Download Crossing over Linkages Sex linkages

Crossing over
Linkages
Sex linkages
How to shuffle the genes
More Patterns of inheritance
• Monohybrid crosses
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Simple dominance
Test crosses
Co dominance
Incomplete dominance
• Dihybrid crosses
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Simple dominance
Linked
Crossing over
Sex linked
A quick reminder
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Genes are segments of DNA
They are a series of base pairs
1 gene codes for 1 protein
We need many proteins we use them for
enzymes, hormones , structural body parts,
pigments, lots of stuff.
• Each chromosome has hundreds of genes
• Each gene has a special place like beads on a
string.
The linkage
• Each chromosme has hundreds of genes on it.
• Each gene is found in a certain place or locus
• If the two genes we are looking at are on the
same chromosome then they are linked.
Linked genes and the Law of
Independent Assortment
• If two genes are on different chromosomes
they sort independently and inherited
combinations show up in a Phenotypic ratio of
9: 3: 3: 1
• If two genes are one the same chromosome
they are linked and give a phenotypic ratio of
3:1 just like a monohybrid cross.
The tetrad
• In Metaphase 1 the two sets of homologous
chromosomes line up at the equator of the
cell. This is called a tetrad because there are 4
side by side.
The chiasma
• Sometimes the chromosomes are sticky and
the homologous pairs in the middle cross over
each other
• This is called
A chiasma
• The DNA can
Switch chromosomes
And shuffle the genes.
Crossing over
or
Recombination
• When there is an equal trade between nonsister chromotids at the chiasma you can get
new combinations of genes
How we can map chromosomes
• The farther apart genes are on a chromosome
the more likely they are to cross over.
• We count the frequency of crossovers
between all the genes on a chromosome and
find out their relative positions.
Sex chromosomes and Autosomes
• You have 23 pairs of chromosomes, one from
your mom and one from your Dad. 22 pairs
are homologous. These are your autosomes.
The 23rd pair is not homologous. They
determine your sex. If you have XX
chromosomes you are a girl. XY you are a
boy. The Y is very small and doesn’t carry
any genes other than sex determination.
These are your sex chromosomes.
Polygenic Inheritance
• So far we’ve only looked at traits that are
caused by 1 gene.
• Some traits are caused by 2 or more genes.
• This is called polygenic inheritance.
• Eye colour ,skin colour, and hair colour are all
polygenic.