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Take me to NZQA Documents relating to this standard AS 90715 Describe the role of DNA in relation to gene expression Determination of Phenotype Part Four…

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Presentation on theme: "Take me to NZQA Documents relating to this standard AS 90715 Describe the role of DNA in relation to gene expression Determination of Phenotype Part Four…"— Presentation transcript:


2 Take me to NZQA Documents relating to this standard AS 90715 Describe the role of DNA in relation to gene expression Determination of Phenotype Part Four…

3 Monhybrid and Dihybrid crosses Incomplete/Codominant Multiple Alleles and Lethal Genes Linked Genes Recombination Chromosome Mapping Sex Linked genes Pedigree charts Pleiotropy and Polygeny Epistasis Epigenetics Contents

4 Some words you may recognise Phenotype Genotype Recessive Dominant Homozygous Heterozygous Punnet Square Allele Gene Red hair Rr RR R Hair Colour R r

5 MONOHYBRID CROSSES The cross of the F1 generation: Gametes Offspring PP is homozygous dominant pp is homozygous recessive Pp is heterozygous Also called “pure breeding” The genotype is a description of the genes contained in the individual The phenotype is a description of its physical appearance (e.g. Purple)

6 In bears, white ears are recessive to black Momma bear (white) What is the genotype of Momma Bear? Momma bear=__________ Poppa bear (black) What genotypes could Poppa Bear have? Poppa bear=________ or _______ This is baby bear (white eared). What is his genotype? Baby bear = Can we say something more about Poppa’s genotype? L2 Revision

7 DIHYBRID CROSSES Not linked (not on the same chromosome). Page 115 In “Quarks” Two eyes (E) is dominant to one eye and Triangular shape (T) is dominant to Pentagonal. 2 Quarks both EeTt are crossed:

8 Alleles that are not dominant and recessive! In yr 10, yr 11 and yes again in yr 12 we lead you to believe that alleles were either recessive or dominate. But now that you are big grown up yr 13s – it is time we expanded you understanding! Yeah sure some alleles are dominant and recessive Some alleles are: Co-dominate Incomplete dominant Lethal Multiple Linked Sex linked

9 INCOMPLETE DOMINANCE AND CODOMINANCE In Incomplete Dominance an intermediate phenotype is produced: In Codominance both alleles are expressed at the same time:

10 MULTIPLE ALLELES It is possible to have more than 2 alleles for a particular trait. Example – In humans the blood group is determined by 3 alleles A common example is the ABO blood groups in humans: O is non-functional A forms a protein with A antigen B forms a protein with B antigen A and B are codominant

11 LETHAL GENES Lethal genes are ones that cause death in the individual. The lethal gene may be dominant or recessive. In the heterozygous individual there may be some observed difference, e.g. Manx (tailless) cats. Even when dominant the lethal gene may be passed on if it does not have onset until after reproductive age (e.g. Huntington’s).

12 LINKED GENES Linked genes are on the same chromosome. This means that when cell division occurs the 2 genes are likely to stay together. So where we might expect a offspring phenotype ratio of 1:1:1:1, we actually get something else. Two genes B (Bent) and D (Dark) are linked. For a cross between BbDd and bbdd… Draw the gametes each could form. Draw a punnet square for the cross. Explain these results: Bent Dark: Bent Light: Straight Dark: Straight Light Page 110 bd BDBbDd BdBbdd bDbbDd bdbbdd 241322 B and D (and b and d) are linked. The 1 Bbdd and 3 bbDd individuals are due to crossing over. The different numbers are due to random chance.

13 Recombination (crossing over) during meiosis increases gamete variation. The closer the genes, the more likely they stay together RECOMBINATION Recombinants AA A BB B a a a b b b Expected gamete A B B a A b a b Page 111 1. Homologous chromosomes line up 2. Chiasma forms, segments swap 4. Mitotic division 3. Meiotic division

14 CHROMOSOME MAPPING We can “map” the distance between 2 genes by comparing how often the cross over. Crossover value (%) = No. of recombinants No. of offspring  100 Low cross over value = not often recombined = close together Page 114

15 SEX LINKED GENES X is normal, Xc is colour blind. Y carries no information. So…XX is a normal female What other possibilities can you have? Explain why it is less common for women to be colour blind. XcX = normal femaleXcXc = colour blind female XY= normal maleXcY= colour blind male Carried on the X-chromosome. As opposed to autosomal. Males only have 1 allele – more susceptible to recessive diseases, e.g. colour blindness. DO NOT SAY WHAT YOU SEE.

16 In cats one aspect of coat colour is controlled by a sex-linked gene with alleles that are codominant. These 2 females (XoXo) and (XbXb) are crossed with a male (XbY)… Draw Punnett squares to find the offspring of each cross. What is the XbXo offspring called? How do we get male Tortoiseshell? Page 121-2

17 PEDIGREE CHARTS Males Females Dead Affected (but not dead – yet) Normal Page 128-9

18 GENE-GENE INTERACTIONS Pleiotropy :One gene (  one protein) controls many phenotypes 40% of cats with white fur and blue eyes are deaf. Marfan syndrome: one gene is responsible for thinness, joint hyper mobility, limb elongation, lens dislocation, and increased susceptibility to heart disease. The p53 gene directs damaged cells to stop reproducing, thereby resulting in cell death… helps avert cancer. BUT it also suppresses the division of stem cells, preventing replacement of deteriorating tissues during aging. Polygeny : Many genes control one phenotype (e.g. Human skin colour, cat coat colour) Lab manual page 133 Epistasis : Two genes interact, mask, or modify. There are a number of types…

19 One gene alters the outcome of another SubstanceProduct AProduct B E n z y m e 1 E n z y m e 2 Can we cross 2 white plants to get purple offspring? What will the genotype ratio of the dihybrid cross be? 9:7 Think 9:3:3:1, but group the last 3 sets. PC Pc pC pc PCPcpCpc Complementary genes: Both need to be present for either to work. White Purple G e n e P G e n e C In a flower, 2 genes control the production of a purple pigment. The intermediate product has no colour. EPISTASIS PPcc ppCC PpCc

20 Supplementary genes: the second gene adds more to the first. Coat color in Labrador retrievers: two genes (B and E) A black dog is B_E_ a.What are the genotypes of two black parental dogs that, when mated, produce black puppies, yellow puppies and brown puppies? b. What proportions of black and yellow puppies do you expect from this cross? c. This cross is an example of a what type of gene interaction? Lab manual page 135/6 YellowBrownBlack G e n e E G e n e B

21 COLLABORATION This is where 2 genes interact to make a product different to that which either could make independently. The most common example is comb types in chickens. rrpprrP_R_ppR_P_ Page 132 Summary 138 See more chickens… single rose Walnut G e n e R G e n e P pea G e n e P G e n e R

22 EPIGENETICS This is where gene expression is affected without changing the underlying DNA code. e.g. a zygote (toti-potent) becomes more and more specialised by turning different genes on or off. e.g. Genomic Imprinting.Genomic Imprinting Some genes can be activated or silenced depending on which parent they came from. The ‘switch’ used to silence or activate a gene is often methylation of cytosines – this inhibits that region of the DNA.

23 GENE-ENVIRONMENT INTERACTIONS This is where gene expression is affected by the environment. Some organisms are affected by temperature, altitude or the presence of members of the opposite sex or competitors/predators. Page 125-6 (opt)

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