Linked and Sex-linked genes
Linkage and sex determination

What determines gender?


What is the key feature of sex linked
inheritance?


Record for humans and 2 other examples
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 nonhomologous 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 clot
 1/6000-10000 males
 Royal disease – Queen Victoria
 Examine 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 = 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

Activities for practice
Self check 118 – 121
 2006 Exam Question

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

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
b
 What 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
A B
A b
a B
a b
a b
%
recombinants
or non recombinants

45 %
Nonrecomb.
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
COV =
number of recombinants
total number of offspring x 100%
Copy example from pg 126
 What happens the closer genes are?

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
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Mapping activities
Self check pg 128
 Handout ‘Q5 Crossing over’
 Handout ‘Q32 part c’
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