Genetics 2 Genetic Crosses

2 Contents Gregor Mendel Gregor Mendel Mendel’s experiments Mendel’s experiments Mendel’s results Mendel’s results Mendel’s conclusions Mendel’s conclusions Law of segregation Law of segregation Dihybrid crosses Dihybrid crosses Law of Independent Assortment Law of Independent Assortment Linked genes Linked genes Crosses with linked genes Crosses with linked genes Sex-linked genes Sex-linked genes Inheritance of a sex-linked trait in humans Inheritance of a sex-linked trait in humans Non-nuclear inheritance DNA – more information A nucleotide The four Possible Bases Base Pairing The DNA double helix Protein synthesis – more information How mRNA is made A tRNA molecule Translation The production of protein molecules from DNA

3 Gregor Mendel The Father of genetics. A monk, mathematician & scientist. Did experiments using pea plants in the monestery garden. Published his findings in First to approach genetics in a scientific manner.

4 Mendel’s experiments He noticed that pea plants had contrasting traits e.g. tall versus short, yellow veersus green seed, etc. He used pure breeding plants i.e. when crossed with one another they always produced offspring identical to themselves. He called these plants the P (parental) generation. He examined only one contrasting trait at a time. This is a monohybrid cross.

5 He crossed pure breeding tall with pure breeding short (dwarf) plants. He removed anthers from tall plants and dusted the stigma of these tall plants with pollen from the short plants. (see drawing on next slide) see drawing on next slidesee drawing on next slide This prevented self-pollination (the usual method in pea plants). He collected the seeds produced – planted them – and examined their size. All plants were tall. He called this the F 1 generation. The result was the same whether the male or female plant was tall or short.

6 The structure of a typical flower Back to previous slide

7 This was not the expected result. He then looked to see if the information for short plants had disappeared. How? He let the F1 plants self-fertilise. He collected the seeds produced – planted them – and examined their size. He called this the F2 generation. He got 787 tall and 277 short plants. Analysed these and results from other crosses of contrasting traits and noticed that there was an approximate 3:1 ratio.

8 Mendel’s results (1/2) TraitDominantVRecessive F2 Results Ratio DomRec Flower colour PurpleXWhite :1 Seed colour YellowXGreen6,0222, :1 Seed shape RoundXWrinkled5,4741, :1 Pod colour GreenXYellow :1

9 Mendel’s results (2/2) TraitDominantVRecessive F2 Results Ratio DomRec Pod colour GreenXYellow :1 Pod shape RoundXConstricted :1 Flower position AxialXTerminal :1 Plant height TallXDwarf :1

10 Mendel’s conclusions Each plant contains two ‘factors’ that control each trait. There are two alternative forms of each factor - one is dominant (tall) & the other recessive (small). The dominant factor is always expressed, when present, whether there is one or two copies of it in the organism. The recessive factor is only expressed when there are two copies present in the organism. Could you rewrite these conclusions using modern terms?

11 Mendel suggested that factors are transmitted from parent to offspring via the gametes. the F 1 plants had one copy of each factor (one factor coming from each parent). This led to his first law, the Law of Segregation

12 The Law of Segregation Mendel stated that Mendel stated that organisms contain two factors for every trait which separate during gamete formation producing gametes with only one copy of each factor. The modern definition states that The modern definition states that characters (traits) are controlled by pairs of genes (e.g. Tt) that separate (segregate) at gamete formation, and each gamete carries only one gene for the trait.

13 The modern explanation for Mendel’s F 1 cross

14 The modern explanation for Mendel’s F 2 cross

15 Dihybrid crosses These involve the study of the inheritance of two pairs of contrasting traits e.g. he crossed a pure breeding tall plant with purple flowers with a pure breeding small plant with white flowers. All the F1 plants were tall with purple flowers. These were allowed to self fertilise. The F2 produced four different phenotypes -

16 The phenotypes produced - Tall purple 96 - Tall white 31 - Short purple 34 - Short white 11 This gives a ratio of 9:3:3:1 From this mendel formulated his second law, the Law of Independent Assortment.

17 The explanation (1/2)

18 The explanation (2/2)

19 Law of Independent Assortment Mendel stated that Mendel stated that alleles of any one gene are transmitted independently of any other pair of alleles. The modern definition states that The modern definition states that during the formation of gametes each member of a pair of genes may combine randomly with either of another pair.

20 Worked example In pea plants yellow seed colour is dominant to green and round seed shape is dominant to wrinkled. What results would be expected in a cross between a pea plant heterozygous for seed colour and seed shape and a plant homozygous recessive for both traits?

21 ParentsHeterozygous Yellow & round Green+ wrinkled Parental genotype gametes F 1 genotype F 1 phenotype Ratio YyRryyrr YRYryR yr All yr YyRrYyrryyRr yyrr Yellow round Yellow wrinkled Green round Green wrinkled

22 Linked genes (1/2) An example of this is seen in the fruit fly. They can have long (L) or vestigial / short (l) wings and broad (B) or narrow (b) abdomens. If a heterozygous long winged, broad abdomen is crossed with another similar fly all the F 1 offspring would be long winged with broad abdomen or vestigial winged with narrow abdomen. No other types of offspring are produced.

23 Linked genes (2/2) The reason for this is that the genes for both traits are on the same chromosome and will be inherited together. These are linked genes. Definition: Linked genes are genes on the same chromosome that are not separated at gamete formation and are inherited together.

24 A cross demonstrating linked genes

25 Another example (1/2) In the fruit fly, normal antennae and grey body are linked genes. The recessive alleles are also linked and produce twisted antennae and black bodied flies. A grey fly with normal antennae was crossed with a black bodied fly with twisted antennae. All the resultant flies were grey with normal antennae.

26 Another example (2/2) When these F 1 flies were crossed with black flies with twisted antennae, an equal number of black, twisted antennae flies and grey, normal antennae flies were produced. Explain these results. The genes are linked. Use a chromosome diagram. The parental genotypes are:

27 Diagram representing F 1 cross

28 Explanation (1/2) This fits in with Mendel’s expected results. But, when we cross these F 1 flies with the black, twisted antennae flies, Mendel would have expected four possible results. This does not happen as the genes do not assort independently of each other into the gametes. N and G travel together as do n and g.

29 Explanation (2/2) There are only two types of gamete from the F1 flies. What are they? As a result there are only two possible types of flies in the F2 generation (not four as would be predicted by Mendel).

30 Diagram representing F 2 cross

31 Sex-linked Genes XX – Female XY – Male The X chromosome carries more genes than the Y chromosome. A male has only one copy of many genes on his X chromosome. He has no matching gene (allele) on the Y chromosome. These genes are sex-linked or X-linked.

32 Sex-linked genes – definition & examples Are genes found on the X chromosome with no corresponding gene (allele) on the Y chromosome. Examples of sex-linked traits are: - Haemophilia, and Haemophilia, and Red/green colour blindness in humans. Red/green colour blindness in humans. Eye colour in Drosophila melonagaster (Fruit fly). Eye colour in Drosophila melonagaster (Fruit fly).

33 Sex-linked genes – some facts If a male has just one recessive gene for a sex- linked trait he will express the phenotype of that trait. A male can only pass this gene on to his daughters. There is no male to male transmission of sex-linked traits. Males with a sex-linked condition got the recessive gene from their mother. Females with one recessive gene for the trait are carriers of the condition and are phenotypically normal.

34 Inheritance of a sex-linked trait in humans

35 Question Can females be colour blind? Explain your answer using chromosome diagrams.

36 Non-nuclear inheritance (1/2) A male gamete (sperm) is little more than a motile nucleus. A female gamete (egg) contains a cell as well as a nucleus. The new individual inherits this cell also at fertilisation. DNA is found in cellular organelles other than the nucleus e.g. mitochondria. These structures are inherited from the female only.

37 Non-nuclear inheritance (2/2) When the cell divides the mitochondrial DNA is replicated and passed on to the next generation. Non-nuclear DNA does not undergo meiosis or fertilisation during sexual reproduction.  so some parts of the offspring’s cells get all of their genetic information from the mother only.

38 DNA – more information DNA is composed of nucleotides. A nucleotide is composed of three basic chemicals A five carbon sugar – deoxyribose A five carbon sugar – deoxyribose A phosphate group, and A phosphate group, and One of four possible nitrogenous bases One of four possible nitrogenous bases

39 A nucleotide

40 The four Possible Bases AdenineA Purine bases – Double ringed GuanineG CytosineC Pyramidine basses – Single ringed ThymineT

41 A double strand of DNA showing bonding

42 A simple diagram of DNA

43 Base pairing Because of the structure of each base, bonding between bases is specific i.e. A only with T and G only with C. These are known as complimentary base pairs. The double strand of DNA coils around to form a double helix.

44 The DNA double helix

45 Protein Synthesis – more info. It is the order of the bases in the DNA that determine the order of the amino acids in a protein. It is the order of the bases in the DNA that determine the order of the amino acids in a protein. Each group of three bases is a triplet. Each group of three bases is a triplet. Each triplet codes for a particular amino acid. Each triplet codes for a particular amino acid. DNA is found in the nucleus only. DNA is found in the nucleus only. Proteins made at ribosomes in cytoplasm. Proteins made at ribosomes in cytoplasm. Another type of nucleic acid, messenger RNA (mRNA), carries the message (instructions) from the nucleus to the ribosomes. Another type of nucleic acid, messenger RNA (mRNA), carries the message (instructions) from the nucleus to the ribosomes.

46 How is mRNA made? Enzymes unwind or unzip the part of the DNA molecule containing the information needed to make the protein. Enzymes unwind or unzip the part of the DNA molecule containing the information needed to make the protein. Transcription occurs next i.e. RNA nucleotide bases (A, G, C & U) bond with one strand of exposed DNA. Transcription occurs next i.e. RNA nucleotide bases (A, G, C & U) bond with one strand of exposed DNA. The enzyme RNA polymerase assembles these bases to form mRNA. The enzyme RNA polymerase assembles these bases to form mRNA. mRNA, therefore, has a series of bases that are complimentary to those in DNA. mRNA, therefore, has a series of bases that are complimentary to those in DNA.

47 What happens next? mRNA leaves the nucleus via the nuclear pores and travels to and attaches itself to the ribosomes (made of ribosomal RNA - rRNA) mRNA leaves the nucleus via the nuclear pores and travels to and attaches itself to the ribosomes (made of ribosomal RNA - rRNA) At the ribosome the mRNA code is matched by nucleotides of transfer RNA (tRNA). At the ribosome the mRNA code is matched by nucleotides of transfer RNA (tRNA). Each tRNA carries a specific amino acid in the correct sequence to the ribosome. Each tRNA carries a specific amino acid in the correct sequence to the ribosome. They are attached by their ‘binding site’ to complementary mRNA already attached to the ribosome. They are attached by their ‘binding site’ to complementary mRNA already attached to the ribosome.

48 A tRNA nucleotide

49 Translation This ensures the amino acids are aligned in the sequence determined by the codons of the mRNA. This ensures the amino acids are aligned in the sequence determined by the codons of the mRNA. The amino acids are then bonded together to form the new protein molecule. The amino acids are then bonded together to form the new protein molecule. This process of manufacture of the proteins is called translation. This process of manufacture of the proteins is called translation. tRNAs continue to move to the ribosome, until a stop codon on the mRNA is reached. tRNAs continue to move to the ribosome, until a stop codon on the mRNA is reached. The protein is released when the mRNA code sequence is complete and the protein is synthesised. The protein is released when the mRNA code sequence is complete and the protein is synthesised.

50 The production of protein molecules from DNA

51 END