Cambridge Pre-U Biology - 1.6 Genes and Protein Synthesis PART 2 Sample

MAKING SENSE OF
Icons CC – The Pink Group
Copyright©2017HenryExham
Icons CC – The Pink Group
Copyright©2017HenryExham

MAKING SENSE OFMAKING SENSE OF
Gene expression
Variation
Inheritance
2
Mutations
The genetic code and protein synthesis
PART1PART2

Copyright © 2017 Henry Exham
h) Define and use the terms allele, locus, phenotype, genotype,
dominant, recessive and codominant.
i) Use genetic diagrams to solve dihybrid crosses, including
those involving sex linkage, autosomal linkage, epistasis,
codominance and multiple alleles.
j) Use and interpret the chi-squared test to test the significance
of the difference between observed and expected results.
3

• So you have already studies the
structure and function of DNA, this is
called molecular genetics.
• But in this section you will study the
transmission of DNA between whole
organisms which is know as
Mendelian genetics after Gregor
Mendel (1822-1884) who was the
monk turned scientist that
pioneered this area of biology.
4

• In order to be able to explain
how genes are passed on you
need to understand all the
necessary vocabulary first.
• When answering questions its
very important to use these
terms correctly and also to
set out any genetic problems
in a very specific way which
will be explained later in this
PowerPoint.
5

6By Emw - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8025119
Your normal body cells contain 23
homologous pairs of chromosomes.
Here is one pair.
The position of each gene on a chromosome is
called its locus.
The genes at the same loci on each
chromosome will code for the same trait but
they may be different versions (alleles).
The combination of alleles for a particular trait
is called the person’s genotype.
In this case this person has one of each allele
for this gene and so they are called
heterozygous, and the genotype is Aa.
As ‘A’ is dominant over ‘a’ (shown by capital
letter) then this allele is the one expressed in
the phenotype.
A a
t t
Cw
CR
The person is homozygous recessive
for this particular trait and their
genotype will be tt
Sometimes one allele isn't dominant
over the other and they can both
contribute to the phenotype, these
alleles are codominant.

• You need to be able to understand and work out the following types of
genetic cross:
7
One gene for one characteristic (a monohybrid cross)
• Two alleles, dominant and recessive e.g. T,t
• Two alleles, codominant e.g. CR, CW.
• Multiple (more than two) alleles, e.g. IA, IB, IO
• Sex linkage: the gene is on the X chromosome so males only have a single copy e.g XHXh x XHY-
Two genes for two characteristics (a dihybrid cross)
• Two genes on different chromosomes
• Two linked genes on the same non-sex chromosome (“autosome”)
Two genes for one characteristic (epistasis / interaction)
• Epistasis: one gene masks (is “epistatic” to) another e.g. in mouse fur colour.
• Interaction: phenotype is determined by the combination of dominant and recessive alleles of two
genes.
• We will now go through these one at a time using specific examples.

• Lets start with the easiest
one, this should be revision.
• A monohybrid cross is the
inheritance pattern for a
single gene.
• Here is an example:
8
• The allele for brown is dominant and
blue is recessive.
• A man is heterozygous for brown eyes
and he mates with a woman who is
heterozygous for brown eyes, what
percentage of their children will have
blue eyes?
Parents Genotype:
Parents Phenotype:
Parents gametes:
F1 genotypes:
F1 phenotypes:
Brown Brownx
Bb x Bb
Father Mother
1 x BB, 2 x Bb, 1 x bb
¾ Brown ¼ Blue
BB Bb
Bb bb
B bB bB b B b

9
• Good examples include the genetic diseases of cystic
fibrosis and Huntington's disease.
• Cystic fibrosis is a recessive condition where as
Huntingdon's is caused by a dominant allele.
ByOpenStaxCollege-Anatomy&Physiology,ConnexionsWebsite.
http://cnx.org/content/col11496/1.6/,Jun19,2013.,CCBY3.0,
https://commons.wikimedia.org/w/index.php?curid=30148626

10
• You should know that the first level of
offspring from the parents is called
the F1 generation and the second is
called F2.
• This example shows true-breeding
(homozygous recessive) pea plants
that produce either yellow or green
seeds which are bred together.
• The F1 generation are all yellow in
phenotype but still carry the recessive
green allele.
• When these F1 offspring are bred
together with each other the green
phenotype is shown again in the F2
generation.
ByCNXOpenStax-http://cnx.org/contents/GFy_h8cu@10.53:rZudN6XP@2/Introduction,CCBY4.0,
https://commons.wikimedia.org/w/index.php?curid=49926348

11
• Geneticists use pedigree
charts to show the family
history of a genetic
condition.
• You should know how to
interpret one.
• Can you work out genotypes
of the individuals labeled 1,2
and 3?
Imagecredit“ThePunnettsquareapproachforamonohybridcross:Figure3,"byOpenStaxCollege,Biology(CCBY3.0).
Alkaptonuria is a recessive genetic disorder in
which two amino acids, phenylalanine and
tyrosine, are not properly metabolized.
Affected individuals may have darkened skin
and brown urine, and may suffer joint damage
and other complications.

12
• Where alleles show dominance it is
not always possible to to tell the
genotype of an organism.
• For example the yellow pea seed
could have either YY or Yy as its
genotype.
• In order to work out what it is, you
would do a test cross.
• This is when you cross the unknown
individual with a known genotype,
usually homozygous recessive.
• Looking at the offspring will tell you
what the genotype of the parent is.
ByKatieAnn127-Ownwork,CCBY-SA4.0,https://commons.wikimedia.org/w/index.php?curid=37309605

13
One gene for one characteristic (a monohybrid cross)
• Two alleles, dominant and recessive e.g. T,t
• Two alleles, incomplete and codominant e.g. CR, CW.
• Multiple (more than two) alleles, e.g. IA, IB, IO
• Sex linkage: the gene is on the X chromosome so males only have a single copy e.g XHXh x
XHY-
Two genes for two characteristics (a dihybrid cross)
• Two genes on different chromosomes
• Two linked genes on the same non-sex chromosome (“autosome”)
Two genes for one characteristic (epistasis / interaction)
• Epistasis: one gene masks (is “epistatic” to) another e.g. in mouse fur colour.
• Interaction: phenotype is determined by the combination of dominant and recessive
alleles of two genes

14
• So as already discussed one allele can be dominant over another. But sometimes
both alleles actually contribute to the phenotype neither one is fully dominant over
the other.
Notice carefully that in
codominance both have
alleles have capital letters
and are shown as
superscript.

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Cambridge Pre-U Biology - 1.6 Genes and Protein Synthesis PART 2 Sample