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

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The genetic code and protein synthesis

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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, includingthose involving sex linkage, autosomal linkage, epistasis,codominance and multiple alleles.

j) Use and interpret the chi-squared test to test the significanceof the difference between observed and expected results.

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• So you have already studies thestructure and function of DNA, this iscalled molecular genetics.

• But in this section you will study thetransmission of DNA between wholeorganisms which is know asMendelian genetics after GregorMendel (1822-1884) who was themonk turned scientist thatpioneered this area of biology.

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• In order to be able to explainhow genes are passed on youneed to understand all thenecessary vocabulary first.

• When answering questions itsvery important to use theseterms correctly and also toset out any genetic problemsin a very specific way whichwill be explained later in thisPowerPoint.

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Copyright © 2017 Henry Exham6By 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

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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.

https://commons.wikimedia.org/w/index.php?curid=8025119


• You need to be able to understand and work out the following types of genetic cross:

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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 easiestone, this should be revision.

• A monohybrid cross is theinheritance pattern for asingle gene.

• Here is an example:

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• The allele for brown is dominant andblue is recessive.

• A man is heterozygous for brown eyesand he mates with a woman who isheterozygous for brown eyes, whatpercentage of their children will haveblue 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

Copyright © 2017 Henry Exham9

• Good examples include the genetic diseases of cysticfibrosis and Huntington's disease.

• Cystic fibrosis is a recessive condition where asHuntingdon's is caused by a dominant allele.

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• You should know that the first level ofoffspring from the parents is calledthe F1 generation and the second iscalled F2.

• This example shows true-breeding(homozygous recessive) pea plantsthat produce either yellow or greenseeds which are bred together.

• The F1 generation are all yellow inphenotype but still carry the recessivegreen allele.

• When these F1 offspring are bredtogether with each other the greenphenotype is shown again in the F2generation.

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• Geneticists use pedigreecharts to show the familyhistory of a geneticcondition.

• You should know how tointerpret one.

• Can you work out genotypesof the individuals labeled 1,2and 3? Im

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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.

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• Where alleles show dominance it isnot always possible to to tell thegenotype of an organism.

• For example the yellow pea seedcould have either YY or Yy as itsgenotype.

• In order to work out what it is, youwould do a test cross.

• This is when you cross the unknownindividual with a known genotype,usually homozygous recessive.

• Looking at the offspring will tell youwhat the genotype of the parent is. B

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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


• So as already discussed one allele can be dominant over another. But sometimesboth alleles actually contribute to the phenotype neither one is fully dominant overthe 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

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