Chromosomes, Linkage and Sex: Quick Revision

A brief note on Chromosomes, Linkage and Sex. Read it before a test. Try each question at the end before you open its answer.

Chromosomes and linkage

• Chromosomal theory: genes are on chromosomes (Sutton and Boveri). Morgan worked on the fruit fly Drosophila melanogaster.
• Linkage: genes on the same chromosome are inherited together. Crossing over makes recombinants.
• Test cross of AB over ab: the parental kinds AB and ab are the common ones, and the recombinant kinds Ab and aB are the rare ones.
• Recombination frequency: recombinants divided by all offspring, in per cent. One per cent is one map unit. It never exceeds 50 per cent.
• Gene map: Sturtevant used the frequencies. The largest distance of three genes is the sum of the other two.

Sex determination

• Humans and the fruit fly: XX female, XY male. The sperm decides.
• Birds: ZZ male, ZW female. The egg decides.
• Honey bee: fertilised egg, 2n = 32, a female. Unfertilised egg, n = 16, a drone. A drone has no father and makes sperm by mitosis.
• Heterogametic sex: the sex that makes two kinds of gamete (XY, XO or ZW).

Sex-linked inheritance

• Haemophilia and colour blindness are X-linked and recessive. A man has one X, so one allele decides.
• Carrier mother × normal father: one quarter of the children are haemophilic sons. No daughter is affected.
• Haemophilic father × normal mother: all daughters are carriers, and all sons are normal.
• Pedigree: a square is a male, a circle is a female, and a filled symbol is an affected person.

A worked example

A test cross gives 100 offspring: 44 AaBb, 46 aabb, 5 Aabb and 5 aaBb. Find the distance between the genes.

1. The recombinants are Aabb and aaBb: $5 + 5 = 10$.
2. The frequency is $\tfrac{10}{100} = 10\%$, so the genes are 10 map units apart.

Practice with new numbers

A program to try

The program finds a recombination frequency and counts the children of a carrier mother.

Mistakes to avoid

• Counting the big classes as recombinants. The recombinants are the small classes.
• Saying that the mother decides the sex of a child. In humans the sperm decides. In birds the egg decides.
• Giving the allele of a father to his son. An X-linked allele passes from a father to his daughters only.
• Mixing up the carrier and the affected woman. A carrier has the allele once and is normal.

Check yourself

1. Two genes show 8 per cent recombination. How far apart are they?

2. How many chromosomes does a drone have in a body cell?

3. A woman has haemophilia. What must be true of her father?

4. In a bird, which parent decides the sex of the chick?

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 4.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 13.