Genetic Variation Today's lesson objectives: First: Revise genetic concepts from yesterday Identify the causes of DNA variation Discuss the differences between somatic and gametic mutations

Do Now What is the relationship between….. DNA Chromosome gene Allele

Genetic Variation Genetic variation is the differences between individual organisms in a population in the combination of alleles. Genetic variation is required for genetic change in a population Genetic variation is caused by: processes during sexual reproduction (independent segregation, independent assortment and crossing over during meiosis AND fertilisation) mutation

DNA Mutation Permanent change to DNA base-pair sequence Can occur randomly during replication Can be caused by mutagens, such as UV light, chemicals, xrays

Types of DNA mutations the fat cat got the rat the efa tca tgo tth era t The fat cat ott her at The fat cat got the mat The fat ca. Insertions Deletions Point mutations Mutations can be silent, beneficial or harmful Progeria

Chromosomal mutations Many genes are changed or affected Usually harmful, mostly lethal

Trisomy 21 causes Down Syndrome in humans

Mitosis = 2 x diploid cell. Meiosis = 4 x haploid cell 46 chromosomes/cell 23 chromosomes/cell Mutations NOT inherited Mutations CAN BE inherited

Meiosis animation http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

Meiosis and Variation Three sources of variation during meiosis: Crossing over Random segregation Independent assortment http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

Crossing Over 1 Maternal chromosome Centromere Chromatids Paternal chromosome Chromatids Crossing over can only occur when homologous chromosomes synapse (come together side-by-side) during the early stages of meiosis

Crossing Over 2 Chiasma in the process of forming chromatids with corresponding gene sequences to become entangled to form chiasma. ‘Equivalent’ segments of homologous chromosomes are able to be exchanged at these points Chromatid of maternal origin Chromatid of paternal origin Mixed types Crossing over point

Crossing Over 3 During the final division of meiosis, the chromatids that were bound together are separated. Each of the four chromatids, with any recombined genes, will end up in one of the four gametes. Chromatid of maternal origin Chromatid of paternal origin Chromatids of mixed maternal and paternal origin Each of these four chromosomes will end up in a separate gamete: Gametes parental types or recombinants

Segregation Segregation describes the separation of the homologous chromosomes (and hence the allele pairs) during the 1st stage of meiosis

Independent assortment Occurs because there are various ways that chromosomes may be aligned Maternal and paternal chromosomes are independent of other homologous pairs new combinations of alleles http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

Independent Assortment Animation http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html http://www.sumanasinc.com/webcontent/animations/content/independentassortmentnm.swf

BUT LINKED GENES do not assort independently. Thus there are fewer allele combinations in the gametes. Linkage reduces genetic variation

Fertilisation Genetic variation is increased by sexual reproduction Fertilisation is the process where genetic information from two genetically different individuals combine Diploid zygote genetically unique 8 milllion possible combinations!

Summary Genetic variation is increased by processes during meiosis crossing over random segregation independent assortment New sources of Genetic variation are produced by mutation. These can be silent, beneficial or harmful mutations Only mutations in gametic cells can be inherited Myotonic goats https://m.youtube.com/watch?v=xsAgNl1p974

Recap of independent assortment of chromosomes During the 1st division of meiosis each pair of homologous chromosomes assorts or lines up at the equator independently of the other pairs. This means that the maternal and paternal chromosome from each homologous pair segregate into the gametes independently of the maternal and paternal chromosomes of all the other homologous pairs. This results in new combinations of unlinked alleles in the gametes and hence in the offspring.

DNA BINGO Some important genetic terms Allele Chromosome Chromatin Centromere Homozygous Heterozygous Recessive allele Dominant allele Linked genes Sister chromatids Homologous chromosomes Crossing over chiasma Autosome sex chromosome Phenotype Genotype DNA mutation

Independent assortment = the process of random organising and separating of chromosomes during meiosis The results of meiosis: production of genetically unique haploid gametes (sperm/eggs in humans)

Fertilisation = the random fusion of gametes Zygote 8 million combinations!! Sexual reproduction results in the rearrangement and shuffling of already existing genetic variation

NCEA exam question The diagram below represents a replicated pair of homologous chromosomes, during meiosis. Draw diagrams to represent the chromosomes in the gametes produced at the end of meiosis when crossing over occurs at point X. (b) Explain how crossing over can contribute to genetic variation that results from sexual reproduction. d d D D __ x__ R R r r

Marking schedule for NCEA exam question Draw diagrams to represent the chromosomes in the gametes produced at the end of meiosis when crossing over occurs at point X. d d D D | | | | R r R r Accept 3 correct, or at least have two recombinant chromosomes (different to parental chromosomes). (b) Explain how crossing over can contribute to genetic variation that results from sexual reproduction. Crossing over increases genetic variation by creating new combinations of genes/alleles, more diversity. For Merit, include an example of genetic advantage such as increased survival chances or ability to adapt to changes in environment etc.

GENETICS AND INHERITANCE MISCONCEPTIONS ACTIVITY

Inheritance of genetic information

Monohybrid Inheritance Today we will be investigating genetic crosses where only one trait is studied We will be using Punnet Squares as a tool to predict the inheritance of alleles

Complete Dominance When information on one allele is ALWAYS EXPRESSED in the phenotype when that allele is present in the genome. The presence of the dominant allele masks the presence of the recessive allele. The recessive allele is only expressed in the phenotype when there is absence of the dominant allele

Examples of hereditary traits in human that show complete dominance Eye Colour

Examples of hereditary traits in human Left and Right handedness

The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. Using Punnett squares you can determine the probability of an offspring having a particular genotype. The Punnett square is a summary of possible combinations of mum allele's with dad’s alleles.

Punnet square activity There is a recessive allele that results in short homozygotes (t). The parents are carriers of the "short" allele Father: heterozygous Tt Mother: heterozygous Tt What gametes that will be produced by each parent Identify the possible genotypes and phenotypes of the offspring What are the ratios of each phenotype?   ?

GENOTYPE PHENOTYPE RATIO Homozygous dominant TT Tall 1   T t TT Tt tT  tt GENOTYPE PHENOTYPE RATIO Homozygous dominant TT Tall 1 Heterozygous Tt Tall/carriers 2 Homozygous recessive tt short 1

Summary Inheritance of different alleles gives rise to genetic variation in the population Monohybrid heterozygote crosses gives rise to offspring with phenotype of 3:1 ratio NEXT: I HAVE, WHO HAS……

Pure Bred/True Breeding Refers to an organism that is known to be homozygous for a given trait ie, homozygous dominant BB homozygous recessive bb

Test Cross To identify whether an organism with a dominant trait is homozygous or heterozygous for a specific gene, a scientist can perform a test cross. The organism in question is crossed with an organism that is homozygous recessive, and the offspring of the test cross are examined.

Incomplete Dominance When neither allele ‘dominates’ the other. When both alleles are present in the heterozygous genotype, both contribute to produce a phenotype that is an intermediate of the homozygote phenotypes Three different phenotypes

100% pink offspring Offspring ratio 1:2:1

Incomplete dominance

Incomplete dominance in humans Examples include Height, skin colour, hand size

Co-dominance When both alleles are equally dominant In heterozygous genotypes both alleles are expressed in the phenotype. Convention is to use capitals for both alleles as both are dominant

Incomplete dominance vs co-dominance These are different things…..

Multiple alleles When genes have more than two different alleles at the same locus Any one Individual only carries two of the alleles in its genotype There are typically more than 3 phenotypes There are no set ratio’s letting you know what kind of monohybrid inheritance you are dealing with

Multiple alleles of ONE GENE in clover plants can produce a range of phenotypes

Human blood groups 3 different alleles - IA IB i Co-dominance of IA IB IA and IB show complete dominance to i The ABO blood groups are the phenotypes

Lethal alleles Lethal alleles effect essential genes. Homozygous condition = death of the individual The phenotype ratio for Aa x Aa crossing is 2:1

Lethal alleles in humans Homozygous AA Embryo doesn't survive