Inheritance 5.4

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The Basics - GCSE recap

A gene is a portion of DNA which codes for a polypepide. Genes can come in different forms and these are called Alleles. Alleles can be dominant or recessive , dominant traits will show themselves even if only one of the alleles is present , whereas recessive alleles must have two copies of that allele. Genotype is what Genes you have for example GG or Gg or gg. If you have two of the same allele then you are said to be homozygous whereas someone who has one of each allele will be hetrozygous.Pheno type is what trait is displayed for example brown or blonde hair.

Make sure that you are happy with basic punett sqaures for example working our eye or hair colour

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Dihybrid Crosses

These show the possible offspring of two parents for two different traits and for this to work the traits must be on different choromosomes. This is so that there is no linkage of the genes so no changes in the ratios.Genes are considered to be linked if they are on the same chromosome.

The ratio that Mendal suggested was 9:3:3:1

there will be nine offspring who have both dominant phenotypes out of 16

Three of the 16 offspring would have one recessive and one dominant trait ( since there are two traits beingn looked at there will be off spring one are dominant for 1 trait and others who will be dominant for the second trait)

1 out of the 16 offspring will have both recessive traits

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Chi squared testing

So we know the ratios that a dihybrid cross should come out with, but how can we prove if our results work ( are significant) in comparison to this ratio.For results to be  significant there must be a difference in the results greater than or equal to 95% . Another way of phrasing this is that only five peercent can be the same/ similar.

x2= (observed - expected)/ expected  if this value is less than or equal to the set value which will be given to you in the exam in table form then the result is significant and any differences are due to  chance and the null hypothesis is accepted.The reverse statements apply if the value is greater than  this set value. A null hypothesis uses the words there is no difference between ( for example) the expected number of offspring of each phenotype and the actual number of offspring with this phenotype.

The degrees of freedom is always one less than the number of classes of phenotype that you have.

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Co dominance

This is where both of the alleles for say petal colour are dominant for example

YY= yellow

RR= Red

RY = orange

The hetrozygote will display a phenotype which is intermediate to that of the other two phenotypes of the Homozygotes. This can be advantagoues for example in the case of sickle cell animea the different shaped cells can not be invaded by the malaria causing Parasite, so a hetrozygote will have a little added protection but not the full blown disease. Sickle cell animea is a point / gene mutation there is a change in one base of the DNA. 

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Sex linked

Some diseases occur mainly in males such as haemophilia. The changes in the genes are carried on the sex chromosomes, in the case of haemophilia on the X chromosome. It is a recessive disease which means that if a male gets a faulty copy of the allele he will display the disease this is becuase he is unable to carry a correct copy his second sex chromosome because the Y chromosome is smaller and carries fewer genes than the X chromosome.Females are able to get this disease but must have two faulty copies of the allele. A pedigree chart will show which memeberrs of a family have a disease so genetic conselling can be given to couples with a chance of passing on a disease and the probabilities worked out. The chance of having a male or a female is always 50%.

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There can be gene mutations which only affect one gene or there can be chromosome mutations which will alter many genes.Gene mutations can occur if the DNA is not copied correctly during replication or same point mutations occur and these can be deletions, additions or subsitutions.Chromosomes can break and then not fix themselves correctly. Organisms which show frequent meiosis are more likely to have mutations occur. As the geneotype of an organisms determines the Phenotype of the organism any change in the DNA will change the way that the organism looks:its phenotype.

CArciogens are mutagens which have been known to cause cancer. They do this by causing mutations in the oncogenes which control cell growth and division. Examples inculde benzene, polycyclic hydrocarbons and ionising radiation.Mutations can be advantageous but are most often harmful. Only mutations which occur on the gametes can be passed between generations to create variation.

Sometimes an Ova can get 2 chromosome 21s and when the cell is fertilised the child has 3 chromosome 21s , this is an example of non disjunction and polypoldity and the cause of downs syndrome.

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