1. Blueprint of life Miss Heretakis
Past HSC questions :
Miss Heretakis

2. recall

3. Sutton and Boveri —the chromosome theory of inheritance
3. Chromosomal structure provides the key to inheritance
outline the roles of Sutton and Boveri in identifying the importance of chromosomes
Walter Sutton ( ), an American cytologist, studied meiosis in cells of grasshoppers.
Sutton’s observations revealed that:
Chromosomes occur in distinct pairs as distinct entities visible during meiosis. One chromosome of each pair is paternal and the other maternal (homologous pairs).
During meiosis, the chromosome number of a cell is halved.
Fertilisation restores the full number of chromosomes in the zygote.

4. Sutton and Boveri —the chromosome theory of inheritance
Sutton concluded that:
Chromosomes arrange themselves independently along the middle of the cell just before it divides.
Chromosomes are units involved in inheritance.
Sutton believed several “Mendelian factors” must be present on one chromosome, and could therefore be inherited as a unit.
Sutton (1902–03)
proposed that genes are carried on chromosomes and behave like Mendel’s factors (observing meiosis in grasshoppers)

5. Sutton and Boveri —the chromosome theory of inheritance
Theodor Boveri, a German biologist, carried out experiments on sea urchin eggs between 1896 and He studied the behaviour of the cell nucleus and chromosomes during meiosis and after fertilisation.
Boveri’s experiments showed that:
The nucleus of the egg and sperm each contribute the same amount of chromosomes to the zygote
When a normal egg and sperm fused, the resulting offspring showed characteristics of both parents.

6. Sutton and Boveri —the chromosome theory of inheritance
Boveri deduced that:
A complete set of chromosomes (in pairs) is required for normal development.
The inheritance “factors” are found on chromosomes within the nucleus – chromosomes are the carriers of heredity.
There are more hereditary “factors” than chromosomes and so there must be many factors (today known as genes) on one chromosome.
Boveri (1903)
demonstrated a connection
between chromosomes and
heredity (by observing sea urchins)

7. Sutton and Boveri —the chromosome theory of inheritance
Even though Sutton and Boveri worked independently, their work became known as the Sutton-Boveri Chromosome Theory, which became hypotheses for testing in future experiments.
With advances in scientific understanding and improved technology, their cytological interpretation of Mendelian inheritance was convincingly confirmed in 1915 by Thomas Morgan and still holds today.

8. Chromosomes, meiosis and gamete formation
Chromosomes are compact coils of thread-like molecules called DNA (deoxyribonucleic acid), organised around proteins called histones.
Every species has a characteristic number of chromosomes in every body cell, e.g. 46 chromosomes in humans
In order to maintain the constant chromosome number from one generation to the next, a mechanism called meiosis (reduction division)occurs to halve the chromosomes when gametes are produced.

9. Chromosomes, meiosis and gamete formation
Every parent cell contains two sets of chromosomes – one paternal and one maternal set, resulting in the diploid number.
During meiosis, homologous pairs of chromosomes segregate so that each gamete receives only one copy of each chromosome – that is, the gametes are haploid. The diploid number is restored when the gametes fuse in fertilisation to form a zygote.

10. Chromosomes, meiosis and gamete formation
Meiosis has the added role of mixing the paternal and maternal chromosomes, and these recombined chromosomes are passed into gametes increasing the genetic variation in offspring produced.

11. The chemical nature of chromosomes and genes
describe the chemical nature of chromosomes and genes
Each chromosome is made up of two chemicals:
DNA, a long, thin thread-like macromolecule, which is the information-carrying part of the chromosome
Proteins (histones) around which the DNA is coiled, to keep it neatly ‘packaged’.
DNA (deoxyribose nucleic acid) molecule meets all the requirements of the hereditary material:
It can carry, in coded form, all the instructions for the formation and functioning of cells, despite the fact that its ‘alphabet’ consists of only four nitrogenous bases.
Its structure allows self-replication.
It can be transferred (packaged in the form of chromosomes) by gametes from one generation to the next.

12. The discovery of dna structure
Read pages
process information from secondary sources to describe and analyse the relative importance of the work of James Watson, Francis Crick, Rosalind Franklin and Maurice Wilkins in determining the structure of DNA and the impact of the quality of collaboration and communication on their scientific research (focus area 4)
Scientist
Work and contribution
Collaboration and communication
James Watson & Francis Crick
In 1953, cracked the DNA code - discovered that the heredity material of living cells is DNA, a giant molecule in the form of a double helix. They also suggested that the pairing of the bases allow the molecule to self replicate.
Awarded Nobel Prize in 1962
Later he increased understanding of the genetic code and how DNA triplet code identifies amino acids and thus controls protein synthesis
Worked closely together
Bounced ideas off everywhere, including Wilkins who showed them the photograph taken by Franklin

13. The discovery of dna structure
Read pages
Scientist
Work and contribution
Collaboration and communication
Rosalind Franklin
Used X-ray crystallography to map the locations of atoms in the DNA molecule
First to state that the phosphates lie on the outside
Described basic double helical structure upon which Watson and Crick based their model on.
Sexism was rife in science at this time (1940s-1950s) and franklin struggled to be respected for her work.
Died of cancer in 1958 aged 37
Franklin did not know Watson and Crick as well as Wilkins did and never truly collaborated with them.
Her work was shown to Watson and Crick by Wilkins
Maurice Wilkins
Successfully extracted some fibres from a gel of DNA in early 1950s
Franklin photographed these DNA fibres and Wilkins showed the photo to Watson and Crick
Awarded Nobel Prize in 1962
Got along with Watson and Crick
Showed Watson and Crick the X-ray crystallography photograph without Franklin’s knowledge or permission

14. The structure of DNA
identify that DNA is a double-stranded molecule twisted into a helix with each strand, comprised of a sugar– phosphate backbone and attached bases—adenine (A), thymine (T), cytosine (C) and guanine (G)—connected to a complementary strand by pairing the bases, A-T and G-C
Watson and Crick revealed that DNA is a double helix or ‘twisted ladder’ consisting of 2 complementary strands (or chains).
Each strand is made up of small building blocks called nucleotides which are held together by weak hydrogen bonds in the centre.
Each nucleotide consists of three parts:
a phosphate
a sugar (deoxyribose sugar)
a nitrogenous base.

15. The structure of DNA
There are four types of nitrogenous bases—adenine (A), thymine (T), guanine (G) or cytosine (C). Chemically, these bases have to pair in a particular manner: adenine with thymine (A-T), and guanine with cytosine (G-C)
A sequence of 3 nitrogenous bases forms a genetic unit.

16. The structure of DNA
A chromosome can be described as a linear sequence of genes.
A gene is considered to be the smallest unit of heredity. Each gene is a portion of DNA, or a specific sequence of genetic units that encodes for a particular trait.
The position of a gene on a chromosome is called its locus.
The total amount of genetic material that an organism has in each of its cells is called its genome.
A gene controls the putting together of amino acids to make proteins.

17. Homologous pair of chromosomes showing corresponding banding patterns
The structure of DNA
Specific staining techniques are used to show up banding patterns on chromosomes.
These bands correspond on homologous chromosomes.
They can be used to identify the positions of particular genes on chromosomes.
Homologous pair of chromosomes showing corresponding banding patterns

18. Modelling meiosis Worksheet with diagrams
process information from secondary sources to construct a model that demonstrates meiosis and the processes of crossing over, segregation of chromosomes and the production of haploid gametes
Worksheet with diagrams
Print page 145 worksheet

19. Meiosis and gamete formation
Read pages
explain the relationship between the structure and behaviour of chromosomes during meiosis and the inheritance of genes
Meiosis is cell division that produces sex cells and halves the number of chromosomes.
During meiosis, genetic variation arises as a result of the behaviour of chromosomes at two stages:
during crossing over
when chromosomes randomly segregate and paternal and maternal chromosomes assort independently of each other.

20. Meiosis and gamete formation
Read pages
During meiosis I:
chromosomes line up in homologous pairs (1 maternal and 1 paternal chromosome in each pair) during prophase I
crossing over (synapsis) occurs—arms of homologous chromosomes exchange genetic material
the chromosomes in each pair of chromosomes randomly segregate (separate) during anaphase I, so that one entire chromosome of each pair moves into a daughter cell. This ensures the chromosome number in the resulting gametes will be half that of the original cell. The manner in which these chromosomes separate is termed independent assortment— the paternal and maternal chromosomes sort themselves independently of each other.
Random Segregation: alleles go through meiosis to create gametes, they will segregate from one another, and each of the haploid gametes will end up with only one allele i.e. Alleles for the same trait (e.g. height) separate/segregate into different gametes. Independent assortment: is how the alleles of two genes separate. The alleles will assort themselves independently of one another when the haploid gametes are formed in meiosis. Each haploid gamete ends up with a different combination of alleles of these two genes i.e. Independent assortment: Alleles for different traits separate/assort independently from each other.

21. Meiosis and gamete formation
Read pages
During meiosis II:
The two daughter cells that result from meiosis I each undergo meiosis II, which is similar to mitosis, and the behaviour of chromosomes in the second meiotic division does not further affect genetic variation.

22. Simplified explanation
See next few slides.

23. Life cycle summary Human Fertilisation (n) (n) Zygote (2n)
Produce gametes
Mitosis
Meiosis
Gametes are the only cell in the body NOT produced by mitosis

24. Meiosis summary Autosomal (body) cells:
23 pairs of chromosomes (1 pair from mum, 1 pair from dad)
Diploid number (2n)
Sex cells (gametes)
23 chromosomes
Haploid number (n)

25. Pair of homologous chromosomes - 1 chromosome from each parent
Meiosis summary
Germ cell (2n)
(before meiosis!)
Pair of homologous chromosomes - 1 chromosome from each parent
Diploid (2n) – 23 chromosomes from mother and 23 chromosomes from father
Each chromosome has only 1 chromatid – they are un-replicated strands of DNA
Number of chromosomes is determined by the number of centromeres
Homologous chromosomes are similar but not identical. Each carries the same genes in the same order, but the alleles for each trait may not be the same.

26. Meiosis summary Chromosomes duplicate (still 2n) (During interphase)
Duplicated pair of homologous chromosomes still in the form of chromatin
Sister chromatids are identical
Each chromosome has 2 chromatids – they are replicated strands of DNA
Blog with diagrams -
Note: they are not aligned in the centre at this point

27. Meiosis summary
Chromosomes condense and separate out into homologous pairs
(Prophase 1)
Homologous pairs of chromosomes align in the centre of the cell
Crossing over – the sharing of genetic material between 2 non-sister chromatids in a homologous pair
(late Prophase 1)

28. Meiosis summary
Homologous pairs of chromosomes align in the centre of the cell (metaphase plate)
Random segregation
Each chromosome attaches to microtubules from only one pole of the spindle
(Metaphase 1)

29. Meiosis summary Chromosome pairs separate
Each chromosome moves to the opposite end (pole) of the cell
(Anaphase 1)

30. Meiosis summary Two daughter cells formed
Chromosomes arrive at opposite ends of the cell
Daughter cells are not identical to each other
Chromosome number halved (n)
(Telophase 1)
Two daughter cells separated b a cell membrane
(Cytokinesis 1)

31. Meiosis summary Chromosomes align in the middle of the cell (again)
(metaphase 2)
AND THEN
Chromatids move apart to opposite ends of the cell
(anaphase 2)

32. Meiosis summary
The 4 resulting daughter cells are not identical to each other and have half the original chromosome number (haploid).
(cytokinesis 2)

33. Variability—gamete formation and sexual reproduction
explain the role of gamete formation and sexual reproduction in variability of offspring
Variability in genetics relates to the different forms of a gene within a population.
Gametes (sex cells) are formed by the process of meiosis.
Genetic variation in individuals (and therefore variability in a population) arises as a result of sexual reproduction.
Sexual reproduction involves the fusion of one male and one female sex cell (fertilisation). Which sex cells join is a random process. New combinations of genes occurring in the offspring lead to greater variability within a population.
The term variability means something different to variation. In the preliminary
Variation is evident in individuals (e.g. differences in colour and height).

34. Variability—gamete formation and sexual reproduction
Evolutionary studies show that greater variability improves the ability of a population to adapt to sudden changes in the environment, resulting in an increased chance of survival.
The term variability means something different to variation. In the preliminary
Variation is evident in individuals (e.g. differences in colour and height).

35. Deviations from Mendelian inheritance and variations of Mendel’s ratios
describe the inheritance of sex-linked genes, and alleles that exhibit co-dominance and explain why these do not produce simple Mendelian ratios
Mendelian ratios of inheritance apply only in situations where conditions are similar to those studied by Mendel. Some crosses do not give the expected Mendelian ratios of offspring.
These include:
Sex-linked inheritance
Co-dominance

36. Deviations from Mendelian inheritance and variations of Mendel’s ratios
SEX-LINKAGE
Sex-linked inheritance results from genes carried on either the male or female chromosome.
Females have a pair of similar sex chromosomes - genotype is XX,
Males have a pair of different sex chromosomes - genotype is XY. The Y chromosome seems to be missing part of its ‘arms’.
The X chromosome also carries a few genes that code for non-sexual body characteristics. These genes are termed sex-linked genes. Since males lack one X chromosome, they have only one allele for each sex-linked gene, rather than a pair of alleles.

37. Thomas Hunt Morgan and sex-linkage
See pages for test crosses
describe the work of Morgan that led to the understanding of sex-linkage
In 1910, Morgan began a series of breeding experiments with a small fruit fly (Drosophila melanogaster), which normally have red eyes, to answer questions about variations in inherited characteristics. He discovered a mutant male fruit fly that had white eyes. Morgan studied crosses between red and white eyed male and female flies and found that the offspring produced could not be explained by the Mendelian pattern of inheritance.
At first Morgan thought that female flies could not have white eyes.
To test this hypothesis he performed a typical ‘test cross’. He crossed a white-eyed male with a hybrid red-eyed female. His results showed in the F2 that both females and males could have white eyes.

38. Thomas Hunt Morgan and sex-linkage
At first Morgan thought that female flies could not have white eyes. To test this he performed a test cross. Between a white-eyed male with a hybrid red-eyed female. His results showed in the F2 that both females and males could have white eyes.
Morgan then arrived at his second hypothesis that the gene for the white-eye characteristic in Drosophila melanogaster is ‘sex-linked’ and carried on the X chromosome. This hypothesis withstood every possible test and explains sex-linked inheritance.

39. Thomas Hunt Morgan and sex-linkage
Morgan’s experiments on the fruit fly showed without any doubt that:
the gene for eye colour in fruit flies is located on the X chromosome
hereditary factors can be exchanged between the X chromosomes of an individual.
In humans: haemophilia and colour blindness are sex linked traits

40. See pages 179-180 for human example: ABO blood system
Co-dominance
explain the relationship between homozygous and heterozygous genotypes and the resulting phenotypes in examples of co-dominance
In genes of some organisms, pairs of alleles do not show dominance of one over the other. In a heterozygote where two different alleles for the same gene are present, both alleles are expressed as separate, unblended phenotypes and so they are termed co-dominant.
EXAMPLE
Pure-breeding (homozygous) cattle may have a red or white coat colour. Hybrid individuals (heterozygotes), which have one allele for red and one for white coat colour, have a roan appearance—both red and white hairs are present, not in patches but interspersed.
EXAMPLE IN HUMANS
Blood cells have proteins on their surfaces called antigens and these allow the person’s immune system to recognise them as ‘self’. Invading cells have different antigens, so the body recognises them as ‘foreign’. If the wrong blood group is given in a transfusion, the antigens on the surface of these blood cells are recognised as ‘foreign’ and the recipient’s body will produce antibodies to attack and fight these cells. The clumping reaction can block blood vessels and prove fatal.
The inheritance pattern of the ABO system of blood antigens shows co-dominance. There are 3 alleles in the population for this particular gene— alleles A and B code for the presence antigens A and B respectively, but allele O codes for no antigen. If both A and B alleles are present, blood cells have both A and B surface antigens—both alleles are expressed in each other’s presence. If neither antigen is present, the blood cells produce no antigen and are said to be group O.

41. Problems - Co-dominance and sex-linkage
See pages
solve problems involving co-dominance and sex-linkage

42. More examples on pages 184-5
Nature versus nurture - environmental effeCTS on gene expression (phenotype)
More examples on pages 184-5
outline ways in which the environment may affect the expression of a gene in an individual
Some variations in organisms are genetically determined (‘nature’), whereas others are influenced by the environment (‘nurture’). However, many variations arise as a result of an interaction between these two—the environment can influence how genes are expressed.
EXAMPLE
The effect of the environment on plant phenotype is seen in flower colour in hydrangeas. The acidity or alkalinity of the soil influences the colour of the flowers. Hydrangeas growing in acidic soil develop blue flowers, whereas those grown in alkaline soil develop pink flowers.

43. Investigating the effects of environment on phenotype
See page 185
identify data sources and perform a first-hand investigation to demonstrate the effect of environment on phenotype