1. Linked and Sex-linked genes

2. Linkage and sex determination
What determines gender?
Record for humans and 2 other examples
What is the key feature of sex linked inheritance?
Males are more affected
What is a definition and diagram for sex linked genes?
Any gene carried on the non-homologous part of the X chromosome is called sex-linked.

3. Sex linkage
Any gene carried on the non-homologous part of the X chromosome is called sex-linked
The Y chromosome is genetically empty for that characteristic.
Faulty genes on X will show up in males because Y cannot mask effect
Symbols written above X and Y symbols

4. Red-green colour blindness
Allele common, so some females affected.
Cross a woman with normal vision (but her father was colour blind) with a colour blind man. Give the genotypes and phenotypes of the children.

5. Haemophilia Blood fails to clot 1/6000-10000 males
Royal disease – Queen Victoria
Examine pedigree chart pg 116

6. Muscular dystrophy
Muscular dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue.

7. Barr bodies What are they? What can this lead to? Define varigation
Use page 117 to explain
What can this lead to?
Define varigation

8. Tortoise-shell cats Only females can be tortoise shell.
Two alleles B = black and O = orange
Males can be ?
Females can be ?
What is the result of a black male mating with an orange female?
XXY is a male tortoise-shell

9. Activities for practice
Self check 118 – 121
2006 Exam Question

10. Sex-influenced genes
Traits not located on sex chromosomes have different appearances in males and females
Influenced by presence of sex hormones
Eg – pattern baldness in humans
- bulls and milk production
- horns in sheep

11. Linkage activity
Create some chromosomes and follow what the teacher says……

12. Linkage Humans have 23 pairs of chromosomes and about 100,000 genes
therefore
many genes are on the same chromosome

13. Genes on the same chromosome are linked genes
Linked genes cannot segregate independently, they move together during meiosis so inherited together
Results in less genetic variation in gametes than when independent assortment takes place

14. Show how these cells can make gametes: A B a b
What ratio is given when this individual is test crossed?
1:1 is the ratio for linkage with no crossing over

15. Linkage with cross over
Create chiasma by performing crossing over and recombination with your chromosomes
Now what gametes can form?

16. Recombination
This is the exchange of alleles between homologous chromosomes as a result of crossing over
It increases genetic variation by creating new combinations of alleles to be passed on in gametes
Offspring formed from these gametes show new combinations of characteristics and are known as RECOMBINANTS

17. Perform a backcross This gives a ratio 9:1:1:9 A B A b a B a b 45 %
which is almost a 1:1 ratio with a ‘little bit over’ means that it is linkage with crossing over
A B
A b
a B
a b
% recombinants or non - recombinants
45 %
Nonrecomb.

18. Cross over value (COV)
This is the percentage of offspring which show separation of the genes
Copy example from pg 126
What happens the closer genes are?
number of recombinants
COV = total number of offspring x 100%

19. Chromosome mapping
This is the relative position of genes on a chromosome
The further the genes are apart, the more likely they are to break and rejoin

20. Mapping activities Self check pg 128 Handout ‘Q5 Crossing over’
Handout ‘Q32 part c’