Linked and Sexlinked genes Linkage and sex determination

Linked and Sex-linked genes

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

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

Red-green colour blindness l Allele common, so some females affected. l 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.

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

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

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

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

Activities for practice l Self check 118 – 121 l 2006 Exam Question

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

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

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

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

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

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

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

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

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

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

Mapping activities l Self check pg 128 l Handout ‘Q 5 Crossing over’ l Handout ‘Q 32 part c’