Linked and Sex-linked genes
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Linked and Sex-linked genes
Linkage and sex determinationWhat determines gender?Record for humans and 2 other examplesWhat is the key feature of sex linked inheritance?Males are more affectedWhat 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.
Sex linkageAny gene carried on the non-homologous part of the X chromosome is called sex-linkedThe Y chromosome is genetically empty for that characteristic.Faulty genes on X will show up in males because Y cannot mask effectSymbols written above X and Y symbols
Red-green colour blindnessAllele 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.
HaemophiliaBlood fails to clot1/6000-10000 malesRoyal disease Queen VictoriaExamine pedigree chart pg 116
Muscular dystrophyMuscular dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue.
Barr bodiesWhat are they? Use page 117 to explain
What can this lead to?
Tortoise-shell catsOnly females can be tortoise shell.Two alleles B = black and O = orangeMales can be ?Females can be ?What is the result of a black male mating with an orange female?XXY is a male tortoise-shell
Activities for practiceSelf check 118 1212006 Exam Question
Sex-influenced genesTraits not located on sex chromosomes have different appearances in males and femalesInfluenced by presence of sex hormonesEg pattern baldness in humans - bulls and milk production - horns in sheep
Linkage activityCreate some chromosomes and follow what the teacher says
LinkageHumans have 23 pairs of chromosomes and about 100,000 genestherefore many genes are on the same chromosome
Genes on the same chromosome are linked genesLinked genes cannot segregate independently, they move together during meiosis so inherited togetherResults in less genetic variation in gametes than when independent assortment takes place
Show how these cells can make gametes:ABabWhat ratio is given when this individual is test crossed?1:1 is the ratio for linkage with no crossing over
Linkage with cross overCreate chiasma by performing crossing over and recombination with your chromosomes
Now what gametes can form?
RecombinationThis is the exchange of alleles between homologous chromosomes as a result of crossing overIt increases genetic variation by creating new combinations of alleles to be passed on in gametesOffspring formed from these gametes show new combinations of characteristics and are known as RECOMBINANTS
Perform a backcross
This gives a ratio 9:1:1:9 which is almost a 1:1 ratio with a little bit over means that it is linkage with crossing over
Cross over value (COV)This is the percentage of offspring which show separation of the genes
Copy example from pg 126What happens the closer genes are?number of recombinantsCOV = total number of offspring x 100%
Chromosome mappingThis is the relative position of genes on a chromosomeThe further the genes are apart, the more likely they are to break and rejoin
Mapping activitiesSelf check pg 128Handout Q5 Crossing overHandout Q32 part c