Linkage and Crossing Over

1. Linkage and Crossing Over Linkage Mr G Davidson

2. Linkage • Because a chromosome can have hundreds of genes, genes found in the same chromosome are said to be linked. • During a cross, therefore, the linked genes will be transmitted together. • e.g. peas T=Tall t=dwarf R=Red r=white • These genes are linked - i.e. on the same chromosome. G Davidson

3. Diagram of Linked Genes G Davidson

4. Linkage • The position of the gene on the chromosome is called the locus. • e.g. Parents TtRr x TtRr • Gametes TR or tr TR or tr • Because T and R are linked, and t and r are linked, there are no recombinants. G Davidson

5. Cross Overs • In dihybrid crossed, the two genes were inherited independently of each other. • Where the two genes are linked they are inherited together, therefore a cross involving a linkage group should behave like a monohybrid cross, however, life is never easy. G Davidson

6. Cross Overs • Usually crosses involving linked genes produce a small proportion of offspring with new allele combinations (recombinants) in addition to the parental combinations. • This is because there is an exchange of material between homologous pairs during meiosis. • The chromosomes cross over at chiasmata and the exchange takes place. G Davidson

7. A b A b A b a a B B a B Parental chromatids AB and ab cross and breaks occur between them. End portions of adjacent chromatids become attached to opposite chromatids Homologous chromosomes separate with new recombinants. Cross Overs G Davidson

8. Cross Overs • These breaks and swapping of chromatid material are called crossovers. • If two genes are far apart on a chromosome, then the chances of them being separated are greater than two genes which are close together. • Therefore, if in a breeding experiment, a high percentage of offspring show that two genes have separated we can conclude that they are far apart or vice versa, if there is a low percentage. G Davidson

9. Cross Overs • The percentage of F2 recombinants is called the cross over value (COV) and is calculated as: • COV= No. of F2 recombinants x 100 Total F2 Offspring G Davidson

10. A B Chromosome Mapping • Cross overs allow geneticists to work out the relative positions of genes on chromosomes. • The crossover values are used to create chromosome maps. • e.g. G Davidson

11. Sex Linkage • Homologous chromosomes are normally identical in appearance and are called autosomes. • Exceptions to this are the sex chromosomes called heterosomes. • These determine the sex of the organism concerned. G Davidson

12. Sex Linkage • In mammals, the possession of 2 different sex chromosomes, i.e. X and Y results in male characteristics. • 2 identical chromosomes, both XX, results in female characteristics. • However, in birds, the reverse is true, with females being the heterogametic sex (XY). G Davidson

13. Sex Linkage • Experiments and observations have shown that other genes in addition to sex determination are carried in the sex chromosomes. • As the Y chromosome is smaller than the X, it carries less information. G Davidson

14. Sex Linkage • This means in females, the genes on the X chromosomes have two alleles in the normal way. • However, in males, some of the genes on the X chromosome have only one allele, because these genes are not present on the Y chromosome. G Davidson

15. Sex Linkage • Where that allele is recessive, the recessive trait will be expressed. • In males, there are many examples where a recessive trait shows in the phenotype. G Davidson

16. Sex Linkage • In a female carrying the same recessive allele, its effect might not show in the phenotype because it could be masked by a dominant allele on the other X chromosome. • As the genes found in the sex chromosomes are linked to those determining sex, they are said to be sex-linked. G Davidson

17. Sex Linkage • In humans, colour blindness is a sex linked trait. • The gene for colour blindness is carried on the non-homologous portion of the X chromosome. • The convention for describing a cross involving sex linkage is as follows: • The chromosome is shown as X or Y and the allele as superscript, e.g. Xn or XN or Y. G Davidson

18. a a A Non-homologous portion of the X chromosome b B b B X X Y X female male Sex Linkage G Davidson

19. Example • Colour blindness (n) is recessive to the normal sight gene (N). • If a man with normal sight marries a woman with normal sight, but a “carrier” for colour blindness, what are the likely phenotypes of any children they have? G Davidson

20. Example Female Male • Parents XNXn x XNY • Gametes XN or Xn XN or Y G Davidson

21. Example Therefore: XNXN = girl with normal sight XNXn = girl with normal sight but a carrier XNY = boy with normal sight XnY = colour blind boy G Davidson