

2. Inherited forms of rickets

Question 18

What leads to red green colour blindness?

3 steps

Answer 18

1. Gene loci for the 3 pigments in the eyes cone cells are on the X chromosome
2. Cone cell pigments are needed for colour vision
3. If one or more pigment genes are faulty/mutated or absent, leads to colour blindness

Question 19

When writing genotypes, do you put letters on the X and Y chromosome?

Answer 19

Only X has letters, Y never has letters

Question 20

What do write when writing phenotypes?

2 points

Answer 20

1. male/female (X/Y)

+

2. whether they have the disease, are unaffected or are an unaffected carrier

e.g.
So this is how the alleles should be written for red-green colour blindness

X^N : normal allele (N) for cone pigment

X^n : mutant allele (n) for cone pigment (no pigment made)

Question 21

Mother (X^N X^n ) and father (X^N Y)

How would you calculate the probability of their child having full colour vision?
2 steps

Answer 21

1. Punnett square showing offspring genotypes and phenotypes
2. (how many have colour vision / total number of possibilities) * 100 = probability of their child having full colour vision

Question 22

Key Summary: Explain why more men than women are colour blind (or have haemophilia or have any sex linked condition).
(6 steps)

Answer 22

1. Colour blindness is caused by a recessive allele on the X chromosome
2. Males inherit only one X chromosome (XY) so only one copy of the gene
3. So if the recessive mutated allele which causes colour blindness is found on this chromosome (X^n), it will always be expressed – genotype X^n Y
4. Females inherit two X chromosomes (XX)
5. So if they inherit one dominant allele on one X chromosome (X^N) and one recessive allele on the other X chromosome (X^n) , the recessive allele which causes colour blindness will not be expressed – genotype X^N X^n
6. For a female to be colour blind they would need to inherit 2 recessive alleles, one from each parent, which is rare – X^n X^n

Question 23

A couple had children using IVF because the father has haemophilia, a sex-linked recessive condition.

Only female embryos were used.

Explain whether or not future generations of this family would be free of haemophilia (3 points)

Answer 23

1. Genotype of father is X^h Y – so female embryos must contain the X^h allele.
2. The family would not be free of haemophilia because their daughters would be carriers (heterozygous X^H X^h).
3. Half of the sons of the carrier daughters would be expected to have the condition.