GENETICS The nucleus of a cell carries the genetic information which allows it to control all of the activities of the cell and determines the overall characteristics of the organism nucleus
nucleus chromosomes The nucleus of a living cell contains threadlike structures called chromosomes. A chromosome is a threadlike structure which carries genetic information. All the nuclei of the body cells of a living organism contain identical copies of the chromosomes. nucleus chromosomes
Chromosome complement The number of chromosomes present in the cells of a living organism is called the chromosome complement and depends on the species. Humans have 46 chromosomes in every body cell.
Species Chromosome complement Horse 64 Sheep 54 Human 46 Mouse 40 Maize 20 Pea plant 14 Drosophila fruit fly 8
Chromosomes and Genes Chromosomes are made from tightly coiled molecules of DNA. DNA is a long chain made up of a backbone with bases attached. DNA = deoxyribonucleic acid!!
Label the following diagram DNA of 1 gene uncoiling DNA backbone Centromere bases Positions of individual genes DNA coiled into a chromosome = Adenine (A) = Guanine (G) = Thymine (T) = Cytosine (C)
There are 4 different types of base within a strand of DNA. - Adenine (A) - Thymine (T) - Guanine (G) - Cytosine (C) The DNA carries pieces of coded genetic information. An individual section of DNA with a single piece of genetic information is a gene. Chromosomes are thought of as lots of genes in a chain
The Function of DNA DNA molecules carry genetic instructions which allow the cell to make specific protein molecules. Proteins are made from amino acid units linked together to form long chains. The order of DNA bases encodes the information for the sequence of amino acids in proteins
An amino acid is a unit of protein structure. Sets of 3 BASES on a DNA strand carry the codes for…………. DNA backbone ….a chain of amino acids which join up to make a protein molecule. AMINO ACID R AMINO ACID S AMINO ACID T AMINO ACID S Protein molecule forming An amino acid is a unit of protein structure. A base is a part of DNA structure
DNA base sequence Amino acid coded for P This table shows the information about the base sequences for some amino acids Q R It is your job to decode the next few diagrams of pieces of DNA and to draw the chain of amino acids they encode. S T
1. 2. 3. 4. 5.
1. P Q R 2. R S T 3. T S P Q Q P 4. 5. T R P
Protein Structure & Function The chains of amino acids are folded and twisted to give the molecules 3-D shapes. The sequence of amino acids is determined by the sequence of the DNA bases. The sequence of the amino acids dictates the structure and function of the protein produced.
e.g. Enzymes Enzymes are made of protein. The folding of the chains of amino acids allows the formation of the active sites which makes the enzymes specific to their substrate.
Relationship between proteins present in a cell and the organisms characteristics Inherited characteristics are the result of many biochemical processes controlled by enzymes (which are made of protein!) In humans, enzymes control the reactions that lead to the formation of hair or a certain texture, eye irises of a certain colour etc.. The protein haemoglobin gives red blood cells their red colour. The body also possesses many hormones which are also made of protein. Hormones are chemical messengers around our body.
MEIOSIS Meiosis is the name given to the process which produce gametes (sex cells).
Chromosome revision centromere one chromatid single chromosome Double chromosome In cells that are not about to divide, chromosomes are found as single chromosomes. In cells that are about to divide, the DNA makes an extra copy of itself (shown above as the dark strand) and the chromosomes become double chromosomes. Each strand is called a chromatid and the chromatids are held together by a centromere.
meiosis fertilisation meiosis sperm cell zygote Sperm mother cell Egg mother cell Egg cell Zygote ready to divide
The Process of Meiosis 1) Gamete mother cell containing 4 double chromosomes 2) Matching chromosome pair and line up across the middle
3) The pairs separate to either end of the cell and the cell divides into 2 4) The chromosomes turn and line up. Each cell then divides again.
5) The centromeres split and the chromatids are pulled apart 5) The centromeres split and the chromatids are pulled apart. After the chromatids are separated from each other they are known as single chromosomes 6) 4 gametes are produced, each with only 1 set of single chromosomes.
Meiosis reduces the total number of sets of double chromosomes from 2 matching sets in the gamete mother cell to 1 set of single chromosomes in each gamete. SO: Gamete mother cell = 2 sets chromosomes Gametes = 1 set chromosomes The 2 sets of chromosomes are restored at fertilisation.
Chromosome shuffling The different ways that the matching chromosomes can pair increases the total number of gamete varieties. Any process which increases the number of different gametes must also increase the variety of offspring. The random assortment of chromosomes during meiosis leads to variation in offspring.
So instead of this arrangement You get this instead
Which results in 4 gametes like this
Because…..
Confused??!!!??!!
Sex Determination In humans, each male gamete has an X or a Y chromosome. So males are XY. Each female gamete has an X chromosome. So females are XX. The sex chromosomes of an individual determine their sex.
Genetic Symbols = male symbol = female symbol He’s a man, man! Stick in the ‘Sex chromosomes’ cut out
All egg cells will contain an X chromosome. Half of sperm cells will be X and half will be Y. It is the sperm cells that determine the sex of the baby – the egg will always be X but the sperm will either be X or Y.
female male XX XY X Y X X Baby girl XX XY Baby boy
Collect a sex determination grid and complete the blank squares to show the 4 possible combinations in the offspring. Use a crayon lightly to shade the boxes to show the male and female offspring. Complete the ratio information below the grid
The ratio of males to females is 1:1 But the process of fertilisation is random so it is a matter of chance which sperm will fertilise the egg – X or Y. It is for this reason that there will be roughly a 1:1 ratio of males to females.
Genetics for Beginners! Genes are parts of chromosomes Alleles are the different forms of a gene. Each gamete will carry one allele of the gene.
E.g. Gene for height in pea plants. Pea Plants can be tall or dwarf. Each plant will carry 2 copies of a gene, one from each parent. The alleles are represented by letters and will be T for tall and t for dwarf. A tall plant will either be TT or Tt A dwarf plant will be tt.
If a tall plant and a small plant cross, the offspring are all tall. This means that ‘Tall’ is dominant. ‘Dwarf’ is recessive. The dominant form of the gene always gets a capital letter e.g. T = tall. The recessive form of the gene always gets the same letter but lower case e.g. t = dwarf
X tall dwarf All tall
Complete the Symbols for Alleles Table Organism Gene Dominant allele Recessive allele Word Symbol Pea plant Height Tall T Dwarf t Human Eye colour Brown B Blue b Drosophila Wing length Long L Short l Maize Grain colour Purple P Yellow p Guinea pig Coat colour Black White
An individual with 2 of the same allele is said to be HOMOZYGOUS. (e.g. tt or TT) An individual with 2 different alleles of a gene it is said to be HETEROZYGOUS. (e.g. Tt) The genetic symbols an individual has is its GENOTYPE, e.g. Tt The physical appearance an individual has is its PHENOTYPE e.g. Tall
E.g. flower colour in pea plants A pea plant with lilac flowers was crossed with a white flowered plant. All offspring were lilac. x All lilac
Which is the dominant characteristic? What letter would we give the dominant allele? What letter would we give the recessive allele? What is the recessive characteristic? Lilac L l white
If a pea plant had the alleles ll – would the individual be homozygous or heterozygous? What colour would it be? If a plant had the alleles Ll – would the individual be homozygous or heterozygous? If a plant had the alleles LL – would the individual be homozygous or heterozygous? homozygous White heterozygous Lilac homozygous Lilac
Genotype and Phenotype An organism can have the same phenotype but have a different genotype. Example: Organism: Pea plants Gene height flower colour TT = tall LL = lilac Tt = tall Ll = lilac
True Breeding X X X X True breeding lilac strain True breeding white strain X Parents (P) X 1st generation (F1) X X Members of F1 cross All lilac All white 2nd generation (F2)
When 2 lilac parent plants cross, the offspring are all lilac When 2 lilac parent plants cross, the offspring are all lilac. When the lilac offspring cross, all their offspring are lilac. When 2 white parent plants cross, the offspring are all _______. When the ______ offspring cross, all their offspring are _______. So, when the flower colour of the offspring is identical to the parent flower colour, the members of the strain are true breeding. (they are always homozygous, e.g. LL or ll)
Terms for Monohybrid Crosses P = parents F1 = 1st filial generation. F2 = 2nd filial generation
Monohybrid Crosses A monohybrid cross is a cross that involves only one difference between the original parents, e.g. flower colour or height. Parents in monohybrid crosses are usually true breeding and show different phenotypes
How to do Monohybrid Crosses Question: A true breeding black mouse was crossed with a true breeding white mouse. All of the offspring were black. Show this as a monohybrid cross using appropriate symbols right through to the F2 generation X
X X P B b B b Black mouse White mouse Black white phenotype genotype F1 phenotype All Black Bb genotype F2 X Black Black phenotype Bb genotype Bb B b B b gametes
Punnet Square 1BB:2Bb:1bb 3 black: 1 white F2 genotypic ratio Sperm B b Eggs BB Bb bb 1BB:2Bb:1bb F2 genotypic ratio 3 black: 1 white F2 phenotypic ratio
Try for yourself…. A true breeding pea plant with round seeds was crossed with a true breeding pea plant with wrinkled seeds. All the F1 generation had round seeds. Show this as a monohybrid cross using appropriate symbols right through to the F2 generation. X
X X P phenotype genotype F1 phenotype All ________ genotype F2 gametes
Punnet Square Sperm Eggs : : F2 genotypic ratio : F2 phenotypic ratio
X X P R r R r Round seeds Wrinkled seeds Round wrinkled phenotype genotype RR rr F1 phenotype All Round Rr genotype F2 X Round Round phenotype Rr genotype Rr R r R r gametes
Punnet Square 1RR:2Rr:1rr 3 round: 1 wrinkled F2 genotypic ratio Sperm R r Eggs RR Rr rr 1RR:2Rr:1rr F2 genotypic ratio 3 round: 1 wrinkled F2 phenotypic ratio
Now do the Possible monohybrid crosses in mice sheet.
Observed vs Predicted Ratios Monohybrid crosses that we have seen so far, always produce a 3:1 ratio in the F2 generation. However, there is often a difference between the observed and predicted numbers of the different types of offspring as an exact 3:1 ratio rarely happens in nature as you can see from the table below…(stick in table) This is due to the fact that fertilisation is a random process involving an element of chance. We can show this by experiment…..
Bead Experiment ….flower colour in pea plants Pick a female and a male gamete at random and record your results in the table, then work out the ratio – is it 3:1? PP Pink Pp pp yellow
Co-dominance When 2 alleles of a gene are codominant this means that neither allele is dominant to the other. Both alleles are expressed equally in the phenotype of an organism with the heterozygous genotype. e.g. Coat colour in horses and cattle, feather colour in domestic fowl, flower colour in carnations.
x Black stallion White Mare All offspring grey roan Black coat is codominant to white coat. They are expressed equally and so offspring have coats with black and white hairs, these are called grey roans.
Genotypes in Co-Dominance Neither allele in co-dominance is recessive so neither symbol has a small letter. Both are capital letters since both alleles are equally dominant. Phenotype Genotype Black coat BB White coat WW Grey roan BW Red coat RR Red roan RW
Time to do some co-dominance problems…. Remember co-dominant alleles are expressed equally. They are equally dominant. Neither is recessive so both alleles have a capital letter.
Polygenic Inheritance This is when characteristics are controlled by the alleles of more than one gene. E.g. Skin colour in humans, seed mass in plants. The characteristics arise due to the interaction of the alleles of several genes.
Remember back to continuous and discontinuous variation?……. Discontinuous variation is controlled by a single gene and is an example of single-gene inheritance. Continuous variation is controlled by the alleles of more than one gene and is an example of polygenic inheritance.
Example of Polygenic Inheritance When a characteristic is controlled by 2 genes there may be 4 alleles working together. In maize, kernel colour is controlled by several genes, we will say 2 genes for this example. (Each gene will have 2 alleles)
Each gene has a dominant allele giving a red colour to the kernel and a recessive allele giving a white colour to the kernel. R1 = red R2 = red r1 = white r2 = white If a maize inherits all dominant alleles, (R1 R1 R2 R2), it will have very dark red kernels
If a maize inherits all recessive alleles (r1 r1 r2 r2) then it will have white kernels genotype R1 R1 R2 R2 x r1 r1 r2 r2 Gametes R1 R2 r1 r2 Parent Phenotypes Very dark red white R1 r1 R2 r2 F1 Genotype F1 Phenotype Medium red
If 2 of the F1 generation are crossed…. x R1 r1 R2 r2 R1 r1 R2 r2 Medium red Medium red ?? See your diagram of polygenic inheritance in maize and complete the missing genotypes of the offspring.
So….. The more genes there are for a particular characteristic, the more different phenotypes there are.
FAMILY TREES Family tree diagrams are set out in a standard way. Male = Female = The squares and symbols can be shaded in or left, depending on the phenotype. Parents are joined by a horizontal line Offspring are connected by a branched line Parents are joined to offspring by a vertical line.
B = brown eyes b = blue eyes Brown-eyed male Brown-eyed female Blue-eyed male Blue-eyed female 1 2 3 4 5 6 7 8 9 10 11 12 13 B = brown eyes b = blue eyes
COMPLETE THESE STATEMENTS a) The phenotype of person 2 is:- b) The phenotype of person 3 is:- c) The genotype of person 1 is:- d) The genotype of person 4 is:- e) Person 7 is likely to be homozygous dominant because…. f) The genotype of person 8 is…. g) The genotype of person 9 cannot be stated with certainty because… Blue-eyed female Brown-eyed male Bb Bb All offspring are brown eyed. bb Could be Bb or BB
Environmental Impact on Phenotype The final appearance of an organism is the result of its genotype and the effects of the environment. If organisms of identical genotype are subject to different environmental conditions they show considerable variation (differences). These changes are not genetic so they are not passed on from one generation to the next.
Twins reared together Twins separated at birth 1 twin fed healthy diet – reached full potential height 1 twin fed poor diet – did not reach full potential height Both twins fed same healthy diet – both reach full potential height.
So: The environment has an effect on our overall phenotype… Genotype + Environment = Phenotype