1. 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

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

3. 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

4. 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

5. 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

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

7. Trisomy 21 causes Down Syndrome in humans

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

9. Meiosis animation

10. Meiosis and Variation Three sources of variation during meiosis:
Crossing over
Random segregation
Independent assortment

11. 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

12. 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

13. 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

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

15. 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

16. Independent Assortment Animation

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

18. 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!

19. 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

20. 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.

21. 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

22. 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)

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

24. 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

25. 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.

26. GENETICS AND INHERITANCE MISCONCEPTIONS ACTIVITY

27. Inheritance of genetic information

28. 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

29. 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

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

31. Examples of hereditary traits in human
Left and Right handedness

34. 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.

35. 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? ?

36. 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

37. 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……

38. 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

39. 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.

40. 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

41. 100% pink
offspring
Offspring ratio 1:2:1

42. Incomplete dominance

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

44. 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

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

47. 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

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

49. 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

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

51. Lethal alleles in humans
Homozygous AA
Embryo doesn't survive