Sex Determination and sex linkage

Question 1

What are the 2 other main strategies of plants that aren’t hermaphrodite?

Answer 1

Monoecious plants have separate male and female flowers on the same plant
Dioecious have separate male and female individuals.

Question 2

What factors control whether an individual is male or female? (6)

Answer 2

• temperature: crocodile and alligator eggs hatch as male above 32 degrees and female below.
• sequential hermaphroditism: mollusc makes stacks of individuals. Top ones are males, as more males join the ones below become female
• species becoming hermaphrodite and fertilise itself if no females available
• hierarachies: clownfish - if dominant female dies the dominant male changes sex
• ploidy level: eg bee eggs not fertilised are male, fertilised are female
• chromosome structure: XX/XY

Question 3

What are homologous pairs?

Answer 3

Each pair contains chromosomes of the same size and shape, with genes in the same order coding for the same characteristics. Contains one chromosome from each parent.

Question 4

What is the karyotype?

Answer 4

The arrangement of homologous para in decreasing size order

Question 5

What is an autosome?

Answer 5

22 in humans. They have identical genes but different alleles.

Question 6

What is a heterosome?

Answer 6

The sex chromosome; XX= female XY= male. Y is shorter.

Question 7

What is the psuedoautosomal region?

Answer 7

Two regions on the X and Y human chromosomes that are homologous and can pair with each other at meiosis. PAR1 and PAR2.

Question 8

What does homogametic sex mean?

Answer 8

All the female secondary oocytes contain the X chromosome so they are the homogametic sex meaning the gametes are identical in relation to sex chromosomes

Question 9

What does heterogametic sex mean?

Answer 9

One secondary spermatocyte has X and one has Y. Gametes are of different types with respect to the sex chromosomes.

Question 10

Why is the probability of conceiving a male or female equal?

Answer 10

The oocyte may be fertilised by an x carrying sperm or y carrying sperm.

Question 11

What gene does the Y have that the X doesn’t?

Answer 11

SRY - the sex determining region on the Y chromosome. It switches on genes in other chromosomes (on autosomes) which are responsible for the expression of male characteristics. An individual only needs one Y chromosome with an SRY gene to be male.

Question 12

Why is the expression of X-linked genes different in males and females?

Answer 12

Females need 2 recessive alleles for the phenotype to be expressed, but males only need 1 as they only have 1 X chromosome.

Question 13

What causes haemophilia?

Answer 13

When the individual can’t produce enough of one of the blood clotting proteins, so blood clots slowly or not at all causing persistent bleeding.

Question 14

What is the gene and symbol for the protein related to haemophilia?

Answer 14

The x-linked gene codes for the blood clotting protein factor VIII. the allele coding for the normal version has the symbol XH and the mutant version is Xh. Females = XHXH, XHXh, XhXh. Males = XH/Xh. It is therefore more common in males.

Question 15

Define a sex linked gene.

Answer 15

When a gene is carried by a sex chromosome so a characteristic it encodes for is seen predominantly in one sex.

Question 16

Why can’t fathers pass x- linked conditions to their sons?

Answer 16

Because their sons inherit their Y chromosome.

Question 17

What is DMD causes by?

Answer 17

An x-linked recessive allele of the dystrophin gene. The gene codes for the protein dystrophin which is a component of a glycoprotein that stabilises the cell membranes of muscle fibres.

Question 18

What are symptoms of DMD?

Answer 18

Start at 2-3 years old, including loss of muscle mass and progressive muscle weakness.

Question 19

What are the alleles for the normal dystrophin protein and mutant one? Describe the results of crosses of carriers/unaffected/affected.

Answer 19

XD and Xd.
Carrier female and normals male = 50% chance of affected sons, females carriers.
Affected males + unaffected females = all daughters carriers.
To be an affected female - affected/Carrier mother and affected father.

Question 20

What should you do to analyse a pedigree diagram?

Answer 20

Assign the genotypes I.e affected males must be XdY, unaffected males XDY. Mothers and grandmothers of affected must be XDXd. If it’s males only = sex linked, if it’s inherited through the mother = x linked.