Linked and Sex-linked genes

Linked and Sex-linked genes

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.

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

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.

Haemophilia

Blood fails to clot1/6000-10000 malesRoyal disease – Queen VictoriaExamine pedigree chart pg 116

Muscular dystrophy

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

Barr bodies

What are they? Use page 117 to explain

What can this lead to?

Define varigation

Tortoise-shell cats

Only 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 practice

Self check 118 – 1212006 Exam Question

Sex-influenced genes

Traits not located on sex chromosomes have different appearances in males and females

Influenced by presence of sex hormonesEg – pattern baldness in humans - bulls and milk production - horns in sheep

Linkage activity

Create some chromosomes and follow what the teacher says……

Linkage

Humans have 23 pairs of chromosomes and about 100,000 genes

therefore

many genes are on the same chromosome

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

Show how these cells can make gametes:

A B

a bWhat ratio is given when this individual is

test crossed?1:1 is the ratio for linkage with no crossing

over

Linkage with cross over

Create chiasma by performing crossing over and recombination with your chromosomes

Now what gametes can form?

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

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

A B A b a B a b

a b

% recombinants or non - recombinants

45 %Nonrecomb.

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 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

Mapping activities

Self check pg 128Handout ‘Q5 Crossing over’Handout ‘Q32 part c’